Genetic Variation and Evolution

Source of Genetic Variation

Genetic variation is essential for evolution, as it provides the raw material for natural selection to act upon. Variation arises from several mechanisms within populations.

• Mutations: Changes in the DNA sequence that can introduce new alleles into a population.

  • Only mutations in sex cells are passed to the next generation in multicellular organisms.

  • Point mutations: Change in a single base pair.

    • Most mutations are harmful because organisms are adapted to their environments.

    • Some mutations are neutral or beneficial, especially if they increase survival or reproduction.

    • Recessive alleles can be masked in diploids, allowing harmful mutations to persist in populations.

  • Non-harmful mutations:

    • Silent mutations: Base changes with no amino acid change.

    • Mutations in non-coding regions: Often neutral.

    • Amino acid changes that do not impact protein shape or function can be neutral.

  • Altering gene number or position:

    • Chromosomal mutations can delete, disrupt, or rearrange genes.

    • Duplication of DNA increases genome size; duplicated genes can evolve new functions.

    • Example: Ancestral odor detecting gene duplication in mammals.

• Sexual Reproduction: Shuffles existing alleles into new combinations.

  • Crossing over and independent assortment of chromosomes during meiosis increase genetic diversity.

  • Fertilization combines alleles from two parents.

  • Populations with more genetic variation can better withstand environmental changes.

Mutation Rates

• Average: One mutation per 100,000 genes per generation.

• Higher mutation rates in prokaryotes and viruses due to short generation times.

Microevolution

Microevolution refers to changes in allele frequencies in a population over generations. Three main mechanisms cause these changes:

• Natural Selection: Differential reproductive success leads to certain alleles being passed on more frequently.

  • Directional selection: Favors one extreme phenotype.

  • Disruptive selection: Favors both extremes, not the average.

  • Stabilizing selection: Favors the average phenotype.

  • Sexual selection: Selection based on traits that increase mating success; can result in sexual dimorphism.

  • Heterozygote superiority (overdominance): Heterozygotes have higher fitness than either homozygote.

  • Underdominance: Heterozygotes have lower fitness than either homozygote.

• Genetic Drift: Random changes in allele frequencies, especially significant in small populations.

  • Founder effect: Small group establishes a new population with a different gene pool.

  • Bottleneck effect: Sudden reduction in population size alters allele frequencies.

  • Genetic drift can lead to loss of genetic variation and fixation of alleles.

  • Example: Prairie chicken populations reduced by habitat loss.

• Gene Flow: Movement of alleles between populations via migration or gamete transfer.

  • Can increase or decrease genetic variation.

  • Example: Mating between Dutch island populations (cursus major).

  • Gene flow can also spread alleles for resistance to insecticides.

Population Genetics and Hardy-Weinberg Principle

Determining if a Population is Evolving

Population genetics studies allele frequencies within populations. The Hardy-Weinberg equation describes a non-evolving population.

• Hardy-Weinberg Equation:

  • Describes expected genotype frequencies in a non-evolving population.

  • Equation:

  • = frequency of dominant allele; = frequency of recessive allele.

  • = frequency of homozygous dominant genotype.

  • = frequency of heterozygous genotype.

  • = frequency of homozygous recessive genotype.

• Conditions for Hardy-Weinberg Equilibrium:

  • No mutations

  • Random mating

  • No natural selection

  • Extremely large population size

  • No gene flow

• Calculating Allele Frequencies:

  • For diploids, total number of alleles = 2 × number of individuals.

  • Allele frequencies must add to one.

  • Example: Incomplete dominance in flower color (CR, CW alleles).

Speciation and the Origin of Species

Speciation

Speciation is the process by which one species splits into two or more distinct species. It helps explain shared features among organisms and the diversity of life.

• Microevolution: Changes in allele frequency within a population over time.

• Macroevolution: Broad patterns of evolutionary change above the species level (e.g., origin of flowering plants).

Species Concepts

• Biological Species Concept: A group of populations whose members can interbreed and produce viable, fertile offspring.

  • Reproductive isolation is key.

  • Limitations: Does not apply to asexual organisms, fossils, or organisms with little known about gene flow.

• Morphological Species Concept: Based on physical characteristics.

• Ecological Species Concept: Based on ecological niche.

• Phylogenetic Species Concept: Smallest group sharing a common ancestor.

Reproductive Isolation

Reproductive isolation prevents gene flow between species and is essential for speciation.

• Prezygotic Barriers: Prevent fertilization.

  • Habitat isolation

  • Temporal isolation

  • Behavioral isolation

  • Mechanical isolation

  • Gametic isolation

• Postzygotic Barriers: Prevent hybrid survival or reproduction after fertilization.

  • Reduced hybrid viability

  • Reduced hybrid fertility

  • Hybrid breakdown

Process of Speciation

• Allopatric Speciation: Geographic separation leads to divergence.

• Sympatric Speciation: Occurs without geographic isolation, often via polyploidy, sexual selection, or habitat differentiation.

Polyploidy and Hybrid Zones

• Polyploidy: Extra sets of chromosomes can lead to instant speciation.

  • Autopolyploid: Multiple chromosome sets from one species.

  • Allopolyploid: Chromosome sets from different species.

• Hybrid Zones: Regions where different species meet and produce hybrids.

  • Possible outcomes: reinforcement, fusion, stability.

  • Reinforcement: Hybrids less fit than parents, reproductive barriers strengthen.

Summary Table: Mechanisms of Microevolution

Key Terms and Definitions

• Allele: Alternative form of a gene.

• Genotype: Genetic makeup of an organism.

• Phenotype: Observable traits of an organism.

• Population: Group of individuals of the same species in a given area.

• Speciation: Formation of new species.

• Polyploidy: Condition of having more than two sets of chromosomes.

Additional info: Some explanations and examples have been expanded for clarity and completeness.