Human Evolution - Speciation

I don't have a lot of time for reply at the moment. However, I'd like to take a few sentences to clarify what a mutation is and what can effect/cause them.Mutuation is simply a change in the DNA of a cell. It can be caused by simple error (the DNA replication process is fairly sloppy, and the cellular repair mechanisms don't always work). In addition, environmental mutagens (certain chemicals, ionizing radiation, etc) can increase the "normal" error rate causing more mutations than can be attributed to copying mistakes. Finally, certain virii, can insert copies of themselves (or at least bits) into the genome of an organism and ride down the generations. Some of the retroviral insertions can cause mutation. There are some other factors, and other not-strictly-mutation things that can change the genome or development process, but the ones I listed are the most common of what are usually called mutation.Two things to take away:1. Mutations do not occur in response to environmental stimuli - except in the case of an increase in environmental mutagens. Critters don't change to suit the environment. When an environment changes, either the population already contains variants which can survive in the new condition, or the population goes extinct.2. Individual organisms don't adapt. Populations adapt over generations (or they don't and disappear). Mutations occurring in individual organisms have no effect on the population as a whole unless the mutation occurs in a germline cell AND the mutation isn't deleterious to the point that it renders the organism unable to reproduce.

Inre positive mutations: the way you describe it isn't strictly correct. In the first place, the determination of what constitutes "positive" or "negative" mutation is heavily dependent on the environment in which the organism finds itself. What might be a beneficial mutation in one environment may be highly deleterious in another context. Secondly, as I pointed out in my previous response, the environment doesn't cause mutation per se, except in the case where mutagens in the environment may effect the rate of mutation. Variants continuously arise in any population. Natural selection acts on those variants, sort of like a filter. If the variant has a net advantage in the given environment, eventually (all other things being equal and disregarding drift, etc.) ultimately the frequency of that particular variant is likely to increase in the population. By the same token, a disadvantagous variant is likely to either die out or decrease (again, all other things being equal) in frequency, simply because it is less able to compete effectively with the other members of the population.This is a fairly simplistic explanation, and glosses over quite a bit, but should give you a feel for the subject. Basically, a "positive" mutation gives its posessor some type of advantage. Since the variant that contains this positive mutation (in the particular environmental context) is likely to pass on these positive factors to its progeny AND thus increase the frequency of that variant in the population while natural selection weeds out those with a deleterious mutation, we see the net effect as beneficial. Hope this helps.