1. Mechanisms of species formation
Darwin (and others): geographic separation followed by adaptation to different ecological circumstances eventually leads to reproductive isolation (allopatric and slow)
Two alternatives that emphasis strong selection:
FOOD: Sympatric speciation due to shifts in host (e.g., in specialized herbivores/parasites) (sympatric and fast)
SEX: Speciation due to divergent sexual selection (evolution of mate preference) (allopatric but very fast)

2. Movement of Pacific plate
Decrease in Age of Island

6. Alpheus snapping shrimp
Alpheus snapping shrimp
Knowlton et al. 1993

7. Model of sympatric speciation via shifts to new host (FOOD)2 2 1 2
Host shift 1 1 1 2
New population founded via shift to derived host 2
Populations on two hosts become reproductively isolated
Population uses ancestral host 1

8. Eurosta solidaginis – an example of sympatric speciation via host-plant shifts
Illustration from Abrahamson and Weis (1997)
Photo courtesy of Warren Abrahamson

9. Signatures of sympatric speciation via host/habitat shift (after Via 2001)
Sympatric overlap of host patches
Mating on the host causes reproductive isolation between races
Host phenology differs
Divergent selection on different hosts/habitats
The article by Sara Via illustrates well how we usually approach the study of speciation, by defining a theory at odds with the prevailing one, and then marshalling evidence from theory that it is possible, and from empirical work that is has occurred, hopefully often. Sara reviews that ups and downs of the theory of sympatric speciation (speciation without geographical isolation as revived by Guy Bush’s work in the late ’60s) then reviews of number of signatures of cases in which this mode has been important

10. Speciation via SEXual selection
Upon recontact, populations reproductively isolated due ONLY to female preference
Females in ancestral populations prefer blue males
Geographic barrier divides populations
Female preference for male trait diverges due to genetic drift

11. Signatures of speciation via sexual selection (after Panhuis et al
Populations within species vary in sexually selected traits and associated preferences, resulting in partial pre-mating isolation
Closely related species differ markedly in mating signals and preferences, which constitute the primary barrier to gene exchange
Species differ in few other traits besides those involved in mate choice
Pahuis et al. do the same with speciation via sexual selection.

12. North American Enallagma
37 spp. North America
Species restricted either to lakes with or without fish (F and D lakes)
Distribution mediated by differences in anti-predator behavior
Species richness 3-5x higher in fish lakes

13. Brown, McPeek and May.2000 Syst. Biol. 49:697-712.
Phylogeny of 34 N.A. species
Shows origin of fishless species
Suggests difference in species richness in two habitats causes by recency of invasion (i.e., not enough time to generate as many species). Also note fishless species arise from only one of two clades (diffs in propensity to swim), occur multiply and are VERY recent, suggesting that extinction rates are higher in fishless habitats.

15. Robertson and Paterson, 1982. Evolution.

16. Brown, McPeek and May.2000 Syst. Biol. 49:697-712.
Phylogeny of 34 N.A. species
Shows origin of fishless species
Suggests difference in species richness in two habitats causes by recency of invasion (i.e., not enough time to generate as many species). Also note fishless species arise from only one of two clades (diffs in propensity to swim), occur multiply and are VERY recent, suggesting that extinction rates are higher in fishless habitats.

17. boreale
laterale
clausum
cyathigerum
hageni
davisi
recurvatum
minusculum
ebrium
Images from Westfall and May, 1996

18. Selection and the Enallagma radiation
Four damselfly-lake species generated by habitat shifts and adaptive evolution (morphological, behavioral and physiological)
Many fish-lake species generated during recent radiations (associated with glacial retreat?), possibly via founder effects on mechanical mate recognition system