Speciation and Phylogenies

• Where genetically distinct populations meet, mate and reproduce • Forming between populations that are not yet ‘true species’ • Distinct populations overlap and there is mixing of genetic material

Remain completely distinct (reinforcement) Merge together (fusion) Form a stable hybrid zone (stability) Form a new hybrid species

• Tell us about the process of speciation • Can tell us about historical patterns e.g. where populations were in the past compared to where they are now • Range of genotypes show genetic differences and selection pressures that separate the taxa

Different Mitochondrial DNA genotypes Relationships between haplotypes to see how similar they are Refugia post glacial recolonisation

• A cline → change in the allele frequency over a geographical transect • Graph of differences in allele frequency over a geographical transect • Width of cline - how long it takes to get from one allele being completely fixed to another • more dispersal = wider cline

ABO blood groups in humans (high dispersal) Stronger selection = steeper cline

Selection purely due to the environment

lower intrinsic fitness than either parental individual Heterozygotes have disadvantageous intermediate phenotypes

When hybrids are really unfit

primary or secondary (common after ice age)

Predators learn to recognise different warning colours Hybrids with mixed colour patterns will have lower fitness

Populations have never separated from one another Natural selection alters alleles in a continuous population populations start to diverge Environment affects different loci in different places

Population that lives on a mountain diverge with altitude population higher up the mountain is better adapted to cold conditions population at the bottom is better adapted for warmer conditions middle of the mountain = hybrid population

A random change in allele frequencies from the parent population that occurs when a small founding group establishes a new population

movement of genes from one species or population into another by hybridisation and backcrossing.

Different colour morphs controlled by 1 gene Maintained by strong selection pressures Primary hybridisation

Even the neutral genes show some selection (not strong) Secondary hybridisation steep cline

Secondary hybrid zones can look like primary if they are old enough Primary hybrid zones can look like secondary if multiple clines have become trapped in a density trough If enough of the genome is under selection (non-selected regions diverge due to hitchhiking)

Selection maintains steep clines at some loci could be a tension zone only if character differences are favoured by different environments could move: - area of low density or differences in migration rates

• Fitness of hybrids not lower than the original populations • introgression • Variation and distinction between 2 populations lost

Strengthening of barriers to gene exchange Large areas of genome protected from introgression Mechanisms controversial - reinforcement?

Hybrids become reproductively isolated from original populations new species

They tend to congregate in areas of low density and can be barriers to gene exchange

Barriers before the egg and sperm meet