Transcript Lecture 25 (4-6-11)

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Speciation = the origin of species
Factors that have accelerated speciation:
In plants: use of different animal pollinators
In animals: modifications in sexual selection
Reinforcement
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Speciation and its Mechanisms
Most animal speciation is visualized as lineage splitting.
typically messy
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Basic speciation models require separation of gene pools.
Darwinian idea: slow accumulation of genetic differences.
But there can be large, rapid effects from modest genetic
changes (e.g., in developmental pathways).
• A new species typically originates from only a small
segment of an established population.
Elephants: how many species?
Traditional: Loxodonta africana and Elephas maximus
But, African elephants are morphologically different
Savannah and forest populations
Essentially geographically isolated….BSC
Phylogenetic species concept applied
Sampled: 195 elephants from 21 populations
4 genes sequenced for each of the 195 individuals
Genetic distances used to construct a phylogeny
Conclusion: two species (L. cyclotis and L. africana)
Polyploidization
Galeopsis tetrahit: 2n = 32
Suspected of being an allotetraploid
Candidate progenitors: G. pubescens (2n = 16) and G. speciosa (2n = 16)
Diploid F1 hybrids (2n = 16) produced and crossed
One rare 3n hybrid produced.
Triploid hybrid backcrossed to G. pubescens
One rare 4n hybrid produced.
The recreation of G. tetrahit, with which it was interfertile.
Therefore; an artificially produced species
Evidence of allopatric speciation by vicariance: genetic divergence in refuges
mtDNA clades: 3-4 mya
H: freshwater refuges formerly
separated by salt water barrier
Conservation of climatic niche space: cross predictions of the ranges of two species
Peripatric Speciation
by dispersal
Representative Hawaiian
Drosophila diversity
Sympatric Speciation
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Flies (Rhagoletis pomonella)
(1) Larvae develop in hawthorns (Crataegus)
Native to NE U.S.
(2) Larvae develop in apples (Pyrus)
Apples introduced c. 300 ya
H0: The flies belong to the same species; there is
phenotypic plasticity in use of hosts.
• H1: Speciation has occurred; each species of fly adapted to
one host species.
• Since hawthorns and apples are both within the range of
Rhagoletis, this would represent an example of sympatric
speciation.
• Flies using the two types of fruit cannot be
distinguished morphologically (cryptic).
• But, evidence of lineage splitting:
• 1. Allele frequency differs for 6 different proteins;
therefore can be distinguished genetically.
• 2. Flies imprint on fruit within which they
developed.
– Mating takes place on the fruit.
– Provides some degree of physical segregation.
– Only c. 6% of matings are between misimprinted flies.
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Plus: flies are diverging because of natural selection
Selective agent = timing of fruit ripening.
Apples ripen “early.”
Larvae in apples
– Selected to develop slowly.
– Prevents emergence of adults prior to winter.
• Hawthorns ripen ca. 3 weeks later than apples.
• Larvae in hawthorn fruit
– Selected to develop rapidly.
– Enables pupation prior to winter.
• A few mistakes are made: speciation nearing
completion.