Thursday, November 18, 2010

Chapter 10: On the Geological Succession of Organic Beings

In this chapter, Darwin explores the known evidence for evolution in the fossil record. Through the fossil record, Darwin notes that the rate of change is not the same for every species, and that over long periods of time some forms seem not to change at all.

Darwin claims that land dwelling creatures seem to have changed more than those in the sea, citing that "higher" creatures change more than "low" creatures. That is to say that Darwin suspected that more complex creatures, due to their complexity and opportunities for variation are more likely to change due to environmental factors than more primitive creatures. This isn't necessarily due to Darwin's qualitative assessment of high and low creatures, but rather that highly complex creatures are often very specialized and would necessarily have to change in response to natural selection. The most basic and primitive bacterial extremophiles have not changed tremendously because they can live just about anywhere without too much selective survival.

One rule that Darwin states is that groups of species cannot simply reappear on the fossil record after a lapse of time. Explanations for this could be poor preservation or geographic migration in and out of a region. Basically once something is totally extinct, it's not going to magically reappear. In this chapter Darwin addresses the concept of extinction in detail. Darwin concluded from fossil evidence that most species gradually disappear over time as opposed to disappearing in large number due to catastrophes. While there have been several mass extinction events in earth's history, the appear to be more the exception than the norm. Most mass extinctions were not sudden, and instead drawn out over a long period of time. The Discovery Channel has an interactive website detailing mass extinctions and their likeliest causes.

Darwin predates the theory of plate tectonics by about a century, however in his time it was recognized that certain rock deposits and formations were located in different, seemingly unconnected areas and having similar fossil groups within them. Darwin saw this as parallelism, because it was probably he (and other geologists of the time) had no concept of the continents being in different positions throughout the ages. Instead of convergent evolution, it is more likely that similar forms do share a common ancestor and the population was geographically isolated from each other during continental drift, causing two new branches in the lineage.

Darwin also summarizes the idea of ancient forms being "lower" than modern forms. However, he notes that naturalists were still arguing the concept of what constituted high and low. Darwin simply decided to define it as modern forms were increasingly complex over their ancient forms.

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