- Alternative meaning Natural Selection (computer game).
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. Other mechanisms of evolution include genetic drift, gene flow and mutation.
Overview
The basic concept of natural selection 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 prolific. 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.
Mechanisms of natural selection
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):
- IF there are organisms that reproduce, and
- IF offspring inherit traits from their progenitor(s), and
- IF there is variability of traits, and
- IF the environment cannot support all members of a growing population,
- THEN those members of the population with less-adaptive traits (determined by the environment) will die out, and
- 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.
Scope of natural selection
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. In this chapter he wrote:
- It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation, even the
slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and
wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of
life. We see nothing of these slow changes in progress, until the hand of time has marked the long lapses of ages, and then so
imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they
formerly were.
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."
Impact of the idea
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.
See also
|