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诸位，百科全书开始，百科全书结束，如何？ -- Anonymous - (796 Byte) 2005-2-19 周六, 上午12:08 (282 reads) |
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作者：Anonymous 在 罕见奇谈 发贴, 来自 http://www.hjclub.org
我还想等到周末侃侃我信奉的那种不可救药的自然主义 --- 既不借助于上帝，也不借助于先验的“道德直觉”，单单用生物本能和进化论，来解释人类道德的起源和演变。我觉得这种自然主义道德观很有力，所以到底没能受洗成为教徒。
自然主义，我把它看作人类的智慧之旅，试试不依靠任何神力，人的智慧能够引导我们走多远。这并非狂妄自大，也不是企图把自己当神。当初在伊甸园里，上帝只说知识果不能吃，可没说不能摸。夏娃理解成知识果不仅不能吃，连摸都不能摸，恐怕非上帝之意。上帝把知识树种在伊甸园里，只禁人吃，却不禁人摸，我想是有深意的。不让我们去吃它，大约是因为人的理性有其限度。假如人无视理性的限度，失去了对自然秩序的敬畏，一定要把自己当神，跟神一样眼睛亮(蛇引诱夏娃的话)，那就难免堕入罪恶。但是上帝给人的使命，是要人管理这个世界，自然需要人运用自己的智慧，去理解这个世界的规律，包括理解人类社会自身的规律。上帝并没有说人的理性限度在哪里，自然不禁止人去探索。有些教徒批判自然主义，有点象是要自己去立个限度，说是超过这个限度，你就不能去探索。我很不喜欢这一点。
后面是大英百科全书的“进化”词条。里面说，关于生命的起源，早期的神学家，如阿奎那，并不排斥生命起源于无机物。他们把这留给科学家去探索，并不以为这违背圣经。
关于“进化作为一个事实”，大英百科全书的说法与你的转述大同小异：The evolutionary origin of organisms is today a scientific conclusion established with the kind of certainty attributable to such scientific concepts as the roundness of the Earth, the motions of the planets, and the molecular composition of matter. This degree of certainty beyond reasonable doubt is what is implied when biologists say that evolution is a "fact"; the evolutionary origin of organisms is accepted by virtually every biologist.
下面是全文：
evolution
[thistle.gif] Encyclopædia Britannica Article
evolution
theory in biology postulating that the various types of animals and
plants have their origin in other preexisting types and that the
distinguishable differences are due to modifications in successive
generations. The theory of evolution is one of the fundamental
keystones of modern biological theory. (See also human evolution.)
The diversity of the living world is staggering. More than 2,000,000
existing species of plants and animals have been named and described;
many more remain to be discovered--from 10,000,000 to 30,000,000
according to some estimates. What is impressive is not just the
numbers but also the incredible heterogeneity in size, shape, and way
of life: from lowly bacteria, measuring less than one-thousandth of a
millimetre in diameter, to the stately sequoias of California, rising
300 feet (100 metres) above the ground and weighing several thousand
tons; from bacteria living in the hot springs of Yellowstone National
Park at temperatures near the boiling point of water to fungi and
algae thriving on the ice masses of Antarctica and in saline pools at
-9° F (-23° C); and from the strange wormlike creatures discovered in
dark ocean depths at thousands of feet below the surface to spiders
and larkspur plants existing on Mt. Everest more than 19,868 feet
above sea level.
The virtually infinite variations on life are the fruit of the
evolutionary process. All living creatures are related by descent from
common ancestors. Humans and other mammals are descended from
shrewlike creatures that lived more than 150,000,000 years ago;
mammals, birds, reptiles, amphibians, and fishes share as ancestors
aquatic worms that lived 600,000,000 years ago; all plants and animals
are derived from bacteria-like microorganisms that originated more
than 3,000,000,000 years ago. Biological evolution is a process of
descent with modification. Lineages of organisms change through
generations; diversity arises because the lineages that descend from
common ancestors diverge through time.
The 19th-century English naturalist Charles Darwin argued that
organisms come about by evolution, and he provided a scientific
explanation, essentially correct but incomplete, of how evolution
occurs and why it is that organisms have features--such as wings,
eyes, and kidneys--clearly structured to serve specific functions.
Natural selection was the fundamental concept in his explanation.
Genetics, a science born in the 20th century, reveals in detail how
natural selection works and led to the development of the modern
theory of evolution. Since the 1960s a related scientific discipline,
molecular biology, has advanced enormously knowledge of biological
evolution and has made it possible to investigate detailed problems
that seemed completely out of reach a few years earlier--for example,
how similar the genes of humans and chimpanzees might be (they differ
in about 1 or 2 percent of the units that make up the genes).
This article discusses evolution as it applies generally to living
things. For a discussion of human evolution, see the article human
evolution. For a more complete treatment of a discipline that has
proved essential to the study of evolution, see human genetics and
heredity. Specific aspects of evolution are discussed in the articles
coloration and mimicry. Applications of evolutionary theory to plant
and animal breeding are discussed in the articles plant breeding and
animal breeding. A detailed discussion of the life and thought of
Charles Darwin is found in the article Darwin, Charles.
