DEVELOPMENT OF EVOLUTIONARY THEORY
(Rewritten from M J Farabee, 2001)
The Ancient Greek philosopher Anaxiamander (611-547 B.C.) and the Roman philosopher
Lucretius (99-55 B.C.) coined the concept that all living things were related and that they
had change over time. The classical science of their time was observational rather than
experimental. Aristotle (384-322 B.C.) developed his Scala Naturae, or Ladder of Life, to
explain his concept of the advancement of living things from inanimate matter to plants,
then animals and finally man.
Post-Aristotlean “scientists” were constrained by the prevailing thought patterns of the
Middle Ages – the literal creation of the earth in six days as laid out in the book of Genesis.
According to Archbishop James Ussher of Ireland, the Earth was formed on October 22,
4004 B.C., and his ideas were readily accepted because they posed no threat to the social
order of the times; comfortable ideas that would not upset the linked applecart’s of church
and state.
Prior to the sixteenth century dissection of human bodies was either illegal or severely
limited so that most of the comparative anatomy was pure guesswork or was taken from
the dissection of animals or during surgery, and it wasn’t until dissection laws had been
relaxed and Andreas Vesalius (1514-1584) began to document his dissections culminating
in his masterwork “The Seven Books on the Structure of the Human Body”. Through his
work and that of many other anatomists, they gradually began to recognize humans as
being one species among many, with a few unique traits but many others shared in
common with other animals. Some 300 years later, Darwin used this vast stock of
anatomical knowledge to build his theory of evolution.
In the 1600’s the study of life changed forever. After relying on the authority of ancient
writes like Aristotle, European naturalists began to look at life for themselves. Anatomists
discovered that familiar organs, such as the heart, did not work the way that had previously
been thought, and the English inventor, Robert Hooke, was able to look through a
microscope to view previously unimaginable complexity hidden in tiny animals such as the
flea.
Some clergymen worried that this approach to life smacked of atheism but many
naturalists, themselves religious, believed that they were on a religious mission to help
demonstrate and appreciate God’s benevolent design. Natural theology dominated English
thinking for nearly two centuries.
The mid 1600’s heralded the birth of paleontology. Before this time fossils were thought to
have fallen from the sky but an Italian, Nicholas Steno, made the leap and declared that
“tongue stones” that had been previously found resembled shark teeth and did indeed
come from the mouths of once-living sharks. Steno proposed that fossils belonged to living
creatures and argued that fossils were snapshots of life at different moments in Earth’s
history and that rock layers formed slowly over them over time. Fossils ultimately became
some on the key evidence for how life evolved on Earth over the past four billion years.
Homo sapiens, Tyrannosaurus rex, Escherichia coli – our English conversation is littered
with pairs of Latin names for animals, plants, and microbes. Aristotle had begun to
organize life in his ladder-like hierarchy with plants on the bottom, animals in the middle,
and humans on the top. Swedish botanist Carl Linne (1707-1778; more popularly known as
Linneus, after the common practice of the day which was to Latinize names of learned
men), attempted to pigeon-hole all known species of the time into immutable categories.
Many of these categories are still used in biology today.
Georges-Louis Leclerc, Comte de Buffon (1707-1788) was the first to propose that
species’ could change. This was a major break from earlier concepts that species were
created by a perfect creator and therefore could not change because they were perfect.
Buffon also provided evidence of “descent with modification” and speculated on various
causative mechanisms. In his work, Buffon mentioned several factors could influence
evolutionary change including: influences of the environment, migration, geographical
isolation, overcrowding, and the struggle for existence. However, Buffon vacillated as to
whether he believed or not in evolutionary descent, and professed to believe in special
creation and the fixity of species.
Erasmus Darwin (1731-1802; grandfather of Charles Darwin) a physician and poet,
proposed that life had changed over time. His writings suggested the possibility of common
descent based on changes undergone by animals during development, artificial selection by
humans, and the presence of vestigial organs. However, he offered no mechanism to
explain evolutionary descent.
Georges Cuvier (1769-1832) was among the foremost proponents of catastrophism, the
theory that the earth and geological events had formed suddenly, as a result of some great
catastrophe (eg. Noah’s flood). This view was comfortable for the times and was widely
accepted. Cuvier eventually proposed that there had been several creations that occurred
after catastrophes. Louis Agassiz (1807-1873 – remember his name for the Africa unit)
proposed 50-80 catastrophes and creations.
Jean Baptiste de Lamarck (1744-1829) developed one of the first theories on how species
changed. Lamarck, concluded that organisms of higher complexity had evolved from
preexisting, less complex organisms. He was struck by the similarities of many of the
animals he studied and was impressed with the ever-growing fossil record. It led him to
argue that life was not fixed. When environments changed, organisms had to change their
behavior to survive. If they began to use an organ more than they had in the past, it would
increase in its lifetime. If a giraffe stretched its neck for leaves, for example, a “nervous
fluid” would flow into its neck and make it longer. Its offspring would inherit the longer
neck, and continued stretching would make it longer still over several generations.
Meanwhile organs that organisms stopped using would shrink. Lamarck was proposing
that life took on its current form through natural processes, not through miraculous
interventions (ie. God, supernatural forces etc). His ideas were rejected and he was
shunned and he died in poverty and obscurity. Despite all Lamarck got wrong, however, he
can be credited with envisioning evolutionary change for the first time.
