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species

a group of organisms with the capacity to breed and produce live, fertile offspring

2 steps for speciation

• two populations become isolated from each other

• genetic differences build up between the two populations due to different selection pressures and/or genetic drift

once genetic differences are great enough to prevent reproduction, the two groups are considered separate species

allopatric speciation

“different fatherland”

genetic divergence occurs during physical (geographic) isolation

3 possible outcomes when contact is reestablished:

1. genetic divergence is sufficient to prevent cross-mating; speciation is complete

2. hybridization is successful and populations merge back into one; no speciation

3. hybrids are inferior (lower survival/fecundity); example: too few mules; selection may “complete” the speciation process through reinforcement

reinforcement

process that enhances pre-zygotic isolation (ex. divergence of mating signals) to minimize hybrid matings

leads to character displacement (signals and/or preferences are more divergent in areas of sympatry than in areas of allopatry; can happen in male signal, female preference, or both)

reinforcement may facilitate speciation by reducing frequency of mis-matched matings

sympatric speciation

same fatherland

reproductive isolation arises due to differences in mating behavior

• mate in different parts of habitat,, at different times, or with divergent mating signals

genetic divergence occurs due to drift or different selection pressures on different sup-populations

differences in signals and/or preferences

may arise due to reinforcement during allopatric speciation

may initiate sympatric speciation