Fitness is the ability of an individual to survive and reproduce relative to other individuals in the population. An adaptation is a trait that increases an individual’s fitness in a specific environment.
The principal tenet of natural selection is that individuals with inherited traits that increase their reproductive output or survival will increase in frequency in the population relative to other competing individuals. Darwin’s argument for natural selection was based on three empirical observations – heritable variation within species, high but unrealized reproductive potential, and competition for scarce resources.
Evolution is the genetic change in a population over time. The phenotype is the characteristic morphology, physiology, and behavior of the organism. The phenotype is the product of the organism’s genotype, the sum total of its genes. The variation among individuals so important to Darwin’s theory arises by mutation, a random change in the DNA sequence of a gene. Beneficial gene variants provide the basis for natural selection. Evolution by natural selection affects a population of individuals. The study of these changes is the province of population genetics, in which is the focus is on the entire population and the gene pool, the sum total of all alleles in the population. We characterize the gene pool for any given trait by the allele frequencies, the proportion of each allele represents in the population. We model evolutionary change using a system known as the Hardy-Weinberg equilibrium, a mathematical representation of the genotype frequencies of a population in which the allele and genotype frequencies are not changing.
A population in Hardy-Weinberg equilibrium will remain there if 1) there is no differential success of genotypes, 2) the population is large, 3) there is no net movement of alleles in or out of the population, and 4) there are no new mutations. The violations of these four respective conditions represent potential mechanisms of evolution: a) natural selection, b) genetic drift (random changes in allele frequencies), c) gene flow (the net gain or loss of certain alleles by movement of individuals), and d) mutation pressure (the evolutionary change resulting from new mutants). The selection coefficient is the proportion of a genotype that is not represented in the next generation due to death or reproductive failure. Directional selection is a form of selection in which one tail of the phenotypic bell curve is favored. Stabilizing selection occurs when individuals in both tails of the curve are at a selective disadvantage and selection favors individuals with intermediate characteristics. Evolutionary trade-offs are features that confer an advantage in one respect but may have a cost in some other important way. Disruptive selection occurs when the tails of the distribution are favored over the intermediate phenotypes. Genetic drift is also known as non-Darwinian evolution (evolution that does not occur by means of natural selection). Loss and fixation of alleles are the inevitable consequences of genetic drift. A population that has lost variation is susceptible to changes in the environment. Gene flow may reinforce or oppose the changes that occur by natural selection and drift. The more isolated the population, the less gene flow and the more effective natural selection can be.
The success of a genotype is measure by its fitness. Fitness determines the relative ability of a genotype to obtain genetic representation in the next generation. The frequency of a genotype is reduced by selection against (by the value of the selection coefficient). Those genotypes that are less fit decrease in fitness by the amount of the selection coefficient each generation. If no other factors are operating, natural selection sorts the genotypes in the population according to their fitness. Because the environment imposes the selective force, the ecology of the organism is central to the adaptive process. There is a quantitative relationship between variation and natural selection. According to Fisher’s Fundamental Theorem, the increase in fitness of the population is directly proportional to the amount of genetic variation in the population. The variation among individuals in a population has two possible bases: genetic differences among individuals and phenotypic plasticity (the developmental or physiological variation among phenotypes induced directly by the environment). The heritability of a trait is the proportion of the phenotypic variation that is due to genetic differences among individuals. The higher the heritability, the tighter the genetic control of development. The rate of evolution depends on the combination of the intensity of selection and the heritability of the trait. Selection leads to geographic variation within species. Genetically distinct populations that are locally adapted to a particular environment are known as ecotypes. Not all organisms are perfectly adapted. Several factors contribute to non-optimal design features. First, adaptations are derived from the modification of existing structures. Another reason for imperfections in nature is that evolution is an ongoing process. The Red Queen hypothesis states that for some organisms the environment changes faster than adaptations can arise by natural selection. Finally, there are formal constraints on the adaptive response to the environment (such as the basic laws of physics and chemistry). Like gene flow, genetic drift may oppose the effects of natural selection or it may reinforce selection. An adaptive landscape is a graphical representation of the fitnesses associated with different genotypes in a population. Selection can only move the population uphill, to higher fitness. Drift could potentially take the population downhill, where selection could move it up to the higher peak.
Ecology and evolution are intimately linked. The evolution of each species is a response to its ecology; ecological processes are the direct result of the adaptive evolution of coexisting species.
—June 2021
Lance Jones 