I INTRODUCTION
Evolution,
in biology, the complex of processes by which living organisms originated on
earth and have been diversified and modified through sustained changes in form
and function. The earliest known fossil organisms are single-celled forms
resembling modern bacteria; they date from about 3.4 billion years ago.
Evolution has resulted in successive radiations of new types of organisms, many
of which have become extinct, but some of which have developed into the present
fauna and flora of the world. Extinction and diversification continue today.
I EARLY
EVOLUTIONARY STUDIES
The notion that populations of organisms can be
transformed over generations into descendant populations of different kinds has
been suggested repeatedly since the early recorded history of ideas (see
Heredity). In various forms it was held by some schools of ancient Greek
philosophy and discussed by philosophers and theologians through the succeeding
ages of history, but no scientific explanations of evolutionary processes were
attempted until the 18th century. The growth of natural history then led to an
increasingly detailed knowledge of living and fossil organisms, and the concept
of evolution attracted serious students. The most outstanding evolutionist in
the early 19th century was Jean Baptiste de Lamarck, who argued that the
patterns of resemblance found in various creatures arose through evolutionary
modifications of a common lineage—for example, that lions, tigers, and other
animals of the cat family had all descended from a catlike ancestor. Naturalists
had already established that different animals are adapted to different modes of
life and environmental conditions; Lamarck believed that environmental changes
evoked in individual animals direct adaptive responses that could be passed on
to their offspring as inheritable traits. This generalized hypothesis of
evolution by acquired characteristics was not tested scientifically during
Lamarck's lifetime.
III DARWINIAN
THEORY
A successful explanation of evolutionary processes was
proposed by Charles Darwin. His most famous book, On the Origin of Species by
Means of Natural Selection (1859), is a landmark in human understanding of
nature. Pointing to variability within species, Darwin observed that while
offspring inherit a resemblance to their parents, they are not identical to
them. He further noted that some of the differences between offspring and
parents were not due solely to the environment but were themselves often
inheritable. Animal breeders, he observed, were often able to change the
characteristics of domestic animals by selecting for reproduction those individuals with the
most desirable qualities—speed in racehorses, milk production in cows, trail
scenting in dogs. This is change by artificial selection. Darwin reasoned that,
in nature, individuals with qualities that made them better adjusted to their
environments or gave them higher reproductive capacities would tend to leave
more offspring; such individuals were said to have higher fitness. Because more
individuals are born than survive to breed, constant winnowing of the less fit—a
natural selection—should occur, leading to a population that is well adapted
to the environment it inhabits. When environmental conditions change,
populations require new properties to maintain their fitness. Either the
survival of a sufficient number of individuals with suitable traits leads to an
eventual adaptation of the population as a whole, or the population becomes
extinct. Thus, according to Darwin's theory, evolution proceeds by the natural
selection of well-adapted individuals over a span of many generations.
The parts of Darwin's theory that were the most
difficult to test scientifically were the inferences about the heritability of
traits, or characteristics, because heredity was not understood at that time.
The basic rules of inheritance became known to science only at the turn of the
century, when the earlier genetic work of Gregor Mendel came to light. Mendel
had discovered that characteristics are transmitted across generations in
discrete units, now known as genes (see Genetics) that are inherited in a
statistically predictable fashion. The discovery was then made that inheritable
changes in genes, termed mutations, could occur spontaneously and randomly
without regard to the environment. Since mutations were seen to be the only
source of genetic novelty, many geneticists believed that evolution was driven
onward by the random accumulation of favorable mutational changes. Natural
selection—that is, evolution directed by adaptive fitness—was reduced to a
minor role by mutationists such as Hugo de Vries, Thomas Morgan, and William
Bateson, whose ideas were prominent well into the 1930s.
IV POPULATION
GENETICS
Even while mutationism was replacing Darwinism, the
leading evolutionary theory, the science of population genetics was being
founded by Sewall Wright, J. B. S. Haldane, and several other geneticists, all
working independently. They developed arguments to show that even when a
mutation that is immediately favored appears, its subsequent spread within a
population depends on such variables as the following: (1) the size of the
population; (2) the length of generations; (3) the degree to which the mutation
is favorable; and (4) the rate at which the same mutation reappears in
descendants.
Furthermore, a given gene is
favorable only under certain environmental conditions. If conditions change in
space, then the gene may be favored only in a localized part of the population;
if conditions change over time, the gene may become generally unfavorable.
