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作者：Anonymous 在 罕见奇谈 发贴, 来自 http://www.hjclub.org
The origin of species
Reproductive isolation
In sexual organisms individuals able to interbreed belong to the same
species. The biological properties of organisms that prevent
interbreeding are called reproductive isolating mechanisms (RIM's).
Oaks on different islands, minnows in different rivers, or squirrels
in different mountain ranges cannot interbreed because they are
physically separated, but not necessarily because they are
biologically incompatible. Geographic separation, therefore, is not an
RIM, since it is not a biological property of organisms.
There are two general categories of reproductive isolating mechanisms:
prezygotic (those that take effect before fertilization) and
postzygotic (those that take effect after). Prezygotic RIM's prevent
the formation of hybrids between members of different populations
through ecological, temporal, ethological (or behavioral), mechanical,
and gametic isolation. Postzygotic RIM's reduce the viability or
fertility of hybrids or their progeny.
Ecological isolation
Populations may occupy the same territory but live in different
habitats and so not meet. The Anopheles maculipennis group consists of
six mosquito species, some of which are involved in the transmission
of malaria. Although the species are virtually indistinguishable
morphologically, they are isolated reproductively, in part because
they breed in different habitats. Some breed in brackish water, others
in running fresh water, and still other in stagnant fresh water.
Temporal isolation
Populations may mate or flower at different seasons or different times
of day. Three tropical orchid species of the genus Dendrobium flower
for a single day; the flowers open at dawn and wither by nightfall.
Flowering occurs in response to certain meteorological stimuli, such
as a sudden storm on a hot day. The same stimulus acts on all three
species, but the lapse between the stimulus and flowering is eight
days in one species, nine in another, and 10 or 11 in the third.
Interspecific fertilization becomes impossible because at the time
when the flowers of one species open, those of the other species have
already withered or are not yet mature.
A peculiar form of temporal isolation exists between pairs of closely
related species of cicadas, in which one species of each pair emerges
every 13 years, the other every 17 years. The two species of a pair
may be sympatric (live in the same territory), but they have an
opportunity to form hybrids only once every 221 (13 ´ 17) years.
Ethological (behavioral) isolation
Sexual attraction between males and females may be weak or absent. In
most animal species, members of the two sexes must first search for
each other and come together. Complex courtship rituals then take
place, with the male often taking the initiative and the female
responding. This in turn generates additional actions by the male and
responses by the female, and eventually there is copulation (or, in
the case of some aquatic organisms, release of the sex cells for
fertilization in the water). These elaborate rituals are specific to a
species and play a significant part in species recognition. If the
sequence of events in the search-courting-mating process is rendered
disharmonious by either of the two sexes, then the entire process will
be interrupted. Courtship and mating rituals have been extensively
analyzed in some mammals, birds, and fishes, and in a number of insect
species.
Ethological isolation is often the most potent RIM to keep animal
species from interbreeding. It can be remarkably strong even among
closely related species. The vinegar flies Drosophila serrata, D.
birchii, and D. dominicana are three sibling species (that is, they
are nearly indistinguishable morphologically) that are endemic in
Australia and on the islands of New Guinea and New Britain. In many
areas these three species occupy the same territory, but no hybrids
are known to occur in nature. The strength of their ethological
isolation has been tested in the laboratory by placing groups of 10
females of one strain and 10 males of another together for several
days. In groups in which the two strains were of the same species but
from different geographic origins, a large majority of the females
(usually 90 percent or more) were fertilized; but no inseminations or
very few (less than 4 percent) took place when males and females were
of different species, whether from the same or different geographic
origins.
It should be added that the rare interspecific inseminations that did
occur among the vinegar flies produced hybrid adult individuals in
very few instances, and the hybrids were always sterile. This
illustrates a common pattern: reproductive isolation between species
is achieved by several RIM's in succession; if one breaks down, others
are still present. In addition to ethological isolation, hybrid
inviability and hybrid sterility prevent successful breeding between
members of the three Drosophila species and many other animal species
as well.
Species recognition during courtship involves stimuli that may be
chemical (olfactory), visual, auditory, or tactile. Pheromones are
specific substances that play a critical role in recognition between
members of a species; they have been chemically identified in ants,
moths, butterflies, and mammals. The "songs" of birds, frogs, and
insects (which produce them by vibrating or rubbing their wings) are
species recognition signals. Some form of physical contact or touching
occurs in many mammals, but also in Drosophila flies and other
insects.
Mechanical isolation
Copulation is often impossible between different animal species
because of incompatible shape and size of the genitalia; in plants,
variations in flower structure may impede pollination. In two species
of sage from California, the two-lipped flowers of Salvia mellifera
have stamens and style in the upper lip, whereas S. apiana has long
stamens and style and a specialized floral configuration. S. mellifera
is pollinated by small or medium-sized bees that carry pollen on their
backs from flower to flower. S. apiana, however, is pollinated by
large carpenter bees and bumblebees that carry the pollen on their
wings and other body parts. Even if the pollinators of one species
visit flowers of the other, pollination cannot occur because the
pollen does not come into contact with the style of the alternative
species.
Gametic isolation
Marine animals often discharge their eggs and spermatozoa into the
surrounding water, where fertilization takes place. Gametes of
different species may fail to attract one another. For example, the
sea urchins Strongylocentrotus purpuratus and S. franciscanus can be
induced to release their eggs and sperms simultaneously, but most of
the fertilizations that result are between eggs and sperms of the same
species. In animals with internal fertilization, spermatozoa may be
unable to function in the sexual ducts of females of different
species. In plants, pollen grains of one species typically fail to
germinate on the stigma of another species, so that the pollen never
reaches the ovary and fertilization cannot occur.
Hybrid inviability
Occasionally, prezygotic mechanisms are absent or break down so that
interspecific zygotes are formed. These zygotes, however, often fail
to develop into mature individuals. The hybrid embryos of sheep and
goats, for example, die in the early developmental stages before
birth. Hybrid inviability is common in plants, whose hybrid seeds
often fail to germinate or die shortly after germination.
