31.4:
Limits to Natural Selection
Natural selection can produce stunning adaptations that seem to perfectly fit organisms to their environments, such as the camouflage that protects these moths from predatory birds. However, natural selection does not always result in such optimal adaptations. In fact, several factors impose serious limitations on natural selection, hindering the evolution of a perfect match between an organism and its environment.
First, natural selection can only act on the available phenotypic variation. For example, a population of birds cannot evolve to fly faster if a faster allele is not available in the gene pool or is not introduced by gene flow or mutation.
Second, natural selection is limited by developmental features established by distant ancestors. For instance, birds evolved from non-flying vertebrates with four limbs, inheriting a developmental plan from their ancestors that made it very unlikely that wings would arise from scratch. Instead, continuous modification of an existing pair of limbs allowed for the evolution of powered flight.
Third, selection acts at the level of the organism and not on individual features. This often results in compromises in which adapting one characteristic comes at the expense of a different one. The ostrich, for example, has long legs and a large body that is favorable in its environment. The massive body, however, is too large for flight.
31.4:
Limits to Natural Selection
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.