Microevolution: How Populations Change Notes

Microevolution: How Populations Change

Natural Selection

  • Definition: Natural selection is a process where due to variation in traits among organisms, some individuals are more likely to survive and reproduce than others.
    • As a result, the selection of heritable traits leads to a change in populations, a process referred to as evolution.
  • Population: A population is defined as a group of individuals of the same species.
    • The change in form that occurs over generations within a population is termed evolution.

Populations: The Units of Evolution

  • Population: A group of interbreeding individuals of the same species.
    • For asexually reproducing species, it refers to groups or lineages with high genetic similarity, sharing a common ancestor and ecological niche.
    • A population can consist of genetically identical or highly similar organisms (clones) living and interacting in a specific area.
  • Key Point: Populations are the smallest biological unit that can evolve.
  • Gene Pool: All the alleles present in a population at any one time comprise the gene pool.
    • This serves as a reservoir for alleles that the next generation will draw from.

Allele Frequencies and Natural Selection

  • Natural selection leads to changes in allele frequencies over time.
  • A hypothetical example illustrates allele distribution:
    • Initial allele frequencies: 50% A and 50% a.
    • Over time, as natural selection acts, the frequencies may adjust (displaying visual representation in graphs).

Outcomes of Natural Selection

  • Three types of natural selection outcomes:
    • Directional Selection: Favors one homozygote and thus shifts the makeup of the population.
    • Example: Frequency of individuals changes favoring homozygotes (AA).
    • Can lead to extreme forms as different phenotypes adapt to specific environments (e.g., camouflage).
    • Disruptive Selection: Favors both extreme phenotypes, decreasing the intermediate forms.
    • Example: The Black-bellied seed crackers exhibit two phenotypes with large and small bills adapted to different types of seeds.
    • Over time, this could lead to speciation.
    • Balancing Selection (Stabilizing Selection): Favors heterozygotes leading to a high proportion of intermediate phenotypes in the population.
    • Example: Human birth weights mostly range from 6.6 to 8.8 pounds, with disadvantages for lower or higher weights.

Sexual Selection

  • Definition: Sexual selection is exemplified through mate choice usually performed by females.
  • Types of Sexual Selection:
    • Intersexual Selection (Mate Choice): Phenotypic differences between males and females (sexual dimorphism), where individuals of one sex are choosy.
    • Intrasexual Selection: Competition for mates among members of the same sex, typically male-male competition.

Mechanisms of Evolution

  • While selection is the most profound mechanism affecting evolutionary change, other processes also contribute:
    • Genetic Drift: This refers to changes in the gene pool of small populations due to random chance events, not selection.
    • Example: Genetic frequencies before (AA = 3/9 = 0.33) and after (AA = 3/7 = 0.43).
    • Gene Flow: Migration that introduces new alleles to populations, reducing differences between them.

Bottleneck Effect

  • Definition: A population bottleneck occurs when a drastic reduction in population size happens, affecting genetic variability.
    • Causes can be natural (hurricane, disease) or anthropogenic (overhunting).
  • Consequences:
    • Reduction of genetic variability.
    • Potential loss of individual variation affecting adaptability.
  • Example: The cheetah population has faced bottlenecks leading to a decreased ability to adapt to environmental challenges.

Founder Effect

  • Describes a situation where a small group colonizes a new habitat, leading to possible genetic drift.
  • Effects: Certain inherited disorders become more common, as exemplified in isolated populations like the Amish experiencing higher rates of polydactyly.

Non-Random Mating

  • Assortative Mating: Individuals show preference for mates with similar phenotypes (e.g., deafness in humans).
  • Disassortative Mating: Preference for mates with different phenotypes, such as the major histocompatibility complex (MHC) detected by pheromones.

Conclusion

  • Different mechanisms including natural selection, genetic drift, founder effects, and non-random mating play crucial roles in the evolutionary processes modifying populations over generations. Each mechanism not only provides explanations for observed phenomena in nature but also highlights the importance of genetic variation in adaptation and survival.