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Charles Darwin and the Theory of Evolution
Summary
Charles Darwin was born in 1809. The grandson of Erasmus Darwin and Josiah
Wedgewood, the young Darwin took a keen interest in science and nature. He trained
as a Clergyman in Christ's College, Cambridge before securing a position on the HMS
Beagle as the ship's naturalist. The Beagle's five year cruise around the world took
Darwin to South America, Australia and New Zealand, where he recorded in
excruciating detail how species changed and sent back to England a multitude of
organisms and fossils.
On returning to England in 1836, he began studying his collections, observing
similarities between fossils and living organisms in the same geographical region. The
Galapagos Islands typified his findings; the islands are geographically isolated enough
from each other to form differentiated endemic species, and their commonalities lead
to the conclusion that the birds on different islands were evolutionarily descended
from the same species but adapted to life on different islands in different ways.
In 1839 Darwin published The Origin of Species, in which his theory of
evolution was detailed with such vast supporting evidence it was difficult to discredit.
Today, the overwhelming majority of biologists accept Darwin's basic theory, that
organisms with an inherited survival advantage over competing organisms of the
same species will have either a higher fertility rate or lower mortality rate and thus
those populations with the advantage will increase and the relative (or absolute, if
competition is great enough that the total population must remain approximately
constant) population of those organisms without the survival advantage will fall. The
advantageous trait will propagate throughout the species (since the probability of
acquiring it increases as it propagates), and thus the species evolves. If the variant
species is sufficiently mutated as to preclude interbreeding, a new species has
evolved.
Darwin's Yearly Life
Born in 1809 in Shrewsbury, Shropshire, Charles Darwin was a lazy young man and a
slow learner in school. However, he was a strong athlete and developed an interest in
the natural sciences from the many hikes he went on in North Wales. His brother built
a small chemistry lab when Darwin was 13, which was his first taste of scientific
method. Lacking academic direction in his life, his father enrolled him in Edinburgh
University to study medicine. In summers, Darwin would assist his father (a doctor),
which he enjoyed. However, studying medicine proved a bore for Darwin, who also
loathed the sight of blood. He once again neglected his studies and pursued other
interests, spending much of his time in Edinburgh's Natural History museum, where
he furthered his interest in the natural sciences.
After dropping out for good in 1827, Darwin's father enrolled him in Christ's
College, Cambridge, to become a clergyman. He (once again) neglected his studies,
but made some very influential friends, who would later inspire him to further his
academic interest in the natural sciences. One such man was Prof. Rev. John
Henslow, who tutored him in Maths and Theology. Their discussions of science
greatly broadened Darwin's knowledge of such diverse topics as geology,
mathematics, entomology, mineralogy, chemistry, and botany.
Darwin's Voyage
After graduating, Darwin found a place aboard the HMS Beagle in 1831 as a paying
passenger on its five-year voyage around the world. The voyage allowed Darwin to
study geological features such as rock strata and continental and island development
as well as the biology of the numerous species and fossils after being made the
official naturalist when the surgeon and previous naturalist left at Rio de Janeiro. The
Beagle sailed along the Eastern coast of South America to the continents tip and the
Falkland Islands, then North up the Western coast towards the Galapagos Islands, and
finally to New Zealand, Australia and Mauritius returning to England. Throughout the
journey, many on board The Beagle collected samples, which they sent back to
Cambridge University and the British Museum. These collections were significant
contributions to science in themselves and were studied in detail at Cambridge.
Darwin himself kept immaculately methodical records and wrote thorough and
detailed reports on the biology, anthropology, sociology and politics of the various
regions he came into contact with.
Of particular note to Darwin were the Galapagos Islands. The Galapagos
Islands are an archipelago of some 13 islands in the Pacific Ocean about 1,000 km off
the western coast of South America. They are home to a wide variety of endemic
species (i.e. species who are indigenous to the Galapagos Islands and nowhere else)
due to their isolation from the mainland. The islands themselves are also fairly
isolated from each other, and Darwin discovered that species on one island are very
similar to species on another, but with subtle differences. This differentiation of
species was the spark for Darwin's theory of evolution.
Darwin's Theory of Evolution
Before the 19th Century, the process by which species were created and destroyed was
largely biblically consistent. It was believed that species were destroyed by
catastrophes (such as the Great Flood of "Noah's Ark") and created from nothing
either by God or by Spontaneous Generation (whereby organisms such as bacteria
were thought to have naturally popped into existence – a theory disproved by Pasteur
in the microbiological sense and discredited by many in the evolutionary sense). The
theory was used to explain geological phenomena as well as speciation, and
prominent geologists such as Charles Lyell (who's work Darwin had read prior to
reaching South America) had begun to discredit it, favouring a more gradual
approach. The contemporary overlap between geology and biology was an important
inspiration for Darwin, who by the time of the Beagle's voyage was well versed in
both.
By the time Darwin took an interest in science, many scientists had put
forward theories of evolution, most notably Jean-Baptiste Lamarck. Lamarck
proposed that individuals adapt during their own lifetimes and pass on advantageous
traits they have acquired to their offspring. Through disuse, he believed, superfluous
traits were discarded and through intense use, beneficial traits were enhanced and
passed on. This form of evolution implied a natural driving force or purpose; he
believed that life innately strived towards perfection, and that this was the driving
force behind his evolution. Although Lamarck's theory was ultimately wrong, it
formed the basis for Darwinian evolution; indeed, Darwin often praised Lamarck for
his contribution to biology.
