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