Karl von Baer (1792-1876) was not a fan of evolution but his work on embryology
provided Darwin with the most compelling evidence for evolution. Baer’s work on
embryology showed that vertebrates all share an anatomy that invertebrates never acquire
- humans bear a resemblance to fish in their early embryonic stages - and although over
time organisms evolve further away from each other, natural selection modifies embryos in
various ways, but some vestiges of common ancestry survive. (See embryo comparisons at
the end of the handout)
Evolution by Natural Selection
The idea that species could change over time was not immediately acceptable to many: the
lack of a mechanism hampered the acceptance of the idea as did its implications regarding
the biblical views of creation. Charles Darwin (1809-1882) and Alfred Wallace (18231913) both worked independently of each other, traveled extensively, and eventually
developed similar ideas about the change in life over time as well as a mechanism for that
change: Natural Selection.
Charles Darwin
Darwin set sail on the Beagle, serving as the ship’s naturalist and spent much time ashore
collecting plant, animal and fossil specimens, as well as making extensive geological
observations. On his return to England in 1836, Darwin began to catalog his collections and
ponder the seeming “fit” of organisms to their mode of existence. He eventually settled on
four main points of the theory:
1. Adaptation: All organisms adapt to their environments.
2. Variation: all organisms are variable in their traits.
3. Over-reproduction: all organisms tend to reproduce beyond their
environment’s capacity to support them. This is based on the work of
Thomas Malthus (1766-1834) who studied how populations of organisms
tended to grow exponentially until they encountered a limit on their
population size).
4. Since not all organisms are equally well adapted to their environment, some
will survive and reproduce better than others – this is known as natural
selection. Sometimes this is also referred to as “survival of the fittest” (this
phrase originated with Herbert Spencer after he finished reading Darwin’s
Origin of Species). This merely deals with the reproductive success of the
organisms, not solely their relative strength or speed.
Alfred Wallace
Wallace came from the lower class of England and struggled financially all of his life. He left
England in 1854 to study the natural history of Indonesia where he contracted Malaria.
During a fever Wallace managed to write down his ideas on natural selection. In 1858,
Charles Darwin received a letter from Wallace, in which Darwin’s as-yet-unpublished
theory of evolution and adaptation was precisely detailed. Darwin had begun formulating
his theory of natural selection in the late 1830’s but he worked on it quietly for 20 years
and was shocked to see that Wallace’s theory nearly replicated his own. Wallace’s paper
published in 1858 (with the help of Darwin and several other colleagues) was the first to
define the role of natural selection in species formation. As a result Darwin rushed to finish
his book “On the Origin of Species by Means of Natural Selection” which remains one of
the most influential books ever written.
The Wallace-Darwin Theory
1. Individuals in a population have variable levels of agility, size, ability to obtain food,
and different successes in reproducing.
2. Left unchecked, populations tend to expand exponentially, leading to a scarcity of
resources.
3. In the struggle for existence, some individuals are more successful than others,
allowing them to survive and reproduce.
4. Those organisms best able to survive and reproduce will leave more offspring than
those unsuccessful individuals.
5. Over time there will be heritable changes in phenotype (and genotype) of a
species, resulting in a transformation of the original species into a new species
similar to, but distinct from, its parent species.
Neither Darwin nor Wallace could explain how evolution occurred: how were these
inheritable traits (variations) passed on to the next generation? Gregor Mendel had yet to
publish his ideas about genetics.
Gregor Mendel (1822-1884) is known as the “father of modern genetics” and working
with pea plants he came up with his laws of inheritance. Although his work was criticized
at the time it is now considered the foundation for modern genetics.
In the twentieth century, several scientists were able to identify the processes of
inheritance at a molecular level which greatly advanced the idea of evolution by descent
with modification. These include:
Hugo DeVries (Dutch botanist) – rediscovered Mendel’s work and in 1900 and proposed
that gene mutations would create a new species in a single step.
Thomas Hunt Morgan – in the 1920’s showed that gene mutation did not result in a new
species but could alter a trait that could be passed down to the next generation.
Theodosius Dobzhansky – In 1937 he published Genetics and the Origin of Species where
he sketched out an explanation for how species’ actually came into existence. Mutations
naturally crop up all the time. Some mutations are harmful in certain circumstances, but a
surprising number have no effect one way of the other. These neutral changes appear in
different populations and linger, creating variability that is far greater than anyone had
previously imagined. It is this variability that serves as the raw material for making new
species. Dobzhansky collaborated with many other scientists from other science disciplines
(eg. geology, population genetics, paleontology, biology, and genetics) and they came up
with one powerful explanation of evolution, showing how mutations and natural selection
could produce large-scale evolutionary change. Their combined work is known as the
“Modern Synthesis” and it became the foundation for future research.
Ernst Mayr – redefined the definition of species and proposed various methods as to the
creation of a new species (1942) including geographic isolation.
Watson and Crick – 1953 proposed the double helix model for DNA. Evolutionary biology
was revolutionized by the discovery of DNA. Scientists realized that a change in the
“spelling” of a strand of DNA would be copied and those mutations might confer a selective
advantage to an individual. The advantage may become more common over time, and
ultimately these mutant genes may drive the older versions out of existence. As a result of
the discovery of DNA and its structure scientist are now able to identify not just genes but
individual bases.
Embryo Comparisons