As Larry Moran noted with his question 2+ years ago, “What is Evolution?”, there’s a lot of confusion in the general public about what evolution is, and most people who object to it cannot define it. Perhaps the most definitive definition is that offered by biologist and author of the authoritative textbook on evolution Douglas Futuyma:
Biological (or organic) evolution is change in the properties of populations of organisms or groups of such populations, over the course of generations. The development, or ontogeny, of an individual organism is not considered evolution: individual organisms do not evolve. The changes in populations that are considered evolutionary are those that are ‘heritable’ via the genetic material from one generation to the next. Biological evolution may be slight or substantial; it embraces everything from slight changes in the proportions of different forms of a gene within a population, such as the alleles that determine the different human blood types, to the alterations that led from the earliest organisms to dinosaurs, bees, snapdragons, and humans.
-Douglas J. Futuyma (1998) Evolutionary Biology 3rd ed., Sinauer Associates Inc. Sunderland MA p.4
More generally speaking, evolution most simply refers to change over time. Since Darwin, evolution has always been a property of the population, not the individual. And, as alluded to by attempts to define evolution as changes in allele frequencies of populations, evolution of populations revolves around the notion of a collective set of heritable units that determine traits in the individuals carrying those units (i.e. genes).
But definitions of evolution centered on individual populations are limited to phyletic gradualism, a.k.a. ‘microevolution.’ What of evolution above the level of the individual population? How do we include the isolation of incipient species and interspecific competition – processes collectively referred to as ‘macroevolution’ – in our definition of evolution?
In the end, the best definition that I’ve come across, acknowledging both intra- and inter-population dynamics of biological evolution, is from the early draft of a “white paper” on Evolution, Science, and Society, chaired by Futuyma. The paper was written on behalf of eight scientific societies who wanted to make a statement about evolution, and appears to tersely sum up the view of evolution from across the dearth of sub-disciplines in biology.
Biological (or organic) evolution consists of change (modification) in the hereditary characteristics of groups of organisms over the course of generations. Such groups of organisms, termed populations or species, are formed by division of ancestral populations or species, and the descendant groups then change independently. Hence, in a long-term perspective, evolution is the descent, with modification, of different lineages from common ancestors.
This says nothing of mechanisms, and is written as a distinct theory of biology from those of Natural Selection, Sexual Selection, Neutral Theory (Genetic Drift, basically), and modes of speciation and clade sorting, which it is. So I would have to say that, as a definition of evolution, this initial draft of the ‘White Paper’ is precise and satisfactory.
What do you think?