Microevolution

Introduction

• Speciation: the origin of new species; at the focal point of evolutionary theory 
• Evolutionary theory must explain how new species originate and how populations evolve
• Microevolution: consists of changes in a single gene in a population over time; a change in allele frequencies in a population over generations
• Three mechanisms cause allele frequency change
  • natural selection
  • random genetic drift
  • gene flow through migration and nonrandom mating

Populations and Hardy Weinberg Equilibrium

• A population is in Hardy-Weinberg equilibrium if the genotype frequencies are the same in each generation. 

• Conditions that must be met include

  • there is no migration (“gene flow”) in or out of the population
  • natural selection is not occurring
  • mutation is not occurring (specifically in our germ cells)
  • each member of the population is equally likely to breed
  • the population is infinitely large

• As long as a population satisfies biological conditions the allele frequencies (p and q -> dominant and recessive allele, respectively) are the same in each generation.

• p^2 + 2pq + q^2 (Punnett square)

Agents That Change Allele Frequency In A Population

• Natural Selection

  • Some are more successful than others in surviving and reproducing owing to traits that give them a better fit to their environment.
  • Relative fitness: the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
    • Includes intrasexual and intersexual (mate choice) sexual selection
  • Causes adaptive evolution
  • Leads to greater relative fitness
  • Outcomes of natural selection
  • directional selection
  • disruptive selection
  • stabilizing selection
  • balancing selection

• Genetic drift

  • Founder effect: small number of individuals become isolated from a larger population
  • Establishes different gene pool from population due to limited variation
  • Bottleneck: when disaster strikes and chance alone leads to certain alleles being more or less present in survivors 
  • Migration and nonrandom mating: one member of the population is not equally likely to mate with any other member.
  • can reduce genetic variation in populations, potentially reducing its ability to evolve in response to new selective pressures.
  • can have drastic results in smaller populations.
    •  Endangered species.
  • can contribute to speciation. 
    • a small isolated population may diverge from the larger population

• Gene flow

  • Gene flow: the movement of alleles among populations

  • Immigration

  • Emigration

  • Alleles can be transferred through the  movement of fertile individuals or gametes

    • Gene flow tends to reduce differences between populations over time

  • Gene flow is more likely than mutation to alter allele frequencies directly

• Mutation

  • A single mutation can have a large effect, but in many cases, evolutionary change is based on the accumulation of many mutations.
  • It must occur in the germ line

• Sex 

  • Introduces new gene combinations into a population.
  • It’s the type of genetic shuffling that is a source of genetic variation. 

Balancing Selection

• Balancing selection: maintains genetic diversity
• Balanced polymorphism: two or more alleles are kept in balance, and therefore are maintained in a population over \n the course of many generations
• Two common ways
  • For a single gene, heterozygote favored
  • Heterozygote advantage – HS allele
  • Negative frequency-dependent selection
  • Rare individuals have a higher fitness

Causes of Adaptive Evolution

• Natural selection is the only mechanism that consistently causes adaptive evolution
• Evolution by natural selection involves both chance and “sorting”
  • new genetic variations arise by chance
  • beneficial alleles are “sorted” and favored by natural selection
• Only natural selection consistently increases the frequencies of alleles that provide reproductive advantage
• Natural selection brings out adaptive evolution by acting on an organism’s phenotype

Genetic Drift

• Genetic drift: unpredictable changes in allele frequency in a small population
• Genetic drift is significant (and faster) in small populations

1. Causes allele frequencies to change at random

      1. Unrelated to fitness

2. Genetic drift can lead to a loss or fixation of an allele (Frequency = 0 or 100%)
3. Genetic drift can cause harmful alleles to become fixed

Nonrandom Mating

• Forms of nonrandom mating
  • Assortative: individuals with similar phenotypes are more likely to mate
  • Increases the proportion of homozygotes
  • Disassortative: dissimilar phenotypes mate preferentially
  • Favors heterozygosity
  • Inbreeding: does not favor any particular allele but does increase the likelihood the individual will be homozygous
  • May have negative consequences with regard to recessive alleles

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