300C_Lec_10_Variation_Cont

Biol 300C: Evolution

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.

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.

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.

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.

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.

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.

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.

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 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.

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

Roles:
- Promote allele clusters that function together.
- Can lead to reproductive isolation, facilitating speciation.

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.

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.

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