Gene flow is a conservative factor that prevents the divergence of only partially isolated populations and is a major reason for the stability of widespread species and for the stasis of populous species.



...



In a small population alleles may be lost simply through errors of sampling (stochastic processes); this is known as genetic drift. Indeed, such a random loss of alleles may occur even in rather large populations. This is usually of no consequence in widespread species, because such locally lost genes will be quickly replaced by gene flow in subsequent generations. However, small founder populations, beyond the periphery of the range of a species, may have a rather unbalanced sampling of the gene pool of the parental population. This may facilitate a restructuring of the genotype of such populations.



(Mayr, Ernst. 2001. What Evolution Is . New York: Basic Books. p 98, 99).

With drastic selection taking place in every generation, it is legitimate to ask why evolution is normally so slow. The major reason is that owing to the hundreds or thousands of generations that have undergone preceding selection, a natural population will be close to the optimal genotype. The selection to which such a population has been exposed is normalizing or stabilizing selection. This selection eliminates all of those individuals of a population who deviate from the optimal phenotype. Such culling drastically reduces the variance in every generation. And unless there has been a major change in the environment, the optimal phenotype is most likely that of the immediately preceding generations. All the mutations of which this genotype is capable and that could lead to an improvement of this standard phenotype have already been incorporated in previous generations. Other mutations are apt to lead to a deterioration and these will be eliminated by normalizing selection.



(ibid, 135)

Selection is not teleological (goal-directed). Indeed, how could an elimination process be teleological?



(ibid, 121)

