"We have never observed evolution. We have only observed microevolution (variations within a species), not macroevolution (appearance of a new species). No one has ever observed a speciation event."
Rebuttal
Apart from the fact that it's an outright lie by virtue of speciations being artificially induced in the laboratory, this argument suffers from numerous weaknesses:
The first (and most obvious) problem stems from its own terminology: the only difference between "micro" and "macro" is one of scale, not concept. It is a straightforward deduction to say that if small genetic changes are possible, then a sufficiently large number of such changes would add up to a large change. To say that one does not demonstrate the possibility of the other is utterly specious.
The fact that macroevolution has not been directly observed is rather misleading. Since macroevolution must involve large numbers of successive small changes, it must by definition require a very long time. Triumphantly pointing out that we have not observed it after a short time is disingenuous to say the least.
No defined mechanism has ever been produced by creationists to explain why the process of evolution would work but then abruptly stop at some imaginary barrier, in order to make microevolution possible but not macroevolution. The closest they've ever come is their belief that DNA already contains all of the "information" it needs to produce all of the variation within a species, and no new "information" can be added to cause further changes. However, this conjecture requires that DNA is static, which is known to be false: DNA changes occur in every generation.
This argument hinges upon the definition of "species" as opposed to "varieties". But if you were to read "The Origin of Species", you would see that Darwin anticipated this objection more than a century ago, and pointed out (quite accurately) that there is no solid criterion for determining whether two groups of animals are different varieties or different species. Creationists say that the definition is strictly based on the ability to interbreed, but how does that explain why wolves and dogs are classified as different species? How does it explain asexual organisms such as bacteria or many types of plant? The harsh reality is that it is rather difficult to determine whether two groups of organisms are different species as opposed to varieties, because the divisions are not as sharp and distinct as creationists would have you believe.
What would a "speciation event" look like? Are they expecting some kind of miraculous transformation? A speciation, according to the predictions of evolution theory (as opposed to creationist strawman versions thereof), would be a gradual differentiation of two isolated animal populations over time until they were sufficiently dissimilar to be considered two different species. There is no sudden and discrete "event", unless you measure "events" in geologic timescales. So pointing out that we have never seen one of these events suddenly occur in front of us is extremely deceptive; the "event" in question is defined in such a manner that it would inevitably be identified only by monitoring divergent animal populations and characteristics over a period of time and then declaring at some point that they are different enough to be considered separate species (or determining that they have become intersterile: something which has occurred many times despite creationist denials).
It is noteworthy that even if we adopt the creationists' strict "intersterility" definition of species (which produces the ludicrous implication that every individual bacteria is its own species), there have still been numerous speciation events, as alluded to at the beginning of this article. For example:
Primula kewensis was speciated from Primula verticillata and Primula floribunda in 1912 by Digby via hybridization and polyploidization.
It was shown that you could reproduce the existing species Tragopogon mirus by hybridizing Tragopogon dubius and Tragopogon porrifolius, as demonstrated by Owenby in 1950.
In 1969, Pasterniani demonstrated speciation (as defined by reproductive isolation) via artificial selection rather than hybridization. He took two existing varieties of maize, planted them in a field, and over a 5 year period, selected only kernels which were not interbred for the next year's planting. At the end of this 5 year period, the plants' natural likelihood of interbreeding had been reduced by an order of magnitude.
In 1983, Macnair and Christie were able to show that varieties of the Mimululs guttatus flower which had developed a tolerance to copper were no longer able to breed with varieties which had not developed this tolerance.
A much longer list is available at TalkOrigins.
Fallacy watch:
Argument from Ignorance (assumes that if a specific and very narrowly-defined form of proof has not been offered for A, then A must be false).
"Moving the goalposts" (technically not a fallacy, but rather, a very dishonest debate tactic in which you simply reject every attempt to satisfy a demand for evidence by saying it doesn't meet your unstated criteria).
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