Wednesday, September 22, 2010

Chapter Two: Variation Under Nature

Darwin begins to define a species by pointing out that each individual of a species has in it, some form of variation. Darwin also goes on to point out that it's more likely that there is one species with, for instance, 3 varieties, rather than 4 distinct species.

This assumption that there are many varieties rather than many distinct species is correct. It is difficult to define what exactly a species is. A generally taught view is that speciation occurs when varieties of a species cannot contribute genetic information to each other, or in other words, two varieties that cannot breed. A few posts back I posted an article that talked about distinct species breeding. One example of this are lions and tigers (two genetically different big cats from different geographical locations, and pretty much universally accepted as separate species) producing "ligers". One note to make is that many hybrid offspring are non-viable, meaning that they are either weak and/or sterile. Liger's don't really count as tigers contributing to the lion gene pool because they cannot be bred.

For purposes of this blog, I'm defining different species as groups of individuals who do not contribute to each others gene pools. Such reproductive isolation can be mechanical, or geographical, and/or pre/post zygotic non-viability.

"And I look at varieties which are in any degree more distinct and permanent, as steps leading to more strongly marked and more permanent varieties; and at these latter, as leading to sub-species, and to species," (Darwin, 126). Darwin defines a species as a variety that is able to flourish in large numbers, and does not assign the rank of species or even sub-species to each and every variant.

"Where many species of a genus have been formed through variation, circumstances have been favorable for variation; and hence we might expect that the circumstances would generally be still favorable 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," (Darwin, 129). Basically the processes shaping variation long ago are still shaping variation today, and if creationism was applied to each and every species, there would be no explanation for variation.

It is important to note that the Linnaeus system of classification was widely known during Darwin's time and is still used today. This is a system of classifying organisms based on their relatedness. One major advancement to this system is the use of genetic analysis to see just how closely species are related.

One other observation Darwin made in this chapter was that the dominant genera represented in a region was also apt to have the most variety.

Thankfully, chapter 2 of Origin is much shorter than chapter 1 and pigeon free.

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