IS A SIMPLE PROCESS
by Whit Gibbons
September 23, 2007
Darwin's ideas of natural selection, survival of the fittest, and evolution,
which means "change," are elegant in their simplicity. Why anyone
interested in understanding the way the world works would try to challenge
the easily understandable basic concepts of the process is not clear.
Following is a simplistic example of evolution, showing how change in
a lizard species could hypothetically occur in a natural manner. Natural
selection operates because some members of a population have a trait that
makes them more likely to survive than others.
In the simple
example, imagine a species of lizards living on an island in which most
individuals are bright yellow but a rare form of the same species is dull
green. The color is caused by genes that produce either green or yellow,
but because the green individuals are so rare, the species would be considered
to be a population of yellow lizards. The lizards live peacefully on the
island for centuries. Eventually, a type of hawk flies over from the mainland,
and over time the bird predator begins to inhabit the island.
discover that when flying over the thick, dark green vegetation of the
island they see bright yellow lizards. If a green lizard and a yellow
one are sitting side-by-side when a hawk flies over looking for a lizard
meal, the yellow lizard will stand out like a beacon on the green bush
and will be the one to become a meal for the hawk. The green lizard will
escape detection and be more likely to survive, consequently proving to
be "fit" because the green genes are the ones passed on to the
next generation. As the natural process in which hawks select and remove
the yellow lizards continues, proportionately more green genes are inherited
than are yellow ones. Eventually, the hawks will practically eliminate
the yellow lizards while green lizards survive, to the point that the
island lizards have evolved to become a population of green lizards. Basic
evolution has prevailed.
processes can be far more complicated. One concept that has been particularly
difficult to understand when using the simple explanation has been that
of the frequently observed phenomenon of "helper" birds. Helpers
are non-reproductive birds that help feed another couple's young. For
example, red-cockaded woodpecker colonies often have young males that
help feed the young but do not themselves mate with the females. The observation
of helper birds seemed to contradict the evolutionary principle that the
most biologically fit individuals were those that passed their genetic
material on to the next generation.
appear to be altruistic, engaging in a charitable act with no expectation
of a reward. But most evolutionary biologists will tell you that strict
altruism does not exist as a permanent and persistent trait in any species
(including humans) because natural selection would ultimately not favor
individuals who invest in making it easier for someone else to survive
without a benefit to themselves.
a half-century ago an English biologist named William Hamilton proposed
a theory that explained how seemingly altruistic acts fit into the evolutionary
scheme. Being a helper fits quite well into an individual passing on his
genetic line, if the helpful acts are directed toward close kin. Siblings
on average share half the genes inherited from their parents, so assisting
the parents in successfully reproducing offspring that are genetically
close to itself, an individual increases the chances that some of its
own genes are being passed along. The same principle applies to a lesser
degree with cousins, which also share genetic material. The overall phenomenon
has been called "kin selection," meaning that natural selection
is operating but that genes are being passed along by close kin rather
than by the individual directly.
biology is challenging because when scientists unravel one mystery of
life, such as why helper birds fit Darwin's ideas, another phenomenon
calls for explanation. A recent study in Australia addressed the inexplicable
observation in which helper birds did not appear to "help."
That is, nestlings did not grow faster or get larger whether or not helpers
were present to assist the parents with feeding.
a blue wren provides the answer.
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