L9 Gene Flow and Speciation

How can local adaptation through selection cause populations to differ?

different environments/stressors select for a different genotype

Cline

change in allele frequencies that occur as a smooth gradient over space

How does drift cause populations to differ?

causes random changes/shifts in allele frequencies

gene flow

mixing of alleles among populations

What causes gene flow?

migration/dispersal of individuals/gametes

Three Effects of Gene Flow on Alleles

equalizes allele frequencies, erodes genetic differences between populations, introduces new alleles into population (same effect of a mutation in that it increases variation in receiving population)

Migration rate m

tells us how quickly gene flow erodes genetic differences between populations

How do you calculate m?

m = # immigrant individuals in a generation/NEW total number of individuals in population

Δq

difference or change in allele’s frequency before and after migration in given population

How do you calculate Δq?

Δq = m(q_m-q)

m = migration rate

q_m = allele’s frequency in migrants

q = allele frequency in focal population before migration

F_ST

statistic that describes the genetic difference among populations

How do you calculate F_ST?

F_ST = Var(p)/p ̅(1-p ̅)

Var(p) = variance of allele frequency among populations

p ̅ = mean allele frequency across all populations

F_ST of 0 means

two populations genetically same, but…

0% variation caused by differences between populations

100% variation caused by differences within typical population

F_ST of 0.36 means

36% variation caused by differences between populations

64% variation caused by differences within typical population

F_ST of 1 means

100% variation caused by differences between populations

0% variation caused by differences within typical population

populations completely genetically different

Biological Species Concept

species are groups of actually or potentially interbreeding populations, which are reproductively isolated from other such groups

Phylogenetic Species Concept

a phylogenetic species is a basal cluster of organisms diagnosably different from other such clusters, and within which there is a parental pattern of ancestry and descent

Reproductive Isolation

reduction of gene exchange (little to no gene flow) between populations

What are the types of reproductive isolation?

Prezygotic barriers, Postzygotic barriers

What are prezygotic barriers?

features that impede transfer of gametes to members of other species

Five Prezygotic Barriers

ecological isolation - breeding habitats don’t overlap

temporal isolation - breed at different times of year

behavioral/copulatory isolation - individuals have different courtship behaviors, pollinators don’t transfer pollen between species

mechanical isolation - physical differences prevent copulation/pollination

gametic isolation - fertilization fails

What are postzygotic barriers?

hybrids are formed (‘successful mating’) but they have reduced fitness

What are the three results of postzygotic barriers?

reduced hybrid survival + reproduction, and hybrid breakdown where fertile hybrid produces sterile offspring regardless of mating partner

_____ + _____ = speciation

isolation + divergence

Allopatric

geographically isolated populations

peripatric

small population isolated at edge of larger population

parapatric

continuously distributed population

sympatric

within the range of the ancestral population, more integrated speciation

Dispersal

individuals of population disperse across a physical barrier

Vicariance

an existing population is split by the appearance of a new physical barrier (e.g. lava flow, river course shift, earthquake)

Behavioral isolation

when adaptive behaviors result in reproductive isolation

Morphological isolation

when morphological adaptations result in reproductive isolation