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Organisms that are well-adapted to their environment are more likely to survive and reproduce. However, natural selection does not lead to perfectly adapted organisms. Several factors constrain natural selection.

For one, natural selection can only act upon existing genetic variation. Hypothetically, red tusks may enhance elephant survival by deterring ivory-seeking poachers. However, if there are no gene variants—or alleles—for red tusks, natural selection cannot increase the prevalence of red tusks. The allele must first exist or arise through mutation.

Tradeoffs also limit natural selection. While an allele for red tusks may protect against poaching, it might also make tusks brittle and less useful for fighting and foraging.

Tradeoffs at the genomic level exist because natural selection acts upon individuals rather than alleles. Neighboring genes on the same chromosome are often linked and inherited together. If an allele for red tusks is passed on with an allele causing infertility, red tusks could disappear because the inherited combination does more harm than good. 

Intermediate traits can also constrain natural selection. Imagine an elephant population with three variants of tusks: traditional, red tusks, and an intermediate rose. The rose tusks may be coveted by poachers, like traditional tusks, and brittle, like red tusks. The harmfulness of the intermediate phenotype could restrict the transition from traditional to red tusks in elephant populations.

While natural selection generally increases a population’s ability to survive and reproduce, other evolutionary mechanisms might have the opposite effect. Harmful alleles can be introduced and helpful alleles erased by migration (i.e., gene flow) or chance events (i.e., genetic drift), like natural disasters. Evolution is not a movement toward perfection, but a consequence of combined pressures on populations.