DEFINITIONS:● POPULATION: a localized group of individuals

belonging to the same species

● SPECIES: a group of populations whose

individuals have the potential to

interbreed and produce fertile

offspring

● GENE POOL: all alleles at all

gene loci in all individuals in a

population

6 different species of flamingo

A population of flamingo’s

We all belong to the same gene

pool!!!

The Hardy-Weinberg Theorem:

● a tool that describes a gene pool of a non-evolving population

● states that allele frequencies and genotypes in a population’s gene pool remain constant over the generations unless acted upon by agents other than sexual recombination

● for Hardy-Weinberg equilibrium to occur, the following conditions must be met:

1) Large population

2) No mutation

3) No gene flow (no immigration or emigration)

4) Random mating (no mating preference for

particular phenotype)

5) No natural selection (all genotypes have an =

chance of surviving & reproducing)

HOWEVER, in nature: 1) most populations are small & may mate with

one another

2) there are always mutations

(chance with every DNA replication)

3) gene flow often occurs between

populations

4) mating is non-random

5) natural selection is always occurring

**Therefore, in nature there will always be

changes in populations (“microevolution”)

**So why study population genetics?

Why use the H-W Theorem? 1) shows how genetics is related to evolution;

2) provides a benchmark genetic equilibrium against

which change can be noted;

3) permits an estimation of gene frequencies;

especially useful in estimating the number of

carriers of lethal alleles in human populations.

Ex: Brachydactyly - fingers are abnormally short in

heterozygotes; condition is fatal during infancy to

homozygous recessive individuals due to major skeletal

defects

Hardy-Weinberg Equation:

● p = frequency of dominant allele (A)

● q = frequency of recessive allele (a)

● p + q = 1

● frequency of possible diploid

combinations (AA, Aa, aa):

p2 + 2pq + q2 = 1

(AA) (Aa) (aa)

Example Problem:

● If the frequency of a recessive allele is 35% in a population of 1500 people, how many people would you predict would be carriers of this allele, but would not express the recessive phenotype?

q = 35% = 0.35

p = 1 - q = 1 - 0.35 = 0.65

p2 + 2pq + q2 = 1

freq. of Aa genotype = 2pq

= 2(0.65)(0.35)

= 0.455 = 45.5%

# of carriers = (0.455)(1500)

= 683 people

Solution:

Example Problem:

● In a population with 2 alleles for a particular locus, B and b, the allele frequency of B is 0.78. If the population consists of 172 individuals, how many individuals are heterozygous? How many will show the recessive phenotype?

p = 0.78

q = 1 - p = 1 - 0.78 = 0.22

p2 + 2pq + q2 = 1

freq. of Bb genotype = 2pq

= 2(0.78)(0.22)

= 0.343 = 34.3%

# of heterozygotes = (0.343)(172)

= 59 individuals

Solution:

p2 + 2pq + q2 = 1

freq. of recessive phenotype =

freq. of bb = q2 = (0.22)2

= 0.0484 = 4.84%

# of recessive ind. = (0.0484)(172)

= 8.3 individuals

(8 ind.)

Solution:

DEFINITIONS● Microevolution = studies

how pop’s of organisms change from generation to generation; changes in allele frequencies in a population’s gene pool

● Macroevolution = studies changes in groups of related species over long periods of geologic time; determines evolutionary relationships among species

Causes of Microevolution:

1) Natural Selection

2) Genetic Drift (changes in the gene pool of a small population due to chance)

Examples:

-Bottleneck Effect: results from drastic

decrease in population size

-Founder Effect: few individuals in a

population colonize a new habitat

Bottleneck Effect

3) Gene Flow (migration of fertile individuals between populations)

 

4) Mutation (introduces new alleles into a population)

5) Nonrandom Mating (individuals choose mates based upon their traits)

Ways Natural Selection Acts on a Population:

1) Stabilizing Selection: eliminates individuals with extreme or unusual traits; existing population frequencies of common traits are maintained

*Example of Stabilizing Selection in humans:

*human babies most commonly weigh 3-4 kg; babies much smaller or larger have higher infant mortality rates.

2) Directional Selection: favors traits at one extreme of a range of traits; common during periods of environmental change

Examples:

-insecticide resistance

-peppered moth

Peppered Moth example:

● 100 years after the first dark moth was discovered in 1848, 90% of moths were dark;

● the light variety continued to dominate in unpolluted areas outside of London.

3) Diversifying (a.k.a. Disruptive) Selection: occurs when environment favors extreme or unusual traits while selecting against common traits

4) Sexual Selection: differential mating of males in a population; leads to sexual dimorphisms

-females tend to increase their fitness by

increasing the quality of their offspring by

choosing superior male mates (and are therefore

“choosier” or more selective when finding a mate)

Sexual Selection (cont.)

-males increase their fitness by maximizing the quantity of offspring produced

**as a result, in vertebrate species,

the male is typically the

“showier” sex

-colorful plumage

-lion’s mane

-antlers

Sexual Selection:

● INTRASEXUAL SELECTION = direct competition among individuals of one sex (males use antlers, aggressive behavior, etc.)

● INTERSEXUAL SELECTION = “mate choice”; individuals of one sex are choosy (usually the females)