Evolution of Populations and Microevolution

Evolution of Populations

• Populations, not individuals, evolve.
• Natural selection involves differential reproductive success; traits influencing survival and reproduction affect individuals.
• Individuals do not evolve; populations undergo changes and evolve.

Microevolution

• Microevolution is changes in allele frequencies within a population.
• Three major causes of microevolution are natural selection, genetic drift, and gene flow.
• For evolution to occur, genetic variation is required.

Genetic Variation

• Genetic variation refers to the differences among individuals within a population, which is crucial for evolution.

Variation within Populations

• Variation exists within all populations, including different species, like rock pocket mice in different environments or snails with diverse shell patterns.

Geographic Variation

• Populations of a species that are geographically isolated may present genetic differences due to different selective pressures and genetic compositions.

Sources of Genetic Variation

• Formation of New Alleles:
  • Mutations create new genes or alleles; these may or may not be heritable.
• Sexual Recombination:
  • Sexual reproduction leads to new combinations of existing alleles through independent assortment, crossing over, and random fertilization.

Populations

• Populations are groups of individuals of the same species that interbreed and produce viable offspring, largely isolated, making it difficult for interbreeding with other populations.

Gene Pool

• Gene pool = the collection of all alleles within a population at a specific time.
• Alleles in the population are present in varying proportions.
• Fixed alleles mean that everyone in the population is homozygous for a specific allele.

Wildflower Population

• The example depicts a wildflower population with a characteristic, like flower color, controlled by two alleles (red and white).
• Different genotypes with varying proportions exist within a population.

Hardy-Weinberg

• Hardy-Weinberg equilibrium describes a population where allele frequencies and, therefore, genotype frequencies, remain constant from one generation to the next.

• This provides a baseline to determine whether a population is evolving or not.

• Five conditions needed for Hardy-Weinberg equilibrium:

  1. No mutations
  2. Random mating
  3. Extremely large population size
  4. No natural selection
  5. No gene flow

• If any of these five conditions are not met, the population is evolving.

• Hardy-Weinberg Equation: p² + 2pq + q² = 1

  • p = frequency of one allele
  • q = frequency of another allele

Genetic Drift

• A significant microevolutionary mechanism.

• Random fluctuations in allele frequencies, particularly prominent in smaller populations (e.g., bottleneck effect).

• Bottleneck effect occurs when a population is drastically reduced.

• The surviving population's allele frequency is likely to differ from the original population's frequency.

Gene Flow

• Alleles move into or out of populations via the migration of organisms.
• Gene flow typically reduces genetic variation between populations.

Natural Selection

• The environment-driven process in which organisms with traits that allow them to survive and reproduce better in their environment pass on more copies of those advantageous alleles.

• Natural selection has different forms, including:

  • Relative fitness, being an individual's contribution measured against others
  • Ability to find food, preferred territory and camouflage.
  • Adaptations for enhanced success, improving survival in particular environments

• Different types of natural selection:

• Sexual selection – certain traits that increase an individual's reproductive success, like size, color or ornamentation.

Explore the key concepts of how populations, rather than individuals, evolve over time. This quiz covers microevolution, genetic variation, and the geographic differences among species. Understand the factors that drive changes in allele frequencies within populations and the role of natural selection.