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Reproductive Isolating Mechanisms
The biological definition of a species is a group of similar organisms that can interbreed to produce fertile, viable offspring. Some extend this to say that this reproduction must occur under natural, not artificial (e.g., in captivity) situations.
When an ancestral species gives rise to two new species, what
determines whether the two new species can reproduce?
Consider the mechanisms that restrict gene flow: reproductive isolating mechanisms.
Prezygotic Isolating Mechanisms prevent the formation of viable zygotes.
Morphological differences prevent mating/pollination.
- The amazing partnership of the Bucket Orchid and Orchid Bee is so precise that if either one went extinct, the other would follow,. No other orchid can possibly cross-pollinate the Bucket Orchid.
- In some snail species, the direction of shell coiling is controlled by a single (maternal effect) gene. Snails with left-coiling shells cannot mate with snails having right-coiling shells. This could eventually lead to further differentiation and speciation.
In this case, sperm and ova of the two species are chemically (genetically) incompatible, and will not fuse to form a zygote.
Sea urchins do not mate. They broadcast their gametes into the ocean where sperm and eggs fuse to form zygotes and then develop into larvae. The Giant Red Urchin (Strongylocentrotus franciscanus and Purple Urchin (Strongylocentrotus purpuratus) cohabit the rocky intertidal along the western U.S., but they do not interbreed. Their gametes are genetically/chemically incompatible, maintaining species integrity.
Postzygotic Isolating Mechanisms prevent hybrids from passing on their
- Hybrid Inviability
A zygote may form with union of sperm and egg from the two species, but the embryo dies after
a few cell divisions. The genetic information from male
and female parents is insufficient to carry the organism through
Successive generations of hybrids
suffer greatly lowered fertility --> sterility.
they are selected out of the population.
Hybridization Between Species
If there is interbreeding between two closely related species, there are several possible outcomes.
- 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. A narrow hybrid zone can foster constant hybridization with reduced hybrid survival (e.g., Bombina) hybrid zone in Eastern Europe.
- Hybrid Speciation - 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.