

There are actually two issues:
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The most obvious effect of inbreeding is the increase in homozygosity for deleterious mutations, causing more birth defects.
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A subtler effect is the loss of genetic diversity reducing a population’s ability to continue to evolve in response to future selection pressures. This would be especially important when migrating to a new environment with new selection pressures the species has never encountered before.













Wait, what? This is the first I’ve heard of that.