but related theory has it that higher temperatures in and of themselves
lead to higher mutation rates.)
A second theory posits that the tropics hold more species because
tropical species are finicky. According to this line of reasoning, what’s
important about the tropics is that temperatures there are relatively
stable. Thus tropical organisms tend to possess relatively narrow thermal
tolerances, and even slight climatic differences, caused, say, by hills or
valleys, can constitute insuperable barriers. (A famous paper on this
subject is titled “Why Mountain Passes Are Higher in the Tropics.”)
Populations are thus more easily isolated, and speciation ensues.
Yet another theory centers on history. According to this account, the
most salient fact about the tropics is that they are old. A version of the
Amazon rainforest has existed for many millions of years, since before
there even was an Amazon. Thus, in the tropics, there’s been lots of time
for diversity to, as it were, accumulate. By contrast, as recently as twenty
thousand years ago, nearly all of Canada was covered by ice a mile thick.
So was much of New England, meaning that every species of tree now
found in Nova Scotia or Ontario or Vermont or New Hampshire is a
migrant that’s arrived (or returned) just in the last several thousand
years. The diversity as a function of time theory was first advanced by
Darwin’s rival, or, if you prefer, codiscoverer, Alfred Russel Wallace, who
observed that in the tropics “evolution has had a fair chance,” while in
glaciated regions “it has had countless difficulties thrown in its way.”
THE following morning, we all crawled out of our sleeping bags early to
see the sunrise. Overnight, clouds had rolled in from the Amazon basin,
and we watched them from above as they turned first pink and then
flaming orange. In the chilly dawn, we packed up our gear and headed
down the trail. “Pick out a leaf with an interesting shape,” Silman
instructed me once we’d descended into the cloud forest. “You’ll see it for
a few hundred meters, and then it will be gone. That’s it. That’s the tree’s
entire range.”