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Problem : What causes the variability of individuals in a population?

Genetic differences between individuals cause this variability. Different alleles of a gene can arise through mutation and these alleles are recombined randomly in each individual.

Problem : How does "survival of the fittest" effect the inheritance of fit alleles?

An individual with greater fitness is more likely to reproduce and pass this allele on to the next generation. Since more fit individuals produce more offspring, the percentage of individuals in the next generation with the fit allele, or the allelic frequency will increase.

Problem : Why is the fact that more offspring are born than can survive important to evolution?

If all offspring survived, those with fit alleles would be just as common as those with unfit alleles, since there would be no competition to test fitness. The frequency of all alleles would remain relatively constant in the population.

Problem : Describe briefly how an adaptation becomes common in a population.

An adaptation occurs when an allele or combination of alleles occurs that produces a trait that makes an individual better adapted to its environment. This individual is more likely to survive to reproductive age and produce offspring than those without the adaptation. This individual will pass its allele or combination of alleles on to its offspring. Those offspring will also have an advantage over non-adapted individuals and will produce more offspring with the adaptive trait. Gradually, the adaptive trait will become common in the population because individuals that posses it enjoy greater reproductive success.
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