Hybrid sterility
Hybrid zygotes sometimes develop into adults, such as mules (hybrids
between horses and donkeys), but the adults fail to develop functional
gametes and are sterile.
Hybrid breakdown
In plants more than in animals, hybrids between closely related
species are sometimes partially fertile. Gene exchange may
nevertheless be inhibited because the offspring are poorly viable or
sterile. Hybrids between the cotton species Gossypium barbadense, G.
hirsutum, and G. tomentosum appear vigorous and fertile, but their
progenies die in seed or early in development, or they develop into
sparse, weak plants.
A model of speciation
Since species are groups of populations reproductively isolated from
one another, asking about the origin of species is equivalent to
asking how reproductive isolation arises between populations. Two
theories have been advanced to answer this question. One theory
considers isolation as an accidental by-product of genetic divergence.
Populations that become genetically less and less alike (as a
consequence, for example, of adaptation to different environments) may
eventually be unable to interbreed because their gene pools are
disharmonious. The other theory regards isolation as a product of
natural selection. Whenever hybrid individuals are less fit than
nonhybrids, natural selection will directly promote the development of
RIM's. This occurs because genetic variants interfering with
hybridization have greater fitness than those favouring hybridization,
given that the latter are often present in poorly fit hybrids.
These two theories of the origin of reproductive isolation are not
mutually exclusive. Reproductive isolation may indeed come about
incidentally to genetic divergence between separated populations.
Consider, for example, the evolution of many endemic species of plants
and animals in the Hawaiian archipelago. The ancestors of these
species arrived in the Hawaiian Islands several million years ago.
There they evolved as they became adapted to the environmental
conditions and colonizing opportunities found on the islands.
Reproductive isolation between the populations evolving in Hawaii and
the continental populations was never directly promoted by natural
selection because their geographic remoteness forestalled any
opportunities for hybridizing. Nevertheless, reproductive isolation
became complete in many cases as a result of gradual genetic
divergence over thousands of generations.
Frequently, however, the course of speciation involves the processes
postulated by both theories; reproductive isolation starts as a
by-product of gradual evolutionary divergence but is completed by
natural selection directly promoting the evolution of prezygotic
RIM's.
The two sets of processes identified by the two speciation theories
may be seen, therefore, as two different stages in the splitting of
one evolutionary lineage into two species. The process can start only
when gene flow is somehow interrupted between two populations.
Interruption may be due to geographic separation, or it may be
initiated by some genetic change that affects some but not other
individuals living in the same territory. Absence of gene flow makes
it possible for the two populations to become genetically
differentiated as a consequence of adapting to diverse local
conditions and of genetic drift. It is necessary that gene flow be
interrupted, because otherwise the two groups of individuals would
still share in a common gene pool and fail to become genetically
different. The two genetically isolated groups are likely to become
more and more different as time goes on. Eventually some incipient
reproductive isolation may take effect because the two gene pools are
no longer coadapted. Hybrid individuals will carry genes combined from
two gene pools and will therefore have reduced viability or fertility.
The circumstances just described may persist for so long that the
populations become completely differentiated into separate species. It
happens quite commonly, however, in both animals and plants, that
opportunities for hybridization arise between two populations that are
becoming genetically differentiated. Two outcomes are possible. One is
that the hybrids manifest little or no reduction of fitness, so that
gene exchange between the two populations proceeds freely, eventually
leading to their integration into a single gene pool. The second
possible outcome is that reduction of fitness in the hybrids is
sufficiently large for natural selection to favour the emergence of
prezygotic RIM's preventing the formation of hybrids altogether. This
situation may be identified as the second stage in the speciation
process.
How natural selection brings about the evolution of prezygotic RIM's
can be understood in the following way. Assume that there are gene
variants in one of two populations, P1, that increase the probability
that P1 individuals will choose P1 rather than P2 mates. Such gene
variants will increase in frequency in the P1 population, because they
are more often present in the progenies of P1 ´ P1 matings, which have
normal fitness. The alternative genetic variants that do not favour P1
´ P1 matings will be more often present in the progenies of P1 ´ P2
matings, which have lower fitness. The same process will enhance the
frequency in the P2 population of genetic variants that lead P2
individuals to choose P2 rather than P1 mates. Prezygotic RIM's may
therefore evolve in both populations and lead to their becoming two
separate species.
The two stages of the process of speciation can be characterized,
finally, by outlining their distinctions. The first stage primarily
involves the appearance of postzygotic RIM's as accidental by-products
of overall genetic differentiation rather than as express targets of
natural selection; the second stage involves the evolution of
prezygotic RIM's that are directly promoted by natural selection. The
first stage may come about suddenly, in one or a few generations,
rather than as a long, gradual process. The second stage succeeds the
first in time but need not always be present.
Geographic speciation
One common mode of speciation is known as geographic, or allopatric
(in separate territories), speciation. The general model of the
speciation process advanced in the previous paragraphs applies well to
geographic speciation. The first stage begins as a result of
geographic separation between populations. This may occur when a few
colonizers reach a geographically separate habitat, perhaps an island,
lake, river, isolated valley, or mountain range. In another process, a
population may be split into two geographically separate ones by
topographic changes, such as a cessation of water flow between two
lakes, or by an invasion of competitors, parasites, or predators into
the intermediate zone. If these types of geographic separation
continue for some time, postzygotic RIM's may appear as a result of
gradual genetic divergence.
In the second stage, an opportunity for interbreeding may later be
brought about by topographic changes establishing continuity between
the previously isolated territories or by ecological changes making
the intermediate territory habitable for the organisms. If the fitness
of hybrids of the formerly separated populations is sufficiently
reduced, natural selection will foster the development of prezygotic
RIM's, and the two populations may then evolve into two species.
Investigation has been made of many populations that are in the first
stage of geographic speciation. There are fewer well-documented
instances of the second stage, presumably because this occurs fairly
rapidly (in evolutionary time).
