Mechanisms of Evolution

Hardy-Weinberg Principle

A. A population is in genetic equilibrium—all individuals are equally adapted to their environment & contribute equally to the next generation1. Five conditions for this to occur

a. No Genetic Driftb. No Gene Flowc. No Mutationsd. Mating must be randome. No Natural Selection

Genetic Drift

B. Chance occurrences can cause an allele to become more or less common1. Founder Effect: result of migration (Darwin’s finches)

Ex. Amish and Mennonites: people rarely marry outside their own communities; Old Order Amish—high frequency of 6-finger dwarfism—can trace ancestry back to one of the founders of the order

2. Bottleneck: result of a dramatic reduction in population size and then rebounds; might be a possible factor in the potential extinction of a speciesEx. Cheetahs in Africa are so genetically similar they appear inbred; researchers think there was a bottleneck 10,000 yrs ago, and then again 100 yrs ago

Gene Flow

C. Rate at which genes enter or leave the population—random migration naturally increases variation

Nonrandom Mating

D. Organisms usually mate w/individuals in close proximity which promotes inbreeding

Mutation

E. Some are harmful, some are advantageous

Natural Selection

F. Acts to select those individuals that are best adapted for a particular environment

Reproductive Isolation

G. Prevents gene flow among populationsEx. 2 species can mate to produce a hybrid

which is sterile (liger)**Other forms of isolation are more specific forms of this

Speciation

H. Population diverges and new species is reproductively isolated

Patterns of Evolution

A. Adaptive Radiation: many related species evolve from a single species (Darwin’s finches)—usually due to different environments Ex. guppies in a pool on the same river

B. Coevolution: 2 or more species change together

C. Convergent Evolution: 2 unrelated species have similar characteristics

D. Rate of Speciation: usually is gradual, but there have been periods of abrupt transitions

Adaptive Radiation

Coevolution

Acacia ants live in thorns of acacia tree. Plant make substance used by ants as food. Ants defend tree from herbivores by attacking/stinging any animal that brushes up against the tree

Convergent Evolution

Rate of Speciation