Natural selection



Natural selection is an essential mechanism of evolution proposed by Charles
Darwin and generally accepted by the scientific community as the best
explanation of speciation as evidenced in the fossil record.

The basic concept is that environmental conditions (or "nature") determine
(or "select") how well particular traits of organisms can serve the survival
and reproduction of the organism; organisms lacking these traits might die
before reproducing, or be less fecund. As long as environmental conditions
remain the same, or similar enough that these traits continue to be
adaptive, such traits will become more common within populations. Loss of
the species' ecological niche or crowding-out due to population growth can
change drastically the adaptive traits required to survive - in such
conditions, or in any circumstance where survival is determined by ecology
more than by the secondary sexual characteristics, an ecological selection
is taking place (this term is used solely to differentiate processes
irrelevant to mating, and is of modern usage, having grown up with the field
of ecology itself).

Darwin's theory of the evolution of species through natural selection starts
from the premise that an organism's traits vary in a non-deterministic way
from parent to offspring, a process called "individuation" by Darwin. This
theory does not make any specific claims as to how this process works,
although more recent scientific discoveries in genetics explain several
mechanisms that occur in the process of reproduction: in the case of both
asexual and sexual reproduction, random mutation (including DNA
transcription errors); in the case of sexual reproduction (which mixes the
DNA of two parents into an offspring), gene flow and genetic drift are also
important mechanisms. Competition (typically among males to impregnate
females) for mates produces sexual selection - a process which Darwin
considered secondary to ecological in most species.

Natural selection does not distinguish between ecological selection and
sexual selection, as it is concerned with traits, e.g. dexterity of
movement, on which both may operate simultaneously. If a particular
variation makes the offspring which manifest it better suited to survival or
to successful reproduction, that offspring and its descendants will be more
likely to survive than those offspring without the variation. The original
traits, as well as any maladaptive variations, will disappear as the
offspring who carry them are replaced by their more successful relatives.

Therefore, certain traits are preserved due to the selective advantage they
provide to their holders, allowing the individual to leave more offspring
than individuals without the trait(s). Eventually, through many iterations
of this process, organisms will develop more and more complex adaptive traits.

What makes one trait more likely to succeed is highly dependent on
environmental factors, including the species' predators, food sources,
abiotic stress, physical environment, and so on. When members of a species
become separated, such as geographically, they face different environments,
and tend to develop in different directions. After a long period of time,
their traits will have developed along different paths to such an extent
that they can no longer interbreed, at which point they are considered
separate species. This is why a species will sometimes separate into
multiple species, rather than simply being replaced by a newer form of the
species (from this fact Darwin suggested that all species today have evolved
from a common ancestor).

Additionally, some scientists have theorized that an adaptation which serves
to make the organism more adaptable in the future will also tend to supplant
its competitors even though it provides no specific advantage in the near
term. Descendants of that organism will be more varied and therefore more
resistant to extinction due to environmental catastrophes and extinction
events. This has been proposed as one reason for the rise of mammals. While
this form of selection is possible, it is more likely to play an important
role in cases where selection for adaptation is continuous. For example, the
Red Queen hypothesis suggests that sex might have evolved to help organisms
adapt to deal with parasites.

Natural selection can be expressed as the following general law (taken from
the conclusion of The Origin of Species):

  1. IF there are organisms that reproduce, and
  2. IF offspring inherit traits from their progenitor(s), and
  3. IF there is variability of traits, and
  4. IF the environment cannot support all members of a growing population,
  5. THEN those members of the population with less-adaptive traits
     (determined by the environment) will die out, and
  6. THEN those members with more-adaptive traits (determined by the
     environment) will thrive

The result is the evolution of species.

Note that this is a continuing process -- it accounts for how species
change, and can account for both the extinction of one species and the
creation of a new one.

Note also that the above law need not apply solely to biological organisms;
it applies to all organisms that reproduce in a way that involves both
inheritance and variation. Thus, a form of natural selection could occur in
the non-biological realm (see, for example, Genetic programming). Note also
that this formulation does not rule out selection occurring at all
biological levels (e.g. gene, organism, group). Finally, note that the
particular process of introducing new traits does not matter. Darwin first
outlined his theory in two unpublished manuscripts written in 1842 and 1844
and more fully developed it for publication in The Origin of Species,
especially Chapter 4.

Darwin ends his book with an often quoted passage: "There is a grandeur in
this view of life, with its several powers, having been originally breathed
into a few forms or into on; and that, whilst this planet has gone cycling
on according to the fixed law of gravity, from so simple a beginning endless
forms most beautiful and wonderful have been, and are being, evolved."

Perhaps the most radical claim of Darwin's theory of evolution through
natural selection is that "elaborately constructed forms, so different from
each other, and dependent on each other in so complex a manner" have evolved
out of the simplest forms of life and according to a few simple principles.

It is this fundamental claim that has inspired some of Darwin's most ardent
supporters--and that has provoked the most profound opposition. Some groups
prefer to believe in divine intervention or guidance of the process, such as
those favoring the Intelligent design school of thought. In addition, many
theories of Artificial selection have been proposed to suggest that economic
or social fitness factors assessed by other humans or their built
environments are somehow biological or inevitable - Social Darwinism. Others
held that there was an evolution of societies analogous to that of species.
Darwin's ideas, along with those of Freud and Marx, are considered by most
historians to have had a profound influence on 19th century thought, and to
have challenged the rationalist and religious fundamentalist schools of
thought that prevailed in Europe.

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