Both stages of speciation are present in a group of six closely
related species of New World Drosophila that have been extensively
studied by evolutionists for more than three decades. Two of these
sibling species, D. willistoni and D. equinoxialis, consist of groups
of populations in the first stage of speciation and identified as
different subspecies. Two D. willistoni subspecies live in continental
South America; D. w. quechua lives west of the Andes and D. w.
willistoni east of the Andes. They are effectively separated by the
Andes because the flies cannot live at high altitudes. It is not known
whether their geographic separation is as old as the Andes, but it has
existed long enough for postzygotic RIM's to evolve. When the two
subspecies are crossed in the laboratory, the hybrid males are
completely sterile if the mother came from the quechua subspecies; but
in the reciprocal cross all hybrids are fertile. If hybridization
should occur in nature, selection would favour the evolution of
prezygotic RIM's because of the complete sterility of half of the
hybrid males.
Drosophila equinoxialis equinoxialis and D. e. caribbensis are another
pair of subspecies. D. e. equinoxialis inhabits continental South
America, and D. e. caribbensis lives in Central America and the
Caribbean. Crosses made in the laboratory between these two subspecies
always produce sterile males, irrespective of the subspecies of the
mother. Natural selection would, then, promote prezygotic RIM's
between these two subspecies more strongly than between those of D.
willistoni. But laboratory experiments show no evidence of ethological
isolation or any other prezygotic RIM, presumably because the
geographic isolation of the subspecies has forestalled hybridization
between members.
One more sibling species of the group is Drosophila paulistorum, a
species that includes groups of populations well into the second stage
of geographic speciation. Six such groups have been identified as
semispecies, or incipient species, two or three of which are sympatric
(occupying the same territory) in many localities. Male hybrids
between individuals of the different semispecies are sterile;
laboratory crosses always yield fertile females but sterile males.
Whenever two or three incipient species of D. paulistorum have come
into contact, the second stage of speciation has led to the
development of ethological isolation, which ranges from incipient to
virtually complete. Laboratory experiments show that when both
incipient species are from the same locality, their ethological
isolation is complete, so that only individuals of the same incipient
species mate. When the individuals from different incipient species
come from different localities, however, ethological isolation is
usually present but far from complete. This is precisely as the
speciation model predicts. Natural selection effectively promotes
ethological isolation where the incipient species are sympatric; but
the genes responsible for this isolation have not yet fully spread to
populations in which another semispecies is not present.
The eventual outcome of the process of geographic speciation is
complete reproductive isolation as can be observed among the species
of the Drosophila group. D. willistoni, D. equinoxialis, D.
tropicalis, and D. paulistorum coexist sympatrically over wide regions
of Central and South America while preserving their separate gene
pools. Hybrids are not known in nature and are virtually impossible to
obtain in the laboratory; and all males at least are completely
sterile. This total reproductive isolation has evolved, however, with
very little morphological differentiation. Females from different
sibling species cannot be distinguished by experts; the males can be
identified only by small differences in the shape of their genitalia,
unrecognizable except under a microscope.
Adaptive radiation
The geographic separation of populations derived from common ancestors
may continue long enough so that the populations become completely
differentiated species before ever regaining sympatry. As the
allopatric populations continue evolving independently, RIM's develop
and morphological differences may arise. The second stage of
speciation--with natural selection directly stimulating the evolution
of RIM's--never comes about in such situations, because reproductive
isolation takes place simply as a consequence of the continued
separate evolution of the populations.
This form of allopatric speciation is particularly apparent when
colonizers reach geographically remote areas, such as islands, where
they find few or no competitors and have an opportunity to diverge as
they become adapted to the new environment. Sometimes a multiplicity
of new environments becomes available to the colonizers, giving rise
to several different lineages and species. This process of rapid
divergence of multiple species from a single ancestral lineage is
called adaptive radiation.
Many examples of speciation by adaptive radiation are found in
archipelagos removed from the mainland. The Galápagos Islands are
about 600 miles off the west coast of South America. When Darwin
arrived there in 1835, he discovered many species not found anywhere
else in the world--for example, 14 species of finch (known as
Galápagos, or Darwin's, finches). These passerine birds have adapted
to a diversity of habitats and diets, some feeding mostly on plants,
others exclusively on insects. The various shapes of their bills are
clearly adapted to probing, grasping, biting, or crushing--the diverse
ways in which the different Galápagos species obtain their food. The
explanation for such diversity is that the ancestor of Galápagos
finches arrived in the islands before other kinds of birds and
encountered an abundance of unoccupied ecological niches. The finches
underwent adaptive radiation, evolving a variety of species with ways
of life capable of exploiting opportunities that in continental faunas
are exploited by other species.
The Hawaiian Islands also provide striking examples of adaptive
radiation. The archipelago consists of several volcanic islands,
ranging from about 1,000,000 to more than 10,000,000 years in age, far
away from any continent or even other large islands. In their
relatively small total land area, an astounding number of plant and
animal species exist. Most of the species have evolved in the islands.
Among them about two dozen species (about one-third of them now
extinct) of honeycreepers, birds of the family Drepanididae, all
derived from a single immigrant form. In fact, all but one of Hawaii's
71 native bird species are endemic; that is, they have evolved there
and are found nowhere else. More than 90 percent of the native species
of flowering plants, land mollusks, and insects are also endemic, as
are two-thirds of the 168 species of ferns.
There are more than 500 native Hawaiian species of Drosophila
flies--about one-third of the world's total number of known species.
Far greater morphological and ecological diversity exists among the
species in Hawaii than anywhere else in the world. The species of
Drosophila in Hawaii have diverged by adaptive radiation from one or a
few colonizers, which encountered an assortment of ecological niches
that in other lands were occupied by different groups of flies or
insects but that were available for exploitation in these remote
islands.
Quantum speciation
In some modes of speciation the first stage is achieved in a short
period of time. These modes are known by a variety of names, such as
"quantum," "rapid," and "saltational" speciation, all suggesting the
shortening of time involved. They are also known as "sympatric"
speciation, alluding to the fact that quantum speciation often leads
to speciation between populations that exist in the same territory or
habitat. An important form of quantum speciation, polyploidy, is
discussed below.
