Zoology 445
Fall 2006
Speciation 3: Handout
3. Secondary Contact
The biggest questions upon secondary contact are:
1.Do hybrids form?
2.What is the fitness of the hybrids?
No hybrids formed; species still ecologically
similar
No hybrids formed; species ecologically
distinct
Abrupt transition (separate species)
Range expansion into other species range: with
or without character displacement in sympatry
(Separate species)
Reinforcement means that selection will favor evolution of reproductive isolating mechanisms
to complete the speciation process.
Two models of speciation that do not require physical separation of the diverging populations:
1. Parapatric & Sympatric
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2. Polyploidy
1. Parapatric & Sympatric
Speciation is a gradual process in response to disruptive selection and/or sexual selection.
Further, gene flow among the differentiating populations is possible throughout the speciation
process. Thus, selection favors pre-zygotic RIM's during the differentiation process.
Marine snail, Littorina saxitilis
Frequency of morphs:
Upper: 100% ridged
Mid: 42% ridged; 46% small;
12% hybrids
Low: 1% ridged; 94% small;
5% hybrids
Ave genetic distance (D) between morphs in an area = 0.007
Interpreting D:
D = 0 then there are no genetic differences.
D = ∞ then populations are completely different
Ave D same morph between areas = 0.046.
2. Polyploidy
a. Autopolyploidy:
Parent (2N) → 4N New species
b. Alloployploidy:
Parent 1 (2N)
4N New species
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Parent 2 (2N)
2N
X 4N

3N (sterile because chromosomes do not pair properly at mitosis or meiosis)
Phlox example
Rates of speciation
Highly variable
Average time to attain full reproductive isolation is about 3 MY.
Factors that promote speciation.
1.
2.
3.
Low dispersal ability (promotes isolation)
Diverse habitat structure (promotes selection for alternate adaptive peaks)
Sexual selection (promotes pre-zygotic isolation by the development divergent speciesspecific markers for female choice)
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