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作者：Anonymous 在 罕见奇谈 发贴, 来自 http://www.hjclub.org
Molecular biology
The field of molecular biology has emerged during the mid-20th
century. This new discipline has unveiled the nature of hereditary
material and the workings of organisms at the level of enzymes and
other molecules. Molecular biology provides the most detailed and
convincing evidence available for biological evolution.
It is now known that the hereditary material, DNA, and the enzymes
that govern all life processes hold information about an organism's
ancestry. This information has made it possible to reconstruct
evolutionary events that were previously unknown and to confirm and
adjust the view of events that already were known. The precision with
which events of evolution can be reconstructed is one reason the
evidence from molecular biology is so compelling. Another reason is
that molecular evolution has shown all living organisms, from bacteria
to humans, to be related by descent from common ancestors.
A remarkable uniformity exists in the molecular components of
organisms--in the nature of the components as well as in the ways in
which they are assembled and used. In all bacteria, plants, animals,
and humans, the DNA comprises a different sequence of the same four
component nucleotides, and all of the various proteins are synthesized
from different combinations and sequences of the same 20 amino acids,
although several hundred other amino acids do exist. The genetic
"code" by which the information contained in the nuclear DNA is passed
on to proteins is everywhere the same. Similar metabolic pathways are
used by the most diverse organisms to produce energy and to make up
the cell components.
This unity reveals the genetic continuity and common ancestry of all
organisms. There is no other rational way to account for their
molecular uniformity when numerous alternative structures are equally
likely. The genetic code may serve as an example. Each particular
sequence of three nucleotides in the nuclear DNA acts as a pattern, or
code, for the production of exactly the same amino acid in all
organisms. This is no more necessary than it is for a language to use
a particular combination of letters to represent a particular reality.
If it is found that certain sequences of letters--planet, tree,
woman--are used with identical meanings in a number of different
books, one can be sure that the languages used in those books are of
common origin.
Genes and proteins are long molecules that contain information in the
sequence of their components in much the same way as sentences of the
English language contain information in the sequence of their letters
and words. The sequences that make up the genes are passed on from
parents to offspring, identical except for occasional changes
introduced by mutations. To illustrate, assume that two books are
being compared; both books are 200 pages long and contain the same
number of chapters. Closer examination reveals that the two books are
identical page for page and word for word, except that an occasional
word--say one in 100--is different. The two books cannot have been
written independently; either one has been copied from the other or
both have been copied, directly or indirectly, from the same original
book. Similarly, if each nucleotide is represented by one letter, the
complete sequence of nucleotides in the DNA of a higher organism would
require several hundred books of hundreds of pages, with several
thousand letters on each page. When the "pages" (or sequence of
nucleotides) in these "books" (organisms) are examined one by one, the
correspondence in the "letters" (nucleotides) gives unmistakable
evidence of common origin.
The arguments presented above are based on different grounds, although
both attest to evolution. Using the alphabet analogy, the first
argument says that languages that use the same dictionary--the same
genetic code and the same 20 amino acids--cannot be of independent
origin. The second argument, concerning similarity in the sequence of
nucleotides in the DNA or the sequence of amino acids in the proteins,
says that books with very similar texts cannot be of independent
origin.
The evidence of evolution revealed by molecular biology goes one step
further. The degree of similarity in the sequence of nucleotides or of
amino acids can be precisely quantified. For example, cytochrome c (a
protein molecule) of humans and chimpanzees consists of the same 104
amino acids in exactly the same order; but differs from that of rhesus
monkeys by one amino acid, that of horses by 11 additional amino
acids, and that of tuna by 21 additional amino acids. The degree of
similarity reflects the recency of common ancestry. Thus, the
inferences from comparative anatomy and other disciplines concerning
evolutionary history can be tested in molecular studies of DNA and
proteins by examining their sequences of nucleotides and amino acids.
The authority of this kind of test is overwhelming; each of the
thousands of genes and thousands of proteins contained in an organism
provides an independent test of that organism's evolutionary history.
Not all possible tests have been performed, but many hundreds have
been done, and not one has given evidence contrary to evolution. There
is probably no other notion in any field of science that has been as
extensively tested and as thoroughly corroborated as the evolutionary
origin of living organisms.
The process of evolution
Evolution as a genetic function
The concept of natural selection
The central argument of Darwin's theory of evolution starts from the
existence of hereditary variation. Experience with animal and plant
breeding demonstrates that variations can be developed that are
"useful to man." So, reasoned Darwin, variations must occur in nature
that are favourable or useful in some way to the organism itself in
the struggle for existence. Favourable variations are ones that
increase chances for survival and procreation. Those advantageous
variations are preserved and multiplied from generation to generation
at the expense of less advantageous ones. This is the process known as
natural selection. The outcome of the process is an organism that is
well adapted to its environment, and evolution often occurs as a
consequence.
Natural selection, then, can be defined as the differential
reproduction of alternative hereditary variants, determined by the
fact that some variants increase the likelihood that the organisms
having them will survive and reproduce more successfully than will
organisms carrying alternative variants. Selection may be due to
differences in survival, in fertility, in rate of development, in
mating success, or in any other aspect of the life cycle. All of these
differences can be incorporated under the term "differential
reproduction" because all result in natural selection to the extent
that they affect the number of progeny an organism leaves.
Darwin maintained that competition for limited resources results in
the survival of the most effective competitors. But natural selection
may occur not only as a result of competition but also as a result of
some aspect of the physical environment, such as inclement weather.
Moreover, natural selection would occur even if all the members of a
population died at the same age, simply because some of them would
have produced more offspring than others. Natural selection is
quantified by a measure called Darwinian fitness, or relative fitness.
Fitness in this sense is the relative probability that a hereditary
characteristic will be reproduced; that is, the degree of fitness is a
measure of the reproductive efficiency of the characteristic.
Biological evolution is the process of change and diversification of
living things over time, and it affects all aspects of their
lives--morphology, physiology, behaviour, and ecology. Underlying
these changes are changes in the hereditary materials. Hence, in
genetic terms, evolution consists of changes in the organism's
hereditary makeup.
