[nismo1 (OP)]

I watched a doco last night about evolution called Mutant Planet Africa.  It focused onThe Grift Rift Vally where it was highlighted that the cichlids fish split into 500 species in a relatively short period (12,500 year old lake). 

They described how this speedy evolution may have occurred. e.g  where some female fish may have preferred a male fish that built sloping mounds on the lake floor and avoided other males that built flat mounds.  The female then breeds with the sloping mounded males.  In turn, the female's offspring is most likely to prefer sloping mounded males, and may never breed with the flat mounded males.  The narrator explains that this situation can start a new cichlids species.

Now to my question.  It seems to me that although the female fish has preference for sloping mounds, I don't see how that can lead to speciation.  My logic is that even though she chooses/prefers sloping mounded males, she is still biologically able to breed with the flat mounded males and have offspring.  When does this "choice" cross the line into creating a new species that is not able to breed with the old species?

Sorry for the rambling, and if i'm not making any sense! Thanks,

[viola123]

Good one dude..keep doing nice work  newbielink:http://www.thenakedscientists.com [nonactive]

Spam you very much.

[Nizzle]

When does this "choice" cross the line into creating a new species that is not able to breed with the old species?

Hmm, interesting question there.
Here's what I think: In order to drive this speciation, there must be an advantage to this female's choice. One that comes to mind is that flat mound and sloping mound building fish (and the females that choose their corresponding 'best match') also show different behavior that is specifically useful for constructing or defending their mounds.
Therefore, in the beginning of differentiation, when there's not yet a speciation, mixed couples will produce mixed offspring, but since they don't have a mixed mound, only the offspring that inherits the correct behaviour according to the mound they're used to will survive in the long run.
This creates an ever bigger split between flat mounding cichlids and sloped mounding cichlids, which leads to an ever bigger genetic difference up until the point where the mixed couples are unable to breed at all and speciation will occur.

EDIT: The fact that this evolution occurs so fast is because fish generally tend to have large numbers of offspring, which implies bigger competition for space amongst siblings, which in turn stresses the importance of 'inheriting the right behaviour'

[nismo1 (OP)]

Thanks Nizzle for your insightful response. Nature and evolution fascinates me to no end :-)