Thus went the verbal jostling, with the mood swinging perceptibly in favor of recognizing stasis as being a real phenomenon. Gabriel Dover, a geneticist from Cambridge University, England, felt strongly enough to call species stasis "The single most important feature of macroevolution." In a generous admission Francisco Ayala, a major figure in propounding the Modern Synthesis in the United States, said: "We would not have predicted stasis from population genetics, but I am now convinced from what the paleontologists say that small changes do not accumulate."
This quote from Ayala is one that creationists like to use. They claim that it shows that Ayala was denying the fundamentals of evolution - that species change over time due to changes in the genetics of their populations.
In fact, Ayala is saying that population genetics predicts continual and gradual change in genomes. But here he appears to be accepting that the fossil record does not show definite trends in species towards speciation. Instead, he accepts that (at the time, 30 years ago), the fossil record tends to show that many species undergo only minor changes over long periods of time.
Why should this be? Well, that depends on the conditions necessary to spur a species to split into two species.
It is important that Ayala has explicitly said that he does
not believe that "small changes do not accumulate" when it comes to genetics. To believe that would be to deny evolution, something Ayala has never done.
The emerging picture of evolutionary change, therefore, is one of periods during which individual species remain virtually unchanged, punctuated by abrupt events at which a descendant species arises from the original stock. ....
The issue of punctuated equilibrium highlighted the particular problems of drawing together such a disparate array of scientific disciplines. The main point of entanglement was one of scale. Ecologists spend much of their lives being impressed by the subtle ways in which populations of organisms can adapt morphologically or physiologically to local conditions. And the population geneticists' view of the world through a Drosophila bottle teaches them how readily fruit flies (and presumably other organisms) can be modified by changes in selection pressure. So how can paleontologists suggest that species remain the same through most of their existence? And who in their right mind would contemplate speciation occurring in an instant? The resolution of this apparent conflict is this. Species do indeed have a capacity to undergo minor modifications in their physical and other characteristics, but this is limited and with a longer perspective it is reflected in an oscillation about a mean: to a paleontologist looking at the fossil record, this shows up as stasis.
The troubling specter of "instant" speciation is again a product of misunderstandings over scale. What is an instant to a paleontologist is an unimaginable tract of time to either an ecologist or a population geneticist. "I'd be happy to see speciation taking place over, say, 50,000 years," said Gould, "but that is an instant compared with the 5 or 10 million years that most species exist."
Read the above carefully. It is the resolution of the argument over whether speciation happens rapidly or slowly. In short, "rapid" means something rather different to a paleontologist than it does to a population geneticist or an ecologist. And so does "stasis".
However, even the most ardent punctuationists do not dismiss gradual change as a force in evolution.
"We are not saying that population genetics is irrelevant," said Eldredge, countering accusations of monotheism; "The question is over what process is most important in bringing about the major changes we see in evolution. And the answer is punctuated equilibrium." Gould also sees gradual change as an important influence in evolutionary history: "The point is one of the relative frequency of one process as against the other," he explained with deliberate emphasis, betraying some frustration at having been repeatedly misunderstood on this particular issue.
Above are statements from the two "radicals" at the conference. Notice that they are
supporting the fundamentals of evolution. They accept that evolution occurs. They accept that "small changes accumulate" and are a force in evolution. But they say that rapid speciation can and does occur when the environment changes rapidly. That is what punctuated equilibrium is about.
If it is true that most evolutionary change follows the model of punctuated equilibrium, then there is the immediate problem of how to explain morphological trends that are frequently seen in the fossil record. A classic example of such a trend is the evolution of the modern horse, whose distant ancestor Hydracotherium was a three-toed creature no bigger than a dog. The fossil record shows an apparently steady "progress" through time, with gradual changes in body size and form leading eventually to the familiar Equus. Classical gradualism would explain such a trend in terms of a progressive expression of the forces of natural selection within a single lineage: a continuous evolutionary ladder would connect the ancestor Hydracotherium with the modern animal, Equus.
By contrast, punctuated equilibrium would explain the morphological trends in horse evolution (and other such trends) as the result of a differentially pruned bush rather than a directed ladder. Think of the evolutionary history of the horse sketched out as a multiply speciating lineage, with some new species projecting in the direction of bigger bodies and fewer toes and others displaying smaller bodies and more toes. Now, if the species with the bigger bodies and fewer toes (the more "advanced" features) thrived more successfully than those with "primitive" features, then this would produce an asymmetric bush. The center of gravity of morphological change through time would lean steadily toward the bigger species equipped with the single toe. The same principle would apply to any characteristics in a group of related species in which natural selection favors one form against another. Known as species selection, the theory was first proposed by Eldredge and Gould and later elaborated by Steven Stanley.
The thing to notice about these two apparently-competing explanation is that both of them still involve evolution by natural selection, just at slightly different levels.
If theories on the tempo of evolution are contentious, then the question of mode is certainly no less so. ....
It is now clear that many possibilities of genetic change exist, ranging from simple point mutations, through jumping genes and transposable elements, to major chromosomal rearrangements. It is also clear, mainly from experimental work, that all permutations of genetically determined morphological shifts are possible: small genetic changes can give rise to either minor or major morphological modifications; and the same holds for large genetic changes. The issue, as Maynard Smith stated, is which of these possibilities is most important in speciation events in nature. The data are not yet available to provide an answer to this question.
There has been a lot of progress on this matter in the past 30 years.
Notice that there is no denial of genetic change here.
The basis of speciation theory is that a new species should arise from parental stock, probably delimited as a small isolated population. The notion of small populations is important in the gradualist model too, where speciation is allowed for through the steady accumulation of genetic change within a limited number of organisms that are geographically separated from the stabilizing gene flow of the main population. Eventually the isolated group might attain sufficient genetic distance from the parental stock as to be reproductively isolated: hence the establishment of a new species.
The above is a description of what is known as
allopatric speciation. This is an evolutionary theory. There is no creationism-evolution debate here. The article goes on to discuss other possibilities, but all are evolutionary theories.
The article goes on to talk about constraints imposed by evolutionary history:
The scientific argument over the third major area of discussion - that of constraints on evolutionary expression - was edged with tinges of sociological conflict too. At their most extreme, the two opposing technical positions are these. According to the Modern Synthesis, species look the way they do as a consequence of utilitarian adaptation to their environments. This theory also implies that organisms of all sizes, shapes, and forms are possible, and it explains the fact that life is actually restricted to a few very limited basic patterns by saying that there exists only a limited variety of ecological opportunities. Why does there not exist a species of cow with a head at either end of its body? Because, according to this line of argument, no adaptational niche is available for such a creature.
The opposing view is that adaptation, though important, is a secondary factor in shaping species morphology. There are, it argues, fundamental constraints in morphological possibilities imposed by mechanical properties of the building materials, basic forms embodied in the building blueprint that underlie many related species, and conservative rules that govern embryological development. In other words, organisms of all sizes, shapes, and forms are not possible.
Why do most land vertebrates have four legs? The seemingly obvious answer is that this arrangement is the optimal design. This response would ignore, however, the fact that the fish that were ancestral to terrestrial animals also had four limbs, or fins.