CH. 22/23 WARM-UP

1. What is the evidence for evolution?

CHAPTER 23

THE EVOLUTION OF POPULATIONS

WHAT YOU MUST KNOW:

• How mutation and sexual reproduction each produce genetic variation.

• The conditions for Hardy-Weinberg equilibrium.

SMALLEST UNIT OF EVOLUTION

Microevolution: change in the allele frequencies of a population over generations

• Darwin did not know how organisms passed traits to offspring

• 1866 - Mendel published his paper on genetics

• Mendelian genetics supports Darwin’s theory Evolution is based on genetic variation

SOURCES OF GENETIC VARIATION

• Point mutations: changes in one base (eg. sickle cell)

• Chromosomal mutations: delete, duplicate, disrupt, rearrange usually harmful

• Sexual recombination: contributes to most of genetic variation in a population1.Crossing Over (Meiosis – Prophase I)2.Independent Assortment of Chromosomes

(during meiosis)3.Random Fertilization (sperm + egg)

Population genetics: study of how populations change genetically over time

Population: group of individuals that live in the same area and interbreed, producing fertile offspring

• Gene pool: all of the alleles for all genes in all the members of the population• Diploid species: 2 alleles for a gene

(homozygous/heterozygous)• Fixed allele: all members of a population

only have 1 allele for a particular trait• The more fixed alleles a population has,

the LOWER the species’ diversity

HARDY-WEINBERG THEORM

Hardy-Weinberg Theorm: The allele and genotype frequencies of a population will remain constant from generation to generation

…UNLESS they are acted upon by forces other than Mendelian segregation and recombination of alleles

Equilibrium = allele and genotype frequencies remain constant

CONDITIONS FOR HARDY-WEINBERG EQUILIBRIUM

1. No mutations.2. Random mating.3. No natural selection.4. Extremely large population size.5. No gene flow.

If at least one of these conditions is NOTNOT met, then the population is EVOLVINGEVOLVING!

CAUSES OF EVOLUTION

CONDITIONS FOR HARDY-WEINBERG EQUILIBRIUM

1. No mutations.2. Random mating.3. No natural selection.4. Extremely large population size.5. No gene flow.

If at least one of these conditions is NOTNOT met, then the population is EVOLVINGEVOLVING!

Minor Causes of Evolution:Minor Causes of Evolution:#1 - Mutations

• Rare, very small changes in allele frequencies

#2 - Nonrandom mating• Affect genotypes, but not allele

frequencies

Major Causes of EvolutionMajor Causes of Evolution:• Natural selection, genetic drift, gene flow

(#3-5)

MAJOR CAUSES OF EVOLUTION

#3 – Natural Selection#3 – Natural Selection• Individuals with variations better suited

to environment pass more alleles to next generation

MAJOR CAUSES OF EVOLUTION

#4 – Genetic Drift#4 – Genetic Drift• Small populations have greater chance of fluctuations in

allele frequencies from one generation to another• Examples:

• Founder Effect• Bottleneck Effect

Genetic DriftGenetic Drift

FOUNDER EFFECT

• A few individuals isolated from larger population• Certain alleles under/over represented

Polydactyly in Amish population

BOTTLENECK EFFECT

• Sudden change in environment drastically reduces population size

Northern elephant seals hunted nearly to extinction in

California

MAJOR CAUSES OF EVOLUTION

#5 – Gene Flow#5 – Gene Flow• Movement of fertile

individuals between populations

• Gain/lose alleles• Reduce genetic

differences between populations

HOW DOES NATURAL SELECTION BRING ABOUT ADAPTIVE EVOLUTION?

Natural selection can alter frequency distribution of heritable traits in 3 ways:

1.Directional selection2.Disruptive (diversifying) selection3.Stabilizing selection

Directional Selection: eg. larger black bears survive extreme cold better than small ones

Disruptive Selection: eg. small beaks for small seeds; large beaks for large seeds

Stabilizing Selection: eg. narrow range of human birth weight

SEXUAL SELECTION

• Form of natural selection – certain individuals more likely to obtain mates

• Sexual dimorphism: difference between 2 sexes• Size, color, ornamentation, behavior

SEXUAL SELECTION

• IntrasexualIntrasexual – selection within same sex (eg. M compete with other M)

• IntersexualIntersexual – mate choice (eg. F choose showy M)

PRESERVING GENETIC VARIATION

• DiploidyDiploidy: hide recessive alleles that are less favorable

• Heterozygote advantageHeterozygote advantage: greater fitness than homozygotes• eg. Sickle cell disease

NATURAL SELECTION CANNOT FASHION PERFECT ORGANISMS.

1. Selection can act only on existing variations.

2. Evolution is limited by historical constraints.

3. Adaptations are often compromises.

4. Chance, natural selection, and the environment interact.

SAMPLE PROBLEM

Define the following examples as directional, disruptive, or stabilizing selection:

a) Tiger cubs usually weigh 2-3 lbs. at birthb) Butterflies in 2 different colors each

represent a species distasteful to birdsc) Brightly colored birds mate more

frequently than drab birds of same species

d) Fossil evidence of horse size increasing over time