History of evolutionary theory
Early ideas
All human cultures have developed their own explanations for the
origin of the world, man, and other creatures. Traditional Judaism and
Christianity explain the origin of living beings and their adaptations
to their environments--wings, gills, hands, flowers--as the handiwork
of an omniscient God. The philosophers of ancient Greece had their own
creation myths. Anaximander proposed that animals could be transformed
from one kind into another, and Empedocles speculated that they could
be made up of various combinations of preexisting parts. Closer to
modern evolutionary ideas were the proposals of early Church Fathers
like Gregory of Nazianzus and Augustine, who maintained that not all
species of plants and animals were created by God; rather, some had
developed in historical times from God's creations. Their motivation
was not biological but religious: it would have been impossible to
hold representatives of all species in a single vessel such as Noah's
ark; hence, some species must have come into existence only after the
Noachian Flood.
The notion that organisms may change by natural processes was not
investigated as a biological subject by Christian theologians of the
Middle Ages, but it was, usually incidentally, considered as a
possibility by many, including Albertus Magnus and his student Thomas
Aquinas. Aquinas concluded, after detailed discussion, that the
development of living creatures like maggots and flies from nonliving
matter like decaying meat was not incompatible with Christian faith or
philosophy. But he left it to scientists to decide whether this
actually happened in fact.
The idea of progress, particularly the belief in unbounded human
progress, was central to the Enlightenment of the 18th century,
particularly in France among such philosophers as Condorcet and
Diderot and such scientists as Buffon. But belief in progress did not
necessarily lead to the development of a theory of evolution.
Pierre-Louis Moreau de Maupertuis proposed the spontaneous generation
and extinction of organisms as part of his theory of origins, but he
advanced no theory of evolution--i.e., the transformation of one
species into another through knowable, natural causes. Georges-Louis
Leclerc, comte de Buffon, one of the greatest naturalists of the time,
explicitly considered--and rejected--the possible descent of several
species from a common ancestor. He postulated that organisms arise
from organic molecules by spontaneous generation, so that there could
be as many kinds of animals and plants as there are viable
combinations of organic molecules.
The physician Erasmus Darwin, grandfather of Charles Darwin, offered
in his Zoonomia or the Laws of Organic Life some evolutionary
speculations, but they were not further developed and had no real
influence on subsequent theories. The Swedish botanist Carolus
Linnaeus devised the hierarchical system of plant and animal
classification that is still in use in a modernized form. Although he
insisted on the fixity of species, his classification system
eventually contributed much to the acceptance of the concept of common
descent.
The great French naturalist Jean-Baptiste Lamarck held the enlightened
view of his age that living organisms represent a progression, with
humans as the highest form. From this idea he proposed, in the early
years of the 19th century, the first broad theory of evolution.
Organisms evolve through eons of time from lower to higher forms, a
process still going on, always culminating in man. As organisms become
adapted to their environments through their habits, modifications
occur. Use of an organ or structure reinforces it; disuse leads to
obliteration. The characteristics acquired by use and disuse,
according to this theory, would be inherited. This assumption, later
called the inheritance of acquired characteristics, was thoroughly
disproved in the 20th century. Although his theory did not stand up in
the light of later knowledge, Lamarck made important contributions to
the gradual acceptance of biological evolution and stimulated
countless later studies.
Charles Darwin
The founder of the modern theory of evolution was Charles Darwin. The
son and grandson of physicians, he enrolled as a medical student at
the University of Edinburgh. After two years, however, he left to
study at Cambridge University and prepare to become a clergyman. He
was not an exceptional student, but he was deeply interested in
natural history. On Dec. 27, 1831, a few months after his graduation
from Cambridge, he sailed as a naturalist aboard the HMS Beagle on a
round-the-world trip that lasted until October 1836. Darwin was often
able to disembark for extended trips ashore to collect natural
specimens.
The discovery of fossil bones from large extinct mammals in Argentina
and the observation of numerous species of finches in the Galápagos
Islands were among the events credited with stimulating Darwin's
interest in how species originate. In 1859 he published On the Origin
of Species by Means of Natural Selection, a treatise establishing the
theory of evolution and, most important, the role of natural selection
in determining its course. He published many other books as well,
notably The Descent of Man and Selection in Relation to Sex (1871),
which extends the theory of natural selection to human evolution.
Darwin must be seen as a great intellectual revolutionary who
inaugurated a new era in the cultural history of mankind, an era that
was the second and final stage of the Copernican revolution that had
begun in the 16th and 17th centuries under the leadership of men such
as Copernicus, Galileo, and Newton. The Copernican revolution marked
the beginnings of modern science. Discoveries in astronomy and physics
overturned traditional conceptions of the universe. The Earth was no
longer seen as the centre of the universe but as a small planet
revolving around one of a myriad of stars; the seasons, the rains that
make crops grow, destructive storms, and other vagaries of weather all
became understood as aspects of natural processes; the circumvolutions
of the planets were now explained by simple laws that also accounted
for the motion of projectiles on the Earth.
The significance of these and other discoveries was that they led to a
conception of the universe as a system of matter in motion governed by
laws of nature. The workings of the universe no longer needed to be
attributed to the ineffable will of the Creator, but were brought into
the realm of science--an explanation of phenomena through natural
laws. Physical phenomena like tides, eclipses, and positions of the
planets could now be predicted whenever the causes were adequately
known. Darwin accumulated evidence showing that evolution had
occurred, that diverse organisms share common ancestors, and that
living beings have changed drastically over the course of the Earth's
history. More importantly, however, he extended to the living world
the idea of nature as a system of matter in motion governed by natural
laws.