Darwin’s theory of evolution required an important intellectual step: that there
was no innate strive towards perfection, no invisible hand guiding the path of
evolution, and no divine intervention dictating which traits are beneficial and which
are disadvantageous. Darwin proposed that the driving force behind evolution was,
simply, survival. Drawing from Lamarck’s observation that each new generation
inherits traits from its ancestors, Darwin based his theory on the idea that there was a
struggle for survival between individuals within a species (normally caused by a lack
of food resources, a high mortality rate due to lack of food or presence of predators, a
high fertility rate, all of which resulting in a constant population), and that only those
individuals with the most advantageous traits will survive to adulthood (at which
stage they procreate). These advantageous traits will be passed on to the next
generation, and become dominant in the species at large throughout time. Thus, the
species has evolved.
Further, Darwin concluded that, if natural selection is observed over a large
enough number of generations, it could lead to new species. Evidence for this came
from the Galapagos Islands, which had its own species of birds, lizards, tortoises, etc
which were similar to those on the mainland and other islands, but were distinct and
unique species in themselves.
Initial failures of Darwin’s theory and the development of modern synthesis
As much as Darwin could explain with his theory of evolution based on natural
selection, there were still a number of holes in the theory; he could not explain where
variations between individuals within a species came from, or the mechanism through
which traits are passed on to the next generation. These were questions not answered
for some time, but even those answers still lead to more questions. Today, modern
synthesis is a robust theory accepted by the overwhelmingly majority of biologists
which combines Darwinian natural selection with modern developments such as DNA
and Genetics.
Gregor Mendel, an Austrian monk, provided a solution to one of the flaws in
Darwin’s theory. Mendel had been investigating variation in species of plants by
cross- and self-pollination for some 7 years, during which time he cultivated and
tested around 28,000 pea plants, before he came to the conclusion that traits were
passed on to the next generation in whole in discrete units (later termed “genes”). He
found that there was a gene that controlled each physical trait or phenotype such as
height, colour, etc, but that the gene had two bits of information, called alleles, one of
which was inherited from the mother, the other from the father. The allele is
essentially a variation of the specific gene which controls one of the organisms
phenotypes. An allele can be dominant or recessive. If an organism inherits the same
version of the trait from both parents, it is said to be homozygous; conversely if the
organism inherits different alleles, it is said to be heterozygous. If the gene is
homozygous, the inherited trait is obvious – it is simply the trait carried by the
particular inherited allele. If the gene is heterozygous, the trait expressed is the trait
carried by the dominant allele.
For example, consider two alleles of the gene for colour of a plant. The white
allele is recessive (denoted as the common letter w) and the red allele is dominant
(denoted as the capital letter R). Now consider two plants, one possessing two white
alleles (the plant is therefore white, having the gene ww) and one possessing two red
alleles (the plant is therefore red, having the gene RR). Cross pollinating these two
plants will yield one possible combination of alleles: { wR } , and all of the
offspring will be red (since the red allele is dominant). Self- or cross-pollinating these
offspring can yield four possible combinations of alleles: { RR, Rw, Rw, ww, }
with equal probabilities of occurring. Thus, the expected ratio of red to white plants in
this generation would be 3:1.
Mendel had thus discovered the mechanism through which traits are passed on
to subsequent generations. Further work based on Mendel's results lead to the
discovery that genes reside on chromosomes at specific locations. It has also been
shown that variants of a gene arise from random variations (mutations) in the
chemical structure of the gene, which may result from errors in DNA replication, or
perhaps more significantly, in natural variations within regulatory sequences, which
determines the specific function of a cell (by, for example, "turning on or off" a gene
specific to the function of a cell). Even a subtle change in the function of a cell could
have drastic consequences to the survival ability of an organism. A mutated cell will
either spread its gene through natural selection (on a cellular scale) if the mutation
aids in the survival of the cell, or will be removed from the gene pool by the same
process if the mutation is deleterious. It should be noted, however, that the vast
majority of mutations have no real effect; the mutation may, however, accumulate
over time, resulting in 'real' evolution when combined with other mutants. This
provides the solution to the second flaw in Darwin's theory: it describes how
variations in species occur naturally.
The ideas and developments after Darwin's The Origin of Species eventually lead to
the development of modern evolutionary synthesis. It is often referred to as neoDarwinism and uses Mendel's theory of genetics to plug the holes present in
Darwinism. The cellular-level evolution described above can be extrapolated to
provide explanations for the development of species as a whole. Assuming that the
environment cannot support a growing population, the modern theory of evolution is
as follows. First, a gene must mutate in such a way that those organisms with the
mutation are more capable of survival than those without. The gene must then be
allowed to propagate through reproduction, as described by Mendel. If this is true,
then the population of those organisms with the mutation will increase and the
population of those organisms without the mutation will dwindle and eventually die
out. The species is now said to have evolved.
This process can be further extrapolated to the development of new, unique
species and is known as speciation. This occurs when a species is in some way
reproductively isolated from its parent species. This may happen if there is a
geographical barrier between the species and its parent species (for example, in the
Galapagos Islands), which is known as allopatry, or if the variant subpopulation has a
significant mating preference for subpopulations with the same trait, which is known
as sympatry. The variant subpopulations, by virtue of their reproductive isolation,
begin to evolve differently from their parent species. When the variant subpopulation
is sufficiently mutated that the two variants are no longer capable of interbreeding, a
new and unique species has evolved.
Conclusion
Charles Darwin was a scientist like no other. His theories were revolutionary not only
to others in the field, but also to society at large. The elegance and simplicity of his
theories, the subtleties lost on many, underplay the awesome consequences of his
theory to philosophy, theology, biology, sociology and politics. No other scientist has
left such an indelible mark on the way humanity thinks of itself and our place in the
universe.