Because different individuals usually have different assortments of genes—no
two humans (except identical twins) are precisely alike genetically—the total
number of genes available for inheritance by the next generation can be large,
constituting a vast store of genetic variability. This is called the gene pool.
Sexual reproduction ensures that the genes are rearranged in each generation, a
process termed recombination. When a population is stable, the gene frequency—that
is, the frequency of occurrence of each gene in proportion to the total number
of genes in the gene pool—remains the same, even though the genes are
recombined in different ways in each individual. When the gene frequencies in
the pool change in a sustained manner, evolution is occurring. Mutations provide
the gene pool with a continuous supply of new genes; through the process of
natural selection the gene frequencies change so that advantageous genes occur
in greater proportions.
Despite the mathematical
support that was developed for this view of evolution, most evolutionists
adhered to the theory of evolution by random mutations until the late 1930s. At
that time Theodosius Dobzhansky, in Genetics and the Origin of Species,
extended the mathematical arguments with a wide range of experimental and
observational evidence. For example, he demonstrated adaptive genetic changes in
large populations of fruit flies as a result of controlled environmental
changes. Dobzhansky proved that the facts of genetics are compatible with
Darwinian natural selection, which is the chief cause of sustained changes in
gene frequencies and therefore of evolutionary changes in a population's
characteristics. In the ensuing decades outstanding contributions were made to
the revitalized Darwinian theory of evolution from nearly all fields of
biological and paleontological science.
V THE
SYNTHETIC THEORY
As the new evolutionary theory became enriched from
such diverse sources, it became known as the synthetic theory. Three American
scientists made especially important contributions. The German-born Ernst Mayr,
a zoologist, showed that new species usually arise in geographic isolation,
often following a genetic "revolution" that rapidly changes the
contents of their gene pools (see Species and Speciation). George
Simpson, a paleontologist, showed from the fossil record that rates and modes of
evolution are correlated: New kinds of organisms arise from the invasion of a
new adaptive zone, usually evolving rapidly. G. Ledyard Stebbins, a botanist,
showed that plants display evolutionary patterns similar to those of animals,
and especially that plant evolution has demonstrated diverse adaptive responses
to environmental pressures and opportunities. In addition, these biologists
reviewed a broad range of genetic, ecological, and systematic evidence to show
that the synthetic theory was strongly supported by observation and experiment.
The theory has formed the basis of textbook accounts of evolution since the
1950s. It has also led to a renewed effort to classify organisms according to
their evolutionary history. See Classification.
During the establishment of the synthetic theory of
evolution, the science of heredity underwent another profound change in 1953,
when James Watson and Francis Crick demonstrated the way genetic material is
composed of two nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid
(RNA). Nucleic acid molecules contain genetic codes that dictate the manufacture
of proteins, and the latter direct the biochemical pathways of development and
metabolism in an organism. Mutations are now known to be changes in the position
of a gene, or in the information coded in the gene, that can affect the function
of the protein for which the gene is responsible. Natural selection can then
operate to favor or suppress a particular gene according to how strongly its
protein product contributes to the reproductive success of the organism. These
findings have made possible the study of evolution at the molecular level,
tracing the history of changes in particular genes and in gene organization.
Today evolutionary studies
extend into all branches of biology. The present state of all forms of life,
from bacteria to humans, has been achieved by evolution, and to study the
physiology of a leech, the ecology of a seashore community, or the behavior of a
hummingbird is to study features that have been achieved through evolutionary
change. As Dobzhansky has said, nothing in biology makes sense except in the
light of evolution.
VI SPECIATION
One of the clearest examples of natural selection at
work in the modern world is afforded by gene-frequency changes in carefully
studied populations of the peppered moth, a species of the genus Biston
found in England. These moths, originally light grayish but with a small
proportion of dark-colored individuals, are eaten by birds that locate them
visually. When the British countryside near cities became blackened by smoke
from industrial processes, the lighter moths, previously well disguised against
light-colored tree trunks, were easily found by birds and thus became less fit.
The dark moths became common because they were more difficult to discern against
the darker background. A single gene, coding for the dark color rather than the
light color, was spread by means of natural selection and raised to a high
frequency in industrial regions. Subsequent reduction of smoke pollution
resulted in a reduction of the dark moth variety.