Evolution can be seen as a two-step process. First, hereditary
variation takes place; second, selection is made of those genetic
variants that will be passed on most effectively to the following
generations. Hereditary variation also entails two mechanisms: the
spontaneous mutation of one variant to another, and the sexual process
that recombines those variants to form a multitude of variations. The
variants that arise by mutation or recombination are not transmitted
equally from one generation to another. Some may appear more
frequently because they are favourable to the organism; the frequency
of others may be determined by accidents of chance, called genetic
drift.
Genetic variation in populations
The gene pool
The gene pool is the sum total of all of the genes and combinations of
genes that occur in a population of organisms of the same species. It
can be described by citing the frequencies of the alternative genetic
constitutions. Consider, for example, a particular gene (which
geneticists call a locus), such as the one determining the MN blood
groups in humans. One form of the gene codes for the M blood group,
while the other form codes for the N blood group; different forms of
the same gene are called alleles. The gene pool of a particular
population is specified by giving the frequencies of the alleles M and
N. Thus, in the United States the M allele in Caucasoids occurs with a
frequency of 0.539 and the N allele with a frequency of 0.461. In
other populations, these frequencies are different; the frequency of
the M allele is 0.917 in Navajo Indians and 0.178 in Australian
Aborigines.
The necessity of hereditary variation for evolutionary change to occur
can be understood in terms of the gene pool. Assume, for instance,
that at the gene locus that codes for the MN blood groups there is no
variation; only the M allele exists in all individuals. Evolution of
the MN blood groups cannot take place in such a population, since the
allelic frequencies have no opportunity to change from generation to
generation. On the other hand, in populations in which both alleles M
and N are present, evolutionary change is possible.
Genetic variation and rate of evolution
The more genetic variation that exists in a population, the greater
the opportunity for evolution to occur. As the number of gene loci
that are variable increases and as the number of alleles at each locus
becomes greater, the likelihood that some alleles will change in
frequency at the expense of their alternates grows. The British
geneticist R.A. Fisher mathematically demonstrated a direct
correlation between the amount of genetic variation in a population
and the rate of evolutionary change by natural selection. This
demonstration is embodied in his fundamental theorem of natural
selection (1930): "The rate of increase in fitness of any organism at
any time is equal to its genetic variance in fitness at that time."
This theorem has been confirmed experimentally. One study employed
different strains of Drosophila serrata, a species of vinegar fly from
eastern Australia and New Guinea. Evolution in vinegar flies can be
investigated by using "population cages" and finding out how a
population changes over many generations. Experimental populations
were set up, with the flies living and reproducing in isolated
microcosms. Single-strain populations descended from flies collected
either in Popondetta, New Guinea, or in Sydney, Australia; and a mixed
population was established by crossing these two strains of flies. The
mixed population had the greater initial genetic variation, since it
was started by combining two different single-strain populations. To
encourage rapid evolutionary change, the populations were manipulated
in such a way that there was intense competition among the flies for
food and space. Adaptation to the experimental environment was
measured by periodically counting the number of individuals in the
populations.
Two results deserve notice. First, the mixed population had, at the
end of the experiment, more flies than the single-strain populations.
Second, and more relevant, the number of flies increased at a faster
rate in the mixed population than in the single-strain populations.
Evolutionary adaptation to the environment occurred in both types of
population; both were able to maintain higher numbers as the
generations progressed. But the rate of evolution was more rapid in
the mixed group than in the single-strain groups. The greater initial
amount of genetic variation made possible a faster rate of evolution.
Measuring gene variability
Because a population's potential for evolving is determined by its
genetic variation, evolutionists are interested in discovering the
extent of such variation in natural populations. It is readily
apparent that plant and animal species are heterogeneous in all sorts
of ways; in the flower colours and growth habits of plants, for
instance, or the shell shapes and banding patterns of snails.
Differences are more readily noticed among humans--in facial features,
hair and skin colour, height and weight--but morphological differences
are present in all groups of organisms. One problem with morphological
variation is that it is not known how much is due to genetic factors
and how much may result from environmental influences.
Animal and plant breeders select for their experiments individuals or
seeds that excel in desired attributes--in the protein content of
corn, for example, or the milk yield of cows. The selection is
repeated generation after generation. If the population changes in the
direction favoured by the breeder, it becomes clear that the original
stock possessed genetic variation with respect to the selected trait.
The results of artificial selection are impressive. Selection for high
oil content in corn (maize) increased the oil content from less than 5
percent to more than 19 percent in 76 generations, while selection for
low oil content reduced it to below 1 percent. Thirty years of
selection for increased egg production in a flock of White Leghorn
chickens increased the average yearly output of a hen from 125.6 to
249.6 eggs. Artificial selection has produced an endless variety of
dog, cat, and horse breeds. The plants that humans grow for food and
fibre and the animals they breed for food and transportation are all
products of age-old or modern-day artificial selection.
The success of artificial selection for virtually every trait and
every organism in which it has been tried suggests that genetic
variation is pervasive throughout natural populations. But
evolutionists like to go one step further and obtain quantitative
estimates. Only since the 1960s, with the advances of molecular
biology, have geneticists developed methods for measuring the extent
of genetic variation in populations or among species of organisms.
These methods consist essentially of taking a sample of genes and
finding out how many are variable and how variable each one is. One
simple way of measuring the variability of a gene locus is to
ascertain what proportion of the individuals in a population are
"heterozygotes" at that locus. In a heterozygous individual, the two
genes for a trait, one received from the mother and the other from the
father, are different. The proportion of heterozygotes in the
population is, therefore, the same as the probability that two genes
taken at random from the gene pool are different.
Techniques for determining heterozygosity have been used to
investigate numerous species of plants and animals. Typically, insects
and other invertebrates are more varied genetically than mammals and
other vertebrates; and plants bred by outcrossing exhibit more
variation than those bred by self-pollination. But the amount of
genetic variation is in any case astounding. Consider as an example
humans, whose level of variation is about the same as that of other
mammals. The human heterozygosity value is stated as H = 0.067, which
means that an individual is heterozygous at 6.7 percent of his genes.
