Chapter 1
Basic Evolutionary Theory
Douglas J. Futuyma Department of Ecology and Evolution, Stony Brook University, Stony Brook, NY, USA
Abstract
This chapter provides a cursory view of the basic principles of modern evolutionary theory. The major topics treated include the origin and nature of genetic variation, effects of genetic drift and various forms of natural selection on phenotypic traits and the genetic constitution of populations, fitness and its components, models of adaptation, levels of selection, the origin of species by speciation, and some aspects of macroevolution, chiefly phylogenies, evolutionary trends and diversification, gradualism, and the role of development in phenotypic evolution.
Keywords
Adaptation; Adaptive radiation; Divergence; Diversity; Evolution; Evolutionary trend; Fitness; Genetic drift; Genetic variation; Kin selection; Macroevolution; Mutation; Natural selection; Phylogeny; Sexual selection; Speciation
Synopsis
This chapter provides a cursory view of the basic principles of modern evolutionary theory. The major topics treated include the origin and nature of genetic variation, effects of genetic drift and various forms of natural selection on phenotypic traits and the genetic constitution of populations, fitness and its components, models of adaptation, levels of selection, the origin of species by speciation, and some aspects of macroevolution, chiefly phylogenies, evolutionary trends and diversification, gradualism, and the role of development in phenotypic evolution.
Introduction
The modern theory of evolutionary change has grown out of the “Evolutionary Synthesis,” from 1930 to 1950, in which researchers in genetics, zoology, botany, and paleontology united Darwin’s theory of evolution by natural selection with Mendelian genetics. They showed that this formulation accurately described the variation and diversity of animals and plants, both living and extinct, and explained why competing theories, such as neo-Lamarckism and simple mutationism, were inadequate or simply false. Reduced to its simplest elements, the modern theory may be summarized as follows. Elementary evolutionary change consists of changes in the genetic constitution of a population of organisms, or in a group of populations of a species. These genetic changes may be reflected as changes in one or more phenotypic characteristics. Genetic change is based on variation that has originated by mutation and/or recombination of DNA sequences. The most elementary evolutionary process is an increase in the frequency of a mutation, or a set of mutations, within a population, and the corresponding decrease in the frequency of previously common alleles. The major causes of such frequency changes are random genetic drift and diverse forms of natural selection. Successive such changes in one or more characteristics cumulate over time, so that potentially indefinite divergence of a lineage from the ancestral state may result. Different populations of a species may remain similar due to gene flow and perhaps uniform selection, but they can diverge (become different from one another) due to differences in mutation, drift, and/or selection. Some of the genetic differences between them can generate biological barriers to gene exchange, resulting in speciation: the formation of different biological species from their common ancestor. The particulars of these processes in any specific population depend on many aspects of the physical and biological environment, and on the existing features of the population, resulting from its previous evolutionary history (see
Futuyma, 2013, for elaboration).
The Origin of Genetic Variation
Mutational changes in DNA sequences range from single base-pair alterations to insertions, deletions, and rearrangements of genetic material, and even changes in ploidy (the number of sets of chromosomes). Mutations that have no effect on “fitness” (i.e., survival and/or reproduction) are said to be “selectively neutral.” These may include synonymous mutations in protein-coding regions (those that do not alter amino acid sequence), and mutations in pseudogenes and other apparently nonfunctional regions. Nonsynonymous mutations in coding regions and mutations in regulatory sequences are more likely to affect fitness. The rate of mutation (usually on the order of 10
−9 per base pair per gamete) is usually too low to appreciably drive allele frequency change within a population, but it can determine the rate of DNA sequence change in the long term, and can influence the level of genetic variation within a population. Whether or not the supply of suitable mutations often constrains rates and directions of phenotypic evolution is uncertain (
Blows and Hoffmann, 2005;
Futuyma, 2010). There is no known mechanism by which the environment can direct the mutational process in advantageous directions; in that sense, mutation is random with respect to utility.
Variation within Populations
Populations of most species carry substantial sequence variation in many gene loci, and most quantitative traits exhibit some heritable variation. The presence of two or more fairly common alleles or genotypes within a population is referred to as “polymorphism.” Such variation has arisen by mutation. Variation is enhanced by mutation, recombination (often), gene flow from other populations, and some forms of natural selection (see below). Variation is eroded by genetic drift and by most forms of natural selection. Analysis of genetic variation is based on the frequencies (proportions, in a population) of the alleles and genotypes at individual genetic loci (see
Hartl and Clark, 2007). For sexually reproducing populations, the Hardy–Weinberg (H–W) theorem states that the frequency of each allele (
pi for allele
i) will remain constant from generation to generation unless perturbed by mutation, gene flow, sampling error (genetic drift), or natural selection, and that the frequencies of the several genotypes will likewise remain constant, at values given by the binomial theorem (
pi2 for homozygote
AiAi, and 2
pipj for heterozygote
AiAj) if mating occurs at random. Alleles at two or more polymorphic loci are eventually randomized with respect to each other by the process of recombination (a state of linkage equilibrium) so that different alleles at one locus are not associated with those at the other locus. (If they are associated to any degree, the loci are in linkage disequilibrium.) These principles have important consequences; for example, at H–W equilibrium, a rare allele exists mostly in a heterozygous state, and so is concealed if it is recessive. Closely linked mutations can remain in linkage disequilibrium for many generations, enabling geneticists to use detectable mutations as genetic markers for nearby mutations of interest, such as those that cause inherited disease.
Phenotypic variation in most quantitative traits is polygenic, based on segregating alleles at several or many loci, and also includes environmental (e.g., dietary) effects on the development or expression of a character (
Falconer and Mackay, 1996). Thus, the variance in phenotype (
VP) includes a genetic component (genetic variance,
VG) and an environmental component (
VE), and often an interaction effect (
VG.E) as well. Although the individual phenotypic effects of alleles at various segregating loci are difficult to measure, most of the many loci appear to have small effects, and a few have fairly large effects, relative to the range of variation. One component of
VG, the additive genetic variance (
VA), is important for evolution by natural selection because it expresses the correlation between the phenotype of parents and their offspring. This component is attributable to the “additive” effects of alleles, that is, the phenotypic effect of each allelic substitution, averaged over all the genetic backgrounds in which it occurs.
VA depends on the number of loci contributing to the character, on the evenness of allele frequencies at each locus, and on the average magnitude of the phenotypic effect of different alleles. The ratio
VA/VP is termed the “heritability” of a trait (in the narrow sense); it is valid only for the particular population and the particular environment in which it was estimated, since other populations might differ in allele frequencies or in environment (which affects
VE). All else being equal, the higher the
VA (or
VA/VP), the greater the potential rate of evolution of a character, in response to natural selection.
A gene commonly affects two or more characters (pleiotropy), and so can contribute to a genetic correlation (rG) between them. Another possible cause of genetic correlation is linkage disequilibrium, nonrandom association of certain alleles at two or more loci within a population (e.g., an excess of AB and ab combinations and a deficiency of Ab and aB). Genetic correlations are important because if the population mean of one character is altered, perhaps by...
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