What is natural selection?

Natural Selection and Evolution

In 1859, English naturalist Charles Darwin published
On the Origin of Species by Means of Natural Selection
, in which he made two significant contributions to the field of biology: First, he proposed that biological evolution can occur by “descent with modification,” with a succession of minor inherited changes in a lineage leading to significant change over many generations; and second, he proposed natural selection as the primary mechanism for such change. (This was also proposed independently by Alfred R. Wallace and was presented with Darwin in the form of a joint research paper some years earlier.) Darwin reasoned that if an individual organism carried traits that allowed it to have some advantage in survival or reproduction, then those traits would be carried by its offspring, which would be better represented in future generations. In other words, the individuals carrying those traits would be “naturally selected” because of the advantages of the traits. For example, if a small mammal happened to have a color pattern that made it more difficult for predators to see, it would have a better chance of surviving and reproducing. The mammal’s offspring would share the color pattern and the advantage over differently patterned members of the same species. Over many generations, the proportion of individuals with the selected pattern would increase until it was present in every member of the species, and the species would be said to have evolved the color pattern trait.

Natural selection is commonly defined as “survival of the fittest,” although this is often misinterpreted to mean that individuals who are somehow better than others will survive while the others will not. As long as the traits convey some advantage in reproduction so that the individual’s offspring are better represented in the next generation, then natural selection is occurring. The advantage may be a better ability to survive, or it may be something else, such as the ability to produce more offspring.

For natural selection to lead to evolutionary change, the traits under selection must be heritable, and there must be some forms of the traits that have advantages over other forms (variation). If the trait is not inherited by offspring, it cannot persist and become more common in later generations. Darwin recognized this, even though in his time the mechanisms of heredity and the sources of new genetic variation were not understood. After the rediscovery of Gregor Mendel’s principles of genetics in the early years of the twentieth century, there was not an immediate integration of genetics into evolutionary biology. In fact, it was suggested that genetic mutation might be the major mechanism of evolution. This belief, known as Mendelism, was at odds with Darwinism, in which natural selection was the primary force of evolution. However, with the “modern synthesis” of genetics and evolutionary theory in the 1940s and 1950s, Mendelian genetics
was shown to be entirely compatible with Darwinian evolution. With this recognition, the role of mutation in evolution was relegated to the source of variation in traits upon which natural selection can act.

The potential for natural selection of an organism is measured by its “fitness.” In practice, the fitness of an individual is some measure of the representation of its own offspring in the next generation, often relative to other individuals. If a trait has evolved as a result of natural selection, it is said to be an “adaptation.” The term “adaptation” can also refer to the process of natural selection driving the evolution of such a trait. There are several evolutionary forces in addition to selection (for example, genetic drift, migration, and mutation) that can influence the evolution of a trait, though the process is called adaptation as long as selection is involved.

Population Genetics and Natural Selection

Population geneticists explore the actual and theoretical changes in the genetic composition of natural or hypothetical populations. Not surprisingly, a large part of the theoretical and empirical work in the field has concentrated on the action of natural selection on genetic variation in a population. Ronald A. Fisher and J. B. S. Haldane were the primary architects of selection theory beginning in the 1930s, and Theodosius Dobzhansky was a pioneer in the detection of natural selection acting on genetic variants in populations of Drosophila melanogaster
(fruit flies).

The most basic mathematical model of genes in a population led to the Hardy-Weinberg law, which predicts that there would be no change in the genetic composition of a population in the absence of any evolutionary forces such as natural selection. However, models that include selection show that it can have specific influences on a population’s genetic variation. In such models, the fitness of an organism’s genotype is represented by a fitness coefficient (or the related selection coefficient), in which the genotype with the highest fitness is assigned a value of 1, and the remaining genotypes are assigned values relative to the highest fitness. A fitness coefficient of 0 represents a lethal genotype (or, equivalently, one that is incapable of reproduction).

The simplest models of selection include the assumption that a genotype’s fitness does not change with time or context and demonstrate three basic types of selection, defined by how selection acts on a distribution of varying forms of a trait (where extreme forms are rare and average forms are common). These three types are directional selection (in which one extreme is favored), disruptive selection (in which both extremes are favored), and stabilizing selection (in which average forms are favored). The first two types (with the first probably being the most common) can lead to substantial genetic change and thus evolution, though in the process genetic variation is depleted. The third type maintains variation but does not result in much genetic change. These results create a problem: Natural populations generally have substantial genetic variation, but most selection is expected to deplete it. The problem has led population geneticists to explore the role of other forces working in place of, or in conjunction with, natural selection and to study more complex models of selection. Examples include models that allow a genotype to be more or less fit if it is more common (frequency-dependent selection) or that allow many genes to interact in determining a genotype’s fitness (multilocus selection). Despite the role of other forces, selection is considered an important and perhaps complex mechanism of genetic change.

Detecting and Measuring Fitness

Although a great amount of theoretical work on the effects of selection has been done, it is also important to relate theoretical results to actual populations. Accordingly, there has been a substantial amount of research on natural and laboratory populations to measure the presence and strength of natural selection. In practice, selection must be fairly strong for it to be distinguished from the small random effects that are inherent in natural processes.