The light and dark moth varieties belong to the same
species and interbreed freely. If pollution had continued, however, the rural
moth population would have become entirely light and the industrial entirely
dark. Then each population would be subject to somewhat different selective
pressures because the two environments vary. In time, the dark and light
populations would differ by groups of genes, with each group advantageous
locally. The moth populations might eventually become incapable of
interbreeding. At this point, natural selection would have caused them to
diverge from a common ancestor into two species.
Populations of various species
have easily become isolated in different habitats on various islands and have
differentiated into new species. Island chains provide a particularly
well-studied context of speciation. Darwin himself was struck by the finches of
the Galápagos Islands, which radiated into 14 species, each with distinctive
form and habits, following an invasion of the islands by a single species from
the American mainland. In the Hawaiian Islands some 500 species of fruit flies
have descended from one or two founding species in less than 10 million years;
they have had a complex history of island hopping and rehopping, in the process
forming isolated populations that were the start of new species.
Natural selection is not the
only source of genetic change in the evolution of species. Gene frequencies may
also change by the chance failure of progeny to reproduce the exact gene
proportions of their parents. This is termed genetic drift and is most important
in small populations, where genes may be lost from the original gene pool simply
by not being represented in successive matings. Also, failure to carry the full
range of genes in the parent population occurs when a few individuals migrate
and found a new, isolated population, which is thus different from the very
beginning; this is called the founder effect. Mutations can, of course, also
change gene frequencies, but such changes occur at low rates relative to the
changes brought about by the recombination of genes in offspring.
Because all the established
genes in a population have been monitored for fitness by selection, newly arisen
mutations are unlikely to enhance fitness unless the environment changes so as
to favor the new gene activity, as in the gene for dark color in the peppered
moth. Novel genes that cause large changes rarely promote fitness and are
usually lethal. The genes already established by selection are carefully
adjusted to one another so their biochemical effects are coordinated; a new gene
with a major effect is comparable to the insertion of a chance word or
rearrangement of words into a precise set of instructions. Mutations with small
effects provide the basis for the genetic changes that are seen to promote
fitness in experimental laboratory environments. Indeed, natural selection by a
series of mutations appears to be the chief agent of evolution.
VII TRANSSPECIFIC
EVOLUTION
Organisms are classified in a hierarchical scheme that
attempts to reflect their evolutionary relationships. Closely related species
are grouped into genera, closely related genera into families, with orders,
classes, phyla, and kingdoms representing the higher categories of
classification. The origin of the groups of organims in the higher categories
is called transspecific evolution. These groups differ from one another in many
ways besides being reproductively isolated; phyla even differ from one another
in the basic architectural plans of their bodies. At one time some evolutionists
believed that the processes responsible for speciation were not adequate to
account for these great differences, but today speciation and transspecific
evolution are generally considered not fundamentally different.s
Because the biological environment is patchy and
variable, different modes of life are required in different habitats. Seafloor
mud, open ocean water, land, and air present different problems for successful
adaptation; burrowing, swimming, walking, and flying are among the locomotory
solutions to life in these environments. Such widely varying solutions demand
the emergence of novel body structures. The key to the development of these
structures is that the necessary innovations must first have appeared as
adaptations to some aspect of the ancestral mode of life and then proved useful
in the new mode as well. Thus, large modifications evolve over time by a series
of preadaptations. For example, the lungs of air-breathing creatures evolved
from an early lunglike structure developed in certain fish for gulping oxygen
from the surfaces of oxygen-deprived waters. Limbs for terrestrial life evolved
from the stiffened fins of fish that gained further adaptive advantages from
crawling up on shore. Similarly, limb modifications that evolved to prolong
gliding jumps between trees later proved advantageous in the development of
wings for flight. Successful pioneering lineages radiate into a number of
descendant lineages with an array of well-adapted forms, thus representing
distinctive new groups of organisms. The separate vertebrate classes of
amphibians, reptiles, birds, and mammals can thus be traced to evolutionary
adaptations of their early ancestors at the species level.
