Every species has features–adaptations–that are thought to enhance survival in its environment. An adaptation is a characteristic that enhances the survival or reproduction of organisms that bear it, relative to alternative character states. Adaptations have evolved by natural selection. Modern biology views the development, physiology, and behavior of organisms as the results of purely mechanical processes, resulting from interactions between programmed instructions and environmental conditions or triggers.
Today we can cite hundreds of examples of adaptive evolution of morphological, physiological, and behavioral traits that have been directly observed. Adaptive evolution can be rapid, especially in species that have been introduced into new regions or subjected to human alterations of their environment.
Natural selection is any consistent difference in fitness among different classes of biological entities. A simple way to think of fitness is as the number of offspring an individual leaves in the next generation. The components of fitness are survival and reproduction. Fitness is sometimes called reproductive success, which includes survival because organisms do not reproduce when they are dead. If evolution by natural selection is to occur, there must be a change in the population across generations, and this requires that the phenotypic differences among the entities to be inherited. Thus, evolution by natural selection occurs if (1) there is a correlation between an individual’s phenotype and its fitness and (2) variation in the phenotype is correlated between parents and their offspring. It can be useful to think about the fitness of a type of gene (i.e., an allele), and consequently of selection among genes, even though the “entities” that differ in survival and reproduction in most discourse about evolution are individual organisms with different phenotypes (individual selection). Evolution by natural selection in sexually reproducing populations entails changes in the frequencies of alleles at the locus (or loci) that underlies variation in the phenotypic characteristic that influences fitness. “Natural selection” is not synonymous with “evolution”. Natural selection can occur without any evolutionary change. And processes other than natural selection can cause evolution. One of those processes is genetic drift: random fluctuations in the frequencies of genotypes within a population. Fitness differences are average differences, biases, differences in the probability of reproductive success. Natural selection is the antithesis of chance. The environmental factors that impose natural selection on a species are greatly influenced by the characteristics of the species itself: the evolutionary history of a species affects its relationship to the environment.
Natural selection applies to any classes of variable entities that can change in number. Selection can occur among genes, cell types, individual organisms, populations, or species, a hierarchy of levels of selection. Natural selection at the gene level (genetic selection) is illustrated by transposable elements, which replicate and proliferate within the genome, irrespective of whether their proliferation affects the organism for good or for ill. Selection at the gene level may act in opposition to individual selection: it may be harmful to individual organisms, and might even cause the extinction of populations or species. In a sense, any gene that has successfully increased in frequency is a selfish gene. Many altruistic traits are best explained by kin selection. An allele for altruistic behavior can increase in frequency in a population if the beneficiaries of the behavior are usually related to the individual who performs it. We may define kin selection as a form of selection in which alleles differ in fitness by influencing the effect of their bearers on the reproductive success of individuals (kin) who carry the same allele by common descent. An altruistic trait cannot evolve if it reduces the fitness of an individual that bears it, even if it benefits the population of species as a whole. There is one conceivable way a trait could evolve that benefits the population at a cost to the individual, namely by group selection: differential production of survival of groups that differ in genetic composition. The majority view is that few characteristics have evolved because they benefit the population or species, and that cooperation and seeming altruism are most likely to have evolved by other causes, especially kin selection. Selection among groups of organisms is called species selection when the groups involved are species and there is a correlation between some characteristic and the rate or speciation or extinction. Species selection does not shape adaptations of organisms, but it does affect the disparity–the diversity of biological characteristics–of the world’s organisms. The consequence of species selection is that the proportion of species that have one character state rather than another changes over time. A likely example of the effects of species selection is the prevalence of sexual species compared with closely related asexual forms.
“Adaptation” means the evolutionary process by which, over the course of generations, organisms are altered to become improved with respect to features that affect survival or reproduction. “An adaptation” is a characteristic of an organism that evolved by natural selection. Most evolutionary biologists think that for a character to be regarded as an adaptation, it must be a derived character that conferred higher fitness than the ancestral character state from which it evolved. A preadaptation is a feature that fortuitously serves a new function. A feature co-opted for a new function during evolution is sometimes called an exaptation. Exaptation is a very common early stage in the evolution of new adaptations. Natural selection may be considered a sieve that selects for a certain body size, mating behavior, or other feature. There may be incidental selection of other features that are correlated with that feature. When we speak of the function of a feature, we imply that there has been natural selection for the feature itself: that the feature caused its bearers to have higher fitness. Not all traits are adaptations: a trait may be a necessary consequence of physics or chemistry; the trait may have evolved by other mechanisms rather than natural selection; the feature may have evolved not because it conferred an adaptive advantage, but because it was correlated with another feature that did; or a character state may be a consequence of phylogenetic history. Methods that are used to infer that a feature is an adaptation for some particular function include analysis of the feature’s complexity, studies of function and design, experimental studies of the correspondence between fitness and variation within species, and correlations between the traits of species and environmental or other features (the comparative method).
Selection can fix only those genetic variants with a higher fitness than other genetic variants in a particular population at a particular time. It cannot fix the best of all conceivable variants if they do not arise, or have not yet arisen, and the best possible variants often fall short of perfection because of various constraints. Among these constraints are trade-offs. Lack of suitable genetic variation may explain cases of so-called phylogenetic constraints, in which species retain nonadaptive features or are unable to evolve adaptive traits.
The optimal feature, the character that maximizes fitness, depends on the context in which it functions. If different closely related species coexist, those individuals that use different resources from the other species suffer less competition. Character displacement is the divergence of species as a consequence of their interaction.
Selection at the level of genes and individual organisms is inherently “selfish”: the gene or genotype with the highest rate of increase spreads at the expense of others. Indeed, cooperation among organisms requires special explanations.
—January 2022
Lance Jones 