BILD 3 Chapter Notes - Chapter 7: Secondary Sex Characteristic, Adaptation, Stabilizing Selection
BILD3 Reading Notes 10/19/18
Chapter 23: The Evolution of Populations
- 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution
o Evolution by natural selection: both chance and sorting
▪ Chance: create new genetic variations through mutations
▪ Sorting: natural selection favors some alleles over others
o Outcome of natural selection in not random; it increases frequencies of alleles that
provide reproductive advantage, leading to adaptive evolution
o Natural selection: a closer look
▪ Relative fitness: contribution an individual makes to the gene pool of the next
generation relative to the contributions of other individuals
• Can be based on efficiency in collecting food, ability to conceal from predators
long enough to produce more offspring, direct combat, etc.
• Acts on phenotype; acts on genotype indirectly, via how the genotype affects the
phenotype
▪ Directional, disruptive, and stabilizing selection
• Directional selection: conditions favor individuals exhibiting one extreme of a
phenotypes range, shifting population’s frequency curve for phenotypic
character in one direction or the other
o Common when environment changes or when members of population
migrate to new habitat
• Disruptive selection: conditions favor individuals at both extremes of phenotypic
range
o Ex: population of black-bellied seedcracker finches have 2 distinct beak sizes.
Small for soft seeds and large for hard seeds. Intermediates relatively
inefficient at cracking both types of seeds, so have lower relative fitness
• Stabilizing selection: acts against both extreme phenotypes and favors
intermediate variants
o Reduces variation
▪ Selection favors individuals with heritable phenotypic traits that give higher
reproductive success
o The key role of natural selection in adaptive evolution
▪ Natural selection increases frequency of favorable alleles, enhancing survival and
reproduction -> adaptive evolution
▪ Adaptive evolution is continuous dynamic process, depends on changes in
environment
▪ Genetic drift and gene flow may change frequency of alleles, but only natural
selection consistently leads to adaptive evolution
o Sexual selection
▪ Sexual selection: individuals with certain inherited characteristics are more likely
than other individuals of the same sex to obtain mates
▪ Can result in sexual dimorphism: difference in secondary sexual characteristics
between males and females of the same species