And Mutation

Type of animal science: Evolution

Fields of study: Ecology, ethology, genetics, population biology

Nonrandom mating, genetic drift, and mutation are three mechanisms, besides natural selection and migration, that can change the genetic structure of a population.

Principal Terms allele: alternative forms of a gene for a particular trait assortative mating: a type of nonrandom mating that occurs when individuals of certain phenotypes are more likely to mate with individuals of certain other phenotypes than would be expected by chance gamete: a haploid sex cell that contains one allele for each gene; sperm and egg cells are gametes that fuse to form a diploid zygote genetic variation or diversity: the total number and distribution of alleles and genotypes in a population genotype: the complete genetic makeup of an organism, regardless of whether these genes are expressed heterozygote: a diploid organism that has two different alleles for a particular trait homozygote: a diploid organism that has two identical alleles for a particular trait inbreeding: mating between relatives, an extreme form of positive assortative mating phenotype: the expressed genetic traits of an organism

Evolution is a process in which the gene frequencies of a population change over time, and nonrandom mating, genetic drift, and mutation are all mechanisms of genetic change in populations. These mechanisms violate the assumptions of the Hardy-Weinberg model of genetic equilibrium by increasing or decreasing the frequency of heterozygote genotypes in the population.

Nonrandom mating occurs in a population whenever every individual does not have an equal chance of mating with any other member of the population. While many organisms do tend to mate randomly, there are some common patterns of nonrandom mating. Often, individuals tend to mate with others nearby, or they may choose mates that are most like themselves. When individuals choose mates that are phenotypically similar, positive assortative mating has occurred. If mates look physically different, then it is negative assortative mating. Population geneticists use the term "assortative" because it means "to separate into groups," usually in a pattern that is not random. The terms "positive" and "negative" refer to the probability that mated pairs have the same phenotype more or less often than expected by chance. Two color varieties of snow geese (Chen hyperborea), blue and white, are commonly found breeding in Canada, and they show positive assortative mating patterns based on color. The geese tend to mate only with birds of the same color; blue mate with blue and white with white. Since a bird's color (phenotype) is determined by the presence of a dominant blue color allele, mat-ings between similar phenotypes are also matings between similar genotypes. Matings between similar genotypes cause the frequency of individuals that are homozygous for the blue or the white allele to be greater, and the frequency of heterozygotes to be less than if mating were random and in Hardy-Weinberg equilibrium. Negative assor-tative mating increases the frequency of heterozygote genotypes in the population and decreases homozygote frequency. Assortative mating does not change the frequency of the blue or white al-leles in the goose population; it simply reorganizes the genetic variation and shifts the frequency of heterozygotes away from Hardy-Weinberg equilibrium frequencies.

Inbreeding is the mating of relatives and is similar to positive assortative mating because like genotypes mate and result in a high frequency of homozygotes in the population. In assortative mating, only those genes that influence mate choice become homozygous, but inbreeding increases the homozygosity of all the genes. High homozygosity means that many of the recessive alleles that were masked by the dominant allele in heterozygotes will be expressed in the phenotype. Deleterious or harmful alleles can remain hidden from selection in the heterozygote, but after one generation of inbreeding, these deleterious alleles are expressed in a homozygous condition and can substantially reduce viability below normal levels. Low viability resulting from mating of like genotypes is called inbreeding depression.

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