Charles Darwin and the origin of species
In the two centuries before Darwin’s birth in 1809 there was a great expansion in the understanding of
Nature.
For example, Carolus Linnaeus (1707-1778) developed his taxonomy that grouped all life into a rational
nested hierarchical system that is the basis of modern classification. He developed the system of
binomial nomenclature in which each organism has a unique two word name e.g. humans are Homo
sapiens , which means “wise man.” Each organism belongs to a genus and then to progressively larger
and more inclusive groups – family, order, class, phylum, kingdom.
Nicolaus Steno (1638-1686) recognized that the triangular rocks known as “tongue stones” were in fact
shark’s teeth that had been transformed into stone (fossilized).
The new science of paleontology led to the discovery that life on the planet had changed greatly over
time. George Cuvier (1769-1832), a famous French paleontologist, documented the fact that fossils
resemble but are not exactly the same as modern species and that many species had become extinct.
These observations suggested that the fauna and flora of the earth changed over time.
The Age of the Earth
That the earth is old and undergoes constant change was not widely accepted until recently in western
thought. Most people in Darwin’s time considered the earth to be young (only thousands of years old
rather than 4.6 billion).
However, by the time Darwin was a young man the idea that the Earth was young was being challenged.
The opposition to a young Earth was based on the principle of Uniformitarianism. Uniformitarianism,
was first proposed by James Hutton (1726-1797) and championed by Charles Lyell (1797-1875) in his
book Principles of Geology. This is the idea that the geological processes happening today are the same
as have operated in the past. These processes, which include the deposition of sediments and erosion
by wind and waves work very slowly. Hutton and Lyell inferred the Earth must be very old based on their
measurements of rates of ongoing rock forming processes. To explain the existence of deep canyons
such as the Grand Canyon, the earth must be very old.
Uniformitarianism contrasted with Catastrophism which proposed that current geological formations
had resulted from catastrophic events (such as biblical flood) which occurred on scale unknown today.
These developments in geology focused Darwin on the potential importance of gradual change in
shaping structures.
In the early 1800’s the world was readyfor a comprehensive theory of evolution to be developed. In
fact, multiple people put forward the idea that evolution had taken place, what was lacking was a
workable mechanism.
Lamarck’s idea for how evolution worked.
One influential scientist is this period was Jean-Baptiste Lamarck (1744-1829). In 1809 Lamarck
suggested that organisms descended gradually from older less complex species. He considered lineages
of organisms had an inherent tendency to become more complex over time and transformed over time.
Lamarck’s primary mechanism by which evolution occurred was something he called the Inheritance of
Acquired Characteristics (IAC). IAC suggested that traits an organism acquired during its life could be
passed on to its offspring. By this process of transformation, lineages would gradually change over time
as beneficial acquired traits were accumulated and passed on
An example of IAC would be a giraffe stretching its neck during its life to reach higher leaves on a tree.
This would lead to the animal’s neck becoming a little longer during its lifetime and Lamarck believed
the giraffe would then pass this slightly longer neck to its offspring. Over time, giraffe necks would
become longer.
Lamarck’s ideas were widely discussed and influential in his time because there was no understanding of
how heredity occurs. No one understood genetics and how parents passed their traits on to their
offspring
Obviously, Lamarck’s ideas about mechanism of evolution contradict current biological knowledge. We
know that information flows from DNA to phenotype not in the reverse direction. As a result Lamarck’s
method cannot work
Charles Darwin (1809-1882)
Darwin was the son of a wealthy doctor, but he himself dropped out of medical school. He studied
theology at university, but was more interested in natural history. He was famously interested in beetle
collecting.
After graduation Darwin signed on as the Captain’s companion for a round the world voyage (18311836) aboard the surveying ship HMS Beagle. On the voyage Darwin read Lyell’s Principles of Geology
and was greatly influenced by Lyell’s emphasis of two major points.
1. The Earth is very old.
2. Geological features we see around us can be explained by the action of the slow, gradual
processes we can observe every day (e.g. the eroding of rock by waves and wind).
The Beagle mapped the coast of South America and circled the globe and visited South America,
Australia, Pacific Islands and South Africa. Darwin famously visited the Galapagos Islands a group of
volcanic (hence quite young islands) off the coast of Ecuador. Unique animals on Galapagos include giant
tortoises, marine iguanas, and Darwin’s finches.
On the voyage Darwin noted many things about nature that did not make sense if life had been
created.
1. South American fossils resembled living animals found in South America today. If God had created
all life at one time, why were there fossils? In addition, if fossils were the remains of an earlier creation
and today’s organism were the product of a newer creation, why should there be any resemblance
between them? If creation started from scratch there was no reason fossils should look like living
animals.
2. Parts of the world with very similar habitats and climates (e.g. in Australia, South America and
Africa) were populated by very different organisms. Why would God create different (e.g. grass eating)
animals in different parts of the world (kangaroos, deer, and antelope in Australia, South America and
Africa, respectively)? Why not just put the same animal in all parts of the world
3. Many species on remote oceanic islands are found only there (endemic). Islands such as the
Galapagos Islands are home to species found nowhere else on earth (e.g. Darwin’s finches). What is so
special about these little pieces of land that they have such unique faunas and floras? In addition, why
do the endemic species on islands often closely resemble species on adjacent continents? This makes no
sense if the island fauna was created, but makes perfect sense if the island was colonized from the
mainland and the organisms later evolved to fit their new environment.
These observations taken together don’t make sense if organisms are specially created. Together, they
suggested to Darwin that species change over time i.e., evolution occurs.
By the time he returned to England Darwin was certain that evolution occurred. However, he needed a
mechanism to explain how evolution happens.