Before Darwin, the origin of the Earth's living things, with their
marvelous contrivances for adaptation, had been attributed to the
design of an omniscient God. He had created the fish in the waters,
the birds in the air, and all sorts of animals and plants on the land.
God had endowed these creatures with gills for breathing, wings for
flying, and eyes for seeing, and he had coloured birds and flowers so
that man could enjoy them and recognize his wisdom. Christian
theologians, from Thomas Aquinas on, had argued that the presence of
design, so evident in living beings, demonstrates the existence of a
supreme creator; the argument from design was Aquinas' "fifth way" for
proving the existence of God. In 19th-century England, the eight
Bridgewater Treatises were commissioned so that eminent scientists and
philosophers would expand on the marvels of the natural world and
thereby set forth "the Power, wisdom, and goodness of God as
manifested in the Creation."
The British theologian William Paley in his Natural Theology (1802)
used natural history, physiology, and other contemporary knowledge to
elaborate the argument from design. If a person should find a watch,
even in an uninhabited desert, Paley contended, the harmony of its
many parts would force him to conclude that it had been created by a
skilled watchmaker; and, Paley went on, how much more intricate and
perfect in design is the human eye, with its transparent lens, its
retina placed at the precise distance for forming a distinct image,
and its large nerve transmitting signals to the brain.
The argument from design seems to be forceful. A ladder is made for
climbing, a knife for cutting, and a watch for telling time; their
functional design leads to the conclusion that they have been
fashioned by a carpenter, a smith, or a watchmaker. Similarly, the
obvious functional design of animals and plants seems to denote the
work of a Creator. It was Darwin's genius that he provided a natural
explanation for the organization and functional design of living
beings.
Darwin accepted the facts of adaptation: hands are for grasping, eyes
for seeing, lungs for breathing. But he showed that the multiplicity
of plants and animals, with their exquisite and varied adaptations,
could be explained by a process of natural selection, without recourse
to a Creator or any designer agent. He brought the living world into
the realm of natural science, thereby completing the Copernican
revolution. All natural phenomena were henceforth opened to
explanation by natural causes and viewed as the result of physical
processes governed by natural laws. This achievement of Darwin's had
intellectual and cultural implications more profound and lasting than
his multipronged evidence that convinced contemporaries of the fact of
evolution.
Darwin's theory of natural selection is summarized in the Origin of
Species as follows:
As many more individuals are produced than can possibly survive,
there must in every case be a struggle for existence, either one
individual with another of the same species, or with the
individuals of distinct species, or with the physical conditions of
life. . . . Can it, then, be thought improbable, seeing that
variations useful to man have undoubtedly occurred, that other
variations useful in some way to each being in the great and
complex battle of life, should sometimes occur in the course of
thousands of generations? If such do occur, can we doubt
(remembering that many more individuals are born than can possibly
survive) that individuals having any advantage, however slight,
over others, would have the best chance of surviving and of
procreating their kind? On the other hand, we may feel sure that
any variation in the least degree injurious would be rigidly
destroyed. This preservation of favourable variations and the
rejection of injurious variations, I call Natural Selection.
Natural selection was proposed by Darwin primarily to account for the
adaptive organization of living beings; it is a process that promotes
or maintains adaptation. Evolutionary change through time and
evolutionary diversification (multiplication of species) are not
directly promoted by natural selection, but they often ensue as
by-products of natural selection as it fosters adaptation to different
environments. (See below The process of evolution: The concept of
natural selection.)
Modern conceptions
The Darwinian aftermath
The publication of the Origin of Species produced considerable public
excitement. Scientists, politicians, clergymen, and notables of all
kinds read and discussed the book, defending or deriding Darwin's
ideas. The most visible actor in the controversies immediately
following publication was T.H. Huxley, known as "Darwin's bulldog,"
who defended the theory of evolution with articulate and sometimes
mordant words on public occasions as well as in numerous writings.
Evolution by natural selection was indeed a favourite topic in society
salons during the 1860s and beyond. But serious scientific
controversies also arose, first in Britain and then on the Continent
and in the United States.
One occasional participant in the discussion was the naturalist Alfred
Russel Wallace, who had hit upon the idea of natural selection
independently and had sent a short manuscript to Darwin from the Malay
archipelago. On July 1, 1858, one year before the publication of the
Origin, a paper jointly written by Wallace and Darwin was presented,
in the absence of both, to the Linnean Society in London--with
apparently little notice. Greater credit is duly given to Darwin than
to Wallace for the idea of evolution by natural selection; Darwin
developed the theory in considerably more detail, provided far more
evidence for it, and was primarily responsible for its acceptance.
Wallace's views differed from Darwin's in several ways, most
importantly in that Wallace did not think natural selection sufficient
to account for the origin of man, which in his view required direct
divine intervention.
A younger contemporary of Darwin, with considerable influence during
the latter part of the 19th and early 20th centuries, was Herbert
Spencer. He was a philosopher rather than a biologist, but he became
an energetic proponent of evolutionary ideas, popularized a number of
slogans, like "survival of the fittest" (which was taken up by Darwin
in later editions of the Origin), and engaged in social and
metaphysical speculations. His ideas considerably damaged proper
understanding and acceptance of the theory of evolution by natural
selection. Darwin wrote of Spencer's speculations:
His deductive manner of treating any subject is wholly opposed to
my frame of mind . . . . His fundamental generalizations (which
have been compared in importance by some persons with Newton's
laws!) which I dare say may be very valuable under a philosophical
point of view, are of such a nature that they do not seem to me to
be of any strictly scientific use.
