Population genetics & Speciation

Evolution

change in allele frequencies in populations over generations 

Phenotype variation

observable differences between individuals

Genetic variation

differences among individuals in a gene sequence

Population

a group of organisms of the same species living in the same area at the same time

Population genetics 

the study of genetic variation within a population

Gene pool

all alleles of all genes in a population at any time given

Allele frequency

proportion of a specific allele at a locus within a population

Fixed allele 

allele at a frequency of 1 in a population → the only allele at a locus 

Genetic equilibrium

no change in allele frequency from generation to generation

Hardy-Weinberg equilibrium

a population’s allele and genotype frequencies will remain constant from generation to generation if no evolutionary forces act upon it

HW equations 

model allele and genotype frequencies in a non-evolving population

Equation for allele frequencies

p + q = 1

P variable

frequency of the dominant allele in a population

Q variable 

frequency of the recessive allele in a population 

Equation for genotype frequencies 

P^2 + 2PQ + Q^2 = 1 

P^2 variable

frequency of a homozygous dominant offspring

2PQ variable

frequency of a heterozygous offspring

Q^2 variable

frequency of a homozygous recessive offspring

5 conditions required for HW

1. no mutation

2. no migration

3. no natural selection

4. random-mating

5. large population size

Mutation

any heritable change in DNA

Random mating 

individuals in a population are equally likely to mate

Non-random mating 

individuals in a population mate based on traits

Interbreeding 

mating of closely related individuals

Natural selection

better adapted alleles increase in frequency as less beneficial alleles decrease in frequency

Genetic drift

random changes in allele frequency

Bottleneck effect 

a population drastically decreases in size

Founder effect

a small group colonizes a new environment

Gene flow 

movement of alleles between populations 

Speciation

the process by which 1 species splits into 2

Biological species concept (BSC)

members in a population who have the potential to produce viable offspring 

Morphology species concept 

defines species as a group of individuals with similar shapes/features

Ecological species concept

defines species as a group of individuals with the same ecological niche

Ecological niche

how individuals interact with the environment

Reproductive isolation 

prevent members of different species from producing viable/fertile offspring 

Hybrids

offspring of 2 different species

Prezygotic barriers

operate prior to zygote formation → prevent fertilization

5 prezygotic barriers

1. habitat isolation

2. temporal isolation

3. behavioral isolation

4. mechanical isolation

5. gametic isolation

Habitat isolation 

prevents species from interbreeding due to different habitats

Temporal isolation

prevents species from interbreeding due to the time of day / season / year

Behavioral isolation

prevents species from interbreeding due to different behaviors

Mechanical isolation

prevents species from interbreeding due to different sizes / shapes

Gametic isolation 

prevents species from interbreeding due to molecular differences

Postzygotic barriers

barriers that occur after zygote formation

3 postzygotic barriers

1. reduced hybrid viability

2. reduced hybrid fertility

3. hybrid breakdown

Reduced hybrid viability

hybrid offsprings have low survival rates

Reduced hybrid fertility

hybrid offsprings are viable but not fertile

Hybrid breakdown 

F1 hybrids are viable and fertile but F2 and subsequent are not 

Allopatric speciation

species are separated by a geographic barrier

Sympatric speciation

reproductive isolation due to sexual selection and habitat differentiation

Sexual selection

individuals in a population mate based on traits that promote reproductive success

Habitat differentation

individuals in a population evolve to use different habitats/resources that are not being used by the rest of the population