Population Genetics and Speciation

How do you calculate frequency of alleles

For two alleles (Aa) —> f(A) = (# A alleles)/(# A alleles + # a alleles) and f(a)=(# a alleles)/(# A alleles + # a alleles). f(A)+f(a)=1

How do you calculate genotypic frequencies

For two alleles (Aa) —> f(AA) = (#AA alleles)/(# of individuals), f(Aa) = (#Aa alleles)/(# of individuals), f(aa) = (#aa alleles)/(# of individuals)

Hardy-Weinstein Equilibrium (HWE)

The null model, we assume nothing is happening. Allele and genotype frequencies remain constant unless an evolutionary force acts upon population.

HWE requirements

Infinity large population (small effect of genetic drift), no mutation, random mating, isolated from other population (no gene flow), no natural selection so all are equally likely to survive

Why is HWE useful?

If you assume population isn’t evolving, to check. Also allows you to determine if evolutionary agents are acting, if you know alleles frequencies

HWE formula for calculating allele frequency

p + q = 1

HWE formula for calculating genotype frequency

p² + 2pq + q² = 1

How to see if population is in HWE

Determine alleles frequencies by hand, calculate genotype frequencies using HWE formula, then compare and observe genotype frequencies

Species

The lowest on the biological taxonomic hierarchy. It can be interpreted many ways.

How can species be connected

They start from an ancestral population. When that population splits, different characteristics can be developed

Biological Species Concept (BSC)

Most widely used. All members have potential to interbreed under natural conditions and produce viable (can survive) and fertile (can reproduce) offspring

Limits of BSC

It can be difficult to apply as it’s not always clear who has the potential to interbreed. Doesn’t apply to asexual organisms (so is every offspring a different species??) and not to fossils.

Why determining a distinct species can sometimes be difficult

Names change over time, “species complexes”/boundaries are unclear, there are levels within levels

Speciation

One ancestor become two descendant species. Genetic divergence among populations over time and reproductive isolation (something that prevents reproduction of viable fertile offspring)

Allopatric Speciation

Physical barrier divides population

Peripatric Speciation

Small group splits off from a larger population to a new, isolated area

Sympatric Speciation

No Physical Barrier

Parapatric Speciation

Strong divergent selection pushes neighboring populations apart due to differences in environmental conditions (ex: one group has natural selection for heavy metal tolerance while the other doesn’t)

Adaptive Radiation

A population of one species disperses throughout an area, and each finding a distinct role/food source/habitat.

What happens to the founder species in adaptive radiation

Many adaptations from one starting point. Can be caused by sympatric or allopatric speciation

Gene flow in each scenario

Not possible for allopatric, might be possible for parapatric, lots possible for sympatric

When two newly species reconnect

Hybird zone

Fusion

The two species merge back into one

Reinforcement

species diverge further because hybrids are weak

Stability

Hybrid zones stay while two species still stay separate

Pre-zygotic barriers vs post-zygotic barriers

Prevent formation of a zygote vs. prevent development of viable or fertile offspring. Both isolate species

Habitat Isolation

Pre-zygotic. Two species prefer different habitats so don’t mate

Temporal Isolation

Pre-zygotic. Mating seasons are at different times

Behavioral Isolation

Pre-zygotic. Species meet but don’t mate because of differences in courtship or other behaviors

Mechanical Isolation

Pre-zygotic. Physical differences prevent successful mating

Gametic Isolation

Pre-zygotic. Gametes don’t recognize each other due to different receptors

Reduced hybrid viability

Post-zygotic. Hybrid offspring don’t develop OR offspring don’t survive as well

Hybrid infertility

Post-zygotic. Hybrids can survive, but not reproduce

Hybrid breakdown

Post-zygotic. 1st gen hybrids fertile but 2nd generation hybrids are sterile or weak