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refl ects their location. Those on the hotter and sunnier African side are more
resistant to water stress, and in this bright environment many animals and
even some fungi secrete more of the dark UV-protective pigment melanin.
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These dif erences in physiology between the populations of organisms on
the two slopes of the canyon are refl ected in their genetic composition as well.
One of the great strengths of modern evolutionary biology is that it has given
us the tools to plumb the depths of the gene pools of every plant and animal
species, including our own. We can detect and measure genetic dif erences,
even those between adjacent populations that look superfi cially alike.
The gene pool of a species is far vaster and more complex than the geno-
type of an individual of that species. The genes in this collective pool come in
many dif erent forms, called alleles. For each of our genes we as individuals
can carry at the most two alleles, one from each of our parents. The gene
pool of our species, in contrast, may contain dozens of dif erent allelic forms
of the same gene. To get an idea of the range of variation that our gene pool
contains, we must look at many dif erent individuals. Gene sequencing and
other methods now allow us to measure the variation within our own popu-
lation and within the populations of other animals and plants.
Dissecting the process of evolution
Eibi Nevo has applied this approach to the species of Evolution Canyon. He
and his students and collaborators have peered within the gene pools of
many dif erent populations that live in the canyon. In species after species
they have found that the populations living on the two sides of the canyon
show dif erences in allele frequency at many dif erent genes. These dif er-
ences parallel the dif erent physiological, behavioral, and color dif erences
that these populations exhibit (though in most cases the genes that actually
determine these physical dif erences are not known).
Some of the genetic dif erences have resulted from natural selection for
and against dif erent alleles on the two slopes of Evolution Canyon. Natural
selection shifts the frequencies of alleles in gene pools, and it is these shifts
that bring about evolutionary changes in populations.
