1. The geography of speciation
most species form when geographic barriers block genetic exchange

2. Allopatric speciation
arguably the most common way species form
evidence for allopatric speciation is common in biogeography:
related species often occupy nearby, non-overlapping ranges

3. Allopatric speciation
Isthmus of Panama closed ~ 3.1 MYA
Split ~150 “geminate” (twin) species

4. Evidence for allopatric speciation in snapping shrimps
Knowlton et al.(1993) created a phylogeny of Pacific (P) and Carribean (C) species pairs of Alpheus
In 6 out of 7 cases, the closest relative of a species was on the other side of the Isthmus

5. The geography of speciation

6. Allopatric speciation through “dispersal and colonization”

7. Evidence from phylogeny of Hawaiian Drosophila
D. heteroneura
D. silvestris

8. Founder effect speciation
divergence of a small population isolated on periphery of range
thought to explain species radiations on islands

9. Founder effect speciation
the genetic “founder effect” of small population size itself is thought to drive speciation

10. Sympatric speciation no geographic barrier much rarer
from Barluenga et al. (2006) Nature 439:
no geographic barrier
much rarer
but shows how ecology can drive speciation by selecting for assortative mating

11. Sympatric speciation valid cases: cichlids in crater lakes
host races of Rhagoletis
(apple maggot fly)
from Barluenga et al. (2006) Nature 439:

12. Lake Apoyo: an isolated volcanic crater lake, homogeneous habitat < 23,000 years old

13. mtDNA sequences are monophyletic: no Lake Apoyo sequences are found in sister species outside the Lake
This shows that all Lake Apoyo fish evolved in sympatry

14. The two sympatric species are reproductively isolated

15. Morphological and ecological differences evolved sympatrically
benthic
limnetic

16. Sympatric speciation in action
Rhagoletis pomonella: the apple maggot fly
Larvae feed on natural host: hawthorn fruits
A “host race” infesting apple trees appeared in North America ~150 years ago
hawthorns
apples

17. apples
hawthorns

18. Speciation in progress
the apple race and hawthorn races have evolved reproductive barriers
Adults mate and oviposit on their host trees
This leads to limited interbreeding

19. Speciation in progress
the apple race and hawthorn races have evolved reproductive barriers
Adults mate and oviposit on their host trees
Genetic differences between the races mark the early stages of speciation

20. Genetic differences between apple and hawthorn races
the two races show genetic differences at six allozyme loci
included is an aconitase locus called Acon-2
hawthorns
apples
citrate
aconitate
isocitrate

21. Genetic differences between apple and hawthorn races
the frequency of the Acon-2 95 allele is much higher in the hawthorn race
hawthorns
apples

22. The role of natural selection in divergence of Rhagoletis
Feder and coworkers predicted that natural selection opposes migration and creates allozyme differences
One hypothesis: hawthorns ripen fruits 3-4 weeks later than apples
Apple fly larvae experience longer periods of warm temperatures prior to and while pupating
This must select for divergent genes

23. Their experiment...
Collect hawthorn pupae and expose to warm temperatures for a varying duration
Follow with freezing temperatures (“winter”), then by warming (“spring”)
Collect emerging adults and assay allozymes

24. hawthorn pupae exposed to longer periods of “pre-winter” warmth produced adults with allele frequencies approaching those of the apple race

25. The classic “three stage” model of allopatric speciation
1st stage: a geographic barrier creates isolation between two or more portions of a population
2nd stage: the descendant populations diverge genetically (due to drift and selection)
[Rhagoletis has skipped the 1st and is in the 2nd stage]

26. “Ecological speciation” in sticklebacks: more evidence for natural selection during the 2nd stage
Dolph Schluter and coworkers have studied Gasterosteus aculeatus in rivers and lakes of British Columbia for many years
A marine ancestor colonized rivers at least 3 times, independently

27. “Ecological speciation” in sticklebacks shows evidence for natural selection during the 2nd stage
Several lake colonizations have led to independent cases of “ecological speciation”

28. “Ecological speciation” in sticklebacks shows evidence for natural selection during the 2nd stage
In several lakes, two morphs that show differences in diet, morphology, and behavior have diverged:
A smaller limnetic (open water) form
A larger benthic (bottom dwelling) form
Benthic and limnetic forms mate like-with-like

30. Evidence for a role for sexual selection during the “divergence” stage

31. Sexual selection on head width in D. heteroneura
Males with wider heads are chosen by females on leks
And they win contests with males for territories

32. This means that sexual selection is likely to be responsible for the differences in head shape between D. heteroneura and D. silvestris
And that it served a role in their speciation??

33. The classic “three stage” model of allopatric speciation
3rd stage: reproductive isolation is completed, or perfected
this occurs after “secondary contact” between allopatric populations
a crucial step, why?
secondary contact is common
without complete reproductive barriers, species will re-fuse

34. Secondary contact after Pleistocene glaciations: bird hybrid zones
from Futuyma (1998), p. 258

35. Secondary contact: hybrid zones in blue mussels
Hilbish et al. 2000
Blue mussels within the Mytilus edulis complex are global distributed throughout subpolar and temperate intertidal and subtidal habitats. Interestingly, despite at least 2.5 my of genetic divergence within this group - when the ranges of any two of these three species meet and overlap hybrids are formed. These hybridization events have lead to geographically expansive and temporally stable hybrid zones – with one of the most intensively studied zones occurring in Western European populations of Mytilus edulis and M. galloprovinciallis.
Yet in all cases where these hybrid zones occur parental species maintain their respective species boundaries.
So here we have a unique observation of closely related, free-spawning marine invertebrates that are interbreeding, but not fusing - which suggests that these hybridizing species have evolved, or are evolving, mechanisms such as gametic incompatibility or assortative mating to limit hybridization.

36. Pacific mussels reinvaded the North Atlantic during warm, high sea level period about 15-20,000 ya
from Riginos and Cunningham 2005

37. The classic “three stage” model of allopatric speciation
Reproductive isolation can be completed in two distinct ways
As a byproduct of the divergence process, through drift and selection (unrelated to interbreeding)
Via selection against hybridization (reinforcement)

38. Coyne and Orr’s (1997) survey of Drosophila species pairs
Reviewed data on over 150 species pairs
Whether the species are allopatric or sympatric
Genetic distance between the species pair (based on allozymes) as an estimate of age
The amount of postzygotic and prezygotic isolation

39. Prezygotic isolation increases with genetic distance
Their results...
Prezygotic isolation increases with genetic distance
Genes for prezygotic isolation diverge over time, just like the rest of the genome

40. Full isolation evolves in allopatry
Secondary contact is not necessary to complete the speciation process

41. Coyne and Orr’s (1997) survey supports reinforcement
Prezygotic isolation evolves faster in sympatry
Selection against hybridization drives more rapid evolution

42. This is not “sympatric speciation!”
why not?