Natural Selection vs. Populations: Natural selection acts on individuals, but populations evolve. A population is the smallest unit of evolutionary change.
Population Definition: A group of individuals of the same species that live in the same area and interbreed, producing fertile offspring.
Microevolution: Changes in allele frequencies in populations over generations.
Genetics Terminology
Gene (Genetic locus): Consists of two alleles in diploid individuals.
Alleles: Different forms of a gene with distinct DNA sequences.
Inheritance: Alleles from each parent remain separate and do not blend, except in cases of incomplete dominance.
Genetic Variation and Evolution
Variation in heritable traits is essential for evolution.
Genetic Diversity: A key component of biodiversity, enabling adaptation via natural selection.
Genotype and Phenotype: Phenotype results from inherited genotype and environmental influences.
Natural selection acts only on genetic variation.
Sources of Genetic Variation
Mutations: Changes in DNA sequence.
Only heritable if they occur in gametes.
Random occurrence, not driven by selection pressures.
Can be deleterious, neutral, or advantageous.
Sexual Reproduction: Increases genetic variation via new combinations of alleles.
Random mating, fertilization, and recombination during meiosis.
Altering Allele Frequencies
Natural Selection: Only mechanism causing adaptive evolution.
Modes: Directional, Disruptive, Stabilizing.
Genetic Drift: Random changes, significant in small populations.
Includes bottleneck and founder effects.
Gene Flow: Movement of alleles between populations, affecting genetic diversity.
Natural Selection Modes
Directional Selection: Favors one extreme phenotype.
Disruptive Selection: Favors both extreme phenotypes, maintains variation.
Causes loss of genetic variation and can fix harmful alleles.
Bottleneck Effect: Drastic reduction in population size.
Founder Effect: New population started by a small group.
Gene Flow Impact
Increases Variation: Within receiving populations.
Reduces Differences: Between populations.
Can both decrease and increase fitness depending on environmental context.
Evolutionary Changes and Constraints
Natural Selection Limitations: Does not produce perfect organisms due to historical constraints, trade-offs, and the random nature of mutations.
Evolution is influenced by both adaptive and non-adaptive changes.
Maintaining Genetic Variation
Neutral variation, mutation, recombination, and balancing selection maintain genetic diversity.
Balancing Selection: Maintains multiple alleles in a population.
Heterozygote Advantage: Fitness advantage in diverse environments (e.g., Sickle Cell Disease).
Frequency-dependent Selection: Fitness depends on allele frequency.
Conclusion
Evolutionary change is driven by natural selection, genetic drift, and gene flow, each contributing differently to the adaptation and diversity of populations.