They’ve changed a great deal already over a short span of a few thousand years due to domestication. Look at what has happened with canines in just a few thousand years (hundreds, in some cases), though it has not been long enough for reproductive isolation.
And we’re not talking about this cow or that cow changing. We’re simply taking about allele drift and selection pressures on living populations, not individuals, working over millions of years. Speciation has no one cut off point. Populations will diverge and converge again. But eventually with enough separation you’ll have two groups isolated from reproducing by any number of factors, including geographical and behavioral, and perhaps eventually due to genetic incompatibility.
It’s not like any one offspring pops out as an “evolved” form of its parents and a new species. The traits present within a given population change over time, while the given population at any one moment will look very similar among itself, but perhaps different than the ancestral population it diverged from hundreds of thousands (if not millions) of years ago.
Nor are things actively evolving. It’s simply selective pressures. If selective pressures remain constant, a species might very well remain the same for millions of years. And typically the entire population does not undergo trait drift together. It’s more common for a group of a population to become isolated in some way and then experience different selective pressures over hundreds of thousands or millions of years than other parts of the ancestral whole population, which may lead to a divergence as different traits are more advantageous to the new situation resulting in increased dispersion of certain traits due to it being a reproductive boon. Or perhaps a non-advantageous trait becomes advantageous in different circumstances, or a neutral trait which had very small minority representation only in one small subpopulation becomes a common trait for that subpopulation after it becomes isolated from the main population, has a more limited reproductive pool, and has trait divergences in other areas.
This is only the how of part of our history. Certainly in the divine intellect all of this and the results are known and intended, and God remains the ontological cause. And neither do we need to say that simple allele drift over millions of years can account for the rational soul of man.
