I suppose I should show that I know at least a little about the whole question of genetics, as I claimed that Neil displayed a marked lack of understanding on this branch of biology. Now, I would reiterate I am no expert, but an interested lay-person who remembers some high school and college biology, and has done a bit of reading on it since. I also have an older sister whose Masters and Ph.D. are in genetics, and teaches biology as Lincoln University, in southeastern PA. Any mistakes are, obviously my own.
When Gregor Mendel's research on pea plants was rediscovered early in the last century, it became clear over time there was a disjunction between his research on breeding and Darwin's account of evolution by natural selection. After the Second World War, following up research that had been conducted during the 1930's, there emerged what became known as "the synthesis" - a new understanding of the relationship between evolution and genetics. This understanding resulted, in one instance, in the science of population genetics, which embodies the synthesis pretty exclusively. Darwin's theory accounted for change in populations, but not having any inkling of the mechanism. Mendel's gene theory accounted for differences among individuals, but not how those differences occurred within populations. The synthesis simply restated the issue thus - evolution occurs both on the individual level and the level of populations. The way to measure this is through a statistical analysis of the rate of genetic mutation and how those mutations breed true.
The vast bulk - well over 99% - of genetic mutations are detrimental to the survival of the individual. The tiny few that survive are inconsequential - hair color, skin color, height, eye color, that kind of thing - and do not result in speciation. A good example from artificial selection is dogs. Chihuahuas and Great Danes are both the same species, and are virtually identical, genetically, with wild wolves. Yet, how many people looking at the first would consider it breedable with the second? There are different species of fish that are genetically diverse yet appear almost identical! So, physiognomy is no clue to genetic relations.
The rate at which DNA errors occur can be mapped and tracked statistically. Given a large enough breeding population, and controlling for other factors - changes in environment, the physical separation of one breeding population from another, the way a given genetic mutation results in an increased chance of survival - one can also see how the process works, over time, and, roughly speaking (within a given statistical margin of error) how long the process takes.
There is not one magical day, some 45,000 years ago when a bunch of homo habilis suddenly produced a homo sapien that was incapable of breeding true with the rest. That's not science, that's just silly (thus Neil's whole "male/female" question). The process takes thousands, sometimes tens of thousands, of generations, which translate in to tens to hundreds of thousands of years. Indeed, Neil mentions the various "explosions" of speciation that have occurred in geological time, the Cambrian explosion being the best known. Yet that "explosion" occurred over a time frame unfathomable to the human mind - several hundred millennia - as the result of increasingly friendly environmental conditions on the planet.
All this is to say the issue of speciation is far more complicated a process than presented in the "questions" Neil poses. It is a gradual process, occurring over thousands of years. I hope, first, I am generally correct in my discription. I also hope, second, I am relatively clear.