Quantum speciation without polyploidy has been seen in the annual
plant genus Clarkia. Two closely related species, Clarkia biloba and
C. lingulata, are both native to California. C. lingulata is known
only from two sites in the central Sierra Nevada at the southern
periphery of the distribution of C. biloba, from which it evolved
starting with translocations and other chromosomal mutations. Such
chromosomal rearrangements arise suddenly but reduce the fertility of
heterozygous individuals. Clarkia species are capable of
self-fertilization, which facilitates the propagation of the
chromosomal mutants in different sets of individuals even within a
single locality. This makes hybridization possible with nonmutant
individuals and allows the second stage of speciation to go ahead.
Chromosomal mutations are often the starting point of quantum
speciation in animals, particularly in groups such as moles and other
rodents that live underground or have little mobility. Mole rats of
the group Spalax ehrenbergi in Israel and gophers of the group
Thomomys talpoides in the northern Rocky Mountains are well-studied
examples.
The speciation process may also be initiated by changes in just one or
a few gene loci when these alterations result in a change of
ecological niche or, in the case of parasites, a change of host. Many
parasites use their host as a place for courtship and mating; so
organisms with two different host preferences may become
reproductively isolated. If the hybrids are poorly fit because they
are not effective parasites in either of the two hosts, natural
selection will favour the development of additional RIM's. This type
of speciation seems to be common among parasitic insects, a large
group comprising tens of thousands of species.
Polyploidy
The multiplication of entire sets of chromosomes is known as
polyploidy. A diploid organism carries in the nucleus of each cell two
sets of chromosomes, one inherited from each parent; a polyploid
organism has three or more sets of chromosomes. Many cultivated plants
are polyploid: bananas are triploid, potatoes are tetraploid, bread
wheat is hexaploid, some strawberries are octaploid.
In animals, polyploidy is relatively rare because it disrupts the
balance between the sex chromosome and the other chromosomes, a
balance being required for the proper development of sex. Naturally
polyploid species are found in hermaphroditic animals (individuals
having both male and female organs), which include snails, earthworms,
and planarians. They are also found in forms with parthenogenetic
females (which produce viable progeny without fertilization), such as
some beetles, sow bugs, goldfish, and salamanders.
All major groups of plants have polyploid species, but they are most
common among flowering plants (angiosperms), of which about 47 percent
are polyploids. Polyploidy is rare among gymnosperms, such as pines,
firs, and cedars, although the redwood, Sequoia sempervirens, is a
polyploid. Most polyploid plants are tetraploids. Polyploids with
three, five, or some other odd-number multiple of the basic chromosome
number are sterile, because the separation of homologous chromosomes
cannot be achieved properly during formation of the sex cells. Some
plants with an odd number of chromosome sets persist by means of
asexual reproduction, particularly through human cultivation; the
banana is one example.
Polyploidy is a mode of quantum speciation that yields the beginnings
of a new species in just one or two generations. There are two kinds
of polyploids: autopolyploids, which derive from a single species, and
allopolyploids, which stem from a combination of chromosome sets from
different species. Allopolyploid plant species are much more numerous
than autopolyploids.
An allopolyploid species can originate from two plant species that
have the same diploid number of chromosomes. The chromosome complement
of one species may be symbolized as AA, and the other BB. An
interspecific hybrid, AB, will usually be sterile owing to abnormal
chromosome pairing and segregation during formation of the gametes at
meiosis. But chromosome doubling may occur as a consequence of
abnormal mitosis, in which the chromosomes divide but the cell does
not. In a hybrid, this results in a cell with four sets of
chromosomes, AABB. Tetraploid plant cells may proliferate and produce
branches and flowers. Because there are two chromosomes of each kind,
functional diploid gametes with the constitution AB can be produced by
the tetraploid flowers. The union of two such gametes at fertilization
produces a tetraploid individual (AABB). In this way,
self-fertilization in plants makes possible the formation of a
tetraploid individual as the result of a single abnormal cell
division.
Autopolyploids originate in a similar fashion, except that the
individual in which the abnormal mitosis occurs is not a hybrid.
Self-fertilization thus enables a single individual to multiply and
give rise to a population. This population is a new species, since
polyploid individuals are reproductively isolated from their diploid
ancestors. A cross between a tetraploid and a diploid yields triploid
progeny, which are sterile.
Genetic differentiation during speciation
Genetic changes underlie all evolutionary processes. In order to
understand speciation and its role in evolution, it is useful to know
how much genetic change takes place during the course of species
development. It is of considerable significance to ascertain whether
new species arise by altering only a few genes, or whether the process
requires drastic changes--a genetic "revolution," as postulated by
some evolutionists in the past. The issue is best considered
separately with respect to each of the two stages of speciation and to
the various modes of speciation.
The question of how much genetic differentiation occurs during
speciation has become answerable only in recent years, with the
development of appropriate methods for comparing genes of different
species. Genetic change is measured with two parameters: genetic
identity (I), which estimates the proportion of genes that are
identical in two populations; and genetic distance (D), which
estimates the proportion of gene changes that have occurred in the
separate evolution of two populations. The value of I may range
between zero and one, which correspond to the extreme situations in
which no or all genes are identical; D may range from zero to
infinity. D can reach beyond unity because each gene may change more
than once in one or both populations as evolution goes on for many
generations.
Table 3
Table
As a model of geographic speciation, the Drosophila willistoni group
of flies offers the distinct advantage of exhibiting both stages of
the speciation process. About 30 randomly selected genes have been
studied in a large number of natural populations of these species. The
results are summarized in Table 3. The most significant figures are
those given in lines 2 and 3 of the Table, which represent the first
and second stages, respectively, of the process of geographic
speciation. D = 0.230 means that about 23 gene changes have occurred
for every 100 gene loci in the separate evolution of two subspecies;
that is, the sum of the changes that have occurred in the two
separately evolving lineages is 23 percent of all the genes. These are
populations well advanced in the first stage of speciation, as
manifested by the sterility of the hybrid males.