It is not known how many gene loci there are in humans, but estimates
range from 30,000 to 100,000. Assuming the lower estimate, a person
would be heterozygous at 30,000 ´ 0.067 = 2,010 gene loci. An
individual heterozygous at one locus (Aa) can produce two different
kinds of sex cells, or gametes, one with each allele (A and a); an
individual heterozygous at two loci (AaBb) can produce four kinds of
gametes (AB, Ab, aB, and ab); an individual heterozygous at n loci can
potentially produce 2^n different gametes. Therefore, a typical human
individual has the potential to produce 2^2,010, or approximately
10^605 (1 with 605 zeros following), different kinds of gametes. But
that number is much larger than the estimated number of atoms in the
universe, 10^76, which is trivial by comparison.
It is clear, then, that every sex cell produced by a human being is
genetically different from every other sex cell and, therefore, that
no two persons who ever existed or will ever exist are likely to be
genetically identical--with the exception of identical twins, which
develop from a single fertilized ovum. The same conclusion applies to
all organisms that reproduce sexually; every individual represents a
unique genetic configuration that will never be repeated again. This
enormous reservoir of genetic variation in natural populations
provides virtually unlimited opportunities for evolutionary change in
response to the environmental constraints and the needs of the
organisms.
The origin of genetic variation: mutations
Life originated about 3,500,000,000 years ago in the form of
primordial organisms that were very simple and very small. All living
things have evolved from these lowly beginnings. At present there are
more than 2,000,000 known species, which are widely diverse in size,
shape, and way of life, as well as in the DNA sequences that contain
their genetic information. What has produced the pervasive genetic
variation within natural populations and the genetic differences among
species? There must be some evolutionary means by which existing DNA
sequences are changed and new sequences are incorporated into the gene
pools of species.
The information encoded in the nucleotide sequence of DNA is, as a
rule, faithfully reproduced during replication, so that each
replication results in two DNA molecules that are identical to each
other and to the parent molecule. But heredity is not a perfectly
conservative process; otherwise, evolution could not have taken place.
Occasionally "mistakes," or mutations, occur in the DNA molecule
during replication, so that daughter cells differ from the parent
cells in the sequence or in the amount of DNA. A mutation first
appears on a single cell of an organism, but it is passed on to all
cells descended from the first. Mutations can be classified into two
categories: gene, or point, mutations, which affect only a few
nucleotides within a gene; and chromosomal mutations, which either
change the number of chromosomes or change the number or arrangement
of genes on a chromosome.
Gene mutations
A gene mutation occurs when the nucleotide sequence of the DNA is
altered and a new sequence is passed on to the offspring. The change
may be either a substitution of one or a few nucleotides for others or
an insertion or deletion of one or a few pairs of nucleotides.
The four nucleotide bases of DNA are represented by the letters A, C,
G, and T. A gene that bears the code for constructing a protein
molecule consists of a sequence of several thousand nucleotides, so
that each segment of three nucleotides--called a codon--codes for one
particular amino acid in the protein. The nucleotide sequence in the
DNA is first transcribed onto a molecule of messenger RNA (ribonucleic
acid). The RNA, with a slightly different code (represented by the
letters A, C, G, and U), bears the message that determines which amino
acid will be inserted into the protein's chain. Substitutions in the
nucleotide sequence of a structural gene may result in changes in the
amino acid sequence of the protein, although this is not always the
case. The genetic code is redundant in that different triplets may
hold the code for the same amino acid. Consider the triplet UUA in the
messenger RNA, which codes for the amino acid leucine. If the first U
is replaced by C, the triplet will still code for leucine; but if it
is replaced by G, it will code for valine instead.
A nucleotide substitution in the DNA that results in an amino acid
substitution in the corresponding protein may or may not severely
affect the biological function of the protein. Some nucleotide
substitutions change a codon for an amino acid into a stop signal, and
those mutations are likely to have harmful effects. If, for instance,
the second U in the UUA triplet is replaced by A, the triplet becomes
UAA, a "terminator" codon; the result is that the following triplets
in the DNA sequence are not translated into amino acids.
Additions or deletions of nucleotide pairs within the DNA sequence of
a structural gene often result in a greatly altered sequence of amino
acids in the coded protein. The addition or deletion of one or two
nucleotide pairs shifts the "reading frame" of the nucleotide sequence
all along the way from the point of the insertion or deletion to the
end of the molecule. To illustrate, assume that a DNA segment is read
as . . . CAT-CAT-CAT-CAT-CAT. . . . If a nucleotide base, say T, is
inserted after the C of the first triplet, the segment will then be
read as . . . CTA-TCA-TCA-TCA-TCA. . . . From the point of the
insertion onward, the sequence of encoded amino acids is altered. If,
however, a total of three nucleotide pairs is either added or deleted,
the original reading frame will be restored in the rest of the
sequence. Additions or deletions of nucleotide pairs in numbers other
than three or multiples of three are called frameshift mutations.
Gene mutations can occur spontaneously; that is, without being
intentionally caused by humans. They can also be induced by
ultraviolet light, X rays, and other high-frequency radiations, as
well as by exposure to certain mutagenic chemicals, such as mustard
gas. The consequences of gene mutations may range from negligible to
lethal. Mutations that change one or even several amino acids may have
a small or undetectable effect on the organism's ability to survive
and reproduce if the essential biological function of the coded
protein is not hindered. But where an amino acid substitution affects
the active site of an enzyme or modifies in some other way an
essential function of a protein, the impact may be severe.
Newly arisen mutations are more likely to be harmful than beneficial
to their carriers, because mutations are random events with respect to
adaptation; that is, their occurrence is independent of any possible
consequences. The allelic variants present in an existing population
have already been subject to natural selection. They are present in
the population because they improve the adaptation of their carriers,
and their alternative alleles have been eliminated or kept at low
frequencies by natural selection. A newly arisen mutant is likely to
have been preceded by an identical mutation in the previous history of
a population; if the previous mutant no longer exists in the
population, that will be a sign that the new mutant is not beneficial
to the organism and is likely to be eliminated also.
