Analyses of bird distribution patterns throughout
South America by Joel Cracraft, Jürgen Haffer, and
others have demonstrated numerous areas of probable
endemism throughout the continent (figs. 8- 1– 2).
Each of the postulated areas of endemism contains
unique assemblages of species not found in other areas.
This helps account for the unusually high diversity of
birds throughout the continent, and it suggests that
evolutionary patterns have been complex.
Natural Selection
Natural selection is defined as differential reproduction
among genotypes within a population. That means that
not all members of any population survive to the same
age or produce the same number of offspring. This is
mostly because not every member of the population
carries the exact same combination of genes (plate 8-
11). Some combinations of genes are more superior in
certain environments than others. There are, so to speak,
genetic winners and losers in every population at every
generation— kind of the luck of the draw. Genes, the
long, coiled molecules of DNA that contain hereditary
information, were unknown to Charles Darwin,
although he fully understood that many traits evident in
the phenotypes of every individual are inherited. It’s no
secret that siblings tend to look alike as well as resemble
their parents and grandparents. But what Darwin and the
independent co- discoverer of natural selection Alfred
Russel Wallace both realized was that for all the similarity
there is among members of a population, it is the genetic
differences among members of populations that make
up the currency of evolutionary change and natural
selection. When Darwin published his most famous
work, On the Origin of Species, in 1859, he challenged the
reader to grasp this essential point (many, to this day, do
not). As Darwin might be apt to say if he were leading a
field trip in Amazonia: realize that all squirrel monkeys
within a given population are not alike. Some will be
genetically inclined to be heavier, some more lithe; some
will be more resistant to certain pathogens and parasites;
some will be able to better detect potential predators such
as Harpy Eagles. These qualities will determine which
are the animals most likely to survive and reproduce
and thus send their genes into the next squirrel monkey
generation. This is natural selection.
As long as a population contains genetic variability
among its members, natural selection will potentially
act (plate 8- 12). Individuals, because they do not
share exactly the same genes, will respond somewhat
differently to abiotic (such as temperature, water
availability, etc.) and biotic (such as threat of predation,
competition, parasitism) selection pressures imposed
by their environment. This variation inevitably leads to
what Darwin called the “struggle for existence.” Natural
selection acts only on the present, never “planning”
for the future. The survival of the fittest, a term used
to describe natural selection (but coined by Herbert
Spencer, not Darwin), sums up the generation- by-
generation sorting done by natural selection on any
population. Individuals that survive and reproduce
more successfully than others in any particular
generation— in other words those that are fit— do so
because conditions, whatever they may be, suit them
somewhat better than others in the population. They
are, for the moment, evolution’s winners, at least of
that round. Natural selection is a statistical truth.
Individuals with genes that confer reproductive
advantage will tend to leave the most progeny, thus those
genes will proportionally increase generation after
generation relative to others in the population’s gene
pool. And as gene frequencies change, the appearance,
the physiology, and (if an animal) the behavior of the
species will change too. The species will evolve.Plate 8- 11. These butterflies clustering in Brazil (members of
the family Pieridae) are not genetically identical, though they
all look alike. The genetic differences among them account
for why natural selection acts on them as individuals and
ultimately for why this species has evolved as it has. Photo by
John Kricher.
116 chapter 8 evolutionary cornucopia