“On the Tendency of Varieties to Depart Indefinitely from the Original Type,” Alfred Russel Wallace’s title for his original 1858 paper proposing the theory of evolution, clarifies what evolutionists are supposed to show — indefinite departure; or microevolution that is observed to continue on to macroevolution. The claim that this is a central feature of the history of life is what we usually mean by evolution.
The Origin of Species explored what animal breeders had shown in Darwin’s day (he was himself a pigeon fancier). The varieties he described were often flamboyant and extravagant, but reflected only micro-evolutionary development and did not show speciation.
I became interested in these issues years ago when I read Norman Macbeth’s Darwin Retried. What follows are some questions and reflections, offered in the hope of sparking further discussion.
In an early chapter, “What Do the Breeders Show?” Macbeth pointed out that the variation achieved by breeding was quite limited. The species of which they were variants stayed the same. Darwin said they were “incipient species,” but he lacked conclusive evidence. By 1859 the theory of evolution was really a theory of extrapolation. Evolutionists assume, of course, that macroevolution has happened. But that has remained an assumption.
In Chapter 1 of the Origin, Darwin brought up another subject — reversion to the mean. He referred to a statement often made by naturalists, “namely that our domestic varieties, when run wild, gradually but certainly revert in character to their aboriginal stocks. Hence it has been argued that no deductions can be drawn from domestic races to species in a state of nature.”
This was unwelcome to Darwin because it seemed to challenge his theory. He countered:

We may safely conclude that very many of the most strongly marked domestic varieties could not possibly live in a wild state. [But] in many cases we do not know what the aboriginal stock was, and so we could not tell whether or not nearly perfect reversion has occurred.

Darwin accepted, nonetheless, that “our varieties certainly do occasionally revert in some of their characters to ancestral forms.”
In Chapter 2 he argued that species can’t really be distinguished from varieties anyway. First he proposed that “well marked varieties” are “incipient species”; then he accepted that “species are only strongly marked and permanent varieties.” Then he decided that there is “no infallible criterion by which to distinguish species and well marked varieties.” By the end of the chapter he is confident that “varieties have the same general characters as species, for they cannot be distinguished from species.”
If so, his breeder friends weren’t just creating varieties, they were creating species. Darwin wanted to believe that he had discovered a process comparable to a journey that could be extended indefinitely; not one that might go only so far, then turn back and retrace its footsteps.
Darwin’s cousin Francis Galton, an early eugenicist and statistician, formulated the law of “regression to the mean.” Tall parents probably have tall children, but probably not as tall as their parents. The very idea that there is a “mean” fits uncomfortably with the idea of progressive evolution. It’s not necessarily a contradiction. But it is a challenge.
Luther Burbank (1849-1926), who worked in Santa Rosa, California in the early decades of the 20th century, was called “the most competent breeder of all time”; he was funded by Andrew Carnegie. His status as a scientist is questioned today because his record keeping was hit or miss. He formulated a law “of the Reversion to the Average.” (The passage below was quoted by Norman Macbeth; it’s hard to get Burbank’s books today):

I am willing to admit that that it is hopeless to try to get a plum the size of a small pea or one as big as a grapefruit. I have daisies on my farms little larger than my fingernail and some that measure six inches across, but I have none as big as a sunflower and never expect to have. I have roses that bloom pretty steadily for six months in the year, but I have none that will bloom twelve, and I will not have. In short there are limits to the development possible, and these limits follow a law. [Norman Macbeth, Darwin Retried, Dell Publishing 1973, p. 36]

Julian Huxley, the grandson of Darwin’s keenest supporter, also found that breeders encounter limits:

In spite of intensive and long continued efforts, breeders have failed to give the world blue roses and black tulips. A bluish purple and a deep bronze in the tulip are the limits reached. True blue and jet black have proved impossible. [J. Huxley, Evolution: the Modern Synthesis, London, Allen and Unwin, 1942, p. 519]

Huxley attributed this limited variability to “lack of modificational plasticity.” Variation can only go so far, in other words. Different species of plants differ greatly in this respect, Huxley added. Some remain “extremely constant” in a wide range of environments. As for animals, “we have less information on the subject.”
In his recent book The Greatest Show on Earth: The Evidence for Evolution, Richard Dawkins observes airily that human beings are “distant cousins of bananas and turnips.” Yet minutely observant plant breeders, “daily and hourly scrutinizing” their productions (to quote Darwin on natural selection), are unable to turn purple roses into blue ones.
It is as though species are surrounded by a “plateau” of limited variability. By artificial selection, varieties can be pushed to the edge of that plateau, but they cannot be pushed off it and into the terrain of adjacent species.
We might go so far as to say that evolution as a theory has been falsified by these observations; and will remain so until we can demonstrate the “indefinite departure” that Wallace heralded.
At Columbia University, Thomas Hunt Morgan used artificial selection to try to increase the number of bristles in fruit flies. But he found that the bristle number reached a maximum and could not be increased further, no matter how much selection pressure was applied.
Lane P. Lester and Raymond G. Bohlin published The Natural Limits of Biological Change in 1984 (Zondervan Publishing). Here is one of the better citations they collect there:

What is the use of their [E. coli] unceasing mutations if they do not change? In sum the mutations of bacteria and viruses are merely hereditary fluctuations around a mean position; a swing to the right, a swing to the left but no final evolutionary effect. [Pierre Grasse, Evolution of Living Organisms, 1977]

I agree with their conclusion, below, although I think the word “suggest” is too weak:

The available data of biology indicates that in contrast to evolutionary theories there is sufficient evidence to suggest that biological change has limits. [p. 149]

The natural limits question is empirical and testable. We may even be looking at a biological “law.” If that is right, it is a law that is profoundly at odds with neo-Darwinism. That may be why it is not discussed.
Richard Lenski’s experiments at Michigan State University are of interest. Michael Behe has discussed Lenski’s work here at ENV and elsewhere in various articles. Lenski has raised E. coli bacteria in jars for 55,000 generations, or 22 years. The bacteria are subject to selection pressure from each other. News from his experiments is always likely to be reported in the New York Times, which is watching closely and perhaps realizes what’s at stake. Nonetheless, Lenski’s E. coli have to date remained E. coli; no new species have been reported. Meanwhile mutations have subjected the bacteria to loss of function.
In earlier experiments, as Lester and Bohlin reported, “Never has there arisen in a colony of bacteria a bacterium with a primitive nucleus.” Nor has a bacterium in a colony “been observed to make a simple multicellular formation” (p. 88).
On his website, Richard Lenski says this:

Speciation occurs when a population changes sufficiently over time that it becomes convenient to refer to the early and late forms by different names. Speciation also occurs when one population splits into two distinct forms that can no longer interbreed. Reproductive isolation does not generally happen in one generation; it may require many thousands of generations; when, for example, one part of a population becomes geographically separated from the rest and adapts to a new environment. Given time, it is inevitable that two populations that live apart will diverge by mutation, drift, and selection until eventually their genes are no longer compatible for successful reproduction.

Omitted from the above are details of the claims of “allopatric” or “sympatric” speciation. The isolated groups are then “probably going to be reshaped, over time, by the selective environments of their new habitats, leading to enough divergence that speciation occurs.” That’s according to the latest evolutionist scenario.
I believe these claims of speciation by isolation — also made by Ernst Mayr decades earlier — are all suspect, as Jonathan Wells has reported (see also Casey Luskin’s report, “Specious Speciation“). Speciation has obviously been frequent in the past, but the question is whether it happened by Darwinian methods. Almost certainly it did not.