This proposition can be illustrated with an analogy. Consider an
English sentence, whose words have been chosen because together they
express a certain idea. If single letters or words are replaced with
others at random, most changes will be unlikely to improve the meaning
of the sentence; very likely they will destroy it. The nucleotide
sequence of a gene has been "edited" into its present form by natural
selection because it "makes sense." If the sequence is changed at
random, the "meaning" rarely will be improved and often will be
hampered or destroyed.
Occasionally, however, a new mutation may increase the organism's
adaptation. The probability of such an event's happening is greater
when organisms colonize a new territory or when environmental changes
confront a population with new challenges. In these cases, the
established adaptation of a population is less than optimal, and there
is greater opportunity for new mutations to be better adaptive. The
consequences of mutations depend on the environment. Increased melanin
pigmentation may be advantageous to inhabitants of tropical Africa,
where dark skin protects them from the Sun's ultraviolet radiation;
but it is not beneficial in Scandinavia, where the intensity of
sunlight is low and light skin facilitates the synthesis of vitamin D.
Mutation rates have been measured in a great variety of organisms,
mostly for mutants that exhibit conspicuous effects. Mutation rates
are generally lower in bacteria and other microorganisms than in more
complex species. In humans and other multicellular organisms, the rate
typically ranges from about one per 100,000 to one per 1,000,000
gametes. There is, however, considerable variation from gene to gene
as well as from organism to organism.
Although mutation rates are low, new mutants appear continuously in
nature, because there are many individuals in every species and many
gene loci in every individual. The process of mutation provides each
generation with many new genetic variations. Thus, it is not
surprising to see that when new environmental challenges arise,
species are able to adapt to them. More than 200 insect and rodent
species, for example, have developed resistance to the pesticide DDT
in different parts of the world where spraying has been intense.
Although the insects had never before encountered this synthetic
compound, they adapted to it rapidly by means of mutations that
allowed them to survive in its presence. Similarly, many species of
moths and butterflies in industrialized regions have shown an increase
in the frequency of individuals with dark wings in response to
environmental pollution, an adaptation known as industrial melanism.
Chromosomal mutations
The chromosomes, which carry the hereditary material, or DNA, are
contained in the nucleus of each cell. Chromosomes come in pairs, with
one member of each pair inherited from each parent. The two members of
a pair are called homologous chromosomes. Each cell of an organism and
all individuals of the same species have, as a rule, the same number
of chromosomes. The reproductive cells (gametes) are an exception;
they have only half as many chromosomes as the body (somatic) cells.
But the number, size, and organization of chromosomes varies among
species. The parasitic nematode Parascaris univalens has only one pair
of chromosomes, whereas many species of butterflies have more than 100
pairs and some ferns more than 600. Even closely related organisms may
vary considerably in the number of chromosomes; species of spiny rats
of the South American genus Proechimys range from 12 to 31 chromosome
pairs. Changes in the number, size, or organization of chromosomes are
termed chromosomal mutations, chromosomal abnormalities, or
chromosomal aberrations.
Changes in the number of chromosomes may occur by fusion of two
chromosomes into one, by fission of one chromosome into two, or by
addition or subtraction of one or more whole chromosomes or sets of
chromosomes (polyploidy). Changes in the structure of chromosomes may
occur by inversion, when a chromosomal segment rotates 180° within the
same location; by duplication, when a segment is added; by deletion,
when a segment is lost; or by translocation, when a segment changes
from one location to another in the same or a different chromosome.
These are the processes by which chromosomes evolve. Inversions,
translocations, fusions, and fissions do not change the amount of DNA.
The importance of these mutations in evolution is that they change the
linkage relationships between genes. Genes that were closely linked to
each other become separated and vice versa.
Dynamics of genetic change
Genetic equilibrium: the Hardy-Weinberg law
Genetic variation is present throughout natural populations of
organisms. This variation is sorted out in new ways in each generation
by the process of sexual reproduction, which recombines the
chromosomes inherited from the two parents during the formation of the
gametes that produce the following generation. But heredity by itself
does not change gene frequencies. This principle is stated by the
Hardy-Weinberg law, so called because it was independently discovered
in 1908 by the English mathematician G.H. Hardy and the German
physician Wilhelm Weinberg.
The Hardy-Weinberg law describes the genetic equilibrium in a
population by means of an algebraic equation. It states that genotypes
(the genetic constitution of individual organisms) exist in certain
frequencies that are a simple function of the allelic frequencies;
namely, the square expansion of the sum of the allelic frequencies.
If there are two alleles, A and a, at a gene locus, three genotypes
will be possible, AA, Aa, and aa. If the frequencies of the two
alleles are p and q, respectively, the equilibrium frequencies of the
three genotypes will be given by (p + q)2 = p2 + 2pq + q2, for AA, Aa,
and aa, respectively. The genotype equilibrium frequencies for any
number of alleles are derived in the same way. If there are three
alleles, A[1], A[2], and A[3], with frequencies p, q, and r, the
equilibrium frequencies corresponding to the six possible genotypes
(shown in parentheses) will be calculated as follows:
Special Comp
Table
Encyclopædia Britannica, Inc.
Table 1 shows how the law operates in a situation with just two
alleles. At the top and to the left are the frequencies in the
parental generation of the two alleles, p for A and q for a. As shown
at the lower right, the probabilities of the three possible genotypes
in the following generation are products of the probabilities of the
corresponding alleles in the parents. The probability of genotype AA
among the progeny is the probability p that allele A will be present
in the paternal gamete multiplied by the probability p that allele A
will be present in the maternal gamete, or p2. Similarly, the
probability of the genotype aa is q2. The genotype Aa can arise by
getting A from the father and a from the mother, which will occur with
a frequency pq, or by getting a from the father and A from the mother,
which also has a probability of pq; this gives a total probability of
2pq for the frequency of the Aa genotype in the progeny.
There is no change in the allele equilibrium frequencies from one
generation to the next. The frequency of the A allele among the
offspring is the frequency of the AA genotype (because all alleles in
these individuals are A alleles) plus half the frequency of the Aa
genotype (because half the alleles in these individuals are A
alleles), or p2 + pq = p(p + q) = p (because p + q = 1). Similarly,
the frequency of the a allele among the offspring is given by q2 + pq
= q(q + p) = q. These are precisely the frequencies of the alleles in
the parents.
