Evolution,
in biology, is a complex process by which the characteristics of
living organisms change over many generations as traits are passed
from one generation to the next.
The
science of evolution seeks to understand the biological forces that
caused ancient organisms to develop into the tremendous and
ever-changing variety of life seen on Earth today. It
addresses how, over the course of time, various plant and animal
species branch off to become entirely new species, and how different
species are related through complicated family trees that span
millions of years.
Evolution
provides an essential framework for studying the ongoing history of
life on Earth. A central, and historically controversial, component
of evolutionary theory is that all living organisms, from microscopic
bacteria to plants, insects, birds, and mammals, share a common
ancestor. Species that are closely related share a recent common
ancestor, while distantly related species have a common ancestor
further in the past. The animal most closely related to humans, for
example, is the chimpanzee. The common ancestor of humans and
chimpanzees is believed to have lived approximately 6 million to 7
million years ago . On the other hand, an ancestor common to humans
and reptiles lived some 300 million years ago. And the common
ancestor to even more distantly related forms lived even further in
the past. Evolutionary biologists attempt to determine the history of
lineages as they diverge and how differences in characteristics
developed over time.
Throughout
history, philosophers, religious thinkers, and scientists have
attempted to explain the history and variety of life on Earth. During
the rise of modern science in Western Europe in the 17th and 18th
centuries, a predominant view held that God created every organism on
Earth more or less as it now exists. But in that time of burgeoning
interest in the study of fossils and natural history, the beginnings
of a modern evolutionary theory began to take shape. Early
evolutionary theorists proposed that all of life on Earth evolved
gradually from simple organisms. Their knowledge of science was
incomplete, however, and their theories left too many questions
unanswered. Most prominent scientists of the day remained convinced
that the variety of life on Earth could only result from an act of
divine creation.
In
the mid-19th century a modern theory of evolution took
hold, thanks to British naturalist Charles Darwin. In his book On
the Origin of Species by Means of Natural Selection, published in
1859, Darwin described the evolution of life as a process of natural
selection. Life, he suggested, is a competitive struggle to survive,
often in the face of limited resources. Living things must compete
for food and space. They must evade the ravages of predators and
disease while dealing with unpredictable shifts in their environment,
such as changes in climate. Darwin offered that, within a given
population in a given environment, certain individuals possess
characteristics that make them more likely to survive and reproduce.
These individuals will pass these critical characteristics on to
their offspring. The number of organisms with these traits increases
as each generation passes on the advantageous combination of traits.
Outmatched, individuals lacking the beneficial traits gradually
decrease in number. Slowly, Darwin argued, natural selection tips the
balance in a population toward those with the combination of traits,
or adaptations, best suited to their environment.
Animal
Kingdom
While
On the Origin of Species was an instant sensation
and best-seller, Darwin’s theories faced hostile reception by
critics who railed against his blasphemous ideas. Other critics
pointed to questions left unresolved by Darwin’s careful arguments.
For instance, Darwin could not explain the mechanism that caused life
forms to change from generation to generation.
Hostility
gave way to acclaim as scientists vigorously debated,
explored, and built on Darwin’s theory of natural selection. As the
20th century unfolded, scientific advances revealed the detailed
mechanisms missing from Darwin’s theory. Study of the complex
chemistry of all organisms unveiled the structure of genes as well as
how they are duplicated, altered, and passed from generation to
generation. New statistical methods helped explain how genes in
specific populations change over generations.
These
new methods provided insight into how populations remain adaptable to
changing environmental circumstances and broadened our understanding
of the genetic structure of populations. Advances in techniques used
to determine the age of fossils provided clues about when extinct
organisms existed and details about the circumstances surrounding
their extinction. And new molecular biology techniques compare the
genetic structures of different species, enabling scientists to
determine heretofore undetectable evolutionary relationships between
species.
Today,
evolution is recognized as the cornerstone of modern biology. Uniting
such diverse scientific fields as cell biology, genetics,
paleontology, and even geology and statistics, the study of evolution
reveals an exquisitely complex interaction of the forces that act
upon every life form on Earth.
