Nonrandom mating, genetic drift, and mutation (zoology)

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 some organisms mate randomly, others engage in nonrandom mating, including humans, birds, mice, fish, wolves, primates, and some frogs and snails. There are several common patterns of nonrandom mating. Often, individuals mate with others nearby, or they 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, or morphs, of snow geese (Anser caerulescens), 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, matings 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 assortative 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 alleles 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.

Genetic Drift and Mutation

Genetic drift, like positive assortative mating, reduces the frequency of heterozygotes in a population, but with genetic drift, the frequency of alleles in a population changes. Nonrandom mating does not change allele frequency. Genetic drift is sometimes called random genetic drift because the mechanism of genetic change is random and attributable to chance events in small populations, such that allele frequencies tend to wander or drift.

Statisticians use the term “sampling effect” to describe observed fluctuations from expected values when only a few samples are chosen, and it is easy to observe by tossing a coin. A fair coin flipped a hundred times would be expected to produce approximately fifty heads and fifty tails, plus or minus a few heads or tails. Yet, if the coin is flipped only four times, it is not too surprising to get four heads or four tails. The probability of getting either all heads or all tails on four consecutive flips is one out of eight, but the probability of getting all heads or all tails decreases to much less than one in a billion as the sample size increases from four to a hundred tosses. Similarly, it is much easier for nonrandom events to occur in small populations than in large populations. If a population has two alleles with equal frequency for a particular trait, then the result of random mating can be simulated by tossing a coin. The frequency of each allele in the next generation would be determined by flipping the coin twice for each individual, since sexually reproducing organisms have two alleles for each trait and counting the number of heads and tails. In a small population, only a few gametes, each containing one allele for the trait, will fuse to form zygotes. Chance events can cause the frequencies of alleles in a small population to drift randomly from generation to generation; often, one allele is lost from the population.

In small populations with fewer than fifty mating pairs, alleles may be eliminated in fewer than twenty generations by random genetic drift, leaving only one allele for a particular trait in the population. Thus, all individuals would be homozygous for the remaining allele and genetically identical. Theoretically, in any finite population, random genetic drift will occur, but it is usually negligible if the population size is greater than a hundred. Sometimes, disasters or diseases may drastically reduce the population size, causing a bottleneck effect. The bottleneck in population size reduces genetic variability in a population because there are only a few alleles and results in random genetic drift. Many islands and new populations are established by a small group of founders that constitute a nonrandom genetic sample because they have only a fraction of the alleles from the original large population. Examples of the bottleneck effect include the overhunting of the American bison to near extinction in the 1800s which left modern buffalo with few variations in their genetic makeup. Similarly, the greater prairie chicken permanently lost 30 percent of its alleles when it was overhunted in the twentieth century. Founder effects and bottleneck effects are phenomena that result in a loss of heterozygosity and decreased genetic variability because of the chance drift in allele frequency away from Hardy-Weinberg equilibrium values in small populations.

Mutations are changes in genetic material that can be passed to offspring. Some mutations are changes at a single point in the chromosome, while at other times, pieces of the chromosome are removed, extra pieces are added, or pieces are exchanged with other chromosomes. All these changes could result in the formation of new alleles or could change one allele into a different allele. The random mistakes in the chromosomes occur at the molecular level, and only later are the changes in information or alleles translated into phenotypic differences. Thus, mutation is the ultimate source of genetic variability and is random with respect to the needs of the organism. Most mutations are lethal and are never expressed, but nonlethal mutations provide the necessary variation for natural selection. Even though mutations are very important for evolution, they have only a small effect on allele and genotype frequencies in populations because mutation rates are relatively low. If an allele makes up 50 percent of the gene pool and mutates to another allele once for every hundred thousand gametes, it would take two thousand generations to reduce the frequency of the allele by 2 percent. The net effect of mutations is to increase genetic variability, but at a very slow rate.

Studying Genetic Variability

Population geneticists use a wide variety of laboratory, field, and natural experiments to investigate genetic variability. Natural experiments are situations that have developed without a scientist intentionally designing an experiment, but conditions are such that scientists can test a theory. Researchers have used known pedigrees or ancestral histories of zoo animals and have found that mortality rates of inbred young are often two to three times higher than for noninbred young. Population geneticists use pedigrees to calculate the probability that two alleles are identical by descent; this research provides an index of the amount of inbreeding in a population.

