1. Speciation
Speciation
-the central process in evol

2. Species have unique combinations of features
when similar, usually closely related
Species are reproductively isolated
when closely related, can hybridize
general scheme of speciation
-biol diversity reveals 2 things:
1 many spp have unique combinations of features we use to distinguish them
when differences hard to detect, the are usually closely related
2 almost all spp are reprod isolated from each other – closely related can hybridize
-therefore copies of genes in 1 spp cannot reach descendants of indiv from other spp, each spp has its own gene pool (some leakage with closely related pools)
-speciation requires splitting of 1 gene pool into 2+ separated pools and diversification into 2+ phenotypically different forms
-genetic separation and phenotypic differentiation

3. Allopatric & sympatric speciation
Allopatric speciation
populations evolve in geographic isolation
Sympatric speciation
populations evolve in geographic proximity
geographic isolation
-allopatry vs sympatry
allopatric speciation
-popln’s become geographically isolated, are exposed to divergent selection, evolve independently
-accumulate so many genetic differences that they become reprod isolated if they come into contact again
-if isolation is complete, speciation has occurred
-can be geographically isolated by water, mtns, lava, savannah etc
-recap – assumes complete geological separation

4. Allopatric speciation
Geographic isolation produces genetic isolation
origin of reprod isolation
-physical separation of allopatric poplns guarantees zero gene flow
-divergence built up in allopatry has inherent reprod isolation – that is only apparent in 2ary sympatry
-what about allopatric poplns that have only partial reprod isolation in 2ary sympatry?
-like the apple flies, how much gene flow will stop the speciation process?

5. Allopatric speciation
Populations can become geographically isolated when individuals colonize a new habitat

7. Allopatric speciation
Populations can also become geographically isolated when a species’ former range is split into two or more distinct areas
-isthmus of Panama
-lava flow - Kilauea

8. Sympatric speciation
Natural selection can cause populations to diverge even when a small amount of gene flow occurs
sympatric speciation
-geological or ecological conditions that cause partial isolation of subpopln’s
-still have some gene flow with parent popln
-most likely form involves selection to use different resources – may shift hosts
-eg. hawthorn/apple fly, Rhagoletis polmonella
-used to lay eggs on hawthorn
-1864 found in apples
-can now distinguish apple & hawthorn poplns that differ genetically
-females prefer to lay eggs in type of fruit from which they came
-males prefer to mate on type of fruit from they came
-thus self-reinforcing
-but still interbreed freely either apple/hawthorn
-speciation not yet complete, but genetic differentiation and partial reprod isolation of R. polmonella did occur sympatrically
-whether it leads to speciation is still open
-incipient spp

9. Changes in chromosome number isolate populations genetically
Sympatric speciation
Changes in chromosome number isolate populations genetically
Speciation triggered by changes in chromosome number has been especially important in plants (polyploidization)
sudden sympatric speciation by polyploidization
-polyploidization – doubling the complete set of chromosomes due to irregularities in cell division
-further normal cell divisions xfer polyploidy condition to all descendent cells
-once arisen, polyploidy cannot mate with parents, because would produce triploid offspring
-particularly important for flowering plants
-not as much in animals

10. Mechanisms of divergence
Genetic drift
Natural selection
Sexual selection
Reinforcement
polyploidy,dispersal, vicariance produce the conditions for speciation

11. Mechanisms of divergence
Genetic drift
Drift can produce rapid genetic divergence in small, isolated populations
genetic drift
-genetic drift effects most pronounced in small poplns
most spp originate with small popln size
-small # of indiv in colonization event
-model this
-start with small popln of non-random from ancestral popln
-drift in the small popln
-random loss of alleles, random fixation of others + new alleles
-so isolated popln undergoes rapid genetic divergence from ancestral popln
-role of drift is controversial
-bottlenecking eliminates only very rare alleles
-popln has to be extremely small & remain small for a signif period for drift to change allele freqs
-now recognize natural selection as more important

12. Mechanisms of divergence
Natural selection
Natural selection can produce genetic divergence in populations subjected to different pressures
natural selection
-look at apple flies again
-selection acting on preferences for different fruit drives flies apart

13. Mechanisms of divergence
Sexual selection
Sexual selection acts on characters involved in mate choice
sexual selection
sexual selection may cause prezygotic reprod isolation
-eg HI drosophila 800spp in HI, 2800 total worldwide
-eg African cichlids
-differences among indiv in their ability to obtain mates
-females prefer males with certain characters which leads to self-reinforcing loop
-more later

14. Mechanisms of divergence
Sexual selection

15. Mechanisms of divergence
Sexual selection
Changes in sexual selection may isolate populations and cause rapid divergence

16. Mechanisms of divergence
Reinforcement
Reinforcement leads to assortative mating and the prezygotic isolation of populations
reinforcement
-2ary reinforcement – mechanisms would help to keep spp separate
-mechanisms selected to prevent non-productive reprod
-reinforcement may not have an important role

17. Mechanisms of divergence
Reinforcement
Prezygotic isolation
2 reinforcement probably unimportant in generating prezygotic isolation
-prezygotic isolation often stronger when indiv are sympatric vs allopatric
-consistent with idea that prezygotic isolation evolved to prevent production of hybrids
-importance of reinforcement is doubtful
-low genetic distance = recent divergence - sexual isolation greater among sympatric than allopatric forms
Genetic distance
Allopatric
Sympatric