Vicariance underlies allopatric speciation in Snapping Shrimp

By Fred Singer

Allopatric speciation

• 1. Physical barrier causes isolation of one population from a second population.

• 2. Genetic divergence (usually in habitat use, resource use or mate choice).

• 3. Reproductive isolation

Physical isolation

• 1. Dispersal of a population to a new area.

• 2. Vicariance – Changing of a habitat by geological processes into two distinct habitats.

Snapping Shrimp - Alpheus

Nancy Knowlton et al.• Sister species (sibling species) – two species that are

very closely related

• Sister species usually are initially identified by similarities in morphology (morphospecies).

• In the case of snapping shrimp, characters included:– Dorsal spots– Dorsal bands– Antenna color– Movable spines

Knowlton’s hypothesis

• Vicariance (formation of the isthmus 3-4 million years ago) caused physical isolation of populations of these species, leading to allopatric speciation.

• This resulted in seven pairs of sister species

Prediction 1

• Sister species (based on morphology) should also be genetically more closely related to each other, than they are to other species– mtDNA (Cytochrome oxidase 1)– 11 polymorphic allozymes

mtDNA tree

Conclusion

• Based on the mtDNA, sister species identified by morphological characters are also closely related based on mtDNA analysis

Prediction 2

• Because there was one vicariance event 3-4 million years ago, all seven sister species should have similar values separating the two sister species in relation to– mtDNA distance– allozyme distance– Similarities in mating behavior

Genetic Distances between sister species

Taxa Allozymes (Nei’s D) mtDNA (% divergence)

P1, C1 0.028 7.7

P2, C2 0.114 6.5

P3, C3 0.109 7.7

P4, C4 0.121 8.5

P5, C5 0.177 13.4

P6, C6 0.188 10.5

P7, C7 0.272 19.2

Behavioral compatibility measuresTaxa Pairings Passive

ContactsSnaps Aggressive

ContactsOverall compatibility

P1, C1 0.86 1.97 2.18 0.49 1.42

P2, C2 0.67 0.33 1.40 0.48 0.58

P3, C3 0.45 0.66 0.49 0.31 0.47

P4, C4 0.00 0.07 0.33 0.53 0.20

P5, C5 0.33 0.51 0.14 0.13 0.24

P6, C6 0.00 0.01 0.33 0.19 0.10

P7, C7 0.00 0.01 0.14 0.00 0.01

Conclusion• The data do not support prediction 2 of the vicariance

hypothesis. • Rather some sister species (e.g. P1 and C1, and P2 and

C2) are more similar to each other genetically and behaviorally than other sister species (such as P7 and C7).

• These differences are consistent for all three measures (allozymes, mtDNA, behavior)

• Thus P1 and C1 speciated more recently than P7 and C7.

• Thus there was a different timing of speciation for each sister pair.

Huh?

• Doesn’t vicariance predict that each sister pair should show similar levels of genetic and behavioral divergence?

• We need to consider ecology!!

• In particular, different species prefer different hangouts.

• http://www.vtmagazine.vt.edu/spring05/images/bubs.jpg

Conclusion

• Vicariance has created the physical barrier for numerous speciation events in snapping shrimp.

• We need to consider differences in ecology, in order to understand the sequence and timing of these speciation events.