The study of random genetic drift is usually carried out in the laboratory. Scientists often use small organisms that reproduce quickly, such as fruit flies (Drosophila melanogaster), to conserve space and save time. In a 1956 study of eye color conducted by Peter Buri, after only eighteen generations and sixteen fruit flies per population, more than half of the 107 populations started had only one of the two alleles for eye color.

Mutations are so rare that even fruit flies reproduce too slowly for scientists to study the effects of mutations on populations, even though much is known about the mechanism of mutation by studying Drosophila. Small bacterial growth chambers can hold many millions of bacterial cells, and, since they reproduce quickly, even mutations that occur in only one in a million cells can be detected. In 1955, it was found that mutation rates were very low in bacteria until caffeine was added to the growth chamber, whereupon mutation rates increased tenfold. Any chemical or type of radiation that can cause mutations is called a mutagen. Electrophoresis has also been a useful tool for the study of nonrandom mating, genetic drift, and mutations because allele and genotype frequencies can be determined from samples of the population, and unique alleles can be identified.

The Dangers of Inbreeding

Most governments and religions forbid marriages between close relatives because matings between first cousins result in a 20 percent decrease in heterozygosity; for those between brothers and sisters, there is an 80 percent decrease in heterozygosity. The decrease in heterozygosity and genetic variation and increase in homozygote frequency often result in inbreeding depression because deleterious recessive alleles are expressed. All inbreeding is not undesirable; many of the prizewinning bulls and pigs at state fairs have some inbreeding in their pedigrees. Most breeds of dogs were produced by breeding close relatives so that the offspring would have particular traits.

Zookeepers and others that breed and protect rare and endangered species must continually be concerned about the negative effects of both inbreeding and genetic drift. Most zoos are lucky if they have two or three pairs of breeding adults, and total population sizes are usually very small compared to those of natural populations. These conditions mean that inbreeding may reduce the vigor of the population, and genetic drift will reduce the diversity of alleles in the population, thus reducing the chances of survival for the captive species. There is hope for rare and endangered species if independent inbred lines are crossed, thus reducing the effects of inbreeding depression, and if breeding adults from other zoos or populations are traded occasionally, thus increasing the effective population size.

Mutations are the ultimate source of genetic variation and so are very important in the study of evolution, but the population-level effects of one mutation are difficult to study because of the low frequency of natural mutations. Certain nonlethal mutations may have little evolutionary impact but may be important medically because spontaneous mutations result in hemophilia, a blood clotting disorder, or dwarfism (achondroplasia) in more than 3 out of 100,000 cases. In certain populations of humans, greater transmission of psychiatric illnesses like bipolar and schizophrenia, poor social skill development, and lower grades in adolescent education are linked to nonrandom mating. As exposure to background radiation and chemical levels increases, mutation rates are likely to increase, as well as the incidence of mutation-related diseases.

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

Bibliography

Ayala, Francisco J. Population and Evolutionary Genetics: A Primer. Benjamin/Cummings, 1982.

Crow, J. F. Basic Concepts in Population, Quantitative, and Evolutionary Genetics. W. H. Freeman, 1986.

Fisher, R. A. The Genetical Theory of Natural Selection. Dover, 1958.

Hartl, Daniel L. A Primer of Population Genetics. 4th ed., Sinauer Associates, 2021.

---. Essential Genetics and Genomics. 7th ed., Jones & Bartlett Learning, 2020.

Hull, Sara. "Evolution." National Human Genome Research Institute, 14 Sept. 2024, www.genome.gov/genetics-glossary/Evolution. Accessed 15 Sept. 2024.

Mettler, L. E., et al. Population Genetics and Evolution. 2nd ed., Prentice-Hall, 1988.

Nordsletten, Ashley E., et al. “Patterns of Nonrandom Mating within and across 11 Major Psychiatric Disorders.” JAMA Psychiatry, vol. 73, no. 4, 2016, pp. 354-61, doi:10.1001/jamapsychiatry.2015.3192.

Real, Leslie A., editor. Ecological Genetics. Princeton UP, 2017.

Richardson, Joshua. Natural Selection and Genetic Drift. Nova Science Publishers, 2016.

Sunde, H. F., et al. "Genetic Similarity between Relatives Provides Evidence on the Presence and History of Assortative Mating." NatureCommunication, vol. 15, no. 2641, 2024, doi.org/10.1038/s41467-024-46939-9.

Templeton, Alan R. Population Genetics and Microevolutionary Theory. 2nd ed., Wiley Blackwell, 2021.