Select All The Examples Of Nonrandom Mating

Select All the Examples of Nonrandom Mating

Nonrandom mating, a cornerstone concept in population genetics, significantly impacts the genetic makeup and evolution of populations. Unlike random mating (panmixia), where all individuals have an equal chance of mating with any other individual regardless of their genotype, nonrandom mating introduces biases that alter allele and genotype frequencies. This article will delve into various forms of nonrandom mating, providing detailed examples and exploring their consequences.

Types of Nonrandom Mating

Nonrandom mating can broadly be categorized into several types, each with unique mechanisms and evolutionary consequences. The most commonly discussed types are:

1. Assortative Mating

Assortative mating refers to the tendency of individuals with similar phenotypes to mate more frequently than expected under random mating. This can be further subdivided into:

a) Positive Assortative Mating:

Positive assortative mating occurs when individuals with similar phenotypes mate more often. This leads to an increase in homozygosity (the presence of two identical alleles at a locus) for the genes influencing the chosen trait.

Examples:

• Human height: Taller individuals tend to marry taller individuals, while shorter individuals tend to marry shorter individuals. This results in a higher frequency of homozygous genotypes for genes affecting height.
• Flower color: Plants with similar flower colors may be more likely to cross-pollinate, leading to an increase in homozygosity for flower color genes.
• MHC genes: Studies have shown a preference in humans for mating with individuals possessing dissimilar MHC (Major Histocompatibility Complex) genes. This seemingly counterintuitive example highlights the complex interplay of factors influencing mate choice. While it might appear to be negative assortative mating, the underlying mechanism often relates to immune system compatibility and disease resistance. Thus, the “dissimilarity” selection at the MHC gene level might create more positive outcomes for heterozygotes at other related loci involved in immune response.
• Self-fertilization (Autogamy): Found in many plant species, self-fertilization represents the ultimate form of positive assortative mating, where an individual mates with itself, resulting in highly homozygous offspring.

b) Negative Assortative Mating:

Negative assortative mating occurs when individuals with dissimilar phenotypes mate more frequently than expected under random mating. This leads to an increase in heterozygosity (the presence of two different alleles at a locus) for the genes influencing the chosen trait.

Examples:

• MHC genes (continued): While the selection for dissimilar MHC genes is positive assortative mating concerning the specific MHC genes, the outcome at other loci might be negative assortative mating. The mating pairs overall show diverse traits, impacting other genes not specifically involved in MHC-related immunity.
• Flower color in some plant species: Plants might show a preference for pollinators from different plants of contrasting flower color, leading to increased heterozygosity.

2. Disassortative Mating

This is essentially synonymous with negative assortative mating, highlighting the avoidance of mating between similar individuals.

3. Inbreeding

Inbreeding refers to mating between individuals who are more closely related than expected under random mating. This increases the probability of homozygous genotypes, including those carrying recessive deleterious alleles.

Examples:

• Selfing in plants: As mentioned above, self-fertilization is a common example of extreme inbreeding.
• Consanguineous marriages: Marriages between close relatives (e.g., first cousins) are a form of inbreeding in humans, increasing the risk of recessive genetic disorders in offspring. This practice has significant historical and cultural context, with varying prevalence and acceptance across different societies.
• Inbred animal strains: Many laboratory animals are inbred to maintain a consistent genetic background for research purposes. However, this often leads to reduced fitness and increased susceptibility to diseases due to the accumulation of deleterious recessive alleles.

4. Outbreeding (Exogamy)

Outbreeding refers to mating between individuals who are less closely related than expected under random mating. It is the opposite of inbreeding and can increase heterozygosity, potentially leading to increased genetic diversity and fitness.

Examples:

• Migration and gene flow: The movement of individuals between populations promotes outbreeding, introducing new alleles and increasing genetic diversity.
• Active avoidance of close relatives: Many animal species have mechanisms to avoid mating with close relatives, ensuring a wider gene pool.

Evolutionary Consequences of Nonrandom Mating

Nonrandom mating, regardless of its type, has significant evolutionary consequences:

• Changes in genotype frequencies: Inbreeding and positive assortative mating increase homozygosity, whereas negative assortative mating and outbreeding increase heterozygosity.
• Increased expression of recessive alleles: Inbreeding increases the probability of homozygous recessive genotypes, leading to a higher frequency of expression of recessive alleles, including those that are deleterious. This phenomenon, known as inbreeding depression, can lead to reduced fitness, lower reproductive success, and increased susceptibility to diseases.
• Altered allele frequencies: While nonrandom mating doesn't directly change allele frequencies in the overall population, it can impact the genetic variation within sub-populations. It can indirectly affect allele frequencies through its effect on fitness. Certain genotypes might be favored or disfavored based on the mating system, eventually leading to shifts in allele proportions over time.
• Loss of genetic diversity: Extreme inbreeding can lead to a significant loss of genetic diversity, making populations more vulnerable to environmental changes and diseases.
• Maintenance of genetic diversity: On the contrary, outbreeding and negative assortative mating can promote genetic diversity, enhancing a population's adaptability and resilience.

Distinguishing Nonrandom Mating from Natural Selection

It's crucial to distinguish nonrandom mating from natural selection. While both influence the genetic makeup of a population, they operate through different mechanisms:

• Nonrandom mating affects genotype frequencies but not necessarily allele frequencies directly. It simply changes the probability of certain combinations of alleles.
• Natural selection directly alters allele frequencies by favoring certain alleles over others due to their differential contribution to fitness.

However, nonrandom mating can indirectly affect natural selection. For example, inbreeding depression can reduce the fitness of individuals, making them more susceptible to natural selection.

Nonrandom Mating and Human Populations

Nonrandom mating plays a significant role in human populations, influencing the frequency of genetic disorders and the distribution of genetic traits. Factors such as cultural practices, geographical isolation, and social structures contribute to nonrandom mating patterns. The effects are observed in the prevalence of specific genetic disorders in certain populations due to historical patterns of inbreeding.

Conclusion

Nonrandom mating is a powerful force shaping the genetic structure and evolution of populations. Understanding its different forms and consequences is essential for comprehending the intricacies of population genetics and the factors contributing to the genetic diversity and adaptation of species. While the specific examples given illuminate the diverse manifestations of nonrandom mating across different organisms, further research continues to refine our understanding of its complex interplay with other evolutionary processes. The investigation into the intricacies of mate choice and its genetic consequences remains a dynamic and crucial area of study within evolutionary biology.