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 could 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 evolution became 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 years.
Contributions from Experimental Genetics
Several advances made by early experimental geneticists led to the following three points:
- 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 the following three points about natural selection. (Population genetics is discussed in more detail in, unsurprisingly, Population Genetics).
- 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 populations.
- 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 Speciation.
- 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 record.
- 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.