Nonrandom mating changes the frequency of homozygotes

Mating patterns may alter genotype frequencies if individuals in a population choose other individuals of certain genotypes as mates. For example, if they mate preferentially with individuals of the same genotype, then homozygous genotypes will be overrepresented, and heterozygous genotypes underrepresented, in the next generation in comparison with Hardy–Weinberg expectations. Alternatively, individuals may mate primarily or exclusively with individuals of different genotypes. An example of such nonrandom mating is provided by plant species, such as primroses (Primula), that bear flowers of two different types. One type, known as pin, has a long style (female reproductive organ) and short stamens (male reproductive organs). The other type, known as thrum, has a short style and long stamens. Pollen grains from pin and thrum flowers are deposited on different parts of the bodies of insects that visit the flowers. When the insects visit other flowers, pollen grains from pin flowers are most likely to come into contact with stigmas of thrum flowers, and vice versa. In most species with this reciprocal arrangement, pollen from one flower type can fertilize only flowers of the other type. Self-fertilization (selfing), another form of nonrandom mating, is common in many groups of organisms, especially plants. Selfing reduces the frequencies of heterozygous individuals below Hardy–Weinberg expectations and increases the frequencies of homozygotes, without changing allele frequencies.

Natural selection results in adaptation

The evolutionary agents we have just discussed influence the frequencies of alleles and genotypes in populations. As we saw in the previous chapter, major perturbations, such as colliding continents, volcanic eruptions, and meteorite impacts, also have periodically altered the survival and reproductive rates of organisms. All of these agents dramatically affect the course of life’s evolution on Earth, but none of them result in adaptations. For adaptation to occur, individuals that differ in heritable traits must survive and reproduce with different degrees of success. When some individuals contribute more offspring to the next generation than others, allele frequencies in the population change in a way that adapts individuals to the environments that influenced their success. This process is known as natural selection. The reproductive contribution of a phenotype to subsequent generations relative to the contributions of other phenotypes is called its fitness. The word “relative” is critical: The absolute number of offspring produced by an individual does not influence the genetic structure of a population. Changes in absolute numbers of offspring are responsible for increases and decreases in the size of a population, but only the relative success of different phenotypes within a population leads to changes in allele frequencies — that is, to evolution. To contribute genes to subsequent generations, individuals must survive to reproductive age and produce offspring. The relative contribution of individuals of a particular phenotype is determined by the probability that those individuals survive multiplied by the average number of offspring they produce over their lifetimes. In other words, the fitness of a phenotype is determined by the average rates of survival and reproduction of individuals with that phenotype.