Encyclopedia of Evolution.pdf - Online Reading Center

poem before Darwin wrote his book.) The fact of extinction
may have forced scholars to think about the fact that disease
and death—often the result of the struggle for existence—was
much more painful than it needed to be in a perfectly ordered
world. Darwin experienced this firsthand through the death
of his mother when he was young. Even without evolution,
religious scholars had, and still have, to struggle with the
“problem of natural evil” (see essay, “Can an Evolutionary
Scientist Be Religious?”).
Fact 4. Each individual in a population is unique. Each
population contains a tremendous amount of diversity.
Humans can easily observe this diversity in their own populations,
but it is true of the populations of all other species as
well, even if humans cannot recognize it. Darwin learned this
not from other scientists but from animal breeders, particularly
pigeon fanciers. All the different breeds of pigeon had
been selected from ancestral stocks of rock pigeons (Columba
livia). These breeds included some with outlandish feathers,
anatomy (for instance, the ability to inflate their crops
massively), or behavior (such as the ability to tumble during
flight). Since all of these breeds had come from rock doves,
then all of their characteristics had to be present, but hidden,
within the rock dove populations. Breeding brought out these
traits, distilling them as one might distill brandy from wine.
The first two chapters of the Origin described how artificial
selection and the observation of natural populations showed
the tremendous amount of diversity that they contained.
Fact 5. Much of the variability in populations is heritable.
Darwin knew nothing about genetics. He was apparently
unaware of the writings (see Mendel, Gregor) that
explained how traits could be passed from one generation
to another. But every plant and animal breeder knew that
this was the case. There was confusion about the mechanism.
Many scientists held varieties of Lamarckism, or the
inheritance of acquired characteristics. Many also believed in
“blending inheritance” in which parental traits blended permanently
in their offspring, like blending different colors of
paint. Mendel, in contrast, was one of the researchers who
demonstrated that traits were inherited in a particulate rather
than a blending fashion; that is, a trait could disappear in one
generation and reappear in another, like mixing different colors
of marbles rather than of paint. Darwin himself ended up
accepting a variation of Lamarckism, which he called pangenesis,
a theory that today most evolutionary scientists consider
to be his only major blunder.
The first inference, plus facts 4 and 5, give rise to Inference
2, which is natural selection itself. In order to reproduce,
an individual needs to obtain resources beyond just
what is needed for survival. Because of the struggle for existence
(inference 1), not all individuals in the population can
reproduce, or even survive. If all individuals in the population
were alike, then the ones that prevailed would be due
either to chance, or to historical contingency, which might
just be another form of chance. But since individuals are
unique, then some of the individuals have what it takes to
survive and reproduce more than others. For example, in a
moist environment, some of the plants may have larger leaves
and shallower roots than others. The plants with larger leaves
natural selection
could grow faster because they would have greater photosynthesis,
and they would not waste their resources producing
deep roots that are not needed in a moist environment. But if
the trait cannot be passed on to the next generation, the superior
survival and reproduction of the plants with larger leaves
would not matter, since their offspring would not have leaves
any larger than those of other plants.
Natural selection is the differential reproduction of
organisms in a population for reasons not due to chance.
That is all that it is. If some genetically distinct individuals
consistently reproduce more than others, natural selection
occurs. Scientists may be able to determine what caused the
superiority of some individuals over others (see Darwin’s
finches) or they may not (see peppered moths). Either way,
natural selection has occurred. As Darwin described it,
Owing to [the] struggle [for existence], variations,
however slight and from whatever cause proceeding,
if they be in any degree profitable to the individuals
of a species, in their infinitely complex relations to
other organic beings and to their physical conditions
of life, will tend to the preservation of such individuals,
and will generally be inherited by the offspring
… I have called this principle, by which each slight
variation, if useful, is preserved, by the term Natural
Selection…
Inference 2 leads directly to Inference 3, which is that if
natural selection continues long enough, evolutionary change
results in the population.
The traits that confer superiority in natural selection,
therefore in evolution, are not the same everywhere. There
are countless pathways to success, in the natural world as
in the human economy. The above example described successful
plants in moist environments as having large leaves
and shallow roots. In an arid environment successful plants
would have small leaves (which waste less water than large
leaves) and deep roots (that may be able to tap into new
sources of water). In most cases, evolution can go in more
than one direction, depending on the circumstances. To use
one example close to home, the major pattern of evolution
in the human lineage has been the increase in brain size (see
australopithecines; Homo Habilis; Homo ergaster;
Homo sapiens) but in at least one case, human brain size
decreased during evolution (see Flores Island people).
Types of Natural Selection
Evolutionary scientists generally classify natural selection into
three types (see figure on page 284).
1. In stabilizing selection, the average individuals reproduce
better than the extreme individuals. For example, in an
environment with medium humidity, very large-leaved and
very small-leaved individuals in a plant population would
grow more slowly and reproduce less. Stabilizing selection
reinforces the mean or average trait value by eliminating outlying
variation. Stabilizing selection may be common in populations
once a particularly successful combination of genes has
been assembled. According to some evolutionary scientists,