After Charles Darwin published his famous theory in 1959, the general concept of evolution gained
popularity relatively quickly.  However, the process that he suggested was responsible for
evolution, natural selection, was not as popular and was met with harsh criticism.  Many of his
critics thought that the concept of natural selection relied on assumptions about inheritance that
could turn out to be false.  This happened because Darwin was far ahead of his time and thus was
forced to rely on premises about heritability that had not survived empirical tests or scientific rigor.  
At the time DNA had not been discovered and Darwin had no authority to turn to for an explanation
of biological inheritance.  Articles written by Gregor Mendel in 1865, which now form the basis for
the study of genetics, were revived in 1900 and many thought that Darwin’s theory of natural
selection would finally be vindicated.  But a funny thing happened.  Many of the scientists
elaborating on Mendel’s laws of inheritance were concerted anti-Darwinians.

The confusion stemmed from the difference between discrete traits and continuous traits.  Some
heritable traits can be controlled by a single gene (a discrete trait) and others are controlled by the
interaction of multiple genes (a continuous trait).  Natural selection generally acts on continuous
traits, yet Mendel’s laws had not been extended to explain such traits.  These “polygenic” traits
were much more complicated and so Darwin was not truly vindicated until the 1920s when
mathematicians R.A. Fisher, J.B.S. Haldane and Sewall Wright were able to show that continuous
traits, the kind that selection acts on, are truly heritable.  Their success led to a modern synthesis
that combined Mendel’s laws of inheritance along with Darwin’s natural selection and is now
referred to as Neo-Darwinism.

Since the 1920s the fundamental tenets of Neo-Darwinism have remained intact but much has
been contributed.  Some of the most notable contributions came from Ernst Mayr who helped to
explain how different groups of animals -which were all molded and changed by natural selection-
came to become separate species.  He suggested that one way that a subpopulation of animals
could become separated genetically from an ancestral species is by geographical isolation.  Mayr
reasoned that if a subpopulation was kept from interbreeding with the species that it came from,
whether by a mountain range or a river or great distance, then over time that subpopulation would
change in physical appearance and genetic makeup.  When different populations of the same
species are separated for extended time, they can become reproductively incompatible.  It is
generally thought that both natural selection and speciation are responsible for the great diversity
of life that we see on our planet.

To read more about speciation
click here.

To read more about Darwin's life and work
click here.

Polygenic: adj.
Relating to a characteristic that is determined by the interaction of several genes.

Premise: noun
A proposition that is used as the basis of an argument or inference.  Something assumed.

Speciation: noun
into two or more genetically distinct ones.
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