31.4
Limits to Natural Selection
Natural selection can produce striking adaptations that closely fit organisms to their environments, such as camouflage that protects butterflies from predatory birds.
However, natural selection does not always lead to ideal adaptations. Several factors constrain the outcomes of natural selection and prevent organisms from achieving a perfect match with their environment.
First, natural selection acts on heritable phenotypic variation present in a population at a given time. For example, a population of birds cannot evolve faster flight unless heritable variation affecting flight performance is present, with some birds naturally flying faster than others.
Second, natural selection is limited by developmental features inherited from distant ancestors.
For instance, birds evolved from nonflying vertebrates with four limbs. They inherited a developmental plan that made it very unlikely for wings to form from scratch. Instead, repeated changes to an existing pair of limbs allowed powered flight to evolve.
Third, selection acts on the overall phenotype of an organism, so traits are often interconnected, leading to trade-offs.
The ostrich, for example, has long legs and a large body that suit its environment. That large body, however, is too heavy for flight.
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, redtusks may enhance elephant survival by deterring ivory-seeking poachers. However, if there are no gene variants—or alleles—for redtusks, natural selection cannot increase the prevalence of redtusks. The allele must first exist or arise through mutation.
Tradeoffs also limit natural selection. While an allele for redtusks 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 redtusks is passed on with an allele causing infertility, redtusks 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, redtusks, and an intermediate rose. The rose tusks may be coveted by poachers, like traditional tusks, and brittle, like redtusks. The harmfulness of the intermediate phenotype could restrict the transition from traditional to redtusks 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.
Natural selection can produce striking adaptations that closely fit organisms to their environments, such as camouflage that protects butterflies from predatory birds.
However, natural selection does not always lead to ideal adaptations. Several factors constrain the outcomes of natural selection and prevent organisms from achieving a perfect match with their environment.
First, natural selection acts on heritable phenotypic variation present in a population at a given time. For example, a population of birds cannot evolve faster flight unless heritable variation affecting flight performance is present, with some birds naturally flying faster than others.
Second, natural selection is limited by developmental features inherited from distant ancestors.
For instance, birds evolved from nonflying vertebrates with four limbs. They inherited a developmental plan that made it very unlikely for wings to form from scratch. Instead, repeated changes to an existing pair of limbs allowed powered flight to evolve.
Third, selection acts on the overall phenotype of an organism, so traits are often interconnected, leading to trade-offs.
The ostrich, for example, has long legs and a large body that suit its environment. That large body, however, is too heavy for flight.
From Chapter 31:
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31.4 : Limits to Natural Selection
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