The genetic distance between the incipient species is the same, within
experimental error, as that between the subspecies, or 22.6 percent
(line 3). This implies that the development of ethological isolation,
as it is found in these populations, does not require many genetic
changes beyond those that occurred during the first stage of
speciation. Indeed, no additional gene changes were detected in these
experiments. The absence of major genetic changes during the second
stage of speciation can be understood by considering the role of
natural selection, which directly promotes the evolution of prezygotic
RIM's during the second stage, so that only genes modifying mate
choice need to change. In contrast, the development of post-zygotic
RIM's during the first stage occurs only after there is substantial
genetic differentiation between populations, because it comes about
only as an incidental outcome of overall genetic divergence.
Sibling species, such as D. willistoni and D. equinoxialis, exhibit 58
gene changes for every 100 gene loci after their divergence from a
common ancestor (line 4). It is noteworthy that this much genetic
evolution has occurred without altering the external morphology of
these organisms. In the evolution of morphologically different species
(line 5), the number of gene changes is greater yet, as would be
expected.
Genetic changes concomitant with one or other of the two stages in the
speciation process have been studied in a number of organisms, from
insects and other invertebrates to all sorts of vertebrates, including
mammals. The amount of genetic change during geographic speciation
varies among organisms, but the two main observations made in the D.
willistoni group seem to apply quite generally. These are that the
evolution of postzygotic mechanisms during the first stage is
accompanied by substantial genetic change (a majority of values range
between D = 0.15 and D = 0.30) and that relatively few additional
genetic changes are required during the second stage of geographic
speciation.
The conclusions drawn from the investigation of geographic speciation
make it possible to predict the relative amounts of genetic change
expected in the quantum modes of speciation. Polyploid species are a
special case; they arise suddenly in one or a few generations, and at
first they are not expected to be genetically different from their
ancestors. More generally, quantum speciation involves a shortening of
the first stage of speciation, so that postzygotic RIM's arise
directly as a consequence of specific genetic changes (such as
chromosome mutations). Populations in the first stage of quantum
speciation, therefore, need not be substantially different in
individual gene loci. This has been confirmed by genetic
investigations of species recently arisen by quantum speciation. For
example, the average genetic distance between four incipient species
of the mole rat S. ehrenbergi is D = 0.022, and between those of the
gopher T. talpoides it is D = 0.078. The second stage of speciation is
modulated in essentially the same way as in the geographic mode. Not
many gene changes are needed in either case to complete speciation.
Patterns and rates of species evolution
Reconstruction of evolutionary history
Evolution within a lineage and by lineage splitting
Evolution can take place by anagenesis, in which changes occur within
a lineage; or by cladogenesis, in which a lineage splits into two or
more separate lines. Anagenetic evolution has, over the course of
2,000,000 years, doubled the size of the human cranium; in the lineage
of the horse, it has reduced the number of toes from four to one.
Cladogenetic evolution has produced the extraordinary diversity of the
living world, with its more than 2,000,000 species of animals, plants,
fungi, and microorganisms.
The most essential cladogenetic function is speciation, the process by
which one species splits into two or more species. Because species are
reproductively isolated from one another, they are independent
evolutionary units; that is, evolutionary changes occurring in one
species are not shared with other species. Over time, species become
more and more divergent from one another as a consequence of
anagenetic evolution. Descendant lineages of two related species that
existed millions of years ago may now be classified into quite
different taxonomic categories, such as different genera or even
different families.
The evolution of all living organisms, or of a subset of them, can be
seen as a tree, with branches that divide into two or more as time
progresses. Such "trees" are called phylogenies. Their branches
represent evolving lineages, some of which eventually die out, while
others persist in themselves or in their derived lineages down to the
present time. Evolutionists are interested in the history of life and
hence in the topology, or configuration, of phylogenies. They are
concerned as well with the nature of the anagenetic changes along
lineages and with the timing of the events.
Phylogenetic relationships are ascertained by means of several
complementary sources of evidence. First, there are the discovered
remnants of organisms that lived in the past, the fossil record, which
provides definitive evidence of relationships among some groups of
organisms. The fossil record, however, is far from complete and is
often seriously deficient. Second, information about phylogeny comes
from comparative studies of living forms. Comparative anatomy
contributed the most information in the past, although additional
knowledge came from comparative embryology, cytology, ethology,
biogeography, and other biological disciplines. In recent years the
comparative study of informational macromolecules--proteins and
nucleic acids--has become a powerful tool for the study of phylogeny.
Morphological similarities among organisms have probably always been
recognized. In ancient times, Aristotle and later his followers and
those of Plato, particularly Porphyry, classified organisms (as well
as inanimate objects) on the basis of similarities. The Aristotelian
system of classification was further developed by some medieval
Scholastics, notably Albertus Magnus and Thomas Aquinas. The modern
foundations of taxonomy, the science of classification, were laid in
the 18th century by Linnaeus and by the French botanist Michel
Adanson. Lamarck dedicated much of his work to the systematic
classification of organisms. He proposed that their similarities were
due to ancestral relationships--in other words, to the degree of
evolutionary proximity.
The modern theory of evolution provides a causal explanation of the
similarities among living things. Organisms evolve by a process of
descent with modification. Changes, and therefore differences,
gradually accumulate over the generations. The more recent the last
common ancestor of a group of organisms, the less their
differentiation; similarities of form and function reflect
phylogenetic propinquity. Accordingly, phylogenetic affinities can be
inferred on the basis of relative similarity.
Convergent and parallel evolution
A distinction has to be made between resemblances due to propinquity
of descent and those due only to similarity of function.
