The granddaddy theory is Natural Selection – circumstances at any given time change the odds of particular "favored" individuals surviving, reproducing, and passing traits suited to those circumstances on. Changes in circumstances spur changes in species, as the "average individual" changes over time. This explains why species that have <b>not</b> changed much are associated with environments that are very stable over time.

There are many genetically-oriented processes that are equally important – not just mutations that shift protein functions, but crossing-over and genome duplications that provide extra copies of "old" genes that can be shifted to new functions without losing the original; lateral transfer of genes from one individual to another, even crossing species lines. A lot of current work is being focused not on the genes themselves, but on the control mechanisms for genes – humans and chimps have essentially the same genes, but apparently express them to different degrees to get the differences between them.

Some newer work focuses on changes being pushed by shifts in relationships between organisms, especially of a cooperative nature rather than competition.

I’m not an evolutionary biologist, but I have had to attempt to explain it to others on occasion. I see it as an interaction of two “forces.”

1)Organisms reproducing with heritable variation (generation)
2)Many more organisms being born than will survive to reproduce (natural selection)

What you seem to be specifically interested in Nelsson is the process of speciation. This seems to be a rather contentious issue among biologists and they disagree over the hows and whys of it. But I suggest that you do a few searches in the literature on speciation to get an overview of the various models.