The genotype equilibrium frequencies are obtained by the
Hardy-Weinberg law on the assumption that there is random mating; that
is, the probability of a particular kind of mating is the same as the
frequency of the genotypes of the two mating individuals. For example,
the probability of an AA female mating with an aa male must be p2 (the
frequency of AA) times q2 (the frequency of aa). Random mating can
occur with respect to most gene loci even though mates may be chosen
according to particular characteristics. People, for example, choose
their spouses according to all sorts of preferences concerning looks,
personality, and the like. But concerning the majority of genes,
people's marriages are essentially random.
Assortative, or selective, mating takes place when the choice of mates
is not random. Marriages in the United States, for example, are
assortative with respect to racial features, so that Negroes, Asians,
and Caucasoids marry members of their own racial group more often, and
people from a different racial group less often, than would be
expected from random mating. Consider a community in which 80 percent
of the population are white and 20 percent are black. With random
mating, 32 percent (2 ´ 0.80 ´ 0.20 = 0.32) of all marriages would be
interracial, whereas only 4 percent (0.20 ´ 0.20 = 0.04) would be
marriages between two blacks. These expectations depart from typical
observations. The most extreme form of assortative mating is
self-fertilization, which occurs rarely in animals but is a common
form of reproduction in many plant groups.
The Hardy-Weinberg law assumes that gene frequencies remain constant
from generation to generation--that there is no gene mutation or
natural selection and that populations are very large. But these
assumptions are not correct; indeed, if they were, evolution could not
occur. Why, then, is the Hardy-Weinberg law significant if its
assumptions do not hold true in nature? The answer is that the
Hardy-Weinberg law plays in evolutionary studies a role similar to
that of Newton's first law of motion in mechanics. Newton's first law
says that a body not acted upon by a net external force remains at
rest or maintains a constant velocity. In fact, there are always
external forces acting upon physical objects, but the first law
provides the starting point for the application of other laws.
Similarly, organisms are subject to mutation, selection, and other
processes that change gene frequencies, but the effects of these
processes can be calculated by using the Hardy-Weinberg law as the
starting point.
Processes of gene frequency change
Mutation
The allelic variations that make evolution possible are generated by
the process of mutation; but new mutations change gene frequencies
very slowly, since mutation rates are low. Assume that the gene allele
A[1] mutates to allele A[2] at a rate m per generation, and that at a
given time the frequency of A[1] is p. In the next generation, a
fraction m of all A[1] alleles become A[2] alleles. The frequency of
A[1] in the next generation will then be reduced by the fraction of
mutated alleles (pm), or p[1] = p - pm = p(1 - m). After t
generations, the frequency of A[1] will be p[t] = p(1 - m)t.
If the mutations continue,the frequency of A[1] alleles will gradually
decrease, because a fraction of them change every generation to A[2].
If the process continues indefinitely, the A[1] allele will eventually
disappear, although the process is slow. If the mutation rate is 10^-5
(1 in 100,000) per gene per generation, about 2,000 generations will
be required to change the frequency of A[1] from 0.50 to 0.49 and
about 10,000 generations to change it from 0.10 to 0.09.
Moreover, gene mutations are reversible: the allele A[2] may also
mutate to A[1]. Assume that A[1] mutates to A[2] at a rate m, as
before, and that A[2] mutates to A[1] at a rate n per generation. If
at a certain time the frequencies of A[1] and A[2] are p and q,
respectively, after one generation the frequency of A[1] will be p[1]
= p - pm + qn. A fraction pm of allele A[1] changes to A[2], but a
fraction qn of the A[2] alleles changes to A[1]. The conditions for
equilibrium occur when pm = qn, or p = m/(m + n). Suppose that the
mutation rates are m = 10^-6 and n = 10^-5; then, at equilibrium, p =
10^-6/(10^-6 + 10^-5) = 1/(1 + 10) = 0.09, and q = 0.91.
Changes in gene frequencies due to mutation occur, therefore, at even
slower rates than was suggested above, because forward and backward
mutations counteract each other. In any case, allelic frequencies
usually are not in mutational equilibrium, because some alleles are
favoured over others by natural selection. The equilibrium frequencies
are then decided by the interaction between mutation and selection,
with selection usually having the greater consequence.
Migration
Gene flow, or gene migration, takes place when individuals migrate
from one population to another and interbreed with its members. Gene
frequencies are not changed for the species as a whole, but they
change locally whenever different populations have different allele
frequencies. In general, the greater the difference in allele
frequencies between the resident and the migrant individuals, and the
larger the number of migrants, the greater effect the migrants have in
changing the genetic constitution of the resident population.
Suppose that a proportion of all reproducing individuals in a
population are migrants and that the frequency of allele A[1] is p in
the population but p[m] among the migrants. The change in gene
frequency, Dp, in the next generation will be Dp = m(p[m] - p). If the
migration rate persists for a number t of generations, the frequency
of A[1] will be given by p[t] = (1 - m)t(p - p[m]) + p[m].
Genetic drift
Gene frequencies can change from one generation to another by a
process of pure chance known as genetic drift. This occurs because
populations are finite in numbers, and thus the frequency of a gene
may change in the following generation by accidents of sampling, just
as it is possible to get more or less than 50 "heads" in 100 throws of
a coin simply by chance.
The magnitude of the gene frequency changes due to genetic drift is
inversely related to the size of the population; the larger the number
of reproducing individuals, the smaller the effects of genetic drift.
This inverse relationship between sample size and magnitude of
sampling errors can be illustrated by referring again to tossing a
coin. When a penny is tossed twice, two heads are not surprising. But
it will be surprising, and suspicious, if 20 tosses all yield heads.
The proportion of heads obtained in a series of throws approaches
closer to 0.5 as the number of throws grows larger.
The relationship is the same in populations, although the important
value here is not the actual number of individuals in the population
but the "effective" population size. This means the number of
individuals that produce offspring, because only reproducing
individuals transmit their genes to the following generation. It is
not unusual, in plants as well as animals, for some individuals to
have large numbers of progeny while others have none. In marine seals,
antelopes, baboons, and many other mammals, for example, a dominant
male may keep a large harem of females at the expense of many other
males who can find no mates. It often happens that the effective
population size is substantially smaller than the number of
individuals in any one generation.
