The Evolution of Populations

Chapter 21

Microevolution

Evolutionary changes within a population Changes in allele frequencies in a population over

generations Population – all members of a species living

in the same area, interbreed, produce fertile offspring

Example – industrial melanism and the peppered moth

Genetic Variation

Differences among individuals in the composition of the genes

Single gene influence (Mendel) or polygenic

Phenotype – physical traits, can be inherited or influence by environment.

Sources of Genetic Variation

Formation of new alleles Mutation

Altering Gene number or position Chromosomal changes – deletion, translocation,

inversion and duplication Rapid reproduction – prokaryotes Sexual Reproduction

Crossing over, independent assortment and random fertilization

Hardy-Weinberg principle

p2 +2pq + q2 Used to calculate the genotype and gene

frequencies of a population States: equilibrium of allele frequencies in a gene

pool will remain in effect in each generation of sexually reproducing populations as long as:

1. No mutations2. No gene flow3. Random mating4. No genetic drift5. No selection

Hardy-Weinberg principle

Tells us what factors cause evolution The 5 conditions are hardly ever met Allele frequencies do change from one generation

to another Evolution can be detected by seeing any deviation

from a Hardy-Weinberg equilibrium

Practice problems p.406

Causes of microevolution

Opposite of HWP Genetic mutations – cause for multiple

alleles, can be adaptive and include favorable phenotypes

Nonrandom mating – inbreeding or breeding between relatives, decreases the heterozygote

Causes of microevolution that alter allele frequency

directlyGenetic Drift – change in allele frequencies due to chanceBottleneck effect – natural disaster, reduce in population prevents the majority of genotypes from participating in the production of the next generationFounder effect – rare alleles occur at a higher frequency in a population isolated from a general population ex. amish

Microevolution

Gene Flow – transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes.

Natural selection

Not random – adaptive evolution Most traits are polygenic, see bell curve in

allele frequency 3 major types of selection -

Directional – extreme phenotype favored Resistance to antibiotics and pesticides, malaria

Stabilizing – intermediate phenotype is favored Birth weight survival, sickle cell trait

Disruptive – 2 or more extreme phenotypes are favored

Sexual selection

Natural selection in which individuals with certain inherited characteristics are more likely than others to obtain mates. Sexual dimorphism – differences in males and

females (i.e. size, color, …)

Balancing selection

Natural selection maintains two or more forms in a population. Heterozygote advantage – Malaria and sickle cell

anemia Frequency-dependent selection – scale eating

fish. Right and left mouthed

Why doesn’t Natural Selection create perfect organisms? Selection can act only on existing variations Evolution is limited by historical constraints Adaptations are often compromises Chance, natural selection and the

environment interact