CH 16EVOLUTION OF POPULATIONS

Crash Course: Population Genetics https://www.youtube.com/watch?v=WhF

KPaRnTdQ

16-1 Genetic equilibrium

Population genetics: study of evolution from a genetic point of view

Basically how populations of a species evolve

But what is a population? Group of members of the same species

living in the same area

Sources of genetic variation

Three main sources1. Mutations: any change in sequence of

DNA Replication mistakes Radiation/environmental causes

2. recombination: reshuffling of genes3. Random pairing of gametes

Bell curve

Many traits in nature show trends like this

Phenotype continuum

# o

f in

div

iduals

wit

h t

hat

trait

Number of phenotypes produced depends on how many genes control that trait

Single gene traits- have two alleles Two distinct phenotypes

Polygenic traits- controlled by two or more genes

Results in multiple phenotypes

Gene pool- all genes, including all different alleles, that are present in a population

frequency (of an allele)- number of times alleles occur in a gene pool Percentage

Genetic definition of evolution? Change in relative frequency of alleles in

a population over time

Phenotype frequency

How often a specific phenotype is observed in a population

Can be written mathematically

Frequency = # indiv. w/a particular phenotype

total # of indiv. in population

Hardy-Weinberg equilibrium

When evolution is not occurring Allele frequencies remain the same

In order for evolution to not occur, certain conditions must be met.

Evolution Versus Genetic Equilibrium

Hardy-Weinberg principle = Genetic Equilibrium

• Random Mating – Equal opportunity to produce

offspring

• Large Population – Genetic Drift does not effect

Allele Frequency

• No Movement into or out of Population – The

gene pool must be kept together (no new alleles)

• No Mutations – Mutations cause new forms of

alleles changing the frequency

• No Natural Selection – All genotypes must have

equal probability of surviving.

Hardy-Weinberg equilibrium

Allele frequency equation p + q = 1 p = frequency of dominant allele q = frequency of recessive alleleTogether, they make 100% of alleles for a gene in that population If p = 34%, what is q? If q = 19%, what is p?

0.66

0.81

Hardy-Weinberg equilibrium

Genotypic frequency equation

p2 + 2pq + q2 = 1 p2 = homozygous dominant frequency 2pq = heterozygous frequency q2 = homozygous recessive frequency If p = .46, what is p2? If p = .12, what is q2? If q =.31, what is 2pq?

0.21160.01440.4278

16-2 Disruption of genetic equilibrium

Mutation Occur at a relatively constant rate over

time Can be sped up when exposed to mutagens

Gene flow: process of genes moving from one population to another

Immigration: moving into a population Emigration: moving out of a population

Genetic Drift

Alleles can become rare by chance Over time a series of chance

occurrences can cause an alleles to become common in a population

Effects of genetic drift are more dramatic with small population size

Founder effect: change in allele frequencies as a result of migration of a small subgroup of a population

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic DriftSection 16-2

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic DriftSection 16-2

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic DriftSection 16-2

Nonrandom mating

Sexual selection: tendency of individuals to choose a mate with certain traits.

Common in birds Peacock display Tropical birds of paradise - Papua New G

uinea The amazing Lyrebird - Australia

Natural selection

Natural selection on a single gene traits can lead to changes in allele frequencies

Natural selection on polygenic traits 3 possible effects

1. Directional selection2. Stabilizing selection3. Disruptive selection

Directional selection

When individuals at one end of curve have higher fitness than individuals in the middle or the other end

Stabilizing selection

When individuals near the middle have higher fitness than the individuals at either end

Disruptive selection

When individuals at upper and lower ends have higher fitness than individuals near the middle

16-3 Formation of Species

As new species evolve, populations become reproductively isolated from each other

Reproductive isolation: when two members of populations cannot interbreed and produce fertile offspring

Separate gene pools

Isolation Mechanisms

Geographic Isolation:- separation of animals in a

specific region- formation of river, canyon,

mountain

Isolation Mechanisms

Behavioral Isolation:- differences in courtship or reproductive behaviors-meadowlark songsTemporal isolation:

-two or more species reproduce at different times

-orchids

Formation of species

Allopatric speciation: when species arise from geographic isolation Different places

Reproductive isolation

Prezygotic isolation: premating isolation

Species may live in different places Reproduce at different times Have different mating

behaviors Postzygotic isolation:

postmating isolation Hybrids may be weak Hybrids may be sterile

Sympatric speciation

Sympatric speciation: when two subpopulations become isolated while living in the same area

Rates of speciation

Gradualism: speciation at gradual and regular rate

Punctuated equilibrium: periods of sudden, rapid change followed by periods of littelchange