- Genetic Variation: difference in DNA bases or sequences
- Genetic variation enables evolutionary responses to environmental change
- Evidence of genetic variation: diff genotypes or phenotypes
- Natural selection acts on variation among individuals in the population and arises by…
- Mutations: are original source of new variation; invent alleles that didn’t exist in the gene pool
- Natural selection and other mechanisms increase variation by rearranging existing alleles & mutations into new combinations
- All new alleles are the result of nucleotide variability
- Sexual Reproduction creates individuals with new combinations of alleles; Genetic recombinations comes from:
- Crossing Over; Independent assortment of homologous chromosomes; Random joining of gametes
- Diploidy: presence of two copies of each chromosome in a cell
- In heterozygous condition, recessive allele is hidden from natural selection → allows variation to be “stored” for future generations → maintains variation in gene pool
- Outbreeding: mating with unrelated partners increases the possibility of mixing different alleles and creating new combinations
- Balanced Selection selection itself may preserve variation at some loci → helps maintain multiple phenotypic forms in a population
- Often a single phenotype provides the best adaptations while others are less advantageous → favorable allele increases in frequency
- Examples of polymorphism (2 or more diff phenotypes) can be maintained by
- Hybrid Vigor (Heterosis): superior quality of offspring resulting from crosses between two diff inbred strains of plants
- Results from the reduction of loci with harmful homozygous recessive conditions and increase with heteroz advantage
- Ex: a hybrid of corn in more resistant than either inbred strains
- Heterozygous Advantage
- When heterozygotes have a greater fitness than either homozygous type→ both alleles and three phenotypes maintained in the population by selection
- Ex: heterozygotes for sickle cell disease are ~healthy but oxygen-carrying impaired; provides resistance to malaria = higher % in Africa → both alleles are maintained in gene pool
- Frequency-Dependent Selection (minority advantage): the fitness of a phenotype depends on how common it is in the population.
- Rare phenotypes are selected → become common → are selected against
- Ex: rarer prey escape predators
- Maintain multiple phenotypes (and their alleles) that alternate between high and low frequencies
What Decreases Variation
- Stabilizing selection, sexual selection/nonrandom mating, genetic drift
Neutral Variation
Not all variation has selective variation → some (especially at molecular level in DNA and proteins) is neutral variation
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- Ex: differences in fingerprint patterns
- Environment determines whether variation is neutral or has selective value
Humans Impacting Evolution
- Monocultures: only grow one type of crop → reduce genetic variation bcuz only a few varieties of many wild varieties of plants are used
- Monocultures have no genetic variation and are susceptible to changing environmental conditions
- Overuse of Antibiotics reduces variation in bacteria population by eliminating certain individuals
- Absence of susceptible individuals decrease competition and allows pathogenic bacteria to increase in number and dominate population
- Artificial Selection/Selective Breeding: breed individuals to produce desired traits
- Similar to Natural Selection: Needs genetic variation
Different to Natural Selection: Humans (not environment) does the selecting