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4. Other practical problems.

a. Circular species.

3. There are several cases where animals are confined by topography to a circular environment with a gap in it. As we go around the Arctic Circle each local tribe of seagulls breeds with the tribe on either side of it, but when we get to the Bering Strait the tribes on either side will not breed with each other. Presumably this is a cultural matter rather than a question of genetics. The call of the gulls is much the same all round the loop, but as you approach the gap from one side the pitch goes up, while from the other side it goes down.

b. Asexual reproduction.

4. A number of plants are self fertilising, and some plants and animals reproduce asexually, so that there is no mixing of the genes between individuals, and inevitably they accumulate genetic mutations with time. However because no individual can breed with any other individual it is, by definition, a unique species. In Florida there is a group of lilies growing by the innumerable ponds in the Everglades which reproduce asexually, and virtually every little pond has its own unique race.

c. Multiphase species.

It is much easier to study the breeding habits of animals than it is of plants, and there are several well-known cases where a single species can come in several spectacularly different forms. For example the Gouldian Finch has three colour forms, with bright red, black, or, rarely, yellow faces. These are popular cage birds, and their breeding has been studied extensively. While there are variations in fertility between the various forms, any male will breed with any female, and the face colours of the offspring will always be one of the three possibilities. *

d. What is a genus?

Linnaeus split families into genera, and these into species. Unfortunately there is no scientific definition of just what constitutes a genus, and taxonomists create genera and shuffle species around between them with gay abandon, but often with very little logical justification. As the only real justification for having genera is to group things which are fairly similar, once a reasonably satisfactory grouping has been made it should generally be left well alone, as the confusion caused by changing things will far outweigh any theoretical advantages from a slightly more logical grouping.

e. Does it matter?

If two groups are consistently different in a number of significant features, and can each be shown to breed true, then this is probably justifiable to split them. But if there is a gradation between the two groups, or it can be shown that they readily interbreed then it is very difficult to justify splitting them.

* http://www.abc.net.au/science/articles/2009/03/20/2521475.htm?site=science
 © Roger Riordan 2004-2017