Saturday, February 7, 2009

Tonya's Comments on Chapter III

Chapter III - Struggle for existence

p61) I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man's power of selection.

It seems that even the name emphasizes (hu)man's power. Was this something Darwin was thinking about when he came up with the name for his idea? Not that humans had any power in natural selection of course, but that people would associate the power of 'nature' with the power already held by humankind?

p62) We behold the face of nature bright with gladness... :)

p63) There are different kinds/gradients of 'struggle'.

p66) ...the average number of any animal or plant depends only indirectly on the number of its eggs or seeds.

Yes, I understand that he is discussing k- verses r- selected organisms but I think this is quite a generalized statement and a lot of other factors determine the average number of them.

p68) Climate plays an important part in determining the average numbers of a species, and periodical seasons of extreme cold or drought, I believe to be the most effective of all checks.

Will this be a check of humans???

p70) ...that a plant could exist only where the conditions of its life were so favourable that many could exist together, and thus save each other from utter destruction.

This statement stood out for me for some reason. Has this been tested? Is this sort of like the school of fish hypothesis? Can anyone elaborate on this for me?

p73) Food Webs!

Nevertheless so profound is our ignorance, and so high our presumption, that we marvel when we hear of the extinction of an organic being; and as we do not see the cause, we invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life!


humble-bees! :)

"In England, these insects used to be called Humble Bees, and they were the stuff of which legends were made. In The Origin of Species, Charles Darwin related a story that asserted that old maids were the real power behind the British Empire. The reasoning behind this remarkable claim was that the British army lived on beef; to raise beef you must have clover; and clover, particularly red clover, is pollinated only by humble bees. When field mice are abundant, they often break into the nests of humble bees to steal the honey pots, thus killing the bees. Old maids keep cats, and cats kill mice; hence the bee population, the clover crop, and the beef supply all depend on the number of old maids— well, you get the idea. It’s the food chain in action again." B. Meredith

p76) ...but probably in no one case could we precisely say why one species has been victorious over another in the great battle of life.

Any papers on this?

1 comment:

Dr. Fox said...

Still on chapter III? C'mon, keep up Tonya! Just kidding. ;-)

A few comments:

Regarding the name 'natural selection', I think your last sentence is the right answer. Darwin's argument for natural selection is essentially that it's just like what we now call artificial selection, save that nature only ever selects those traits that allow the bearer to be relatively successful at contributing to the next generation. So it's natural (no pun intended) for him to reinforce this argument via his choice of terminology.

p68) In terms of checks on humans, population ecologist and demographer Joel Cohen wrote a big book a few years ago called How Many People Can the Earth Support?, in which he basically showed that the answer is "No one knows." Cohen's question is also a coarse one (as he would readily acknowledge), since it doesn't mention the standard of living we'd like to have, or global variation in living standards and how people respond to that variation (e.g., by migrating). The issue of human population size is an incredibly serious issue, but also an incredibly hard one, as it's tied up with many other issues, none of which are purely scientific.

p70) Darwin's speculations here are interesting as far as they go, but they probably don't go quite far enough. It's true that in the short term one might expect a superabundance of prey to swamp the local predators, causing the prey to experience a low per-capita mortality rate. But that kind of safety in numbers is a short-term phenomenon, in general. In the longer term we'd expect predator numbers to increase as long as they're living in such a bountiful environment. And even in the short term we'd expect predators to move into an area that regularly experiences an abundance of prey (there are many dramatic examples of this phenomenon). For Darwin's idea to work in the long term, high abundances of prey need to be widely and unpredictably distributed in time. It's thought that this is why oak trees produce most of their acorns in 'mast years' that are widely and irregularly spaced in time (once every 3-4 years or so, if memory serves), and are synchronized in space (all the oak trees over 1000s of square km mast at the same time). The resulting superabundance of acorns swamps the predators (mainly squirrels), which then breed well. But the paucity of acorns for the next few years ensures that that increase in squirrel abundance is short-lived. And the spatial synchrony of masting across wide areas ensures that there's no possibility of squirrels moving in from elsewhere to consume a localized bounty of acorns. I know this sounds like what Stephen J. Gould would call a just-so story, but there's actually a fair bit of evidence that this story is true.

p76) There are plenty of mechanistic studies of why particular invasive species are replacing particular local species.