There are three main ways for nonrandom mating to occur. First, positive assortative mating results when individuals and their mates share one or more phenotypic characteristics with themselves.
Negative assortative mating occurs when individuals and their mates are dissimilar phenotypically. Inbreeding is a third form of nonrandom mating that occurs when individuals mate with relatives more often than would be expected by chance.
For both inbreeding and assortative mating, genes combine in such a way that offspring genotypes differ from those that are predicted by the most basic population genetic model, described by the Hardy Weinberg theorem.
One assumption of the Hardy Weinberg theorem, which predicts unchanging equilibrium values for genotype and allele frequencies, is that individuals mate at random. Unlike other violations of this model (such as natural selection, genetic drift, mutation), nonrandom mating affects genotype but not allele frequencies.
Inbreeding is very common in many plant species for two main reasons. First, seed dispersal tends to followa leptokurtic distribution, such that most seed falls near the parent plant. This results in near neighbors that are closely related and increases the probability that short-distance pollen movement will result in mating among relatives.
In small populations with a limited number of potential mates, such matings between relatives are also common. Second, most flowering plants are hermaphroditic or monoecious. Thus, individual plants produce both male and female gametes and are capable of self-fertilization, the most extreme form of inbreeding.
The degree to which inbreeding occurs in a population depends upon the probability that an individual will mate with a relative or with itself. A plant’s mating system is characterized by the degree to which self-fertilization occurs and can range from complete outcrossing to complete self-fertilization, or selfing.
For species with a mixed mating system, and which therefore engage in both selfing and outcrossing, the degree to which individual offspring are inbred is highly variable.
Flowers with multiple ovules within an ovary can produce fruits with both selfed and outcrossed seeds. Some plants, such as violets (Viola) and jewelweed (Impatiens), produce morphologically distinct flowers for selfing and outcrossing.
Consequences of Inbreeding
Inbreeding has a larger evolutionary impact than assortative mating because it can affect all genes in the population. It can have negative consequences for plant survival and reproduction (fitness) because it tends to increase homozygosity and decrease heterozygosity.
In response to these negative effects, collectively known as inbreeding depression, plants have evolved numerous adaptations to reduce levels of inbreeding. Although evidence for inbreeding depression in plants has been found, many species of plants are almost completely self-fertilizing and do not appear to suffer fitness consequences.
Under certain conditions, inbreeding may be advantageous. For example, rare plants or plants with rare pollinators may have few opportunities for outcrossing, and thus, self-fertilization provides a level of reproductive assurance.
Many weedy plant species that tend to colonize disturbed sites are, in fact, capable of self-fertilization. Common crop weeds such as velvetleaf (Abutilon theophrasti) and shepherd’s purse (Capsella bursa-pastoris)predominantly self-fertilize.
In these species, inbreeding may provide benefits that outweigh any associated costs. It has also been suggested that inbreeding species are better able to adapt to local environmental conditions because fewer maladapted genes from other populations would enter through outcrossing.
Reducing Inbreeding Depression
Inbreeding depression (which occurs when alleles that decrease fitness drift to fixation, causing a decrease in average fitness within a population) is reduced when plants are genetically or morphologically unable to self-fertilize.
Genetic self-incompatibility, which is thought to occur in more than forty different plant families (for example, Brassicaceae, Solanaceae, and Asclepiadaceae) prevents mating between individuals that share certain genes that are involved in the interaction between pollen grains and the stigma or style. Morphological adaptations that reduce self-fertilization include those that separate anther and stigma maturation in time (protandry and protogyny) or space (heterostyly).
The individual hermaphroditic flowers of protandrous plants, such as phlox, shed their pollen prior to the time when the stigma on the same flower is receptive. Protogyny, which is less common than protandry, occurs when stigma receptivity occurs first (as in Plantago lanceolata).
Dioecious plant species, such as date palms (Phoenix) and marijuana (Cannabis sativa), avoid selfing by having unisexual male and female flowers on separate individuals. In some hermaphroditic species, selfing is avoided when flower morphology favors crosses between certain flower phenotypes, as in the case of heterostyly.
Assortative mating generally affects only those traits important for reproduction. Many primrose (Primula) species are distylous, having two types of flowers.
Flowers with the pin morphology have a tall style and relatively short stamens, while flowers with thrum morphology have a short style and long stamens. Insect-mediated pollen transfer results in matings between pin and thrumbut not between two pin or two thrum plants.
The result is nonrandom negative assortative mating. Unlike inbreeding, negative assortative mating tends to increase the level of heterozygosity in a population, at least for those traits that are involved inmate choice (such as relative style length).
Positive assortative mating, like inbreeding, results in increased homozygosity and decreased heterozygosity. Positive assortative mating for flowering time, for example, is common in many plant populations because individuals that flower early in the season will tend to mate with other early-flowering individuals.