Correspondence of features in different organisms that is due to
inheritance from a common ancestor is called homology. The forelimbs
of humans, whales, dogs, and bats are homologous. The skeletons of
these limbs are all constructed of bones arranged according to the
same pattern because they derive from an ancestor with similarly
arranged forelimbs. Correspondence of features due to similarity of
function but not related to common descent is termed analogy. The
wings of birds and of flies are analogous. Their wings are not
modified versions of a structure present in a common ancestor but
rather have developed independently as adaptations to a common
function, flying. The similarities between the wings of bats and birds
are partially homologous and partially analogous. The skeletal
structure is homologous, owing to common descent from the forelimb of
a reptilian ancestor; but the modifications for flying are different
and independently evolved, and in this respect they are analogous.
Features that become more rather than less similar through independent
evolution are said to be convergent. Convergence is often associated
with similarity of function, as in the evolution of wings in birds,
bats, and flies. The shark (a fish) and the dolphin (a mammal) are
much alike in external morphology; their similarities are due to
convergence, since they have evolved independently as adaptations to
aquatic life.
Taxonomists also speak of parallel evolution. Parallelism and
convergence are not always clearly distinguishable. Strictly speaking,
convergent evolution occurs when descendants resemble each other more
than their ancestors did with respect to some feature. Parallel
evolution implies that two or more lineages have changed in similar
ways, so that the evolved descendants are as similar to each other as
their ancestors were. The evolution of marsupials in Australia
paralleled the evolution of placental mammals in other parts of the
world. There are Australian marsupials resembling true wolves, cats,
mice, squirrels, moles, groundhogs, and anteaters. These placental
mammals and the corresponding Australian marsupials evolved
independently but in parallel lines by reason of their adaptation to
similar ways of life. Some resemblances between a true anteater
(Myrmecophaga) and a marsupial anteater (Myrmecobius) are due to
homology--both are mammals. Others are due to analogy--both feed on
ants.
Parallel and convergent evolution are also common in plants. New World
cacti and African euphorbias are alike in overall appearance although
they belong to separate families. Both are succulent, spiny,
water-storing plants adapted to the arid conditions of the desert.
Their corresponding morphologies have evolved independently in
response to similar environmental challenges.
Homology can be recognized not only between different organisms but
also between repetitive structures of the same organism. This has been
called serial homology. There is serial homology, for example, between
the arms and legs of humans, among the seven cervical vertebrae of
mammals, and among the branches or leaves of a tree. The jointed
appendages of arthropods are elaborate examples of serial homology.
Crayfish have 19 pairs of appendages, all built according to the same
basic pattern but serving diverse functions--sensing, chewing, food
handling, walking, mating, egg carrying, and swimming. Serial
homologies are not useful in reconstructing the phylogenetic
relationships of organisms, but they are an important dimension of the
evolutionary process.
Relationships in some sense akin to those between serial homologs
exist at the molecular level between genes and proteins derived from
ancestral gene duplications. The genes coding for the various
hemoglobin chains are an example. About 500,000,000 years ago a
chromosome segment carrying the gene coding for hemoglobin became
duplicated, so that the genes in the different segments thereafter
evolved in somewhat different ways, one eventually giving rise to the
modern gene coding for a hemoglobin, the other for b hemoglobin. The b
hemoglobin gene became duplicated again about 200,000,000 years ago,
giving rise to the g (fetal) hemoglobin. The a, b, g, and other
hemoglobin genes are homologous; similarities in their nucleotide
sequences occur because they are modified descendants of a single
ancestral sequence.
There are two ways of comparing homology between hemoglobins. One is
to compare the same hemoglobin--for instance, the a chain--in
different species of animals. The degree of divergence between the a
chains reflects the degree of the evolutionary relationship among the
organisms, because the hemoglobin chains have evolved independently of
one another since the time of divergence of the lineages leading to
the present-day organisms. A second way is to make comparisons
between, say, the a and b hemoglobins of a single species. The degree
of divergence between the different globin chains reflects the degree
of relationship among the genes coding for them. The different globins
have evolved independently of each other since the time of duplication
of their ancestral genes. Comparisons between homologous genes or
proteins within a given organism provide information about the
phylogenetic history of the genes and, hence, about the historical
sequence of the gene duplication events.
Whether similar features in different organisms are homologous or
analogous--or simply accidental--cannot always be decided
unambiguously, but the distinction must be made in order to determine
phylogenetic relationships. Moreover, the degrees of homology must be
quantified in some way so as to determine the propinquity of common
descent among species. Difficulties arise here as well. In the case of
forelimbs, it is not clear whether the homologies are greater between
man and bird than between man and reptile, or between man and reptile
than between man and bat. The fossil record sometimes provides the
appropriate information, even though the record is deficient. Fossil
evidence must be examined together with the evidence from comparative
studies of living forms and with the quantitative estimates provided
by comparative studies of proteins and nucleic acids.
Gradual and punctuational evolution
The fossil record indicates that morphological evolution is by and
large a gradual process. Major evolutionary changes are usually due to
a building up over the ages of relatively small changes. But the
fossil record is discontinuous. Fossil strata are separated by sharp
boundaries; accumulation of fossils within a geologic deposit
(stratum) is fairly constant over time, but the transition from one
stratum to another may involve gaps of tens of thousands of years. New
species, characterized by small but discontinuous morphological
changes, typically appear at the boundaries between strata, whereas
the fossils within a stratum exhibit little morphological variation.
That is not to say that the transition from one stratum to another
always involves sudden changes in morphology; on the contrary, fossil
forms often persist virtually unchanged through several geologic
strata, each representing millions of years.
The apparent morphological discontinuities of the fossil record are
often attributed by paleontologists to the discontinuity of the
sediments; that is, to the substantial time gaps encompassed in the
boundaries between strata. The assumption is that, if the fossil
deposits were more continuous, they would show a more gradual
transition of form. Even so, morphological evolution would not always
keep progressing gradually, because some forms, at least, remain
unchanged for extremely long times. Examples are the lineages known as
"living fossils": the lamp shell Lingula, a genus of brachiopod that
appears to have remained essentially unchanged since the Ordovician
Period, some 450,000,000 years ago; or the tuatara (Sphenodon
punctatus), a reptile that has shown little morphological evolution
for nearly 200,000,000 years since the early Mesozoic.
