What are the theories accounting for species diversity?

I know this is going to most likely get a spat going here, but that's half the fun sometimes.

I think Darwin's "Origin of Species" is the most breakthrough piece of work that really got this field of study moving. In this, Darwin tries to explain how animals evolved so as to create the stunning range and scope we see on the planet today. It was written in 1859 and goes into great detail his observations of wildlife on the remote Galapagos Islands.

Elizabeth Pennisi is one of the modern day writers on this topic and recently mentioned "The interplay between environment and living organisms and between the organisms themselves play key roles in encouraging or discouraging diversity, as do human disturbances, predator-prey relationships, and other food web connections. But exactly how these and other forces work together to shape diversity is largely a mystery."

The links bellow offer a wonderful range of thoughts, ideas and insights on this topic. Good luck!

Not an official theory ,here smine .Why dont we see humans having different looks like living in different cities in the same country ,it is a different environment .

Creatures adapt faster than we do ,they are more prone to mutations than we are .Adapt in the way of nucleic acid ,we have the ultimate adaptation of adaptations IMAGINATIONS .

For those without ,they take in more of their environment than they can .Species are only the start of change ,we let them live like this longer and the change becomes more apparent .

That depends on what you mean. If you're asking why there is diversity within a species, consider your own family. You resemble your parents but are not an exact copy of either one. You get genes from each parent and the combination determines what you look like. Everybody (except identical twins, triplets, etc) differs. The same holds true for other higher opganisms.
If you are asking about the benefits of having a diversity of species, it's like having several souces of income. If one drops, others are likely to pick up the slack. Similarly, the more complex the community, the more stable it is.