Most pernicious was the crude extension of the notion of "struggle for
existence" to human economic and social life that became known as
social Darwinism.
The most serious difficulty facing Darwin's evolutionary theory was
the lack of an adequate theory of inheritance that would account for
the preservation through the generations of the variations on which
natural selection was supposed to act. Current theories of "blending
inheritance" proposed that offspring merely struck an average between
the characteristics of their parents. As Darwin became aware, blending
inheritance (including his own theory of "pangenesis") could not
account for the conservation of variations, because differences among
variant offspring would be halved each generation, rapidly reducing
the original variation to the average of the preexisting
characteristics.
The missing link in Darwin's argument was provided by Mendelian
genetics. About the time the Origin of Species was published, the
Augustinian monk Gregor Mendel was starting a long series of
experiments with peas in the garden of his monastery in Brünn,
Austria-Hungary (now Brno, Czech.). These experiments and the analysis
of their results are by any standard an example of masterly scientific
method. Mendel's paper, published in 1866 in the Proceedings of the
Natural Science Society of Brünn, formulated the fundamental
principles of the theory of heredity that is still current. His theory
accounts for biological inheritance through particulate factors
(genes) inherited one from each parent, which do not mix or blend but
segregate in the formation of the sex cells, or gametes.
Mendel's discoveries, however, remained unknown to Darwin and, indeed,
did not become generally known until 1900, when they were
simultaneously rediscovered by a number of scientists on the
Continent. In the meantime Darwinism, in the latter part of the 19th
century, faced an alternative evolutionary theory known as
neo-Lamarckism. This hypothesis shared with Lamarck's the importance
of use and disuse in the development and obliteration of organs, and
it added the notion that the environment acts directly on organic
structures, which explained their adaptation to the way of life and
environment of the organism. Adherents of this theory discarded
natural selection as an explanation for adaptation to the environment.
Prominent among the defenders of natural selection was the German
biologist August Weismann, who in the 1880s published his germ-plasm
theory. He distinguished two substances that make up an organism: the
soma, which comprises most body parts and organs, and the germ plasm,
which contains the cells that give rise to the gametes and hence to
progeny. Early in the development of an egg, the germ plasm becomes
segregated from the soma; that is, from the cells that give rise to
the rest of the body. This notion of a radical separation between germ
and soma prompted Weismann to assert that inheritance of acquired
characteristics was impossible, and it opened the way for his
championship of natural selection as the only major process that would
account for biological evolution. Weismann's ideas became known after
1896 as neo-Darwinism.
The synthetic theory
The rediscovery in 1900 of Mendel's theory of heredity, by Hugo de
Vries of The Netherlands and others, led to an emphasis on the role of
heredity in evolution. De Vries proposed a new theory of evolution
known as mutationism, which essentially did away with natural
selection as a major evolutionary process. According to de Vries
(joined by other geneticists such as William Bateson in England),
there are two kinds of variation that take place in organisms. One is
the "ordinary" variability observed among individuals of a species,
which is of no lasting consequence in evolution because, according to
de Vries, it could not "lead to a transgression of the species border
even under conditions of the most stringent and continued selection."
The other consists of the changes brought about by mutations,
spontaneous alterations of genes that yield large modifications of the
organism and gave rise to new species: "The new species thus
originates suddenly, it is produced by the existing one without any
visible preparation and without transition."
Mutationism was opposed by many naturalists, and in particular by the
so-called biometricians, led by Karl Pearson, who defended Darwinian
natural selection as the major cause of evolution through the
cumulative effects of small, continuous, individual variations (which
the biometricians assumed passed from one generation to the next
without being limited by Mendel's laws of inheritance).
The controversy between mutationists (also referred to at the time as
Mendelians) and biometricians approached a resolution in the 1920s and
'30s through the theoretical work of geneticists. They used
mathematical arguments to show, first, that continuous variation (in
such characteristics as size, number of eggs laid, and the like) could
be explained by Mendel's laws; and second, that natural selection
acting cumulatively on small variations could yield major evolutionary
changes in form and function. Distinguished members of this group of
theoretical geneticists were R.A. Fisher and J.B.S. Haldane in Britain
and Sewall Wright in the United States. Their work contributed to the
downfall of mutationism and, most importantly, provided a theoretical
framework for the integration of genetics into Darwin's theory of
natural selection. Yet their work had a limited impact on contemporary
biologists because it was formulated in a mathematical language that
most of them could not understand; because it was almost exclusively
theoretical, with little empirical corroboration; and because it was
limited in scope, largely omitting many issues, like speciation, that
were of great importance to evolutionists.
A major breakthrough came in 1937 with the publication of Genetics and
the Origin of Species by Theodosius Dobzhansky, a Russian-born
American naturalist and experimental geneticist. Dobzhansky's book
advanced a reasonably comprehensive account of the evolutionary
process in genetic terms, laced with experimental evidence supporting
the theoretical argument. Genetics and the Origin of Species may be
considered the most important landmark in the formulation of what came
to be known as the synthetic theory of evolution, effectively
combining Darwinian natural selection and Mendelian genetics. It had
an enormous impact on naturalists and experimental biologists, who
rapidly embraced the new understanding of the evolutionary process as
one of genetic change in populations. Interest in evolutionary studies
was greatly stimulated, and contributions to the theory soon began to
follow, extending the synthesis of genetics and natural selection to a
variety of biological fields.
