How a species change can happen though gradual adaptation. My understanding of what constitutes a species revolves around the ability to procreate. Is there a point where offspring cannot procreate with previous generations?
Yes you're correct that species are generally made distinct in terms of procreation. How evolution has created species is by gradual change over long periods of time. In retrospect we think of changes as big jumps but evolution does this gradually so that every generation can procreate with the previous one and the subsequent one at least. Jump 1000 generations and the first animal in the evolutionary history will not be able to mate with the 1000th generation animal even though they have a genetic history but because each animal only differs slightly from the previous generation and the subsequent one it creates an evolutionary chain link.
I think it was in one of Dawkins books he mentioned how this has happened in our world today. There are sea birds on one side of the atlantic (I think it was atlantic) who have over time journeyed across via various islands leaving behind breeding colonies every time to the other side of the atlantic. Over time they have become slightly disimilar from the other birds, the most disimilar being those on either side of the atlantic who can't mate with each other. Going along the chain of birds on their islands however reveals that each flock on neighbouring islands can mate with each other even though those far apart geopraphically can't.
Dawkins mentions that mutations happen on the order of one per thousands of generations. This implies that things with shorter life spans would adapt faster and vice versa. Has the Earth been around long enough for humans to adapt and mutate, one slow adaptation at a time?
Usually when dawkins is talking about generations he's talking about genes and not organisms. When calculating mutation for organisms you have to factor in the size of the genome, so humans have a massive genome susceptable to mutation. Mutation rates can differ from animal to animal due to environmental variables so I'd assume one per thousand is a mean figure to illustrate levels of mutation for all genes.
I hope I've explained that correctly.
Any biology grads can pick me up on what might be wrong.