BIOL 112 Lecture 7

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Natural Selection (revisited)

• Acts on individuals, even though smallest unit for observable change is population
• Not due to chance (happens all the time), even though chance events may determine selective force
• Strictly intra-specific: each species has its own selective pressures (different niche)
• Can't select for future, only selects for present

Modes of Natural Selection

Stabilizing

Selects against extremes in the population.

Any forms outside the "normal" are selected against

Bell-curve of population becomes more narrow about the "desired" phenotype

Directional

Selects against variants of one extreme

Shifts bell-curve of population left or right

Common during adaptation to new habitat.

Disruptive

Acts against the intermediate forms, favoring the extremes

Slices bell-curve down middle into two bell-curves

Sexual Selection

Natural selection for mating process.

adaptations that increase the probability of

• finding a mate
• successful mating & fertility

Selection of traits makes males and females different (morphology and behavior): sexual dimorphism

Types

• inter-sexual selection (between sexes)
  • choosiness of females → selection of males with attractive characteristics
• intra-sexual selection (competition between same sex over a mate)

Dimorphism

1. (primary): difference in structures/function directly related to copulation.
2. (secondary): differences outside of copulatory apparatus (e.g. peacock tail, lion mane)

Secondary has pros and cons:

pros

• Increase reproductive success
• Increase survival? (e.g. bright colors to indicate toxins)

cons

• Decrease survival? (e.g. increase risk of predation or energetically costly)

Maintaining Genetic Variability

How is variability maintained with all of this selection going on?

• diploid organisms can carry recessive genes "quietly"
• Balancing Selection
  • heterozygote advantage (heterosis): heterozygous individuals may be more fit than homozygous "wild type"
    • e.g. heterozygous individuals with sickle-cell allele are resistant to malaria
  • Frequency-dependent selection: frequency of phenotype declines if it becomes too common
    • e.g. scale-eating fish has mouth on left or right side depending on which side it attacks from (fig 23.18)