The effects of genetic drift in changing gene frequencies from one
generation to the next are quite small in most natural populations,
which generally consist of thousands of reproducing individuals. The
effects over many generations are more important. Indeed, in the
absence of other processes of change (such as natural selection and
mutation), populations would eventually become fixed, having one
allele at each locus after the gradual elimination of all others. With
genetic drift as the only force in operation, the probability of a
given allele eventually reaching a frequency of 1 would be precisely
the frequency of the allele; that is, an allele with a frequency of
0.8 would have an 80 percent chance of ultimately becoming the only
allele present in the population. The process would, however, take a
long time, because increases and decreases are likely to alternate
with equal probability. More important, natural selection and other
processes change gene frequencies in deterministic ways, so that no
allele has an opportunity to become fixed as a consequence of genetic
drift alone.
Genetic drift can have important evolutionary consequences when a new
population becomes established by only a few individuals--a phenomenon
known as the founder principle. Islands, lakes, and other isolated
ecological sites are often colonized by one or very few seeds or
animals of a species, which are transported there passively by wind,
in the fur of larger animals, or in some other way. The allelic
frequencies present in these few colonizers are likely to differ at
many loci from those in the population they came from, thus having a
lasting impact on the evolution of the new population. The founder
principle is one reason that species in neighbouring islands, such as
those in the Hawaiian archipelago, are often more heterogeneous than
species in comparable continental areas adjacent to one another.
Climatic or other conditions, if unfavourable, may on occasion
drastically reduce the number of individuals in a population and even
threaten it with extinction. Such occasional reductions are called
population bottlenecks. The populations may later recover their
typical size, but the allelic frequencies may have been considerably
altered, thereby affecting the future evolution of the species.
Bottlenecks are more likely in relatively large animals and plants
than in smaller ones, because populations of large organisms typically
consist of fewer individuals. Primitive human populations of the past
were subdivided into many small tribes that were time and again
decimated by disease, war, and other disasters. Differences among
current human populations in the allele frequencies of many
genes--such as those determining the ABO and other blood groups--may
have arisen at least in part as a consequence of bottlenecks in
ancestral populations. Persistent population bottlenecks may reduce
the overall genetic variation so greatly as to alter future evolution
and endanger the survival of the species. A well-authenticated case is
that of the cheetah, where no allelic variation whatsoever has been
found among the many scores of gene loci studied.
The operation of natural selection in populations
Natural selection as a process of genetic change
Natural selection refers to any reproductive bias favouring some genes
or genotypes over others. Natural selection promotes the adaptation of
organisms to the environments in which they live; any hereditary
variant that improves the ability to survive and reproduce in an
environment will increase in frequency over the generations, precisely
because the organisms carrying such a variant will leave more
descendants than those lacking it. Hereditary variants, favourable or
not to the organisms, arise by mutation. Unfavourable ones are
eventually eliminated by natural selection; their carriers leave no
descendants or leave fewer than those carrying alternative variants.
Favourable mutations accumulate over the generations. The process
continues indefinitely because the environments that organisms live in
are forever changing. Environments change physically--in their
climate, physical configuration, and so on--but also biologically,
because the predators, parasites, and competitors with which an
organism interacts are themselves evolving.
Mutation, migration, and drift are random processes with respect to
adaptation; they change gene frequencies without regard for the
consequences that such changes may have in the ability of the
organisms to survive and reproduce. If these were the only processes
of evolutionary change, the organization of living things would
gradually disintegrate. The effects of such processes alone would be
analogous to those of a mechanic who changed parts in a motorcar
engine at random, with no regard for the role of the parts in the
engine. Natural selection keeps the disorganizing effects of mutation
and other processes in check because it multiplies beneficial
mutations and eliminates harmful ones.
Natural selection accounts not only for the preservation and
improvement of the organization of living beings but also for their
diversity. In different localities or in different circumstances,
natural selection favours different traits, precisely those that make
the organisms well adapted to their particular circumstances and ways
of life.
The parameter used to measure the effects of natural selection is
fitness, which can be expressed as an absolute or as a relative value.
Consider a population consisting at a certain locus of three
genotypes: A[1]A[1], A[1]A[2], and A[2]A[2]. Assume that on the
average each A[1]A[1] and each A[1]A[2] individual produces one
offspring, but that each A[2]A[2] individual produces two. One could
use the average number of progeny left by each genotype as a measure
of that genotype's absolute fitness and calculate the changes in gene
frequency that would occur over the generations (this, of course,
requires knowing how many of the progeny survive to adulthood and
reproduce). Evolutionists, however, find it mathematically more
convenient to use relative fitness values--which they represent with
the letter w--in most calculations. They usually assign the value 1 to
the genotype with the highest reproductive efficiency and calculate
the other relative fitness values proportionally. For the example just
used, the relative fitness of the A[2]A[2] genotype would be w = 1 and
that of each of the other two genotypes would be w = 0.5. A parameter
related to fitness is the selection coefficient, often represented
with the letter s, which is defined as s = 1 - w. The selection
coefficient is a measure of the reduction in fitness of a genotype.
The selection coefficients in the example are s = 0 for A[2]A[2] and s
= 0.5 for A[1]A[1] and A[1]A[2].
The different ways in which natural selection affects gene frequencies
are illustrated by the following examples.
Selection against one of the homozygotes
Suppose that one homozygous genotype, say A[2]A[2], has lower fitness
than the other two genotypes, A[1]A[1] and A[1]A[2]. (This is the
situation in many human diseases, such as phenylketonuria [PKU] and
sickle-cell anemia. The heterozygotes and the homozygotes for the
normal allele have equal fitness, higher than that of the homozygotes
for the deleterious allele.) Call the fitness of these homozygotes 1 -
s (the fitness of the other two genotypes is 1), and let p be the
frequency of the normal allele (A[1]) and q the frequency of the
deleterious allele (A[2]). It can be shown that the frequency of A[2]
will decrease each generation by an amount given by Dq = -spq2/(1 -
sq2). The deleterious allele, A[2], will continuously decrease in
frequency until it is eliminated. The rate of elimination is fastest
when s = 1 (i.e., w = 0); this occurs with fatal diseases, such as
untreated PKU, when the homozygotes die before the age of
reproduction.