The main writers who, together with Dobzhansky, may be considered the
architects of the synthetic theory were the zoologists Ernst Mayr and
Sir Julian Huxley, the paleontologist George G. Simpson, and the
botanist George Ledyard Stebbins. These researchers contributed to a
burst of evolutionary studies in the traditional biological
disciplines and in some emerging ones--notably population genetics
and, later, evolutionary ecology. By 1950 acceptance of Darwin's
theory of evolution by natural selection was universal among
biologists, and the synthetic theory had become widely adopted.
Later developments
The most important line of investigation since 1950 has been the
application of molecular biology to evolutionary studies. In 1953
James Watson and Francis Crick deduced the structure of DNA
(deoxyribonucleic acid), the hereditary material contained in the
chromosomes of every cell's nucleus. The genetic information is
contained within the sequence of nucleotides that make up the
chainlike DNA molecules. This information determines the sequence of
amino acids in the proteins, including enzymes responsible for the
organism's fundamental life processes. Genetic information contained
in the DNA can thus be investigated by examining the sequences of
amino acids in the proteins.
In the mid-1960s techniques like electrophoresis and selective assay
of enzymes became available for the rapid and inexpensive study of
differences among enzymes and other proteins. The application of these
techniques to evolutionary problems made possible the pursuit of
issues that earlier could not be investigated; for example, exploring
the extent of genetic variation in natural populations (which sets
bounds to their evolutionary potential) and determining the amount of
genetic change that occurs during the formation of new species.
Comparisons of the amino acid sequences of proteins in different
species provided quantitatively precise measures of species
divergence, a considerable improvement over the typically qualitative
evaluations obtained by comparative anatomy and other evolutionary
subdisciplines. In 1968 the Japanese geneticist Motoo Kimura proposed
the neutrality theory of molecular evolution, which assumes that at
the level of DNA and protein sequence many changes are adaptively
neutral and have little or no effect on the molecule's function. If
the neutrality theory is correct, there should be a "molecular clock"
of evolution; that is, the degree of divergence between species in
amino acid or nucleotide sequence would provide a reliable estimate of
the time since their divergence. This would make possible a
reconstruction of evolutionary history that would reveal the order of
branching of different lineages, such as those leading to humans,
chimpanzees, and orangutans, as well as the time in the past when the
lineages split from one another.
During the 1970s and '80s it gradually became clear that the molecular
clock is not exact; nevertheless, it has continued to provide the most
reliable source of evidence for reconstructing a history of evolution.
The techniques of DNA cloning and sequencing have provided a new and
more powerful means of investigating evolution at the molecular level.
The fruits of this new development began to accumulate during the
1980s.
The earth sciences have also experienced, in the second half of the
20th century, a conceptual revolution with considerable consequence to
the study of evolution. The science of plate tectonics has shown that
the configuration and position of the continents and oceans are
dynamic, rather than static, features of the Earth. Oceans grow and
shrink, while continents break into fragments or coalesce into larger
masses. The continents move across the Earth's surface at rates of a
few centimetres a year, and over millions of years of geologic history
this profoundly alters the face of the Earth, causing major climatic
changes along the way. These previously unsuspected massive
modifications of the planet's environments have of necessity been
reflected in the evolutionary history of life. Biogeography, the
evolutionary study of plant and animal distribution, has been
revolutionized by the knowledge, for example, that Africa and South
America were part of a single landmass some 200,000,000 years ago and
that the Indian subcontinent was not connected with Asia until recent
geologic times.
Ecology, the study of the interactions of organisms with their
environments, has evolved from descriptive studies--"natural
history"--into a vigorous biological discipline with a strong
mathematical component, both in the development of theoretical models
and in the collection and analysis of quantitative data. Evolutionary
ecology is an active field of evolutionary biology; another is
evolutionary ethology, the study of animal behaviour. Sociobiology,
the evolutionary study of social behaviour, is perhaps the most active
subfield of ethology. It is also the most controversial, because of
its extension to human societies.
Impact and acceptance of evolutionary theory
The theory of evolution makes statements about three different, though
related, issues: (1) the fact of evolution; that is, that organisms
are related by common descent; (2) evolutionary history--the details
of when lineages split from one another and of the changes that
occurred in each lineage; and (3) the mechanisms or processes by which
evolutionary change occurs.
The first issue is the most fundamental and the one established with
utmost certainty. Darwin gathered much evidence in its support, but
the evidence has accumulated continuously ever since, derived from all
biological disciplines. The evolutionary origin of organisms is today
a scientific conclusion established with the kind of certainty
attributable to such scientific concepts as the roundness of the
Earth, the motions of the planets, and the molecular composition of
matter. This degree of certainty beyond reasonable doubt is what is
implied when biologists say that evolution is a "fact"; the
evolutionary origin of organisms is accepted by virtually every
biologist.
But the theory of evolution goes much beyond this first issue, the
general affirmation that organisms evolve. The second and third issues
involve seeking to ascertain the evolutionary relationships between
particular organisms and the events of evolutionary history, as well
as to explain how and why evolution takes place. These are matters of
active scientific investigation. Some conclusions are well
established; for example, that the chimpanzee and gorilla are more
closely related to humans than is any of those three species to the
baboon or other monkeys; or that natural selection, the process
postulated by Darwin, explains the adaptive configuration of such
features as the human eye and the wings of birds. Many matters are
less certain, others are conjectural, and still others--such as the
characteristics of the first living things and when they came
about--remain completely unknown.
