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Lectures 2-5: Evolutionary Forces (Patterns of Selection (Sexual selection…
Lectures 2-5: Evolutionary Forces
Convergence: Unrelated organisms have similar shapes and behaviors
Alfred Russel Wallace and Charles Darwin concurrently developed ideas about evolution
Proposed:
Descent with Modification- divergent species have common ancestors
Natural Selection explains changes over time
Species are not immutable (they change over time)
Darwin's Theory of Evolution
Populations tend to be stable
Resources are limited
Species can increase without bounds
Only some offspring survive
Individuals vary in their characteristics
Some of this variation is heritable
Accumulation of individuals with more favorable traits
Individuals with favorable traits tend to survive and reproduce
Evolution:
Change in the allele frequency of a population across generations.
Change in the genetic composition of a population across generations.
Adaptation: Verb and Noun
Process by which useful characteristics evolve by natural selection
Characteristic that makes it more likely for an organism to survive and reproduce
Quantitative Trait: typically distributed in a bell-shaped curve
Qualitative Trait: Either present or not present
Patterns of Selection
Directional e.g. Bigger horns are better
Stabilizing selection e.g. birth weight
Disruptive selection e.g. bird beaks
Sexual selection: Not always correlated with fitness, e.g. peacock feathers, but selected for by mates
Intra-sexual selection: can beat up competitors
Inter-sexual selection: makes organism more attractive
Artificial selection: human-mediated
Evolution can never create the perfect organism
Physical constraints
Historical constraints
Environments change too quickly
Selection only for
current
environment
Some evolutionary forces are non-adaptive
Evolutionary Forces
Selection: leads to adaptation
Gene Flow: Migration; prevents species from diverging, often constrains local adaptation
Mutation: Generates variation; random, usually deleterious
Genetic Drift: Random change in allele frequency due to imperfect sampling across generations
Especially powerful in small populations (bottlenecks)
Neanderthals: appeared 400,000 y.a, extinct 30,000 y.a. - some interbreeding with humans
Phylogeny: Evolutionary history of relationships among organisms
Splits in branches are called nodes
Branches can be rotated around node without changing meaning of tree
Taxon: Any group of organisms that we name
Monophyletic Group OR Clade: taxon that consists of all the descendants of a common ancestor
Tree of Life: Complete, evolutionary history of life