The gene pools of small populations are reduced compared to those of larger populations primarily due to inbreeding and bottleneck events.
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Inbreeding: In smaller populations, individuals are more likely to breed with close relatives, leading to an increase in homozygosity (the presence of two identical alleles at a locus). This can reduce genetic diversity and increase the chances of expressing deleterious traits or diseases associated with recessive alleles, resulting in a reduced overall fitness of the population.
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Bottleneck Events: These occur when a population experiences a significant reduction in size due to events such as natural disasters, habitat loss, or human activities. This sudden decrease in population size can severely limit the gene pool, as only a small number of individuals contribute their genes to the future generations, reducing genetic variation.
While the other responses—lack of suitable mates and lack of advantageous traits—can also be relevant to small populations, they are generally considered secondary effects stemming from the primary causes of inbreeding and bottleneck events.