Since Darwin, the theory of evolution has gradually extended its
influence to other biological disciplines, from physiology to ecology
and from biochemistry to systematics. All biological knowledge now
includes the phenomenon of evolution. In the words of Dobzhansky,
"Nothing in biology makes sense except in the light of evolution."
The term evolution and the concept of change through time have also
become incorporated into scientific language well beyond biology, and
even into common language. Astrophysicists speak of the evolution of
the solar system or of the universe; geologists, of the evolution of
the Earth's mantle; psychologists, of the evolution of the mind;
anthropologists, of the evolution of cultures; art historians, of the
evolution of architectural styles; and couturiers, of the evolution of
fashion. These and other disciplines share only the slightest
commonality of meaning: the notion of gradual, and perhaps
directional, change over the course of time.
Darwin's notion of natural selection has also been extended to other
areas of human discourse, particularly in the fields of sociopolitical
theory and economics. The extension can only be metaphorical, because
in Darwin's intended meaning natural selection applies only to
hereditary variations in entities endowed with biological
reproduction, that is, to living organisms. That natural selection is
anatural process in the living world has been taken by some as a
justification for ruthless competition and for "survival of the
fittest" in the struggle for economic advantage or for political
hegemony. Social Darwinism was an influential social philosophy in
some circles through the late 19th and early 20th centuries. At the
other end of the political spectrum, Marxist theorists have resorted
to evolution by natural selection as an explanation for mankind's
political history.
These uses and abuses of the terms evolution and natural selection
have in turn stimulated resistance against biological evolution and
natural selection. In addition, the theory of evolution has been seen
by some people as incompatible with religious beliefs, particularly
those of Christianity. The first chapters of the book of Genesis
describe God's creation of the world, the plants, the animals, and
man. A literal interpretation of Genesis seems incompatible with the
gradual evolution of humans and other organisms by natural processes.
Independently of the biblical narrative, the Christian beliefs in the
immortality of the soul and in man as "created in the image of God"
have appeared to many as contrary to the evolutionary origin of man
from nonhuman animals.
Religiously motivated attacks started during Darwin's lifetime. In
1874 Charles Hodge, an American Protestant theologian, published What
Is Darwinism?, one of the most articulate assaults on evolutionism.
Hodge perceived Darwin's theory as "the most thoroughly naturalistic
that can be imagined and far more atheistic than that of his
predecessor Lamarck." He argued that the design of the human eye
evinces that "it has been planned by the Creator, like the design of a
watch evinces a watchmaker." He concluded that "the denial of design
in nature is actually the denial of God."
Other Protestant theologians saw a solution to the difficulty in the
idea that God operates through intermediate causes. The origin and
motion of the planets could be explained by the law of gravity and
other natural processes without denying God's creation and providence.
Similarly, evolution could be seen as the natural process through
which God brought living beings into existence and developed them
according to his plan. Thus, A.H. Strong, the president of Rochester
(N.Y.) Theological Seminary, wrote in his Systematic Theology (1885):
"We grant the principle of evolution, but we regard it as only the
method of divine intelligence." The brutish ancestry of man was not
incompatible with his excelling status as a creature in the image of
God. Strong drew an analogy with Christ's miraculous conversion of
water into wine: "The wine in the miracle was not water because water
had been used in the making of it, nor is man a brute because the
brute has made some contributions to its creation."
Arguments for and against Darwin's theory came from Roman Catholic
theologians as well. Gradually, well into the 20th century, evolution
by natural selection came to be accepted by the majority of Christian
writers. Pope Pius XII in his encyclical Humani Generis (1950; "Of the
Human Race") acknowledged that biological evolution was compatible
with the Christian faith, although he argued that God's intervention
was necessary for the creation of the human soul. In 1981 Pope John
Paul II stated in an address to the Pontifical Academy of Sciences:
The Bible itself speaks to us of the origin of the universe and its
make-up, not in order to provide us with a scientific treatise but
in order to state the correct relationships of man with God and
with the universe. Sacred scripture wishes simply to declare that
the world was created by God, and in order to teach this truth it
expresses itself in the terms of the cosmology in use at the time
of the writer. . . . Any other teaching about the origin and
make-up of the universe is alien to the intentions of the Bible,
which does not wish to teach how the heavens were made but how one
goes to heaven.
The Pope's point was that it would be a blunder to mistake the Bible
for an elementary book of astronomy, geology, and biology. The
argument was clearly directed against Christian Fundamentalists who
see in Genesis a literal description of how the world was created by
God. Biblical Fundamentalists make up a minority of Christians, but
they have periodically gained considerable public and political
influence in the United States. During the decade of the 1920s, more
than 20 state legislatures were influenced by them to debate
antievolution laws, and four states--Arkansas, Mississippi, Oklahoma,
and Tennessee--prohibited the teaching of evolution in their public
schools. A spokesman for the antievolutionists was William Jennings
Bryan, three times the unsuccessful Democratic candidate for the
presidency, who said in 1922, "We will drive Darwinism from our
schools." In 1925 Bryan took part in the prosecution of John T.
Scopes, a high school teacher in Dayton, Tenn., who had admittedly
violated the state's law forbidding the teaching of evolution.
