In the context of genetics, fitness means the ability to pass on genes to the next generation. This may be due to the strength and fecundity of some individuals, but it can also be the result of attraction to mate or pollinator. Selection is the force that favors or disfavors some phenotypes relative to others under a certain set of environ mental circumstances. For selection to act, there must be variation of traits within a population. Without this genetic diversity a population is ill prepared for changes in its environment. Selection can be natural or artificial. Artificial selective forces such as antibiotics and pesticides can quickly increase resistance among bacteria and insects. Human genes reflect the history of the selective forces experienced by our ancestors. For example, some deleterious recessive genetic traits exist in higher proportion than expected due to the heterozygous advantage they confer. Also, some diseases are more prominent than expected, not because of any selective advantage, but because of the founder effect or random genetic drift. Thus, small populations, just because of their size, can contain a high proportion of less-fit individuals.

Try This at Home: Demonstrations of the Effects of Small Population Size

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