Darwin’s mechanism is the process of Natural Selection.
Malthus and the struggle for existence
An important event in developing the idea of natural selection occurred when Darwin read Thomas
Malthus’ “Essay on population.” Malthus emphasized that the reproductive potential of animals and
thus the rate at which populations could increase in size was much greater than the rate at which food
resources might increase. Malthus concluded that high death rates were inevitable as a result as food
production could not match the birth rate.
Darwin realized that Malthus’ ideas applied to animals and plants also. He realized that most organisms
that were born could not survive to reproduce. There just were not enough resources to go around.
Because food supplies were limited there must be intense competition to survive. Darwin called this “a
struggle for existence” in which only some organisms survived to reproduce and the remainder did not.
The importance of variation
Darwin was a rabid collector (of beetles among other things) and like all collectors he sought rare things.
He was well aware that unusual individuals occurred in all species and that individuals in a population
differ from each other (there is lots of variation). Individuals in a species can differ in many ways -- size,
coloration, hunting ability, visual acuity, running speed, etc.
Darwin’s big breakthrough was in realizing that if success in surviving and reproducing was related to
variation (i.e., the traits an individual possessed affected its chances of reproducing) then beneficial
traits would become more common over time because the individuals that possessed those traits would
leave behind more descendants than individuals that lacked those traits. As a result of these differences
in reproductive success populations would change over time. And organisms would become better
adapted to their environment.
Darwin also realized that the process he was envisioning was very similar to what domestic breeders do.
That process is called Artificial Selection.
Humans have selectively bred for desirable traits in domestic animals and plants for millenia. In each
generation only those individuals with the best traits (e.g. wool quality, growth rate, seed size, etc.) are
chosen to reproduce. The process has produced our crop plants, garden plants, pets, and domestic
animals.
Darwin closely studied pigeon breeding. Over many generations pigeon fanciers have by selecting for
particular traits among Common Rock Doves have been able to produce a huge variety of different
breeds of domestic pigeons that look nothing like the original wild form.
Darwin saw that these varieties were so different in appearance that they would have been considered
to be different species if encountered in the wild.
Darwin envisaged a process similar to artificial selection that had produced organisms we see today. He
called it Natural Selection.
Natural Selection
The difference between artificial selection and natural selection was that instead of humans deciding
who would breed, nature would.
The logic of Natural Selection
Darwin proposed evolution was the inevitable outcome of 4 logical postulates or principles:
1. There is variation in populations. Individuals within populations differ in their traits (color, size,
hunting ability, etc).
2. Variation is heritable. Parents pass on their traits to their offspring. If a trait is genetically determined
and helps an organism to survive and reproduce it can be passed on to the individual’s offspring.
3. There is differential reproductive success. In every generation organisms differ in the number of
offspring they produce. Most organisms do not survive long enough to reproduce and those that do
survive produce different numbers of offspring.
4. Survival and reproduction are not randomly determined. It is not luck that decides who survives to
reproduce. Instead the traits that an organism possesses enable it to survive and reproduce. Those
organisms whose traits fit the environment best are ‘naturally selected’ and they pass their genes on to
the next generation. This is the crucial step in process of natural selection because it ties variation and
reproduction together.
Because only some individuals reproduce it is only their genes that make it into the next generation.
Thus the gene pool changes and the traits of the population change in the next generation.
If all four of these postulates are true then the population will change from one generation to the next.
Evolution will occur.
Darwin and his fear of controversy
Darwin realized his ideas would be controversial and delayed publishing them for fear of the backlash
that would result. Instead he continued to gather evidence to support his ideas.
It was not until he received a letter from Alfred Russel Wallace in 1858 in which Wallace outlined the
idea of natural selection was Darwin finally prompted to publish his work.
In 1858 Charles Darwin and Alfred Russel Wallace jointly proposed natural selection as the mechanism
of evolution in a paper presented to the Royal Society. In 1859 the “Origin of Species” was published
The idea that evolution occurred was accepted rapidly. However, a lack of understanding of heredity
and population genetics prevented natural selection being accepted as the primary mechanism until the
1930’s. Gregor Mendel discovered genes at about the same time the origin was published, but Mendel’s
work was ignored until after his (and Darwin’s) death and was not rediscovered until 1900.
The “modern synthesis”
The “Modern Synthesis” of genetics and Darwin’s ideas melded an understanding of population genetics
and natural selection to explain gradual evolution, speciation, and macroevolution.
We now know (which Darwin did not) that genes are passed whole and intact from parent to offspring.
Genes are “particulate.” They do not “blend” with other genes. Sometimes their presence may be
hidden by the action of other genes, but they do not disappear even if their effects are invisible.
We know that genes can change (mutate). Most mutations are harmful, but some mutations are
beneficial and these increase in frequency as a result of selection.
Mutation is a major source of genetic variation, but sexual reproduction also produces massive amounts
of genetic variation. Many traits result from the interaction of multiple genes and some genes affect
more than one trait. Thus, complex changes in phenotype can occur without assuming a large or
sudden genotype change.
Many genotype changes affect the phenotype (physical appearance of an organism). This variation in
phenotype is the raw material for natural selection.
The synthesis showed there was no conflict between modern genetics and Darwin’s idea that evolution
was largely a slow process driven by natural selection.
Darwin’s theory of Natural Selection being the primary engine driving the process of evolution has been
has been expanded to include the importance of Sexual Selection (selection for traits (e.g. the peacock’s
tail) that provide a mating advantage) and Genetic Drift, which is change in frequency of traits due to
chance events, which can be especially important when population sizes are small. We will explore both
of these topics later in the semester.