Relate the idea of reproductive isolation to the biological species concept.?

As an analogy, imagine a population of people who spoke the same language. At a certain point, the population divided into 2 and went their separate ways (each were isolated). Population A moved to an area where they traded with a lot of french traders, and thus, their own language gradually took on some french influence and changed in that direction. Population B moved to an area where they interacted with a lot of english traders, thus their language gradually adopted english influences. Both populations started off with the exact same language, but when they became separated, they each took that same language and changed it into something completely different. So 500 years later, when you bring these 2 populations back together, they realize that they no longer understand each other at all since their language has branched off and evolved into 2 different languages, despite the fact that they started with the same language. At this point, is when 2 ';species'; have arose from a common ancesteral language.





Same with organisms. When you divide up a population, each population will live in different environments, and thus will adapt differently to their own environments. One may select for brown fur because the environment is a wooded area and brown fur may blend in better. The other may select for white fur since the environment is snowy and white would blend in better there. It's these small differences that gradually add up, until each population is so genetically different from each other, that if you were to bring them back together, they would no longer ';understand'; each other, and you thus have speciation. In this case, it means that they are so genetically different now because they've adapted differently to their own respective environments, that they can no longer reproduce with each other.





Note, that the criterion of reproduction is a loose one to define speciation, since many many organisms don't reproduce sexually, and even those that do may have different degrees of success. Some may have sterile children (mule/liger), some may have sterile grandchildren. Some may be able to conceive, but the fetus dies in the womb.