Experiment
1953: Attempt to reproduce the atmosphere of the primitive Earth
nearly 4 billion years ago
Some
evolutionary biologists are trying to understand how life originated
on Earth. This too requires the careful examination and
interpretation of many indirect clues. In one well-known series of
experiments in 1953, American chemists Stanley L. Miller and Harold
C. Urey attempted to reproduce the atmosphere of the primitive Earth
nearly 4 billion years ago. They circulated a mixture of gases
believed to have been present at the time (hydrogen, methane,
ammonia, and water vapor) over water in a sterile glass container.
They then subjected the gases to the energy of electrical sparks,
simulating the action of lightning on the primitive Earth. After
about a week, the fluid turned brown and was found to contain amino
acids―the building blocks of proteins. Subsequent work by these
scientists and others also succeeded in producing nucleotides, the
building blocks of DNA and other nucleic acids.
While
the artificial generation of these molecules in laboratories did not
produce a living organism, this research offers some support that the
first building blocks of life could have arisen from raw materials
that were present in the environment of the primitive Earth. Once
all the raw materials were in place―nucleic acids,
proteins, and the other components of simple cells―it is not clear
how the first self-replicating life forms actually came about.
Recent
theories center on the role of a particular nucleic acid―ribonucleic
acid (RNA), which, in modern cells, carries out the task of
translating the instructions coded in DNA for the assembling of
proteins. RNA also acts as a catalyst―that is, to cause other
chemical reactions―and perhaps most significantly, to make copies
of itself. Some scientists believe that the first self-replicating
organisms were based on RNA.
Lamarck:
Perhaps the most prominent of those who embraced the idea of
progressive change in the living world was the early 19th-century
French biologist Jean-Baptiste Lamarck. Lamarck’s theory, now known
as Lamarckism and based in part on his study of the fossils of marine
invertebrates, was that species do change over time. He believed,
furthermore, that animals evolve because unfavorable conditions
produce needs that animals try to satisfy.
Modern
scientists know that adaptation and natural selection are far more
complicated than Lamarck supposed, having nothing to do with an
animal’s voluntary efforts. Nevertheless, the idea of acquired
characteristics, with Lamarck as its most famous proponent, persisted
for many years.
Cuvier:
French naturalist and paleontologist Georges Cuvier feuded
(disagreed) with Lamarck. Unearthing the fossils of mastodons and
other vanished species, Cuvier produced proof of long-extinct life
forms on Earth. Unlike Lamarck, however, Cuvier did not believe in
evolution. Instead, Cuvier believed that floods and other cataclysms
destroyed such ancient species. He suggested that after each
cataclysmic event, God created a new set of organisms.
Thomas
Robert Malthus:
At around the same time that Cuvier and Lamarck were
squabbling, British economist Thomas Robert Malthus proposed ideas
extremely influential in evolutionary theory. In his 1798 work An
Essay on the Principle of Population,
Malthus theorized that the human population would increase at a much
greater rate than its food sources. This theory introduced the key
idea of competition for limited resources―that is, there is not
enough food, water, and living space to go around, and organisms must
somehow compete with each other to obtain resources necessary for
survival.
Charles
Lyell: Another key idea came from Scottish geologist Charles
Lyell, who supplied a deeper understanding of Earth’s history. In
his book Principles of Geology (1830), Lyell set forth his
case that the Earth was millions of years old rather than only a few
thousand years old, as was maintained by those who accepted the
biblical story of divine creation as fact.
Alfred
Russell Wallace
British
naturalist Alfred Russel Wallace shares credit for the revolutionary
theory
of evolution by natural selection with another British
naturalist, Charles Darwin.
Though the two scientists arrived at
their conclusions independently, excerpts of
their papers were
presented simultaneously at a now famous meeting of the
Linnean
Society in London in 1858.
In
1831 Charles Darwin,
who was intending to become a country minister, had an opportunity to
sail as ship’s naturalist aboard the HMS
Beagle on a
five-year, round-the-world mapmaking voyage.
In
1837, shortly after returning to England, Darwin began a
notebook of his observations and thoughts on evolution. Although
Darwin had developed the major components of his theory of evolution
by natural selection in an 1842 unpublished paper circulated among
his friends, he was unwilling to publish the results until he could
present as complete a case as possible. He labored for almost 20
additional years on his theory of evolution and on its primary
mechanism, natural selection.
