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Evolution in Biology
Biologists distinguish evolution, the fact that populations of organisms change over generations, from the theories that explain it, and argue about how the fact itself should be defined. The theory, natural selection and its synthesis with genetics, is set out under Darwinism.
The population-genetic definition
The definition most often given in textbooks is the one the modern synthesis made possible: evolution is any change in the frequency of alleles within a population from one generation to the next. It is usually credited to Theodosius Dobzhansky’s Genetics and the Origin of Species (1937), which treated evolution as change in the genetic composition of populations and its mechanisms as problems of population genetics.
The argument over it
Biologists have objected that the definition says too little. Ernst Mayr wrote in 1988 that the reductionist definition, “evolution is a change in gene frequencies in populations”, “is not only not explanatory, but is in fact misleading”, and proposed instead that “evolution is change in the adaptation and in the diversity of populations of organisms”. John Endler (1986) held that the definition “often results in an overemphasis on changes in allele frequencies” at the expense of the origin of the variants, and Leigh Van Valen (1973) defined evolution as “the control of development by ecology”. Douglas Futuyma’s textbook Evolution (2005) takes the broad course: evolution is “change in the properties of groups of organisms over the course of generations”, embracing “everything from slight changes in the proportions of different forms of a gene within a population to the alterations that led from the earliest organism to dinosaurs, bees, oaks, and humans”.
Evolution as such
Mayr’s One Long Argument (1991) separated the Origin into five theories: evolution as such, the claim that the world is not constant and organisms are transformed in time; common descent; the multiplication of species; gradualism; and natural selection. The first is what the word names in biology when it names a fact; the others are claims about its pattern and cause, and Mayr argued that they could be accepted or rejected independently, as Darwin’s contemporaries did.
The other root
The word came into biology through embryology before it was applied to species, for the unfolding of a preformed germ. Embryology also supplied the pattern the species sense later took over: Karl Ernst von Baer’s laws of development, in which the embryo passes from the general to the special, were what Spencer generalised into his law of evolution in the 1850s. That history is told on The word.
See also: Evolution (the subject landing) · Darwinism · Integration with genetics · The word · The criterion · Mayr · Dobzhansky · Von Baer