Field of Science

Blogging the Origin: Chapter II: Variation Under Nature

As you may recall in Darwin's long argument, the point of the previous chapter was establish that variation exists within well understood domesticated plants and animals. In this chapter, Darwin extends his examples into the natural world.

I believe Darwin made the conscious decision to start with domesticated organisms first, because his argument is something any educated person could agree with. We have all seen litters of kittens or puppies, raised fish, or grown flowers, or at least known people who have done these things. Even today, which has much greater urbanization than existed in Darwin's time, we are aware that dog breeds are exquisitely different from each other, yet all German Shepherds are not alike. If you want to argue about variation within a population, you do not start with white oak trees, brown bear, and large-mouthed bass. Only a specialist would be aware of the intrinsic variation within distinct members of these organisms. But take a dog or feral cat or farm animal/plant as an example and most people would be inherently familiar with the idea of variation, even if they had never considered it. Darwin makes us consider it. Once we have recognized the diversity within organisms for which we have a great deal of experience, we can then appreciate the diversity that exists in organisms for which we have much less experience.

The first part of this chapter is somewhat derivative with the previous chapter. Variation exists. What I found compelling was Darwin begins to address the species concept in this chapter and notes the difficulties inherent with it. We also discuss varieties aka breeds or sub-species and the subjectiveness of classification.

An example of the first:

Some few naturalists maintain that animals never present varieties; but then these same naturalists rank the slightest difference as of specific value; and when the same identical form is met with in two distant countries, or in two geological formations, they believe that two distinct species are hidden under the same dress. The term species thus comes to be a mere useless abstraction, implying and assuming a separate act of creation. It is certain that many forms, considered by highly competent judges to be varieties, resemble species so completely in character that they have been thus ranked by other highly competent judges. But to discuss whether they ought to be called species or varieties, before any definition of these terms has been generally accepted, is vainly to beat the air. (emphasis mine)
From these remarks it will be seen that I look at the term species as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, for convenience sake. (emphasis mine)
Darwin then gets into a discussion of the amount of variation within common species and within larger genera. The idea here is that members of a species that is wide-spread and colonizes/survives in a variety of different environments has more variation than members of a species that is only found in precise or limited environments. He then takes these arguments to make the following predictions:
From looking at species as only strongly marked and well-defined varieties, I was led to anticipate that the species of the larger genera in each country would oftener present varieties, than the species of the smaller genera; for wherever many closely related species (i.e., species of the same genus) have been formed, many varieties or incipient species ought, as a general rule, to be now forming. Where many large trees grow, we expect to find saplings. Where many species of a genus have been formed through variation, circumstances have been favourable for variation; and hence we might expect that the circumstances would generally still be favourable to variation. On the other hand, if we look at each species as a special act of creation, there is no apparent reason why more varieties should occur in a group having many species, than in one having few.
and the test of these predictions
To test the truth of this anticipation I have arranged the plants of twelve countries, and the coleopterous insects of two districts, into two nearly equal masses, the species of the larger genera on one side, and those of the smaller genera on the other side, and it has invariably proved to be the case that a larger proportion of the species on the side of the larger genera presented varieties, than on the side of the smaller genera. Moreover, the species of the large genera which present any varieties, invariably present a larger average number of varieties than do the species of the small genera. Both these results follow when another division is made, and when all the least genera, with from only one to four species, are altogether excluded from the tables. These facts are of plain signification on the view that species are only strongly marked and permanent varieties; for wherever many species of the same genus have been formed, or where, if we may use the expression, the manufactory of species has been active, we ought generally to find the manufactory still in action, more especially as we have every reason to believe the process of manufacturing new species to be a slow one. And this certainly holds true if varieties be looked at as incipient species; for my tables clearly show, as a general rule, that, wherever many species of a genus have been formed, the species of that genus present a number of varieties, that is, of incipient species, beyond the average. It is not that all large genera are now varying much, and are thus increasing in the number of their species, or that no small genera are now varying and increasing; for if this had been so, it would have been fatal to my theory; inasmuch as geology plainly tells us that small genera have in the lapse of time often increased greatly in size; and that large genera have often come to their maxima, declined, and disappeared. All that we want to show is, that where many species of a genus have been formed, on an average many are still forming; and this certainly holds good. (emphasis mine)
Finally, Darwin leaves us with a synopsis of species differences within large genera that, in my mind, serves as an initial argument for incipient speciation. This is the 'MANY OF THE SPECIES INCLUDED WITHIN THE LARGER GENERA RESEMBLE VARIETIES IN BEING VERY CLOSELY, BUT UNEQUALLY, RELATED TO EACH OTHER, AND IN HAVING RESTRICTED RANGES.' section.

What are your thoughts?


RBH said...

To encourage you to maintain this series: I've just recommended it to a (layman) friend to follow along with as he reads the Origin. So keep it up! :)

Anonymous said...

Please do keep the series going. I am
the friend RBH recommended the series to and I find it very helpful.