Fitness

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Fitness

Fitness is a central concept of evolutionary biology. We will consider individual fitness, followed by fitness as applied to alleles or genotypes.

The direct fitness of an individual is related to the number of offspring that that individual produces. Specifically, it is one-half of the number of offspring produced, because in sexual species, only half of an offspring's genes come from either parent. That proportion, one-half, represents the degree of relatedness, or proportion of genes shared, between parent and offspring.

Indirect fitness derives from shared genes with kin other than the direct offspring of an individual. This might include cousins, nieces, nephews, siblings, and so on. The indirect fitness of an individual is calculated by adding the relations of that individual multiplied by the degree of relatedness. Inclusive fitness represents the sum of direct and indirect fitness.

The concept of indirect fitness was developed by the evolutionary biologist W. D. Hamilton. The idea originated with attempts to explain altruistic behavior in animals. Altruistic behavior is defined as behavior that harms the actor yet benefits the recipient, and includes such actions as alarm calling, which may draw the attention of the predator to the caller.

According to natural selection theory, altruistic behavior should be eliminated from populations because it hampers individual survival and reproduction. However, Hamilton noted that if altruistic behavior benefits the kin of the actor, that behavior can nonetheless be selected for. This is because kin share genes with the actor. Hamilton's Rule dictates when altruistic behavior is beneficial: Altruism is selected for if the cost of a behavior to the actor is less than the benefit to the recipient, multiplied by the recipient's degree of relatedness to the actor. Thus, altruistic acts are more likely if they benefit close kin rather than distant kin, or unrelated individuals.

Kin selection explains a wide variety of altruistic behavior. It also explains the evolution of social systems in which some individuals forego reproduction in order to help parents raise siblings. This is the situation in many pack species, such as wolves. In wolves, packs are often made up of two parents and their offspring from several mating seasons. Only the parents, which are the dominant individuals in the pack, reproduce.

Kin selection also explains more extreme examples of social behavior, such as that found in eusocial insects (species in which there are non-reproductive individuals. The primary groups of eusocial insects are the Hymenoptera (ants and bees) and the termites. Both groups have evolved special genetic systems in order to make kin selection more powerful. The Hymenoptera are characterized by haplodiploidy, a genetic system in which the males are haploid and females are diploid.

One consequence of haplodiploidy is that females (who are the crucial players in the colony) share a greater proportion of genes with their sisters than they would with their own offspring. It therefore benefits females to care for sisters in the colony rather than try to reproduce on their own. Termites are not haplodiploid, but they do go through repeated cycles of inbreeding, which also results in individuals sharing an unusually large proportion of their genes.

Kin selection is more complicated in the real world than Hamilton's Rule suggests because the expected reproductive success of individuals must also be factored in. For example, even though an offspring only shares half its genes with a parent, the parent may protect an offspring more vigorously than expected because reproductive success of the younger offspring may be greater than that of the more aged parent.

So far, this discussion has focused on individual fitness. Fitness can also be defined for alleles or for genotypes rather than for individuals. Allelic or genotypic fitness describes the relative contribution of one allele or geno-type to the next generation as compared to that of possible alternate alleles or genotypes. These forms of fitness are central to population genetics.

Genotypes and alleles with higher fitness are selected for in the next generation, and make up a greater proportion of the total gene pool than other genotypes and alleles. All else being equal, alleles with greater fitness will eliminate and replace alleles of lower fitness. However, the fitness of particular alleles or genotypes may depend on numerous external factors, and changes in the relative fitnesses of alternate alleles/genotypes may help maintain polymorphisms in populations, situations in which a population has multiple alleles for a given locus.

One external factor determining the fitness of alleles and genotypes is the specific environment in which they are found. One well-studied example is that of the sickle-cell anemia allele. This allele is normally disadvantageous because individuals who are homozygous for the allele (that is, carrying two copies of it) have sickle-cell anemia. However, in malaria-prone areas, it has been shown that individuals who are heterozygous (carrying one sickle-cell allele and one normal allele) are more resistant than individuals who have two normal alleles. So, in areas where malaria occurs, the fitness of the sickle-cell allele is higher than in malaria-free areas.

Another external factor determining the fitness of a particular allele or genotype is the alleles an individual possesses for other genes. This is called epistasis.

Yet an additional external factor that may determine the fitness of an allele or genotype is its frequency in the population. This is known as frequency-dependent selection. Frequency-dependent selection is known to operate in mimicry systems, in which there are poisonous individuals as well as non-poisonous individuals of the same species that mimic the appearance of poisonous individuals. The fitness of either type depends on the relative frequencies of poisonous and nonpoisonous individuals in the population.

Jennifer Yeh