Barriers leading to reproductive isolation

Last week we read the first two chapters of Coyne and Orr’s book Speciation. Chapter 1 of Speciation spends quite a bit of time discussing and classifying reproductive isolating barriers. Though I have already focused on reproductive isolation in past blogs based on information given in The Ecology of Adaptive Radiation, I will spend some time now giving a more comprehensive overview of possible barriers leading to reproductive isolation. Coyne and Orr define pre- and postmating isolation, as well as the barriers that cause them, as follows:

1. Premating isolation barriers- barriers that impede gene flow before transfer of gametes to members of other species.
a. Behavioral isolation: Members of different species do not initiate courtship or copulation with each other due to a lack of “cross-attraction”.
b. Ecological isolation: Barriers to reproduction are direct byproducts of adaptation to the local environment. This category includes isolation because of differences in habitat preference, timing of breeding, and pollinator interactions.
c. Mechanical isolation: Members of different species do not reproduce because they have incompatible reproductive structures.
d. Mating system isolation: This barrier refers to the evolution of partial or complete self fertilization or the asexual production of offspring, which can result in the formation of new species.

2. Postmating, prezygotic isolation barriers- barriers that act after transfer of gametes but before fertilization takes place.
a. Copulatory behavioral isolation: The behaviors of individuals during copulation do not allow fertilization to occur.
b. Gametic isolation: The transferred gametes cannot effectively cause fertilization.

3. Postzygotic isolation barriers- barriers resulting in hybrid sterility and inviability.
a. Extrinsic barriers: Sterility and inviability are results of the biotic or abiotic environment. This category includes ecological inviability, where hybrids have a lower fitness because of a failure to find an appropriate ecological niche. Behavioral sterility, where hybrids cannot obtain mates, is also included.
b. Intrinsic barriers: Sterility and inviability are results of developmental problems. Hybrid inviability is a result of developmental problems causing full or partial lethality. Hybrid sterility may be physiological or behavioral; physiological sterility leads to problems in the development of gametes or reproductive organs, while behavioral leads to developmental problems causing hybrids to be incapable of successful courtship.

Which of these potential barriers could be acting currently among populations of lizards in white sands and dark soils? Of the premating isolation category, behavioral and ecological isolation both seem possible. In fact, it seems like a combination of these two types would be most likely. As discussed previously, behavioral isolation could be a result of either a preference for local mates that are phenotypically similar, or a lack of recognition of mates from populations adapted to living in different environments. Ecological isolation is certainly important in terms of habitat preference; if white lizards and dark lizards don’t encounter each other because they live in different areas, this will result in reproductive isolation. I think ecological isolation would be important over most of the habitat, while behavioral isolation may be important at the ecotone.

In terms of postzygotic isolation, extrinsic barriers seem most relevant to the white sands system. Depending on the genetic basis of dorsal coloration (which is different among different lizard species in white sands) hybrids could suffer from ecological inviability. The mutation that causes blanched coloration is dominant in one white sands species and recessive in another. Either way, if a hybrid finds itself not well matched to its substrate and can’t effectively choose a better niche, it won’t do as well (it will probably end up as food for someone else).

From reading about all these different isolation barriers I have come to the realization that as I continue on with my plans to investigate behavioral premating reproductive isolation in these lizards, I can’t overlook the possibility that other reproductive barriers could be playing an important role as well, and I need to be aware that many factors have likely shaped the system as it is today.