VIII CURRENT
EVOLUTIONARY DEBATE Although the fact of
evolution is scientifically accepted as underlying modern biology, theories that
concern themselves with the processes of evolution continue to be debated and
refined. Much of this work involves highly sophisticated mathematical studies,
as required by the complex interactions of the various elements of the modern
synthesis, from gene mutations to population genetics to large-scale ecological
interactions over geological time. Because understanding of the actual
evolutionary events that took place over earth's long history depends largely on
interpretations of an incomplete fossil record, much latitude remains for
differences in such interpretations. One of the issues that is currently being
debated among theorists derives from a notable fact observed in the fossil
record. That is, when a new species appears in the record it usually does so
abruptly and then apparently remains stable for as long as the record of that
species lasts. The fossils do not seem to exhibit the slow and gradual changes
that might be expected according to the modern synthesis. For this reason, in
part, a number of evolutionists—most notably Stephen Jay Gould of Harvard
University and Niles Eldredge of the American Museum of Natural History—have
proposed a variant concept of "punctuated equilibria" for species
evolution. According to this concept, species do in fact tend to remain stable
for long periods of time and then to change relatively abruptly—or rather, to
be replaced suddenly by newer and more successful forms. These sudden changes
are the "punctuations" in the state of equilibrium that give this
concept its name.
Although these proposed
periods of rapid change would be abrupt only in terms of the geological time
scale and would actually occur over periods of thousands of years, most
evolutionists tend to consider the punctuated-equilibrium concept only another
possible mode of evolutionary change that could take place along with the
processes described by the modern synthesis, rather than as a supplanting model
for evolution theory. The very incompleteness of the fossil record does not
permit any such clear choice to be made, because the record of almost any
species is highly selective over geological time. In addition, the small changes
that would make up gradual evolutionary development according to the modern
synthesis are themselves not necessarily of a nature that would be apparent in
the fossil history of a species, however complete it might be over a given
stretch of time. Fossils primarily show gross morphological changes, whereas
changes taking place in genetic makeup could be extensive even though overall
body structures do not reveal these shifts in populations of species.
Arguments from the known
nature of small-scale evolutionary change do not, in fact, necessarily establish
long-term evolutionary events, as following either the model proposed by the
modern synthesis or the one proposed by punctuated equilibrium. Evolution may
just as well have proceeded along both routes.
IX STEPS
IN EVOLUTION
Life originated about 3.5 billion years ago, when the
earth's environment was very different than it is today. Especially important
was the lack of significant amounts of free oxygen in the atmosphere.
Experiments have shown that rather complicated organic molecules, including
amino acids, can arise spontaneously under conditions that are believed to
simulate the earth's primitive environment. Other experiments indicate that
concentration of such molecules may have led to the synthesis of complex
molecules, such as proteins, nucleic acids, and carbohydrates, and eventually to
interactions among these molecules. A rudimentary genetic system eventually
arose and was elaborated by natural selection into the complicated mechanisms of
inheritance known today. Scientists speculate that the earliest organisms fed on
inorganic or organic molecules dissolved in the waters of the early earth.
Later, evolution led to organisms capable of tapping chemical and solar energy
sources to produce organic molecules.
Widely accepted evidence
suggests that the first organisms were archaebacteria, primitive cells without
nuclei. These cells may have evolved in waters with extremely high temperatures
and no oxygen. Cyanobacteria, more advanced organisms, evolved from 2.5 to 3.4
billion years ago. Cyanobacteria carried out photosynthesis, a process that
traps the energy of the sun and uses it to build glucose, an organic molecule,
from water and carbon dioxide. Photosynthesis freed organisms from their
dependence on organic molecules dissolved in water, and also released oxygen so
that the atmosphere and oceans gradually became more hospitable to more advanced
life forms.
Advanced cells (eukaryotes)
may have evolved through the amalgamation of a number of distinct simple cell
types. A large ingesting cell may have incorporated as symbionts (see
Symbiosis) some small blue-green algal cells that evolved into chloroplasts
(cell bodies that photosynthesize) and some tiny aerobic bacteria that evolved
into mitochondria (cell bodies that release energy during respiration). Other
features of advanced cells, such as their large DNA contents, may also have
arisen from prokaryotic symbionts. Single-celled eukaryotes then developed
complex modes of living and advanced types of reproduction that led to the
appearance of multicellular plants and animals. The latter are first known from
about 700 million years ago, and their appearance implies that at least moderate
levels of free atmospheric oxygen and a relatively predictable supply of food
plants had been achieved.