Some paleontologists have proposed that the discontinuities of the
fossil record are not artifacts created by gaps in the record, but
rather reflect the true nature of morphological evolution, which
happens in sudden bursts associated with the formation of new species.
The lack of morphological evolution, or stasis, of lineages such as
Lingula and Sphenodon is in turn due to lack of speciation within
those lineages. The proposition that morphological evolution is jerky,
with most morphological change occurring during the brief speciation
events and virtually no change during the subsequent existence of the
species, is known as the punctuated equilibrium model of morphological
evolution.
Whether morphological evolution in the fossil record is predominantly
punctuational or gradual is a much debated question. The imperfection
of the record makes it unlikely that the issue will be settled in the
foreseeable future. Intensive study of a favourable and abundant set
of fossils may be expected to substantiate punctuated or gradual
evolution in particular cases. But the argument is not about whether
only one or the other pattern ever occurs; it is about their relative
frequency. Some paleontologists argue that morphological evolution is
in most cases gradual and only rarely jerky, whereas others think the
opposite is true.
Art:Figure 2: Rib strength in the evolution of the brachiopod Eocelia.
The horizontal bars ...
Figure 2: Rib strength in the evolution of the brachiopod Eocelia. The
horizontal bars ...
Encyclopædia Britannica, Inc.
Art:Figure 2: Rib strength in the evolution of the brachiopod Eocelia.
The horizontal bars ...
Figure 2: Rib strength in the evolution of the brachiopod Eocelia. The
horizontal bars ...
Encyclopædia Britannica, Inc.
Much of the problem is that gradualness or jerkiness is in the eye of
the beholder. Consider the evolution of rib strength (the ratio of rib
height to rib width) within a lineage of fossil brachiopods of the
genus Eocelia. An abundant sample of fossils from the Silurian Period
in Wales has been analyzed, with the results shown in Figure 2. One
possible interpretation of the data is that rib strength changed
little or not at all from 415,000,000 to 413,000,000 years ago; rapid
change ensued for the next 1,000,000 years, with virtual stasis from
412,000,000 to 407,000,000 years ago; another short burst of change
occurred around 406,000,000 years ago, followed by a final period of
stasis. On the other hand, the record shown in the Figure may be
interpreted as not particularly punctuated but rather as a gradual
process, with the rate of change somewhat greater at particular times.
The proponents of the punctuated equilibrium model propose not only
that morphological evolution is jerky but also that it is associated
with speciation events. They argue that phyletic evolution--that is,
evolution along lineages of descent--proceeds at two levels. First,
there is continuous change through time within a population. This
consists largely of gene substitutions prompted by natural selection,
mutation, genetic drift, and other genetic processes that operate at
the level of the individual organism. The punctualists maintain that
this continuous evolution within established lineages rarely, if ever,
yields substantial morphological changes in species. Second, they say,
there is the process of origination and extinction of species, in
which most morphological change occurs. According to the punctualist
model, evolutionary trends result from the patterns of origination and
extinction of species rather than from evolution within established
lineages.
Species are groups of interbreeding natural populations that are
reproductively isolated from any other such groups. Speciation
involves, therefore, the development of reproductive isolation between
populations previously able to interbreed. Paleontologists recognize
species by their different morphologies as preserved in the fossil
record, but fossils cannot provide evidence of the development of
reproductive isolation because new species that are reproductively
isolated from their ancestors are often morphologically
indistinguishable from them. Speciation as seen by paleontologists
always involves substantial morphological change because
paleontologists identify new species by morphological differences.
This situation creates an insuperable difficulty for resolving the
question whether morphological evolution is always associated with
speciation events. If speciation is defined as the evolution of
reproductive isolation, the fossil record provides no evidence of a
necessary association between speciation and morphological change. But
if new species are identified in the fossil record by morphological
changes, then all such changes will occur concomitantly with the
origination of new species.
Molecular evolution
DNA and protein as informational macromolecules
The advances of molecular biology have made possible the comparative
study of proteins and the nucleic acids, DNA and RNA. The DNA is the
repository of hereditary (evolutionary and developmental) information.
The relationship of proteins to the DNA is so immediate that they
closely reflect the hereditary information. This reflection is not
perfect, because the genetic code is redundant and, consequently, some
differences in the DNA do not yield differences in the proteins.
Moreover, it is not complete, because a large fraction of the DNA
(about 90 percent in many organisms) does not code for proteins.
Nevertheless, proteins are so closely related to the information
contained in the DNA that they, as well as the nucleic acids, are
called informational macromolecules.
Nucleic acids and proteins are linear molecules made up of sequences
of units--nucleotides in the case of nucleic acids, amino acids in the
case of proteins--which retain considerable amounts of evolutionary
information. Comparing two macromolecules establishes the number of
their units that are different. Because evolution usually occurs by
changing one unit at a time, the number of differences is an
indication of the recency of common ancestry. Changes in evolutionary
rates may create difficulties, but macromolecular studies have two
notable advantages over comparative anatomy and the other classical
disciplines. One is that the information is more readily quantifiable.
The number of units that are different is readily established when the
sequence of units is known for a given macromolecule in different
organisms. The other advantage is that comparisons can be made even
between very different sorts of organisms. There is very little that
comparative anatomy can say when organisms as diverse as yeasts, pine
trees, and human beings are compared; but there are homologous
macromolecules that can be compared in all three.
Informational macromolecules provide information not only about the
topology of evolutionary history (cladogenesis) but also about the
amount of genetic change that has occurred in any given lineage
(anagenesis). It might seem at first that quantifying anagenesis for
proteins and nucleic acids would be impossible, because it would
require comparison of molecules from organisms that lived in the past
with those from living organisms. Organisms of the past are sometimes
preserved as fossils, but their DNA and proteins have largely
disintegrated. Nevertheless, comparisons between living species
provide information about anagenesis.
Art:Figure 3: Amount of change in the evolutionary history of three
living species (C, D, and E), ...
Figure 3: Amount of change in the evolutionary history of three living
species (C, D, and E), ...
