Re: If cross breeding is not geneticaly possible why animals show this tendency

Hi Joseph,

You pose an interesting question and I won't pretend to know the answer, but consider this...

Since most males invest few, if any, resources in their offspring after mating, a male animal of species A will maximise the number of offspring he produces (and therefore his evolutionary fitness, and that of the genes which influence his mating behaviour) by mating with as many females of species A as possible. In order to do this he must identify females of species A to mate with. If females of species A are totally different in appearance to females of any other species then this doesn't present a problem. However, if alongside species A there is another species (B) which has females which look similar to those of species A, it may be in the interest of males of species A to mate with any female which resemble those of species A, whether or not they actually belong to species A. In this case he will occasionally mate with a female of species B.

If this occurs there are a number of things that could happen. [1] The sperm fail to fertilise an egg, in which case little harm has been done to either the male or the female. [2] The sperm fertilise an egg but the embryo does not develop beyond a particular stage and is aborted. This will incur a slight cost to the female since she has invested some resources in this embryo which are not available to offspring fathered by males of her own species. [3] The embryo develops to adulthood but is infertile (as in the case of mules - the infertile offspring of horses and donkeys), or has reduced fertility. This will incur a major cost to the female since she will have invested considerable resources in this offspring. In cases [2] and [3] there is a cost to the female but in all cases there is a benefit to the male, therefore there is a conflict of interest between the sexes.

Some fish and salamanders exhibit a form of reproductive parasitism in which a female will mate with a male in order to receive certain chemicals from the sperm which are required to activate her eggs, but the genome from the sperm is not incorporated into the genome of the offspring. This does not require the male and female to be genetically compatible and therefore the male may be, and often is, from a different species to the female. In this system it is the male who is being cheated.

These examples illustrate the importance of conflict between the sexes in evolution and I think that this will be an important element in the answer to your question. Just because a behaviour appears to be harmful to some members of a species does not mean that it will not evolve if it is of benefit to others. There are other types of conflict which are important in evolutionary biology such as the conflict between workers and queens in colonies of social insects or the conflict between parents and offspring. You can read about this in any good evolutionary biology textbook. Of course, that's just my take on things. Ask another biologist and you will get another answer. The things you're asking about are still being actively researched so there is no definitive answer, but I hope what I have had to say goes some way towards answering your question.

For more information about the role of sexual conflict in evolutionary biology check out these references...

Brown, W. D., Crespi, B .J. and Choe, J. 1997. "Sexual conflict and the evolution of mating systems" in Choe, J. C. and Crespi, B. J. (eds.) The Evolution of Mating Systems in Insects and Arachnids, pp. 352-377. Cambridge University Press. (Senior author's web site)

Gavrilets, S. 2000. Rapid evolution of reproductive barriers driven by sexual conflict. Nature 403: 886-889. (Abstract) (Author's web site)

Hosken, D. J., Garner, T. W. J. and Ward, P. I. 2001. Sexual conflict selects for male and female reproductive characters. Current Biology 11: 489-493. (PDF)

...and these web sites...

An interesting article about sexual conflict
The website of Göran Arnqvist, a scientist working on sexual conflict

Regards,
Rob Cruickshank