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424 appendix
Second, individuals may benefit from specializing upon
the male function, or the female function, thus performing
that function more efficiently than could an individual that
attempts to do both. A plant species with flowers that have
both male (stamens) and female (pistils) parts can evolve into
one in which some of the plants specialize on pollen production
and others on fruit and seed production. They can eventually
evolve into a species with separate male individuals (the
flowers of which have only stamens) and female individuals
(the flowers of which have only pistils).
The following two objections that have been raised
against the effectiveness of natural selection:
First, such a large number of offspring die that any good,
new variation that might appear among them would be very
likely to disappear. However, natural selection would still
favor the beneficial variations among the survivors.
Second, any new variation would get swamped by intermixing
with the mediocre majority with which the new variant
crossbreeds. However, a new variation, when it arises,
interbreeds primarily with close neighbors; the new trait is
therefore not likely to get lost in the big pool of the whole
population. It would become established as a local variety. It
is not uncommon to find two varieties of animal which can
interbreed, but which nevertheless remain distinct.
Small, isolated islands usually produce their own, unique
species. These species are seldom able to compete against the
species that have evolved on large continents. For example,
continental placental mammals are displacing the marsupial
mammals of Australia. In fact small, isolated islands often
have species that appear to have not changed for long periods
of time (see living fossils). For reasons described above,
it is important for a new variety to be partially isolated in
order for natural selection to begin favoring it (see isolating
mechanisms). This isolation need not be on an island; a
large continent has isolated habitats (forests, lakes, etc.) that
are as effective as islands in producing new species. Perhaps
most effective of all is a large continental area that experiences
fluctuations of conditions (such as changes in sea level, or
changes in climate): First, species in isolated habitats evolve
uniquely in each habitat, then when conditions change these
species spread, come in contact with each other, and evolve
into even more species in response to one another, then they
become isolated yet again and evolve into yet more species.
The possible lack of isolation on a continent, compared to
an island, is more than compensated by its larger population
sizes, with its accompanying greater heritable variability.
Varieties within a species can evolve into separate species
as a result of the following:
• Competition is stronger between varieties of one species
than between species, because they are competing for
a more similar set of resources. This is indirectly demonstrated
by the fact that the species that successfully invade
a new continent are usually members of genera that are not
already present on that continent.
• Natural selection will favor the individuals within two varieties
that are most different from one another, specializing
on different resources.
This results in what I call divergence of character (see character
displacement).
While divergence has occurred throughout the history
of life, convergence has also occurred: When two species,
very different in structure, begin to adapt to a common environment,
natural selection favors the same characteristics in
each, causing them to become more similar to one another.
Natural selection has also led to advancement in complexity
of organisms over time. Natural selection favors
whatever traits confer advantages, whether the traits are more
complex, or less. There are more ways for a complex trait to
prevail over a simple trait than for the reverse to happen.
The result is an evolutionary diversification of species
that resembles a vast bush, with many branches from a common
ancestral trunk. Many of its branches have been pruned
away by extinction, the remaining ones have diverged, adapting
in different ways to different environments. Some branches
diverge more, some less; some produce more branchlets,
some fewer; some go nowhere, into extinction; we behold
only the outermost twigs and try to figure out the history of
branching that has produced their pattern.
Someday people will recognize that species have arisen
through the slow work of natural selection, rather than sudden
creation, just as they now realize that geological formations
are the result of gradual processes such as erosion and
uplift, rather than due to huge, brief floods. [Darwin was
overly optimistic about both of these; see creationism.]
chapter 5. Laws of Variation
Many factors determine the development of an organism’s
characteristics. The direct effect of the environment can
make individual organisms acclimatize to new conditions,
but heritable variation that is not directly related to environmental
conditions also arises. Natural selection acts
only on the heritable variation. There are constraints upon
this heritable variation. For example, some traits (such as
albinism with deafness in cats) are correlated for unknown
reasons. [This is now called linkage.] Other traits are correlated
because of the “economy” of the organism’s growth:
Greater growth of one part necessitates lesser growth of
another (see allometry). Therefore the degeneration of
some organs may result from natural selection. For example,
cave organisms do not need their eyes, but what harm
would it be if they had eyes anyway? The answer is, eyes are
easily damaged and infected, and if they are unnecessary,
the organism benefits from not having them, and natural
selection gets rid of them.
There is more variability in those parts of organisms that
are (1) multiple, (2) unspecialized, (3) rudimentary, (4) rapidly
evolving. Unspecialized organs need flexibility, since they
cannot specialize on just one use. Rudimentary organs, not
being needed by the organism, are not controlled as much by
natural selection. Rapidly evolving organs have more variability
because they are still in the process of evolving. Those
traits that differ more between species also differ more within
them: This is a pattern that makes sense if the traits are evolving,
but makes no sense in terms of special creation.