Understanding Alleles, Traits, and Microevolution Processes, Slides of Biology

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Processes of Evolution

Chapter 12

12.1 Impacts/Issues

Rise of the Super Rats

• When humans tried to eradicate rats with warfarin, natural selection favored individuals with a mutation for warfarin resistance

Alleles and Traits

• Alleles of the same genes are the main source of variation in a population - Traits with two distinct forms are dimorphic - Traits with several distinct forms are polymorphic - Traits with continuous variation may have interactions of several genes or be influence by environment
• Mutation is the source of new alleles

Mutation Revisited

• Mutations are the original source of alleles, but many are lethal or neutral
• Lethal mutation
  • Mutation that drastically alters phenotype; usually causes death
• Neutral mutation
  • A mutation that has no effect on survival or reproduction

Genetic Equilibrium

• Genetic equilibrium
  • Theoretical state in which a population is not evolving (allele frequencies do not change)
• Only occurs if five conditions are met:
  • Mutations never occur, population is infinitely large, population is isolated from gene flow, mating is random, all individuals survive and reproduce equally

Processes of Microevolution

• Genetic equilibrium does not occur in nature because processes that drive microevolution are always in play - Mutation - Natural selection - Genetic drift - Gene flow

Directional Selection

• Directional selection
  • Mode of natural selection in which phenotypes at one end of a range of variation are favored
  • Allele frequencies shift in a consistent direction in response to selection pressure
• Examples: peppered moths, rock pocket mice, antibiotic-resistant bacteria

Directional Selection

in Rock Pocket Mice

• Mice with coat colors that do not match their surroundings are more easily seen by predators

Stabilizing Selection

in Sociable Weavers

• Body weight in sociable weavers is a trade off between starvation and predation

Disruptive Selection

• Disruptive selection
  • Mode of natural selection that favors extreme phenotypes in a range of variation
  • Intermediate forms are selected against
• Example: African seedcrackers

12.4 Factors That

Affect Variation in Traits

• Interactions within species, and among genes and the environment, also influence natural selection - Sexual selection - Balanced polymorphism - Genetic drift - Gene flow

Sexual Selection

• Sexual selection leads to forms of traits that enhance reproductive success
• Sexual dimorphism is one outcome
• Sexual selection
  • Some individuals of a population outreproduce others because they are better at securing mates

Genetic Drift

• Genetic drift can lead to the loss of genetic diversity (fixation)
• Genetic drift
  • Random change in allele frequencies in a population over time, due to chance alone
• Fixed
  • Refers to an allele for which all members of a population are homozygous

Genetic Drift and Bottlenecks

• Genetic drift is pronounced in small or inbreeding populations, such as those that occur after an evolutionary bottleneck
• Bottleneck
  • Reduction in population size so severe that it reduces genetic diversity
• Inbreeding
  • Nonrandom mating among close relatives