Encyclopædia Britannica, Inc.
The following is an example of such comparison: Two living species, C
and D, have a common ancestor, the extinct species B (Figure 3). If C
and D were found to differ by four amino-acid substitutions in a
single protein, then it could safely be assumed that two substitutions
(four total changes divided by two species) had taken place in the
evolutionary lineage of each species. This assumption, however, could
be invalidated by the discovery of a third living species, E, that is
related to C, D, and their ancestor, B, through an earlier ancestor,
A. The number of amino-acid differences between the protein molecules
of the three living species may be as follows:
Special Comp
Art:Figure 3: Amount of change in the evolutionary history of three
living species (C, D, and E), ...
Figure 3: Amount of change in the evolutionary history of three living
species (C, D, and E), ...
Encyclopædia Britannica, Inc.
Figure 3 proposes a phylogeny of the three living species, making it
possible to estimate the number of amino-acid substitutions that have
occurred in each lineage. Let x denote the number of differences
between B and C, y denote the differences between B and D, and z
denote the differences between A and B as well as A and E. The
following three equations can be produced:
Special Comp
Solving the equations yields x = 3, y = 1, and z = 8.
As a concrete example, consider cytochrome c, a protein involved in
cell respiration. The sequence of amino acids in this protein is known
for many organisms, from bacteria and yeast to insects and humans; in
animals, cytochrome c consists of 104 amino acids. When the amino-acid
sequences of humans and rhesus monkeys are compared, they are found to
be different at position 66 (isoleucine in humans, threonine in rhesus
monkeys), but identical at the other 103 positions. When humans are
compared with horses, 12 amino-acid differences are found; but when
horses are compared with rhesus monkeys there are only 11 amino-acid
differences. Even without knowing anything else about the evolutionary
history of mammals, one would conclude that the lineages of humans and
rhesus monkeys diverged from each other much more recently than they
diverged from the horse lineage. Moreover, it can be concluded that
the amino-acid difference between humans and rhesus monkeys must have
occurred in the human lineage after its separation from the rhesus
monkey lineage.
Molecular phylogenies of species
Art:Figure 4: Phylogeny based on differences in the protein sequence
of cytochrome c in organisms ...
Figure 4: Phylogeny based on differences in the protein sequence of
cytochrome c in organisms ...
Encyclopædia Britannica, Inc.
Table 4
Table 4
Protein sequencing is one of several molecular methods developed for
estimating genetic change during evolution. The effectiveness of this
method can be illustrated by again using as an example the protein
cytochrome c, whose amino-acid sequences are well known. Phylogenies
can be constructed based on the number of amino-acid differences
between species. But the amino-acid sequence of a protein contains
more information than is reflected in the number of amino-acid
differences. This is because the replacement of one amino acid by
another in some cases requires no more than one nucleotide
substitution in the DNA that codes for the protein but, in other
cases, requires at least two nucleotide changes. Table 4 shows the
minimum number of nucleotide differences in the genes of 20 separate
species that are necessary to account for the amino-acid differences
in their cytochrome c. Figure 4 proposes a phylogenetic tree based on
the data in Table 4, showing the minimum numbers of nucleotide changes
in each branch.
Art:Figure 4: Phylogeny based on differences in the protein sequence
of cytochrome c in organisms ...
Figure 4: Phylogeny based on differences in the protein sequence of
cytochrome c in organisms ...
Encyclopædia Britannica, Inc.
Table 4
The relationships between species as shown in Figure 4 correspond
fairly well with the relationships determined from other sources, such
as the fossil record. According to Table 4, chickens are less closely
related to ducks and pigeons than to penguins, and humans and monkeys
diverged from the other mammals before the marsupial kangaroo
separated from the nonprimate placentals. These are known to be
erroneous relationships; but the power of the method is apparent in
that a single protein yields a fairly accurate reconstruction of the
evolutionary history of 20 organisms that started to diverge more than
1,000,000,000 years ago.
Cytochrome c is a slowly evolving protein. Widely different species
have in common a large proportion of the amino acids in their
cytochrome c, making possible the study of genetic differences among
organisms only remotely related. For the same reason, however,
comparing cytochrome c cannot determine evolutionary change in closely
related species. For example, the amino-acid sequence of cytochrome c
in humans and chimpanzees is identical, although they diverged about
10,000,000 years ago; between humans and rhesus monkeys, who diverged
from their common ancestor 50,000,000 to 40,000,000 years ago, it
differs by only one amino-acid replacement.
Other proteins that evolve more rapidly can be studied in order to
establish phylogenetic relationships among closely related species.
Genetic changes in the evolution of such species can also be studied
by DNA sequencing, DNA hybridization, immunology, and gel
electrophoresis.
Molecular phylogenies of genes
It is now possible to obtain the nucleotide sequence of the DNA.
Although the number of genes that have been sequenced is relatively
small, new sequences are being worked out at a fast rate. The use of
DNA sequences in evolutionary research has so far been rewarding,
particularly in the study of gene duplications. The genes that code
for the hemoglobins in humans and a few other mammals provide the best
example.
The amino-acid sequences of the hemoglobin chains and of myoglobin, a
closely related protein, are known for humans as well as for other
organisms. These sequences have made it possible to reconstruct the
evolutionary history of the duplications that gave rise to the
corresponding genes. But direct examination of the nucleotide
sequences of the coding for these proteins has shown that the
situation is more complex, and also more interesting, than it appears
from the protein sequences.
The DNA sequence studies on human hemoglobin genes have shown that
their number is greater than previously thought. The hemoglobins are
tetramers, consisting of two polypeptides of one kind and two of
another kind. One of the two kinds of polypeptide is e in embryonic
hemoglobin, g in fetal hemoglobin, b in adult hemoglobin A, and d in
adult hemoglobin A[2]. (Hemoglobin A makes up about 98 percent, and
hemoglobin A[2] about 2 percent, of human adult hemoglobin). The other
kind of polypeptide is z in embryonic hemoglobin and a in fetal and
adult hemoglobin. The genes cod
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