Because of new mutations, the elimination of a deleterious allele is
never complete. A dynamic equilibrium frequency will exist when the
number of new alleles produced by mutation is the same as the number
eliminated by selection. If the mutation rate at which the deleterious
allele arises is u, the equilibrium frequency for a deleterious allele
that is recessive is given approximately by q = Ö u/[s], which, if s =
1, reduces to q = Öu.
The mutation rate for many human recessive diseases is about 1 in
100,000 (u = 10^-5). If the disease is fatal, the equilibrium
frequency becomes q @ Ö10^-5 = 0.003, or about 1 recessive lethal
allele for every 300 normal alleles. That is roughly the frequency in
human populations of alleles that in homozygous individuals, like
those with PKU, cause death before adulthood. The equilibrium
frequency for a deleterious, but not lethal, recessive allele is much
higher. Albinism, for example, is due to a recessive gene. The
reproductive efficiency of albinos is, on average, about 0.9 that of
normal individuals. Therefore, s = 0.1 and q = Ö u/[s] = Ö
10^-5/[10^-1] = 0.01, or 1 in 100 genes rather than 1 in 300 as for a
lethal allele.
For deleterious dominant alleles, the mutation-selection equilibrium
frequency is given by p = u/s, which for fatal genes becomes p = u. If
the gene is lethal even in single copy, all the genes are eliminated
by selection in the same generation in which they arise, and the
frequency of the gene in the population is the frequency with which it
arises by mutation. One deleterious condition that is caused by a
dominant allele present at low frequencies in human populations is
achondroplasia. Because of abnormal growth of the long bones,
achondroplastics have short, squat, often deformed limbs, along with
bulging skulls. The mutation rate from the normal allele to the
achondroplasia allele is about 5 ´ 10^-5. Achondroplastics reproduce
only 20 percent as efficiently as normal individuals; hence, s = 0.8.
The equilibrium frequency of the allele can therefore be calculated as
p = u/s = 6.25 ´ 10^-5.
Overdominance
In many instances the heterozygotes have a higher degree of fitness
than the homozygotes for one or the other allele. This situation,
known as heterosis or overdominance, leads to the stable coexistence
of both alleles in the population and, hence, contributes to the
widespread genetic variation found in populations of most organisms.
The model situation is:
Special Comp
It is assumed that s and t are positive numbers between 0 and 1, so
that the fitnesses of the two homozygotes are somewhat less than 1. It
is not difficult to show that the change in frequency per generation
of allele A[2] is Dq = pq(sp - tq)/(1 - sp2 - tq2). An equilibrium
will exist when Dq = 0 (gene frequencies no longer change); this will
happen when sp = tq, at which the numerator of the expression for Dq
will be 0. The condition sp = tq can be rewritten as s(1 - q) = tq
(when p + q = 1), which leads to q = s/(s + t). If the fitnesses of
the two homozygotes are known, it is possible to infer the allele
equilibrium frequencies.
A colour polymorphism in the marine copepod Tisbe reticulata is one of
many well-investigated examples of overdominance in animals. Three
colour morphs found in the lagoon of Venice are known as violacea
(homozygous genotype VVVV), maculata (homozygous genotype VMVM), and
violacea-maculata (heterozygous genotype VVVM). The colour
polymorphism persists in the lagoon because the heterozygotes survive
better than either one of the two homozygotes. In laboratory
experiments, the fitness of the three genotypes depends on the degree
of crowding, as shown by the following comparison of their relative
fitnesses:
Special Comp
The greater the crowding--with more competition for resources--the
greater the superiority of the heterozygotes.
A particularly interesting example of heterozygote superiority among
humans is provided by the gene responsible for sickle-cell anemia.
Human hemoglobin in adults is for the most part hemoglobin A, a
four-component molecule consisting of two a and two b hemoglobin
chains. The normal b hemoglobin chain consists of 146 amino acids and
is coded for by the gene Hb^A. A mutant allele of this gene, Hb^S,
causes the b chain to have in the sixth position the amino acid valine
instead of glutamic acid. This seemingly minor substitution modifies
the properties of hemoglobin so that homozygotes with the mutant
allele, Hb^SHb^S, suffer from a severe form of anemia that in most
cases leads to death before the age of reproduction.
The high frequency of the Hb^S allele in some African and Asian
populations formerly was puzzling because the severity of the anemia,
representing a strong natural selection against homozygotes, should
have eliminated the defective allele. The situation was understood
after it was noticed that the Hb^S allele occurred at high frequency
precisely in regions of the world where a particularly severe form of
malaria (caused by the parasite Plasmodium falciparum) was endemic. It
was hypothesized that the heterozygotes, Hb^AHb^S, were resistant to
malaria, whereas the homozygotes Hb^AHb^A were not. In
malaria-infested regions, then, the heterozygotes survived better than
either one of the two homozygotes, which were more likely to die from
either malaria (Hb^AHb^A homozygotes) or anemia (Hb^SHb^S
homozygotes). This hypothesis has been confirmed in various ways. Most
significant is that most hospital patients suffering from severe or
fatal forms of malaria are homozygotes Hb^AHb^A. In a study of 100
children who died from malaria, only one was found to be a
heterozygote, while 22 were expected to be so according to the
frequency of the Hb^S allele in the population.
Table
Table
Table 2 shows how the relative fitness of the three b-chain genotypes
can be calculated from their distribution among the Yoruba people of
Ibadan, Nigeria. The frequency of the Hb^S allele among adults is
estimated as q = 0.1232. According to the Hardy-Weinberg law, the
three genotypes will be formed at conception in the frequencies p2,
2pq, and q2, which are the expected frequencies given in Table 2. The
ratios of the observed frequencies among adults to the expected
frequencies give the relative survival efficiency of the three
genotypes. These are divided by their largest value (1.12) in order to
obtain the relative fitness of the genotypes. Sickle-cell anemia
reduces the probability of survival of the Hb^SHb^S homozygotes to 13
percent of that of the heterozygotes; but the homozygotes for the
normal allele, Hb^AHb^A, have their survival probability reduced by
malaria infection to 88 percent.