Between about 700 and 570
million years ago the basic body plans of modern animals were developed during a
remarkable burst of evolutionary diversification. The earliest body fossils
consist chiefly of impressions belonging to jellyfish and their allies, a
rudimentary group. At about the same time, however, fossil burrows appeared,
signaling the evolution of burrowing worms with considerably more advanced body
structures. Then, beginning just before 570 million years ago, skeletons
developed independently in a number of animal lineages. Fish arose from a
wormlike lineage of early invertebrates that evolved a stiff dorsal cord and,
eventually, an articulated internal skeleton that improved swimming efficiency.
In order for complex animal
communities to develop, plants must first become established to support
herbivore populations, which in turn may support predators and scavengers. Land
plants appeared about 400 million years ago, spreading from lowland swamps as
expanding greenbelts. Arthropods (some evolving into insects) and other
invertebrate groups followed them onto land, and finally land vertebrates
(amphibians at first) rose from freshwater fish nearly 360 million years ago. In
general, the subsequent radiations of land vertebrates made them increasingly
independent of water and increasingly active. Dinosaurs and mammals shared the
terrestrial environment for 135 million years; dinosaurs may well have been more
active, and certainly were larger, than their mammalian contemporaries, which
were small and possibly nocturnal. The mammals, however, survived a wave of
extinction that eliminated dinosaurs about 65 million years ago, and
subsequently diversified into many of the habitats and modes of life that
formerly had been dinosaurian.
Mammals among vertebrate
animals and insects among invertebrates dominate the terrestrial faunas today. See
also Animal Distribution; Plant Distribution.
X HUMAN
EVOLUTION
Humans belong to an order of mammals, the primates,
which existed before the dinosaurs became extinct. Early primates seem to have
been tree dwelling and may have resembled squirrels in their habits. Many of the
primate attributes—the short face, overlapping visual fields, grasping hands,
large brains, and perhaps even alertness and curiosity—must have been acquired
as arboreal adaptations. Descent from tree habitats
to forest floors and
eventually to more open country, however, was associated with the development of
many of the unique features of the human primate, including erect posture and
reduced canine teeth, which suggest new habits of feeding. A shift to
cooperative hunting and gathering, with concomitant requirements for a high
level of intelligence and social organization, accompanied the rise of the
modern human species. See Human Evolution.
XI EVOLUTIONARY
PATTERNS
The history of life as inferred from the fossil record
displays a wide variety of trends and patterns. Lineages may evolve slowly at
one time and rapidly at another time; they may follow one pathway of change for
some time only to switch to another pathway; and they may diversify rapidly at
one time and then shrink under widespread extinctions.
The key to many of these
patterns is the rate and nature of environmental change. Species become adapted
to the environmental conditions that exist at a given time, and when change
leads to new conditions, they must evolve new adaptations or become extinct (see
Endangered Species). When the environment undergoes a particularly rapid or
extensive change, waves of extinction occur; these are followed by waves of
development of new species. The times of mass extinction are not yet well
understood. Although the most famous one is that of the dinosaurs, about 65
million years ago, such events appear in the fossil record as far back as
Precambrian time, when life first arose. In fact, five mass extinctions on the
scale of that at the end of the age of dinosaurs are known over the past 600
million years. Some scientists also claim to have demonstrated a definite
periodicity to smaller periods of mass extinction, and in particular a
26-million-year cycle of eight extinctions over the past 250 million years.
Controversy has arisen over
the proposal made by some geologists that mass extinctions are related to
periodic catastrophes such as the striking of the earth's surface by a large
asteroid or comet. Many paleontologists and evolutionary theorists reject such
hypotheses as unjustified; they feel that periods of mass extinctions can be
accounted for by less spectacular evolutionary processes and by more earthbound
events such as cycles of climatic change and volcanic activity. Whatever
proposals may eventually prove true, however, it seems fairly certain that
periodic waves of mass extinction do occur.
Species adapted to live in
environments that are changeable in the short term have broad tolerances, which
may better enable them to survive extensive changes. Human beings are uniquely
adapted in that they make and use tools and devices and invent and propagate
procedures that give them extended control over their environments. Humans are
significantly changing the environment itself, however. The effects are most
complex and cannot be predicted, and yet the likelihood is that evolutionary
patterns in the future will reflect the influence of the human species. See
Adaptation; Natural Selection; Sexual Selection.
For a theological
interpretation of the origin of life, see Creation.
Contributed By:
James W. Valentine
