Allopatric Speciation—Drift

• Drift is important in evolution• Just because an allele is common doesn’t

mean selection favored it

Speciation via drift?Probably not.

• Flies in allopatry, same environment never RIM

• Drift may facilitate speciation, but probably cannot often cause speciation on its own

Ecological Speciation

• Sister species in the same lake• Big is benthic, small is limnetic• No interbreeding in nature– habitat isolation or pre-

mating RIM?

Low probability of spawning between different ecomorphs, even when closely related (A). Ecology is important in RIM.

C = control (same species, same population, high probability of spawning)D = same ecotype, distantly related (act like same species)A = sympatric, closely related, different ecotype (act like different Biological species)B = allopatric, distantly related, different ecotype

Reproductive compatibility determined more by ecotype than by genetic relatedness

No-choice mating trials in the lab

• RIM appears to be body size—did divergent natural selection on body size speciation?

Speciation via Sexual Selection

• Many sister lineages with same ecological niche, but different secondary sexual characters

Greater species diversity in lineages with greater promiscuity. Due to stronger sexual selection?

Fig. 24-12

EXPERIMENT

Normal lightMonochromatic

orange light

P.pundamilia

P. nyererei

Speciation by Sexual Selection

Under manipulated lighting, females made “wrong” mate choice

Genetics and Timing of Speciation

Time to SpeciationVaries

Fig. 24-19

One-gene speciation

RIM’s can be generated by simple differences in genetics

Ancestral species:

Triticummonococcum(2n = 14)

AA BB

WildTriticum(2n = 14)

Product:

AA BB DD

T. aestivum(bread wheat)(2n = 42)

WildT. tauschii(2n = 14)

DD

Speciation may involve hybridization, so it can be quick for many species

Speciation involves a stochastic element…

• Medium ground finches on Daphne major (Gallapagos Island)

Top to bottom: A to F show successive generations of the hybrids, which now mate only with each other.Grant and Grant, PNAS, doi/10.1073/pnas.0911761106

Immigrant

F1

F5 F6

Immigrant Geospiza fortis: large body, wide beak, unusual song (bad mimicry)

• The Grants followed the fate of the immigrant over 7 generations (28 years)

• The immigrant imitated (imperfectly) the local song and mated with a large female

• In generation 4, severe drought, lineage reduced to a single brother and sister, which mated

• From then on, this lineage was reproductively isolated—premating RIM

• RIM due to song – culturally transmitted to sons (learned)?– sound may also be a consequence of bill shape