Though Darwin published his Origin of Species
in 1859, and its picture of
the long term process of evolution was widely accepted within 15 years, the
mechanism of natural selection was not accepted for decades: Darwin
provide no evidence that such a mechanism could work. The field of
genetics, which has provided
a large portion of this evidence,
was in its infancy at time Darwin was writing Origin. Gregor Mendel, the
father of modern genetics, did not publish his famous findings on inheritance in
pea plants until 1866, and even then he was largely ignored for nearly 40 years.
However, once genetics began to move forward into its modern form, natural
selection became a much more viable mechanism for evolution, though with new
scientific knowledge some modifications needed to be made to Darwin's original
idea. In the 1930's and 40's several major works on evolution were published,
including Genetics and the Origin of Species by Theodosius Dobzhansky,
Systematics and the Origin of Species by Ernst Mayr, and Evolution: a
Modern Synthesis by Julian Huxley (brother to Aldous Huxley, author of
A Brave New World
). These bookst attempted to make
sense of Darwin's theory in light of the evidence for evolution found in
genetics and other fields (see Evidence for
Evolution). The resulting theory of
known as neo-Darwinism, the synthetic theory of evolution, or the modern
synthesis. Below are the main tenets of the modern synthesis. Today most of
these are still accepted, though some, most notably the gradual rate of
evolution, have come under
fire in recent
Contributions from Experimental Genetics
Several advances made by early experimental geneticists led to the following
- Genotype, the genetic make-up of an individual, differs from
phenotype, or the traits that individual displays. Phenotype results from
the interaction of the environment with the individual's genotype.
- The environment may change phenotype, but it does not affect genotype. There
is no Lamarckian inheritance.
- Hereditary variation is due to genes. One or several genes and how they are
passed from parent to offspring will determine the inheritance of a given trait.
Most traits are polygenic, involving several genes.
- Genes can change through mutation. This process takes place slowly.
Mutation and recombination of alleles give rise to genetic variability.
- Environmental factors may effect the rate of mutation, but they do not
direct mutation toward adaptation.
Contributions from mathematical model of population genetics
Mathematical modeling of the genetic make-up of populations led to useful
models such as those dictated by the Hardy-Wienberg
Law and gave us
three points about natural selection. (Population genetics is discussed in more
detail in, unsurprisingly, Population
- Evolutionary change is a populational process. It is dependent on the
balance of genotypes within a population rather than an individual's phenotype,
as Lamarck believed.
- Mutation occurs too slowly to shift a population from one genotype to
another. Rather, this occurs through natural selection, random genetic
drift, or both acting at once.
- Genetic differences do not need to be large to cause evolution in a short
period of time. Only a small slight fitness advantage is needed to cause
selection to occur.
Contributions from population geneticists and natural historians
Evidence gathered by scientists who observe natural populations rather than
artificial systems in the lab contributed the following six ideas:
- Selection pushes recombination further. A greater variation and combination
of traits is found than can be explained by normal rates of recombination.
- Natural populations are genetically variable.
- Populations of species in different locations may vary genetically. This
observation further divides "species" into genetically distinct individual
- Differences between species and populations can be experimentally shown to
have a genetic component. Most of these differences are polygenic,
supporting Darwin's claim that evolution takes place in small steps rather than
by individual mutations.
- Natural selection does occur in natural populations.
- Differences among populations of a species are often related to
environmental differences and, thus, are adaptive.
Contributions from systematists and taxonomists
The work of scientist attempting to classify organism based on comparative
anatomy and other techniques provided the following four points.
- Species represent different gene pools rather than groups that differ in
one or more characters. Genotype, not phenotype, determines species. This point
is discussed further in
- There is a continuum of genetic difference and reproductive isolation among
populations, providing support for the gradual, small step view of evolution
rather than the single mutation view.
- Speciation occurs when geographically separate populations become
genetically different. See the section on
Speciation for a detailed discussion
of this topic.
- Gradations in phenotypic variation between species, genera, orders, and
higher divisions show that evolutionary change occurs gradually rather than
through the sudden appearance of radically new "types".
Contributions from paleontologists
The fossil record, as discussed in Paleontology, the Evidence of
Evolution, has provided ample support for
evolution. Paleological findings can be summarized in the following two points:
- The fossil records show sudden jumps in the forms of species as well as
gradual change. The jumps are explainable as missing portions of the fossil
- All observations of the fossil record are consistent with evidence for
evolution from other fields. Every event in the fossil record can be explained
by evolution through natural selection.