In
1858 he received a letter from British naturalist Alfred Russel
Wallace, a professional collector of wildlife specimens. Much to
Darwin’s surprise, Wallace had independently hit upon the idea of
natural selection to explain how species are modified by adapting to
different conditions. Not wanting Darwin to be unfairly deprived of
his share of the credit for the theory, some of Darwin’s scientific
colleagues presented extracts of Darwin’s work along with Wallace’s
paper at a meeting of the Linnean Society, a London-based science
organization, in June 1858. Wallace’s paper stimulated Darwin to
finish his work and get it into print. Darwin published On
the Origin of Species by Means of Natural
Selection on November 24, 1859. All 1,250 copies of the
first printing were sold on that day.
Gregor
Mendel
Known
as the father of modern genetics, Austrian monk Gregor Mendel
developed
the principles of heredity by studying the variation and
heredity of seven pairs of
inherited characteristics in pea plants.
Although the significance of his work was
not recognized during
his lifetime, it became the basis for the present day
field of
genetics.
Mendel
performed hundreds of experiments and produced precise statistical
models and principles of heredity, now known as Mendel’s Laws,
showing how dominant and recessive traits are expressed over
generations. However, no one appreciated the significance of Mendel’s
work until after his death. But his work ultimately gave birth to the
modern field of genetics.
Hugo
Marie de Vries:
In 1900, Dutch botanist Hugo Marie de Vries and others
independently discovered Mendel’s laws. The following year, de
Vries’s book The
Mutation Theory
challenged Darwin’s concept of gradual changes over long periods by
proposing that evolution occurred in abrupt, radical steps.
Having
observed new varieties of the evening primrose plant coming into
existence in a single generation, de Vries had subsequently
determined that sudden change, or mutation, in the genetic material
was responsible. As the debate over evolution continued in the early
20th century, some scientists came to believe that mutation, and not
natural selection, was the driving force in evolution. In the face
of these mutationists, Darwin’s central theory
threatened to fall out of favor.
Theodosius
Dobzhansky
American
geneticist and zoologist Theodosius Dobzhansky was an important
contributor to the field of population genetics. Dobzhansky studies
of fruit
flies and other organisms helped explain the role of
adaptation in the
evolution of races and species. As the
science of genetics advanced during the
1920s and 1930s,
several key scientists forged a link between Mendel laws
of
inheritance and the theory of natural selection proposed by Darwin
and Wallace.
British
mathematician Sir Ronald Fisher, British geneticist J.B.S. Haldane,
and American geneticist Sewall Wright pioneered the field of
population genetics. By mathematically analyzing the genetic
variation in entire populations, these scientists demonstrated
that natural selection, and not just mutation, could result in
evolutionary change.
Modern
synthesis: Further investigation into population genetics and
such fields as paleontology, taxonomy, biogeography, and the
biochemistry of genes eventually led to what is called the modern
synthesis. This modern view of evolution integrated discoveries and
ideas from many different disciplines.
In
so doing, this view reconciled the many disparate ideas about
evolution into the all-encompassing evolutionary science studied
today.
In
1942, American paleontologist George Gaylord Simpson demonstrated
from the fossil record that rates and modes of evolution are
correlated: New kinds of organisms arise when their ancestors
invade a new niche, and evolve rapidly to best exploit the conditions
in the new environment.
In
the late 1940s American botanist G. Ledyard Stebbins 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, biologists reviewed a broad range of genetic,
ecological, and anatomical evidence to show that observation and
experimental evidence strongly supported the modern synthesis. The
theory has formed the basis of evolutionary science since the 1950s.
Watson
and Crick: In
1953, American biochemist James Watson and British
biophysicist Francis Crick described the three-dimensional shape of
DNA, the molecule that contains hereditary information in nearly all
living organisms. In the following decade, geneticists developed
techniques to rapidly compare DNA and proteins from different
organisms. In one such procedure, electrophoresis, geneticists
evaluate different specimens of DNA or proteins by observing how they
behave in the presence of a slight electric charge. Such techniques
opened up entirely new ways to study evolution. For the first time
geneticists could quantitatively determine, for example, the genetic
change that occurs during the formation of new species.
In
1968 Japanese geneticist Motoo Kimura proposed that much of the
variation at the molecular level results not from the forces of
natural selection, but from chance mutations that do not affect an
organism’s fitness. Not all scientists agree with the neutral
gene theory.
