Species are evolutionary units wihtin which gene flow occurs.
Natural populations belong to the same species only if they can
interbreed and leave fertile offspring. Different species are
reproductively isolated from each other. Most new species originate
geographically.
Importance of species: Species characteristics are passed from parents to
offspring. Species were long considered real and important because these
characteristics seemed to be constant and unchanging.
Morphological (typological) species definition: Each species is
defined by characteristics thought to be "essential" or "typical".
Biological species definition: Species are groups of interbreeding
populations that are reproductively isolated from other species.
Reproductive isolating mechanisms that act prior to mating:
- Ecological isolation: Potential mates do not meet because
they live in different habitats or breed at different times or seasons.
- Behavioral isolation: Mating calls or mating rituals differ.
- Mechanical isolation: "Lock and key" mismatch of genitalia.
Reproductive isolating mechanisms that act after mating:
- Gametic mortality: Gametes die before fertilization.
- Zygotic mortality: Fertilized eggs fail to divide properly.
- Embryonic or larval mortality: They die prematurely.
- Hybrid inviability: Hybrids never reach reproductive age.
- Hybrid sterility: Hybrids cannot reproduce, as in mules.
- F2 breakdown: Offspring of hybrids are inviable.
Geographic speciation: Most speciation occurs geographically.
- A species develops geographic variation over its range.
- Geographic barriers prevent contact between populations.
- Reproductive isolation may now evolve.
- Geographic isolation ends, with two possible outcomes:
- No reproductive isolation-- still a single species.
- Reproductive isolation is effective-- two species now exist;
selection will enhance differences between them.
Other models of speciation (sometimes controversial):
- By doubling or tripling of chromosome number in plants (polyploidy).
- "Semigeographic" or "parapatric" speciation if adjacent ranges touch.
- "Sympatric" or "stasipatric": no geographic separation.
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