BSC 314 Lecture Notes - Lecture 38: Allele Frequency, Selective Breeding, Botany
27 Jun 2018
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Darwin's Theory of Evolution
Evolution, as understood by biologists, is the change through time that occurs
in populations of organisms in response to changing environments. The changes,
coded in the molecules of DNA, are transmitted from generation to generation and over
the history of the Earth have resulted in progressively more complex life forms. The
name of Charles Darwin and his theory of natural selection are inexorably attached
to evolution and, together with the mechanisms of genetics, form the basis of the
modern theory of evolution.
Simplifying and paraphrasing from Darwin's book, On the Origin of Species by Means of
Natural Selection, and adding current interpretations, the main points of his theory are:
all life came from one or a few kinds of simple organisms; new species arise gradually
from preexisting species; the result of competition among species is extinction of the
less fit; gaps in the fossil record account for the lack of transitional forms. These
assertions set the stage for the next part of the theory, why life evolves: the number of
individuals increases at a geometric rate; populations of organisms tend to remain the
same size because the resources are limited, and only the fittest survive; the survivors
are variable, and those that survive reproduce, perpetuating the favorable traits.
Natural selection, according to Darwin, is similar to artificial selection. The
environment acted as the selecting force in natural selection. Unlike the relatively rapid
selection pressures instituted by breeders, however, natural selection took long periods
of time to accomplish change. Darwin was familiar with the then new conclusions of
geologists that the Earth was far older than previously thought, which gave his theory of
natural selection sufficient time in which to work.
A major problem was an explanation for how the favorable selections were perpetuated.
In the 1860s the idea that offspring were blends or mixtures of the traits of their parents,
the so called “blending theory of inheritance,” was unable to accommodate transmittal ‐
of favorable adaptations from one generation to the next. With botanist Gregor Mendel's
ideas and the development of genetics, the inheritance portion of Darwin's theory no
longer posed a problem.
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