Genetic variability originates through the random
process of mutation, a sudden and unpredictable change
in a gene. Mutation is nondirectional: environments do
not cause or produce useful mutants in response to need.
The variability resulting from mutation is enhanced by
recombination of genes (more accurately called alleles)
in sexually reproducing species. Selection can act only
on whatever genetic variants are present. Selection,
unlike mutation, is not a random process, because only
certain members of a population are best suited for a
given environment, and thus these individuals have a
nonrandom chance of survivorship and reproduction.
In recent years both geneticists and molecular biologists
have established beyond doubt that large amounts of
genetic variability exist in most populations. Thus,
in most cases, there is ample raw material for natural
selection to act upon. Evolution continues.
Natural selection does not guarantee survival. Most
species that have ever inhabited Earth are now extinct,
and because of all the various human influences on Earth
at the current time, more species are moving toward
what may become the sixth major extinction event in
the history of our planet. But that said, the amazing
biodiversity of Earth so evident in tropical latitudes
remains the remarkable result of natural selection.Adaptations
An adaptation is any anatomical, physiological, or
behavioral characteristic shown to enhance either the
survival or the reproductive ability of an organism. Such
traits, as they translate into successful reproduction,
make up what is called evolutionary fitness. Fitness is
reflected in suites of adaptations, all of which result
from the action of natural selection.
For instance, various opossums, Neotropical
porcupines, and the Kinkajou (Potos flavus), as well
as many monkeys of the American tropics, possess
a prehensile tail (plate 8- 13). (And some snakes do
too.) Such a structure functions effectively as a fifth
limb, lending security and mobility to the animal as it
moves through the canopy. It is easy to see intuitively
that the prehensile tail is an adaptive structure. Tailless
monkeys or opossums would face a smaller lifetime
reproductive success because of the added risk of
falling. But note that many Neotropical monkeys
(sakis, for example) do not possess prehensile tails
and yet are fit within their arboreal environments.Plate 8- 12. This photo shows a flock of Scarlet Ibises
(Eudocimus ruber) feeding before roosting at Caroni Swamp
in Trinidad. The mangroves in which they roost and the birds
themselves are each members of populations experiencing
natural selection, along with everything else alive in the
mangrove forest. Charles Darwin and Alfred Russel Wallace
recognized, in this profound reality of nature, how nature
really works. Photo by John Kricher.Plate 8- 13. This Yellow- tailed Woolly Monkey (Oreonax
[Lagothrix] flavicauda), found in South America, has a prehensile
tail and is demonstrating how to use it. Photo by John Kricher.chapter 8 evolutionary cornucopia 117