Selection in finite populations leaves hidden variation.

This diagram depicts a population consisting of 14 haploid individuals who carry plus or minus alleles at each of four sites in their genome (left panel). In a new environment favoring the plus alleles, selection will, over time, increase the frequency of the plus alleles throughout the genome (right panel). For example, in a hotter climate, alleles conferring tolerance to higher temperatures would rise in frequency. Selection favors the good gene combinations (here, the ones containing two plus alleles) and eliminates the bad gene combinations. In the absence of sex, the only variation that remains after several rounds of selection is hidden in the sense that plus alleles at the first site are found with minus alleles at the second site (or vice versa). This problem is irrelevant in an infinitely large population, because mutation will immediately create beneficial combinations (e.g., ++++), which are then amplified in frequency by selection. Sex and recombination acting in the population on the right would produce ++++ offspring, allowing adaptation to proceed further.