I was recently asked a few questions about evolutionary pressures in Darwinism. Here’s the majority of my response (certain personal parts have been removed from the original, and some of the grammar has been edited):When it comes to evolutionary pressures, it is important to keep in mind that even Darwinists must acknowledge the limitations of such pressures. While you listed the example of a giraffe’s neck or a cheetah’s speed, even those run into the problem of “islands” of fitness. While we think of evolution as progressing to better and better organisms, the fact is that this sort of concept is highly anthropomorphic because even secularists want humans to occupy the “highest” niche. For ease of understanding, however, I believe it’s better to reverse the direction.Suppose that you have a ball on the top of a steep hill. If you release the ball, it will roll down the hill because of the force of gravity. For this illustration, let’s assume that the ball is a species’ genetic characteristics, gravity is evolutionary pressures (such as the environment, competition, and anything else that would result in survival of the fittest), and the farther down the hill you progress the more highly suited the species is for the environment.Now, if the hill was perfectly smooth, the ball would roll all the way to the base and you’d have the perfect organisms being produced at sea level. But that’s not a realistic perspective. I live in Colorado, and in my backyard stands Pikes Peak. Suppose we were to start with a ball on the top of Pikes Peak and we were going to try to roll the ball to the ocean using nothing but gravity. Would that work?Obviously not. For while the ball could roll quite some distance from the top of the Peak, eventually it will encounter a section that isin all directions before it gets to the ocean. In fact, there are countless uphill sections from Pikes Peak to the sea, no matter which way you try to roll the ball. If you’re lucky, you might be able to get the ball to roll from the summit (at just over 14,000 feet elevation) down to maybe 10,000 feet, although in most cases you probably wouldn’t get it to roll more than a few dozen feet before it would be stopped by some rocks creating a local valley.While the actual distance isn’t really that important, the point is that eventually the ballsettle into a localized valley where every direction around the ball is uphill. Let’s stipulate the ball we drop runs into this problem at 10,000 feet elevation. Now obviously, 10,000 feet is long way above sea level, where the ball “wants” to go. If it had the means, it would roll down to sea level—but locally, it is trapped. Every direction is uphill. It cannot go lower using gravity alone, because it is stuck in its valley.Evolution runs into the same problems. There are valleys that a species will fall into, where they could develop into an even more “advanced” species, but they are so adapted to their niche that any movement the species makes isnatural selection. That is, once in the valley, natural selection will keep the organism in the valley, and ultimately natural selection itself will selectfurther evolution.Ernst Mayr pointed this out when talking about gene flow and genetic drift:Notice, therefore, that in the vast majority of populations, natural selection will seek to keep organisms. In fact, Mayr says:So, on the one hand, natural selection is supposed to be the driving force behind evolution, but on the other hand, natural selection is the very thing keeping organisms from changing. Indeed, as far as the second method goes, I firmly accept the validity of natural selection! We can actually observe that process in work. Natural selectionvariety; it does not create it. Indeed, Mayr acknowledges this, calling selection “an elimination process”:So, whether one accepts Darwinism or Creationism, it only makes sense that organisms right now are as close to their “peak” (or, given our illustration, as close to sea level) as possible. Under Creationism, it’s because they were created that way; under Darwinism, it’s because they’ve had millions of years to evolve to their environment. In both instances, natural selection now seeks toorganisms the way they are, because they are already as adapted as they can be to their environment. The only way to change this is to change the environment in some radical way.But notice that most of the radical alterations of an environment do not grant us any new species, but rather only makes certain other species go extinct. For instance, the dodo bird was easy prey when Westerners arrived with their pets. The dodo didn’t evolve—it died out. On the other hand, when rabbits were introduced into Australia, their population exploded. In the process, they’ve driven several native species to the brink of extinction too. But notice that neither the rabbit themselves, nor the organisms that are dying out, have significantly altered their genotype in the face of this extremely different environment. The changes are too abrupt, and even granting Darwinism every advantage, there simply isn’t enough time for the species to garner sufficient mutations to avoid doom. The only way they could would be to have the environmentchange, giving each species time to adapt to thechanging environment. Abrupt changes only result in mass extinction.As far as your argument from incredulity, that it’s unlikely that “mistakes” would result in these types of happy occurrences, I believe that is a fairly strong argument, actually. First of all, lifelike it is designed. This is the strength of Paley’s watch argument. The default assumptions any reasonable person would make, when looking at life, would be to conclude that it is designed. As a result, the impetus really is on the Darwinist tothat this sort of thing can happen without teleology. When something appears to be designed, the default is to assume that it is designed until it is proven otherwise, so the Darwinism does have the burden of proof here.Furthermore, the Darwinist himself cannot escape teleological terminology. In fact, Darwinism iswith teleological terms. The very fact that they say, “Naturethe fittest organism” is itself a teleological claim. First of all, there is the claim that nature is actually doing a selection, and selection implies a choice. Secondly, what is selected is the “fittest” and “fittest” implies that the organism has a specific role to play, and it plays that role “the best.” So, “natural selection” itself is a term loaded with teleology—it is claiming that nature is choosing the best organism to play a specific role. But that simplyan affirmation of design. Darwinists try to get around this by claiming they are just using language metaphorically, but I have yet to findwho can explain Darwinism without resorting to teleological statements. And I daresay that if you cannot describe a process without reference to design, then you’re better just admitting that what you’re looking atdesign.I mean, put it this way. There are certain crystals that have properties that make them take on very cool geometrical shapes and patterns. We know that these processes occur naturally without any apparent design in nature, and we can mimic this in a lab and create these crystal shapes ourselves, etc. A physicist may use teleological language at some point to talk about it—say, “A salt crystal wants to be cubic.” But the same physicist can also describe the chemical bonds and how the sodium and chlorine react to create this structure without using any teleological language at all, if the physicist so chooses.But I have yet to read a biologist who can explain Darwinism in that way. Now, some may very well claim that trying to explain an entire ecosystem is vastly more complicated than trying to explain the interactions of two elements forming a salt crystal, so the biologist is forced to use teleological metaphors. But given that biologyto deny teleology, the impetus really is on them to stop using teleological language.So, as I said, I see nothing wrong with you arguing from incredulity. You are perfectly justified in being incredulous. The Darwinist has told you a counter-intuitive story that he supports with language that he specifically says is disallowed. Whyyou be incredulous?More to the point, there are many examples that seem to affirm Behe’s irreducibly complexity argument. It is incumbent upon the Darwinist to explain how those types of systems can arise. The Darwinist likes to just assume Darwinism is true, and then fit all observation into that theory. But that is begging the question. To be scientific, he must derive his theory from the observations, not force the observations to fit his theory.