REPRODUCTIVE ISOLATING MECHANISMS

When one species gives rise to two new species (cladogenesis), what is it that determines whether or not the two can reproduce, if allowed to regain physical contact? We can help define separate species by considering the mechanisms that restrict gene flow between them.

Two related species may be separated by one or more of these types of reproductive isolating mechanisms, which may evolve--once again--due to the five factors already discussed that can change allele and genotype frequencies in a population.

PREZYGOTIC ISOLATING MECHANISMS prevent the formation of viable zygotes.

• environmental/spatial isolation - the geographic ranges of two species overlap, but their microhabitats or breeding conditions differ enough to cause reproductive isolation.

  e.g. - Rana aurora (Red-legged frog) breeds in fast-moving, ephemeral streams, whereas its relative Rana catesbiana (Bullfrog) breeds in permanent ponds. (The metamorphosis times of the tadpoles are correspondingly different.)
  

  

• temporal isolation - two species whose ranges overlap have different periods of sexual activity (or breeding season)

  Rana aurora - breeds January - March
  

  

  Rana boylii - breeds late March - May
  

  

  another example -

  Drosophila persimilis - breeds in early morning

  Drosophila pseudoobscura - breeds in the afternoon
  

behavioral isolation - In species with courtship rituals (breeding calls, mating dances, etc.), there is usually a complex, give-and-take "ritual" before actual mating takes place. This prevents "wasted effort" with a partner who will not produce fertile offspring with you!

mechanical isolation - morphological differences prevent mating/pollination.

gametic isolation - sperm and ova are chemically incompatible, and will not fuse to form a zygote.

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POSTZYGOTIC ISOLATING MECHANISMS prevent hybrids from passing on their genes.

• hybrid inviability - zygote forms, but dies after a few series of cell divisions (the genetic information from male and female parent were insufficient to carry the organism through morphogenesis)
  

• hybrid sterility - viable hybrid is produced (often physically more vigorous than either parent), but is unable to reproduce due to meiotic problems.
  Examples: liger or mule

• hybrid breakdown - successive generations of hybrids suffer greatly lowered fertility --> sterility. Eventually, they are selected out of the population.
  

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Three possible scenarios:

• Species Reinforcement - hybrids have lower fitness than either parent species; reproductive isolation is maintained due to lack of hybrid survival/reproduction.
• Species Fusion - two species in a hybrid zone may have weak reproductive isolating barriers, and the two species may, over time, eventually share a common gene pool.
• Species Stability/Hybrid Equilibrium - Hybrids are continually produced by the two parent populations in a hybrid zone.
  • narrow hybrid zone can foster constant hybridization without much hybrid survival (e.g., Bombina
  • hybrids may actually be reproductively superior to parent populations, and if they tend to breed with each other, this can result in what could be termed "hybrid speciation."