Heritability and Phenotype in Evolution

Heritability and Phenotype

• Heritability is crucial for Darwinian evolution. Phenotypes must be inheritable for natural selection to operate.
• Darwin recognized the importance of heritability but lacked a mechanism for inheritance. This was discovered shortly after his work.
• Phenotypes (measurable traits) are determined by genotypes (genes), but a precise phenotype doesn't always result from a specific genotype.
• Genetic variation is the basis of phenotypic variations. Without this variation, there would be no selection pressure from the environment.
• Genes are "selfish," striving for replication and transfer to the next generation; phenotypes are the tools genes use to achieve this.

Genetic Definitions

• Genome: All hereditary information in an organism (including non-gene parts).
• Gene pool: All alleles (versions) of all genes in a population.
• Genotype: All alleles within an individual (or a specific set of alleles).
• Genes: DNA segments coding for polypeptides (traits). They can also regulate other genes.
• Locus: A gene's location on a chromosome.
• Alleles: Different versions of a gene at the same locus.

Genotype vs. Phenotype

• Similar genotypes can produce different phenotypes due to environmental influences.
• Identical twins have slightly different phenotypes because developmental environments vary.
• Similar phenotypes can arise from distinct genotypes; convergent evolution provides an example (gray wolf and Tasmanian wolf).
• Exact identical phenotypes require identical genotypes and homogeneous developmental conditions (nearly impossible in organisms outside a lab).

Variation in Eukaryotes

• Chromosomes: DNA and associated proteins; occur in sets, each with unique chromosomes.
• Ploidy: The number of chromosome sets in a cell (e.g., diploid [2n] has two sets).
• Homologous chromosomes: Matching chromosome pairs (maternal and paternal) with the same genes, but potentially different alleles.
• Haploid (1n): One set of chromosomes.
• Diploid (2n): Two sets of chromosomes. Example: Most animals are diploid; humans are diploid. Some organisms have haploid males and diploid females, like ants, bees & wasps.
• Dominance: One allele obscuring the effect of another in a diploid organism.
• Meiosis: The process of gamete (egg & sperm) formation. It halves the chromosome number to produce haploid gametes.

Variation in Meiosis

• Meiosis involves two divisions: Meiosis I and Meiosis II; both are important for further variation
• Crossing over: Homologous chromosomes exchange genetic material during meiosis I, creating new combinations of alleles – a key source of variation.
• Random alignment: Homologous chromosomes randomly align during meiosis, further shuffling genetic combinations in daughter cells.

Other Sources of Variation

• Mutations: Changes in DNA sequences (e.g., from radiation, chemicals) important but typically not deleterious. Used to study relationships between types of organisms.
• Nondisjunction: During meiosis, chromosomes may fail to separate properly, resulting in gametes with too many or too few chromosomes. Down syndrome is an example.

Asexual Reproduction

• Asexual reproduction (e.g., bacteria, budding) typically creates genetically identical offspring.
• Three mechanisms that create variation in asexually reproducing organisms:
  • Transformation: Ingestion of external DNA by bacterial cells.
  • Transduction: Transfer of genetic material by viruses.
  • Conjugation: Transfer of genetic material between bacterial cells.