Sociobiology:
In recent decades, another branch of evolutionary theory has
appeared, as
researchers have explored the possibility that not only physical
traits, but behavior itself, might be inherited.
Behavioral
geneticists have studied how genes influence behavior, and more
recently, the role of biology in social behavior has been explored.
This field of investigation, known as sociobiology, was
inaugurated in 1975 with the publication of the book Sociobiology:
The New Synthesis by American evolutionary biologist Edward O.
Wilson.
Sociobiologists
examine animal behaviors that are called altruistic―that
is, unselfish, or demonstrating concern for the welfare of others.
When birds feed on the ground, for example, one individual may
notice a predator and sound an alarm. In so doing, the bird also
calls the predator’s attention to itself. What can account for the
behavior of such a sentry, who would seem to derive no evolutionary
benefit from its unselfish behavior and so seem to defy the laws of
natural selection?
Darwin
was aware of altruistic social behavior in animals, and of how this
phenomenon challenged his theory of natural selection.
Evolutionary
theory has undergone many further refinements in recent years. One
such theory challenges the central idea that evolution proceeds by
gradual change.
In
1972 American paleontologists Stephen Jay Gould and Niles Eldredge
proposed the theory of punctuated equilibria. According to this
theory, trends in the fossil record cannot be attributed to gradual
transformation within a lineage, but rather result from quick bursts
of rapid evolutionary change.
In
the last several decades, scientists have questioned the role of
extinction in evolution. Of the millions of species that have
existed on this planet, more than 99 percent are extinct.
Historically,
biologists regarded extinction as a natural outcome of competition
between newly evolved, adaptively superior species and their older,
more primitive ancestors. Recently, however, paleontologists have
discovered that many different, unrelated species living in large
ecosystems tend to become extinct at nearly the same time. The cause
is always some sort of climate change or catastrophic event that
produces conditions too severe for most organisms to endure.
Moreover, new species evolve after the wave of extinction removes
many of the species that previously occupied a region for millions of
years. Thus extinction does not result from evolution, but
actually causes it.
Scientists
have identified several instances of mass extinction, when species
apparently died out on a huge scale. The greatest of these episodes
occurred during the end of the Permian Period, some 245 million years
ago. At that time, according to estimates, more than 95 percent of
species―nearly all life on the planet―died out. Another
extensively studied extinction took place at the boundary of the
Cretaceous Period and the Tertiary Period, roughly 65 million years
ago, when the dinosaurs disappeared. In all, more than 20 global mass
extinctions have been identified. Some scientists theorize that such
events may even be cyclical, occurring at regular intervals.
Advances
in medical technology may also affect natural selection. The
study from the mid-20th century showing that babies of medium birth
weights were more likely to survive than their heavier or lighter
counterparts would be difficult to reproduce today. Advances in
neonatal medical technology have made it possible for small or
premature babies to survive in much higher numbers.
Recent
genetic analysis shows the human population contains harmful
mutations in unprecedented levels. Researchers attribute this to
genetic drift acting on small human populations throughout history.
They also expect that improved medical technology may exacerbate the
problem. Better medicine enables more people to survive to
reproductive age, even if they carry mutations that in past
generations would have caused their early death. The genetic
repercussions of this are still unknown, but biologists speculate
that many minor problems, such as poor eyesight, headaches, and
stomach upsets may be attributable to our collection of harmful
mutations.
Humans
have also developed the potential to affect evolution at the
most basic level―the genes. The techniques of genetic engineering
have become commonplace. Scientists can extract genes from living
things, alter them by combining them with another segment of DNA, and
then place this recombinant DNA back inside the
organism.
Genetic
engineering has produced pest-resistant crops as well as larger cows
and other livestock. To an increasing extent, genetic engineers fight
human disease, such as cancer and heart disease. The investigation of
gene therapy, in which scientists substitute functioning copies of a
given gene for a defective gene, is an active field of research. The
way this tinkering with genetic material will affect evolution
remains to be determined.
Comments:
Though lot of thinkers have put forward different theories on the
process of evolution, there are many un-answered questions still
waiting for answers. We shall try to see some more 21st
century thinkers' views in the articles to follow.
Vijay
R. Joshi.
