Download Causes of Evolution

Causes of Evolution Chapter 16-1 & 16-2
Forces that cause populations and species to evolution to occur are:
 Natural selection
 Migration
 Genetic drift
 Mutation
Causes of Evolution
1. Gene Shuffling= Most common cause of offspring variation from sexual
reproduction/gamete formation
2. Mutations = changes or random errors in DNA (by chance)
 Mutations are constantly occurring and causing variation.
 Even identical twins are not because of mutations.
 Frameshift and point mutations are constantly acting on populations and
are constantly affecting the evolution of a population
3. Gene flow (Migration)
4. Genetic Drift = random change in allele freq.
• affects small, isolated populations most
• Ex: Amish - stay together & isolated from society
Genetic drift-this can change allele frequency by chance and not
natural selection. The allele frequency is determined by the
founding population & may differ from the orig. pop.
Founder Effect-A change in allele frequency due to migration of
small subgroup of pop. Ex. if a small group of gray birds leaves
a population of black and gray birds. The newly founded
population will consist of only gray bird alleles.
5. Nonrandom mating
• mates selected based on how “fit” they are
• more fit = better chance of offspring surviving
6. NATURAL SELECTION!!
 Acts on phenotype not genotype
 Initaited by Darwin
 Darwin was known for the phrase "Survival of the fittest" Implying that
healthiest and those with most favorable traits are more likely to survive
Will Evolution Ever Not Happen?
Genetic Equilibrium = no change in a pop. over many generations
 Hardy-Weinberg principle that allele freq. will remain the same unless 1 or
more factors change
To achieve genetic equilibrium:
1. No mutations occur
2. No gene flow/migration in or out
3. No natural selection
4. Yes Individuals mate randomly
5. Yes large gene pool
Gene Pool = ALL the alleles that make up a population’s genes (includes all the 2 or
more alleles for each trait)
Genetic equilibrium is very, very rare because gene pools are always changing!!!!
Types of Natural Selection
1. STABILIZING Selection = favors average individuals in a population
• reduces variation in organisms
Ex: lizards – large captured easily & small cannot run fast enough
2. DIRECTIONAL Selection = favors one of the extreme variations of a trait
• can lead to rapid evolution of a new species
• Ex: Anteaters – long tongues are best to get food
3. DISRUPTIVE Selection – favors both extreme variations of a trait
• no intermediate forms survive
• can lead to evol. of two new species
• Ex: Limpets – white colored mimic white barnacles & dark colored camouflage on
dark rocks