Evolution: Counter-argument for hybrids being infertile?

What is a good counter argument for when someone says that when two different species breed they produce offspring that is infertile therefore one species could not evolve into a different species? Like say one species evolves into a new species through a mutation, the new creature then can not mate with anything else to produce offspring since it is the only one of its kind. What is the appropriate response?

Posted: October 20th 2011

George Locke

This is a classic straw-man argument, many times debunked.

A person who believes that the argument described above is a real challenge to evolutionary science is terribly ignorant of basic biology. Either this person has no idea what he’s talking about, or he has trained himself enough in creationist apologetics to blind himself to the nonsense he’s spouting. I’d hesitate to spend my time debating such a person unless you have reason to suspect that their ignorance comes from lack of exposure rather than self-deceit.

The idea that two deer might give birth to an antelope that can never find a mating partner is a gross misunderstanding of the way evolution occurs.

Sexual species share genes with one another when they breed, causing the gene pool of a given population to mix. When that population splits in two for whatever reason, the two child groups cease to share genes with one another. While the two child populations are separated, their physiology, reproductive patterns, etc., are likely to change through genetic drift. If the environments of the two populations are very different, then natural selection will encourage the genetic changes driving the populations apart.

Whether the source of genetic changes is due to differing selection pressures or merely random drift, mutations within a population spread throughout that population through breeding. If the populations remain apart long enough, the accumulated mutations will alter reproductive processes enough to prevent members of either group from mating with one another. That is speciation.

Wikipedia explains this all very well. There’s no excuse for this kind of ignorance when it’s so easy to educate oneself on the basic science.

Posted: October 25th 2011

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Blaise www

From a debate perspective, a good counter argument would be to point out that the statement betrays a woeful misunderstanding of how evolution works. If pressed for details, see SmartLX’s post.

Posted: October 24th 2011

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Galen Rose www

SmartLX has provided an excellent answer, and I would merely add the concept of “ring species” to help you answer your question.

There are 2 particular species of gull in England which are related but cannot breed. It has been found that one of those species can breed with similar gulls found in northern Scandinavia. The Scandinavian birds can in turn breed with similar gulls found in northern Russia. The northern Russian birds can in turn breed with similar gulls found in northern Alaska. The Alaskan gulls can breed with similar gulls found in Northeastern Canada. The Canadian gulls can breed with those English gulls which CANNOT breed with the other English gulls. Thus, as one travels around the globe, one finds very closely related gulls which are not categorized as different species. Yet, the two ends of this loop cannot breed and are indeed different species. The point here is that species categorizations are in part subjective because there will be many steps of evolution between two species. Thus, it is in the end, in part, a subjective call at which point to say a new species has arrived.

Ring species have also been found ringing large valleys or bodies of water, although in these cases they tend to be very small animals or plants which are unlikely to be able to travel around the complete ring.

Posted: October 24th 2011

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SmartLX www

Species develop as groups, not individuals. They change very slowly, generation to generation, so that the offspring in a group can always mate with their nearby cousins. (That sounds icky, but we’re all cousins umpteen times removed.) By the time the group has changed enough that its members could not have bred with the original members, the original members are long dead and the current animals are all somewhat alike.

Closely related animals only lose the ability to interbreed properly when two groups have been isolated from each other for many generations. The two gene pools have then had time to grow apart and accumulate enough unique mutations to cause problems. Depending on how long the segregation has lasted before the groups encounter each other again, the offspring may be sterile, conception of any sort may be impossible or the groups may not even try to mate.

The point is that infertile hybrids are the result of a special case which in no way represents the normal process of speciation, and in fact can only happen after a speciation event as a side-effect.

Besides, animal breeders knew about sterile hybrids like mules centuries before Darwin. If such a common occurrence contradicted his theory, it would never have got off the ground.

Posted: October 24th 2011

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