300C_Lec_10_Variation_Cont

Biol 300C: Evolution

Microevolution: Genetic Variation

• Focuses on the genetic changes within populations over time.

Mutation

• Source of Genetic Variation: Mutation is the original source of all genetic variation.

• Rarity and Variation: Although rare, mutation rates vary across different species.

• Mechanism: Arises from random errors occurring during DNA replication, recombination, or repair, leading to permanent changes in the DNA sequence.

• Scope of Changes: Changes can be as small as a single base change to alterations affecting entire chromosomes.

Types of Mutation in Evolutionary Context

• Beneficial Mutations: Enhance the fitness of the organism.

• Neutral Mutations: Have no significant impact on fitness.

• Deleterious Mutations: Decrease the overall fitness of the organism.

• Environmental Dependence: The effect of a mutation is heavily influenced by environmental circumstances.

Rates of Mutation Substitutions Across Organisms

• Range of substitution rates from various species observed through direct sequencing:

  • RNA viruses: 10^-11

  • Retroviruses: 10^-10

  • Various other organisms: Rates increase to 10^-3 and higher, with mammals showing lower substitution rates compared to RNA viruses.

Point Mutations: Substitutions

• Substitutions: Replacement of one nucleotide pair with another.

  • Synonymous Mutations: Do not change the resulting amino acid due to the redundancy of the genetic code.

  • Nonsynonymous Mutations:

    • Missense Mutations: Change one amino acid for another, affecting protein function.

    • Nonsense Mutations: Convert an amino acid codon to a stop codon, resulting in a nonfunctional protein.

  • Frameshift Mutations: Caused by insertions or deletions that alter the reading frame, usually leading to nonfunctional proteins.

Effects of Mutations on Populations

• Through mutation accumulation experiments, the effects of mutations can be studied under controlled conditions without natural selection.

• Under normal selection, nearly all deleterious mutations are removed from the population.

Discovery of Selective Pressures on Mutations

• Under novel selection pressures, some mutants can enhance population performance, as demonstrated with Drosophila birchii, where irradiated lines showed variability in fitness and growth.

• Mutation vs. Genetic Drift: Mutation consistently adds genetic diversity, while genetic drift can limit diversity within a population.

Take-home Messages on Point Mutations

• Mutation alone tends to reduce genetic quality over generations but can also introduce beneficial mutations.

• The combination of mutation and natural selection drives evolutionary processes, retaining or improving population fitness.

Intraspecific Variation Due to Mutation

• Levels of Variation:

  • Variation in alleles: Different versions of a gene with distinct nucleotide sequences.

  • Variation in gene arrangements: The structural organization of genes on chromosomes can vary.

  • Gene copy number variation: Changes in how many copies of a gene are present within the genome.

Chromosome Rearrangements and Fitness Consequences

Mechanisms and Impacts

• Chromosome rearrangements can occur when break points do not recombine properly, affecting meiotic processes.

• Large rearrangements can lead to partial sterility due to improper segregation of homologous chromosomes.

• Impact of Inversions: Alleles can be linked together, causing deviations from expected inheritance patterns.

Crossover and Linkage Effects

• Rearrangements inhibit recombination, influencing genetic linkage and potentially leading to the formation of supergenes, where multiple linked genes work cohesively.

Importance of Inversions in Evolution

• Roles:

  • Promote allele clusters that function together.

  • Can lead to reproductive isolation, facilitating speciation.

Gene Duplication Mechanisms and Evolutionary Impact

Mechanisms

• Gene duplication can occur through polyploidization, aneuploidy, or unequal crossing over, leading to multiple copies of genes.

• Fitness advantages can be conferred by increased gene expression from these duplications.

Outcomes of Gene Duplication

• Duplicated genes may retain similar functions or diverge into specialized functions, leading to increased adaptation and efficiency in various physiological processes.

• Duplications can lead to subfunctionalization, where duplicated genes share ancestral responsibilities, or neofunctionalization, introducing entirely new gene functions.

Conclusion on Gene Duplication

• Significant evolutionary force in the development of complex traits and enhanced biological roles, facilitating diversification within gene families and organismal structure.