1. Genetic Drift:
* Founder Effect: When a small group of individuals breaks away from a larger population to establish a new colony, they carry only a subset of the original population's genetic diversity. This can lead to a loss of alleles and a decrease in genetic diversity in the new population.
* Bottleneck Effect: This occurs when a population undergoes a drastic reduction in size due to a sudden event like a natural disaster or disease outbreak. The surviving individuals may not represent the full genetic diversity of the original population, leading to a loss of alleles.
2. Non-Random Mating:
* Inbreeding: When individuals with similar genetic backgrounds mate, it increases the frequency of homozygous genotypes and reduces heterozygosity, leading to a decrease in genetic diversity.
* Assortative Mating: This occurs when individuals with similar traits (e.g., size, coloration) mate more frequently than randomly, leading to a decrease in genetic diversity at loci associated with those traits.
3. Natural Selection:
* Directional Selection: Favors one extreme phenotype, reducing the diversity of traits within a population.
* Stabilizing Selection: Favors the average phenotype, leading to a reduction in the frequency of extreme variants and decreased genetic diversity.
* Artificial Selection: This is a form of directional selection driven by humans, where individuals with desired traits are selectively bred, leading to a decrease in the diversity of genes associated with those traits.
4. Gene Flow:
* Low Gene Flow: When there is limited migration between populations, genetic diversity within each population can decline due to the lack of new alleles being introduced.
5. Mutation:
* Deleterious Mutations: While mutations are the ultimate source of new genetic variation, deleterious mutations can be lost from a population through purifying selection, which can reduce genetic diversity. However, this effect is generally less significant compared to other mechanisms listed above.
It's important to note that these mechanisms can act in combination, and their impact on genetic diversity can vary depending on factors like population size, environmental conditions, and the specific genes involved.
In summary, while genetic diversity is essential for a species' adaptation and survival, several evolutionary processes can decrease it, making populations more vulnerable to extinction.