In 1968 the Supreme Court of the United States declared
unconstitutional any law banning the teaching of evolution in public
schools. Since that time Christian Fundamentalists have introduced
bills in a number of state legislatures ordering that the teaching of
"evolution-science" be balanced by allocating equal time to
"creation-science." Creation-science maintains that all kinds of
organisms abruptly came into existence at the Creation, that the world
is only a few thousand years old, and that the Noachian Flood was an
actual event which only one pair of each animal species survived. In
the 1980s Arkansas and Louisiana passed acts requiring the balanced
treatment of evolution-science and creation-science in the schools,
but opponents successfully challenged the acts as violations of the
constitutionally mandated separation of church and state.
The evidence for evolution
Darwin and other 19th-century biologists found compelling evidence for
biological evolution in the comparative study of living organisms, in
their geographic distribution, and in the fossil remains of extinct
organisms. In the 20th century the evidence from these sources has
become considerably stronger and more comprehensive. New biological
disciplines--genetics, biochemistry, physiology, ecology--have
supplied powerful additional evidence. Molecular biology, the most
recent and successful of all biological disciplines, furnishes
extensive, consistent, and detailed confirmation of evolution. The
amount of information about evolutionary history stored in the DNA and
proteins of living things is virtually unlimited. Only a want of
resources now precludes the reconstruction of every detail of the
phylogenetic history of life on Earth.
Evolutionists are no longer concerned with obtaining evidence to
support the fact of evolution, but rather with what sorts of knowledge
can be obtained from different sources of evidence. The following
sections identify the most productive of these sources and illustrate
the types of information they have provided.
The fossil record
Art:Figure 1: The geologic time scale from 700,000,000 years ago to
the present, showing major ...
Figure 1: The geologic time scale from 700,000,000 years ago to the
present, showing major ...
Encyclopædia Britannica, Inc.
Paleontologists have recovered and studied the fossil remains of many
thousands of organisms that lived in the past. The fossil record shows
that many kinds of extinct organisms were very different in form from
any now living. It also shows successions of organisms through time,
manifesting the transition from one form to another (see Figure 1).
When an organism dies, it is usually destroyed by other forms of life
and by weathering processes. On rare occasions, some body
parts--particularly hard ones like shells, teeth, or bones--are
preserved by being buried in mud or protected in some other way from
predators and weather. Eventually they may become petrified and
preserved indefinitely with the rocks in which they are embedded.
Methods such as measuring radioactive decay make it possible to
estimate the time period when the rocks, and the fossils associated
with them, were formed.
Radioactive dating indicates that the Earth was formed about
4,500,000,000 years ago. The earliest fossils resemble microorganisms
such as bacteria and blue-green algae; the oldest ones appear in rocks
3,500,000,000 old. The oldest animal fossils, about 700,000,000 years
old, come from small wormlike creatures with soft bodies. Numerous
fossils belonging to many living phyla and exhibiting mineralized
skeletons appear in rocks about 570,000,000 years old. These organisms
are different from organisms living now and from those living at
intervening times. Some are so radically different that
paleontologists have created new phyla in order to classify them. The
first vertebrates, animals with backbones, appeared about 400,000,000
years ago; the first mammals less than 200,000,000 years ago. The
history of life recorded by fossils presents compelling evidence of
evolution.
The fossil record is incomplete. Of the small proportion of organisms
preserved as fossils, only a tiny fraction have been recovered and
studied by paleontologists. But the succession of forms over time has
been in some cases reconstructed in detail. One example is the
evolution of the horse. It began with the dawn horse (genus
Hyracotherium), an animal the size of a dog, with several toes on each
foot and dentition appropriate for browsing, which evolved over
50,000,000 years ago; the most recent form is Equus, the modern horse,
much larger in size, one-toed, and with teeth appropriate for grazing.
The transitional forms are well preserved as fossils, as are many
other kinds of extinct horses that evolved in different directions and
left no living descendants.
Paleontologists have also been able to recover and reconstruct radical
transitions in form and function. The lower jaw of reptiles contains
several bones, that of mammals only one; the other bones in the
reptile jaw evolved into bones now found in the mammalian ear. This
would seem an unlikely transition. A bone being either in the jaw or
in the ear, it is hard to imagine what function it could have during
the intermediate stages. Yet paleontologists discovered two
transitional forms of therapsids (mammal-like reptiles) with a double
jaw joint--one joint consisting of the bones that persist in the
mammalian jaw, the other composed of the quadrate and articular bones,
which eventually became the hammer and anvil of the mammalian ear.
For skeptical contemporaries of Darwin, the "missing link"--the
absence of any transitional form between apes and humans--was a battle
cry, as it remained for uninformed people afterward. Not one but many
creatures intermediate between living apes and humans have since been
found as fossils. Australopithecus, a hominid that lived 3,000,000 or
4,000,000 years ago, had an upright human stance but a cranial
capacity of less than 500 cubic centimetres--comparable to that of a
gorilla or chimpanzee and just about one-third that of humans. Its
head displayed an odd mixture of ape and human characteristics: a low
forehead and a long, ape-like face, but with teeth proportioned like
those of humans. Along with increased cranial capacity, other human
characteristics have been found in Homo habilis, which lived about
1,500,000 to 2,000,000 years ago and had a cranial capacity of more
than 600 cubic centimetres, and Homo erectus, which lived between
500,000 and more than 1,000,000 years ago and had a cranial capacity
of 800 to 1,100 cubic centimetres.
