Rucete ✏ Campbell Biology In a Nutshell
Unit 4 MECHANISMS OF EVOLUTION — Concept 23.4 Natural Selection Is the Only Mechanism That Consistently Causes Adaptive Evolution
Natural selection uniquely drives adaptive evolution, consistently increasing allele frequencies that enhance survival and reproduction. Unlike genetic drift or gene flow, natural selection reliably leads to adaptations by favoring traits that improve reproductive success.
Adaptive Evolution through Natural Selection
Evolution by natural selection involves two processes:
Chance: random genetic variation (mutations, sexual recombination).
Sorting: natural selection consistently favors advantageous traits.
Adaptive evolution occurs when traits that increase reproductive success accumulate over generations, improving a population’s fit to its environment.
Relative Fitness
Relative fitness is the contribution of an individual to the gene pool of the next generation compared to others.
Fitness does not always involve direct competition; subtle advantages (e.g., camouflage, energy efficiency) significantly influence reproductive success.
Selection acts directly on phenotypes, indirectly affecting genotypes.
Types of Selection
Natural selection can alter phenotype distributions in three main ways:
Directional Selection:
Favors individuals at one extreme of a trait.
Example: Finch beak size increases after drought conditions select for larger seeds.
Disruptive Selection:
Favors individuals at both extremes of a trait.
Example: Black-bellied seedcracker finches in Cameroon develop distinctly small or large beaks, with intermediate sizes less efficient.
Stabilizing Selection:
Favors intermediate phenotypes and selects against extremes.
Example: Human birth weights; babies of average weight have higher survival rates.
Sexual Selection
Sexual selection occurs when individuals with certain traits are more likely to obtain mates.
Can lead to sexual dimorphism, notable differences between sexes (e.g., male peacock plumage).
Two types of sexual selection:
Intrasexual selection: competition among same-sex individuals (usually males) for mates.
Intersexual selection (mate choice): individuals (usually females) select mates based on attractive traits, potentially indicating superior genetics.
Balancing Selection
Maintains genetic diversity through mechanisms such as:
Frequency-dependent selection:
Fitness of a phenotype depends on its frequency in a population.
Example: Left- and right-mouthed scale-eating fish; rarer mouth phenotype is favored.
Heterozygote advantage:
Heterozygotes have higher fitness than homozygotes.
Example: Sickle-cell trait provides malaria resistance, maintaining sickle-cell allele frequency in malaria-prone regions.
Why Natural Selection Doesn't Produce Perfect Organisms
Natural selection has limitations:
Selection acts only on existing variation: It can't produce ideal traits on demand.
Evolution is historically constrained: Evolution modifies existing structures rather than creating new ones from scratch.
Adaptations often involve trade-offs: Compromise between competing demands (e.g., seals optimized for swimming, not walking).
Chance, selection, and environment interact unpredictably: Sudden environmental changes can influence evolutionary outcomes.
In a Nutshell
Natural selection consistently drives adaptive evolution, favoring traits enhancing reproductive success. Through directional, disruptive, stabilizing, sexual, and balancing selection, populations adapt to changing environments. Despite its powerful effects, natural selection is constrained by genetic variation, historical factors, compromises, and unpredictable interactions with the environment.