Ideally, a researcher would measure the total selection on organisms over their entire life cycles, but in some cases this may be too difficult or time-consuming. Also, a researcher may be interested in discovering what specific parts of the life cycle selection influences. For these reasons, many workers choose to measure components of fitness by breaking down the life cycle into phases and looking for fitness differences among individuals at some or all of them. These components can differ with different species but often include fertility selection (differences in the number of gametes produced), fecundity selection (differences in the number of offspring produced), viability selection (differences in the ability to survive to reproductive age), and mating success (differences in the ability to successfully reproduce). It is often found in such studies that total lifetime fitness is caused primarily by fitness in one of these components, but not all. In fact, it may be that genotypes can have a disadvantage in one component but still be selected with a higher overall fitness because of greater advantages in other components.

There are several empirical methods for detection and measurement of fitness. One relatively simple way is to observe changes in gene or genotype frequencies in a population and fit the data on the rate of change to a model of gene-frequency change under selection to yield an estimate of the fitness of the gene or genotype. The estimate is more accurate if the rate of mutation of the genes in question is taken into account. In the famous example of “industrial melanism,” it was observed that melanic (dark-colored) individuals of the peppered moth


Biston betularia became more common in Great Britain in the late nineteenth century, corresponding to the increase in pollution that came with the Industrial Revolution. It was suggested that the melanic moths were favored over the lighter moths because they were camouflaged on tree trunks where soot had killed the lichen and were therefore less conspicuous to bird predators. Although it is now known that the genetics of melanism are more complex, early experiments suggested that there was a single locus with a dominant melanic allele and a recessive light allele; the data from one hundred years of moth samples were used to infer that light moths have two-thirds the survival ability of melanic moths. Later studies also showed that peppered moths do not rest on tree trunks, calling into question the role of bird predation in the selection process. Nevertheless, selection of some sort is still considered the best explanation for the changes observed in peppered moth populations, even though the selective factor responsible is not known.

Later, a second method of fitness measurement was applied to the peppered moth using a mark-recapture experiment. In such an experiment, known quantities of marked genotypes are released into nature and collected again some time later. The change in the proportion of genotypes in the recaptured sample provides a way to estimate their relative fitnesses. In practice, this method has a number of difficulties associated with making accurate and complete collections of organisms in nature, but the fitness measure of melanic moths by this method was in general agreement with that of the first method. A third method of measuring fitness is to measure deviations from the genotype proportions expected if a population is in Hardy-Weinberg equilibrium. This method can be very unreliable if deviations are the result of something other than selection.

Units of Selection

Darwin envisioned evolution by selection on individual organisms, but he also considered the possibility that there could be forms of selection that would not favor the survival of the individual. He noted that in many sexual species, one sex often has traits that are seemingly disadvantageous but may provide some advantage in attracting or competing for mates. For instance, peacocks have a large, elaborately decorated tail that is energetically costly to grow and maintain and might be a burden when fleeing from predators. However, it seems to be necessary to attract and secure a mate. Darwin, and later Fisher, described how such a trait could evolve by sexual selection if the female evolves a preference for it, even if natural selection would tend to eliminate it.

Other researchers have suggested that in some cases selection may act on biological units other than the individual. Richard Dawkins’s
The Selfish Gene (1976) popularized the idea that selection may be acting directly on genes and only indirectly on the organisms that carry them. This distinction is perhaps only a philosophical one, but there are specific cases in which genes are favored over the organism, such as the “segregation distorter” allele in Drosophila that is overrepresented in offspring of heterozygotes but lethal in homozygous conditions.

The theory of kin selection
was developed to explain the evolution of altruistic behavior
such as self-sacrifice. In some bird species, for example, an individual will issue a warning call against predators and subsequently be targeted by the predator. Such behavior, while bad for the individual, can be favored if those benefiting from it are close relatives. While the individual may perish, relatives that carry the genes for the behavior survive and altruism can evolve. Kin selection is a specific type of group selection in which selection favors attributes of a group rather than an individual. It is not clear whether group selection is common in evolution or limited to altruistic behavior.

Impact and Applications

The development of theories of selection and the experimental investigation of selection have always been intertwined with the field of evolutionary biology and have led to a better understanding of the history of biological change in nature. More recently, there have been medical applications of this knowledge, particularly in epidemiology. The specific mode of action of a disease organism or other parasite is shaped by the selection pressures of the host it infects. Selection theory can aid in the understanding of cycles of diseases and the response of parasite populations to antibiotic or vaccination programs used to combat them.

Although the idea of natural selection as a mechanism of biological change was suggested in the nineteenth century, artificial selection in the form of domestication of plants and animals has been practiced by humans for many thousands of years. Early plant and animal breeders recognized that there was variation in many traits, with some variations being more desirable than others. Without a formal understanding of genetics, they found that by choosing and breeding individuals with the desired traits, they could gradually improve the lineage. Darwin used numerous examples of artificial selection to illustrate biological change and argued that natural selection, while not necessarily as strong or directed, would influence change in much the same way. It is important to make a clear distinction between the two processes: Breeders have clear, long-term goals in mind in their breeding programs, but there are no such goals in nature. There is only the immediate advantage of the trait to the continuation of the lineage. The application of selection theory to more recent breeding programs has benefited human populations in the form of new and better food supplies.

Key terms



adaptation

:

the evolution of a trait by natural selection, or a trait that has evolved as a result of natural selection





artificial selection


:

selective breeding of desirable traits, typically in domesticated organisms




fitness

:

an individual’s potential for natural selection as measured by the number of offspring of that individual relative to those of others




group selection

:

selection in which characteristics of a group not attributable to the individuals making up the group are favored





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