Structural similarities
The skeletons of turtles, horses, humans, birds, and bats are
strikingly similar, in spite of the different ways of life of these
animals and the diversity of their environments. The correspondence,
bone by bone, can easily be seen in the limbs, but also in every other
part of the body. From a purely practical point of view, it is
incomprehensible that a turtle should swim, a horse run, a person
write, and a bird or bat fly with structures built of the same bones.
An engineer could design better limbs in each case. But if it is
accepted that all of these skeletons inherited their structures from a
common ancestor and became modified only as they adapted to different
ways of life, the similarity of their structures makes sense.
Comparative anatomy investigates the homologies, or inherited
similarities, among organisms in bone structure and in other parts of
the body. The correspondence of structures is typically very close
among some organisms--the different varieties of songbirds, for
instance--but becomes less so as the organisms compared are less
closely related in their evolutionary history. The similarities are
less detailed between mammals and birds than they are among mammals,
and less yet between mammals and fishes. Similarities in structure,
therefore, not only manifest evolution but also help to reconstruct
the phylogeny, or evolutionary history, of organisms.
An explanation of why most organismic structures are not perfect is
also revealed by comparative anatomy. Like the forelimbs of turtles,
horses, humans, birds, and bats, an organism's body parts are less
than perfectly adapted because they are modified from an inherited
structure rather than designed from completely "raw" materials for a
specific purpose. The imperfection of structures is evidence for
evolution and against design.
Embryonic development and vestiges
Darwin and his followers found support for evolution in the
comparative study of embryology--the science that investigates the
development of organisms from fertilized egg to time of birth or
hatching. Vertebrates, from fishes through lizards to humans, develop
in ways that are remarkably similar during early stages, but they
become more and more differentiated as the embryos approach maturity.
The similarities persist longer between organisms that are more
closely related (man and monkey) than between those less closely
related (man and shark). Common developmental patterns reflect
evolutionary kinship. Lizards and humans share developmental patterns
inherited from their remote common ancestor; the inherited pattern was
modified only as the separate descendant lineages evolved in different
directions. The common embryonic stages of the two creatures reflect
the constraints imposed by this common inheritance, which prevents
changes that have not been necessitated by their diverging environment
and way of life.
Human and other nonaquatic embryos exhibit gill slits even though they
never breathe through gills. These slits are found in the embryos of
all vertebrates because they share as common ancestors the fish in
which these structures first evolved. Human embryos also exhibit by
the fourth week of development a well-defined tail, which reaches
maximum length when the embryo is six weeks old. Similar embryonic
tails are found in other mammals, such as dogs, horses, and monkeys;
in humans, however, the tail eventually shortens, persisting only as a
rudiment in the adult coccyx.
A close evolutionary relationship between organisms that appear
drastically different as adults can sometimes be recognized by their
embryonic homologies. Barnacles are sedentary crustaceans with little
apparent likeness to such crustaceans as lobsters, shrimps, or
copepods. Yet barnacles pass through a free-swimming larval stage, the
nauplius, which is unmistakably similar to other crustacean larvae.
Embryonic rudiments that never fully develop, such as the gill slits
in humans, are common in all sorts of animals. Some, however, like the
tail rudiment in humans, persist as adult vestiges reflecting
evolutionary ancestry. The most familiar rudimentary organ in humans
is the vermiform appendix. This wormlike structure attaches to a short
section of intestine called the cecum, which is located at the point
where the large and small intestines join. The human vermiform
appendix is a functionless vestige of a fully developed organ present
in other mammals, such as the rabbit and other herbivores, where a
large cecum and appendix store vegetable cellulose to enable its
digestion with the help of bacteria. Vestiges are instances of
imperfections that argue against creation by design but are fully
understandable as a result of evolution.
Biogeography
Darwin also saw a confirmation of evolution in the geographic
distribution of plants and animals, and later knowledge has reinforced
his observations. For example, there are about 1,500 species of
Drosophila vinegar flies in the world; nearly one-third of them live
in Hawaii and nowhere else, although the total area of the archipelago
is less than one-twentieth the area of California. There are also in
Hawaii more than 1,000 species of snails and other land mollusks that
exist nowhere else. This unusual diversity is easily explained by
evolution. The Hawaiian Islands are extremely isolated and have had
few colonizers; those species that arrived there found many unoccupied
ecological niches, or local environments suited to sustain them and
lacking predators that would prevent them from multiplying. In
response, they rapidly diversified; this process of diversifying in
order to fill in ecological niches is called adaptive radiation.
The continents of the world have their own distinctive fauna and
flora. In Africa there are rhinoceroses, hippopotamuses, lions,
hyenas, giraffes, zebras, lemurs, monkeys with narrow noses and
nonprehensile tails, chimpanzees, and gorillas. South America, which
extends over much the same latitudes as Africa, has none of these
animals but has different ones: pumas, jaguars, tapirs, llamas,
raccoons, opossums, armadillos, and monkeys with broad noses and large
prehensile tails.
These vagaries of biogeography are not due solely to the suitability
of the different environments. There is no reason to believe that
South American animals are not well suited to live in Africa or those
of Africa to live in South America. The Hawaiian Islands are no better
suited than other Pacific islands for Drosophila flies, nor are they
less hospitable than other parts of the world for many absent
organisms. In fact, although no large mammals are native to the
islands, pigs and goats have multiplied there as wild animals since
being introduced by humans. This absence of many species from a
hospitable environment in which an extraordinary variety of other
species flourishes can be explained by the theory of evolution, which
holds that species can exist and evolve only in geographic areas that
were colonized by their ancestors.
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