Frequency-dependent selection
The fitness of genotypes can change when the environmental conditions
change. White fur may be protective to a bear living on the Arctic
snows, but not to one living in a California forest; there an allele
coding for brown pigmentation may be favoured over one that codes for
white. The environment of an organism includes not only the climate
and other physical features, but also the organisms of the same or
different species with which it is associated.
Changes in genotypic fitness are associated with the density of the
organisms present. Insects and other short-lived organisms experience
enormous yearly oscillations in density. Some genotypes may possess
high fitness in the spring, when the population is rapidly expanding,
because such genotypes yield more prolific individuals. Other
genotypes may be favoured during the summer, when populations are
dense, because these genotypes make for better competitors, more
successful at securing limited food resources. Still others may be at
an advantage during the long winter months, because they increase the
population's hardiness, or ability to withstand the inclement
conditions that kill most members of the other genotypes.
The fitness of genotypes can also vary according to their relative
numbers, and genotype frequencies may change as a consequence. This is
known as frequency-dependent selection. Particularly interesting is
the situation in which genotypic fitnesses are inversely related to
their frequencies. Assume that two genotypes, A and B, have fitnesses
related to their frequencies in such a way that the fitness of either
genotype increases when its frequency decreases and vice versa. When A
is rare, its fitness is high, and therefore A increases in frequency;
but as it becomes more and more common, the fitness of A gradually
decreases, so that its increase in frequency eventually comes to a
halt. A stable polymorphism occurs at the frequency where the two
genotypes, A and B, have identical fitnesses.
In natural populations of animals and plants frequency-dependent
selection is very common and may contribute importantly to the
maintenance of genetic polymorphism. In the vinegar fly Drosophila
pseudoobscura, for example, three genotypes exist at the gene locus
coding for the enzyme malate dehydrogenase: the homozygous SS and FF
and the heterozygous SF. When the SS homozygotes represent 90 percent
of the population, they have a fitness about two-thirds that of the
heterozygotes, SF; but when the SS homozygotes represent only 10
percent of the population, their fitness is more than double that of
the heterozygotes. Similarly, the fitness of the FF homozygotes
relative to the heterozygotes increases from less than half to nearly
double as their frequency goes from 90 to 10 percent. All three
genotypes have equal fitnesses when the frequency of the S allele, p,
is about 0.70, so that there is a stable polymorphism with frequencies
p2 = 0.49 for SS, 2pq = 0.42 for SF, and q2 = 0.09 for FF.
Frequency-dependent selection may arise because the environment is
heterogeneous and because different genotypes can better exploit
different subenvironments. When a genotype is rare, the
subenvironments that it exploits better will be relatively abundant.
But as the genotype becomes common, its favoured subenvironment
becomes saturated. That genotype must then compete for resources in
subenvironments that are optimal for other genotypes. It follows,
then, that a mixture of genotypes exploits the environmental resources
better than a single genotype. This has been extensively demonstrated.
When the three Drosophila genotypes mentioned above were mixed in a
single population, the average number of individuals that developed
per unit of food was 45.6. This was greater than the numbers of
individuals that developed when only one of the genotypes was present,
which averaged 41.1 for SS, 40.2 for SF, and 37.1 for FF. Plant
breeders know that mixed plantings are more productive than single
stands, although farmers avoid them for reasons such as increased
harvesting costs.
Sexual preferences can also lead to frequency-dependent selection. It
has been demonstrated in some insects, birds, mammals, and other
organisms that the mates preferred are precisely those that are rare.
People also experience this rare-mate advantage; blonds may seem
attractively exotic to Latins, or brunets to Scandinavians.
Types of selection
Stabilizing selection
Natural selection can be studied by analyzing its effects on changing
gene frequencies; but it can also be explored by examining its effects
on the observable characteristics--or phenotypes--of individuals in a
population. Distribution scales of phenotypic traits such as height,
weight, number of progeny, or longevity typically show greater numbers
of individuals with intermediate values and fewer and fewer toward the
extremes (the so-called normal distribution). When individuals with
intermediate phenotypes are favoured and extreme phenotypes are
selected against, the selection is said to be stabilizing. The range
and distribution of phenotypes then remains approximately the same
from one generation to another. Stabilizing selection is very common.
The individuals that survive and reproduce more successfully are those
that have intermediate phenotypic values. Mortality among newborn
infants, for example, is highest when they are either very small or
very large; infants of intermediate size have a greater chance of
surviving.
Stabilizing selection is often noticeable after artificial selection.
Breeders choose chickens that produce larger eggs, cows that yield
more milk, and corn with higher protein content. But the selection
must be continued or reinstated from time to time, even after the
desired goals have been achieved. If it is stopped altogether, natural
selection gradually takes effect and turns the traits back toward
their original intermediate value.
As a result of stabilizing selection, populations often maintain a
steady genetic constitution with respect to many traits. This
attribute of populations is called genetic homeostasis.
Directional selection
The distribution of phenotypes in a population sometimes changes
systematically in a particular direction. The physical and biological
aspects of the environment are continuously changing, and over long
periods of time the changes may be substantial. The climate and even
the configuration of the land or waters vary incessantly. Changes also
take place in the biotic conditions; that is, in the other organisms
present, whether predators, prey, parasites, or competitors. Genetic
changes occur as a consequence, because the genotypic fitnesses may be
shifted so that different sets of alleles are favoured. The
opportunity for directional selection also arises when organisms
colonize new environments where the conditions are different from
those of their original habitat. In addition, the appearance of a new
favourable allele or a new genetic combination may prompt directional
changes, as the new genetic constitution replaces the preexisting one.
The process of directional selection takes place in spurts. The
replacement of one genetic constitution with another changes the
genotypic fitnesses at other loci, which then change in their allelic
frequencies, thereby stimulating additional changes, and so on in a
cascade of consequences.
Directional select
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