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 a…
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.
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Q1: Why can't natural selection produce perfect adaptations in organisms?
Natural selection is constrained by several factors. It can only act on existing heritable genetic variation within a population at a given time. Additionally, organisms inherit developmental features from distant ancestors that limit evolutionary possibilities. For example, birds evolved wings from existing limbs rather than developing them from scratch, constraining their flight capabilities.
Q2: How do trade-offs limit the effectiveness of natural selection?
Trade-offs occur because natural selection acts on an organism's overall phenotype, making traits interconnected. An adaptation beneficial for one function may be harmful for another. The ostrich's large body suits its environment but prevents flight. Similarly, an allele for red tusks might deter poachers but make tusks brittle, reducing their usefulness for fighting and foraging.
Q3: What role does genetic variation play in natural selection?
Natural selection can only increase the frequency of traits if heritable genetic variation already exists in the population. A population cannot evolve a trait unless alleles for that trait are present. For instance, a bird population cannot evolve faster flight without existing heritable variation in flight performance among individuals.
Q4: How can linked genes on chromosomes constrain natural selection?
Neighboring genes on the same chromosome are often inherited together as a unit. If a beneficial allele is linked to a harmful allele, both are passed together to offspring. An allele for red tusks inherited with an allele causing infertility would reduce in frequency because the combined effect harms the organism more than the individual allele benefits it.
Q5: Why might intermediate traits prevent evolution toward a new adaptation?
Intermediate phenotypes can be less favorable than either the original or target trait. An elephant population with traditional tusks, red tusks, and intermediate rose tusks might find rose tusks both brittle and coveted by poachers. This harmfulness of the intermediate form restricts the population's ability to transition from traditional to red tusks.
Q6: How do evolutionary mechanisms other than natural selection affect adaptation?
Gene flow and genetic drift can introduce harmful alleles or eliminate helpful ones, opposing natural selection's effects. Migration introduces new alleles into populations, while chance events like natural disasters cause random changes in allele frequency. Evolution results from combined pressures on populations, not a movement toward perfection.
Q7: What developmental constraints inherited from ancestors limit evolution?
Organisms inherit developmental plans from distant ancestors that restrict evolutionary possibilities. Birds evolved from nonflying vertebrates with four limbs, inheriting a body plan that made wings unlikely to form independently. Instead, repeated modifications to existing limbs allowed powered flight to evolve, demonstrating how ancestral constraints shape evolutionary outcomes.