Natural selection is the process by which heritable traits that increase an organism's chances of survival and reproduction become more common in a population over generations. Organisms with traits better suited to their environment are more likely to survive and pass on those traits, gradually changing the population over time. The peppered moth example demonstrates how a population adapted as the environment changed during the industrial revolution. Natural selection is a key driver of evolution and helps explain the diversity of life on Earth.

2. Natural selection is the gradual
process by which heritable biological
traits become either more or less
common in a population as a function
of the effect of inherited traits on the
differential reproductive success of
organisms interacting with their
environment. It is a key mechanism
of evolution.

3. General principles
Natural variation occurs among the individuals of any population of
organisms. Many of these differences do not affect survival or
reproduction, but some differences may improve the chances of survival
and reproduction of a particular individual. A rabbit that runs faster than
others may be more likely to escape from predators, and algae that are
more efficient at extracting energy from sunlight will grow faster.
Something that increases an organism's chances of survival will often
also include its reproductive rate; however, sometimes there is a trade-
off between survival and current reproduction. Ultimately, what matters
is total lifetime reproduction of the organism.
The peppered moth exists in both light and dark colors in the United
Kingdom, but during the industrial revolution, many of the trees on which
the moths rested became blackened by soot, giving the dark-coloured
moths an advantage in hiding from predators. This gave dark-coloured
moths a better chance of surviving to produce dark-coloured offspring,
and in just fifty years from the first dark moth being caught, nearly all of
the moths in industrial Manchester were dark. The balance was reversed
by the effect of the Clean Air Act 1956, and the dark moths became rare
again, demonstrating the influence of natural selection on peppered moth
evolution.

4. If the traits that give these individuals a reproductive advantage are also heritable,
that is, passed from parent to child, then there will be a slightly higher proportion of
fast rabbits or efficient algae in the next generation. This is known as differential
reproduction. Even if the reproductive advantage is very slight, over many
generations any heritable advantage will become dominant in the population. In this
way the natural environment of an organism "selects" for traits that confer a
reproductive advantage, causing gradual changes or evolution of life. This effect was
first described and named by Charles Darwin.
The concept of natural selection predates the understanding of genetics, the
mechanism of heredity for all known life forms. In modern terms, selection acts on
an organism's phenotype, or observable characteristics, but it is the organism's
genetic make-up or genotype that is inherited. The phenotype is the result of the
genotype and the environment in which the organism lives (see Genotype-
phenotype distinction).
This is the link between natural selection and genetics, as described in the modern
evolutionary synthesis. Although a complete theory of evolution also requires an
account of how genetic variation arises in the first place (such as by mutation
and sexual reproduction) and includes other evolutionary mechanisms (such as
genetic drift and gene flow), natural selection appears to be the most important
mechanism for creating complex adaptations in nature.

6. How natural selection helps
few species to survive !!
Natural selection is the engine that
drives evolution. The organisms best suited to
survive in their particular circumstances have a
greater chance of passing their traits on to the
next generation. But plants and animals interact
in very complex ways with other organisms and
their environment. These factors work together
to produce the amazingly diverse range of life
forms present on Earth.

7. By understanding natural selection, we can learn why some
plants produce cyanide, why rabbits produce so many offspring,
how animals first emerged from the ocean to live on land, and
how some mammals eventually went back again. We can even
learn about microscopic life, such as bacteria and viruses, or
figure out how humans became humans.
Charles Darwin coined the term "natural selection." You'll
typically hear it alongside the often misunderstood evolutionary
catchphrase "survival of the fittest." But survival of the fittest
isn't necessarily the bloody, tooth-and-claw battle for survival
we tend to make it out to be (although sometimes it is). Rather,
it is a measure of how efficient a tree is at dispersing seeds; a
fish's ability to find a safe spawning ground before laying her
eggs; the skill with which a bird retrieves seeds from the deep,
fragrant cup of a flower; a bacterium's resistance to antibiotics.

8. A great deal of evolution by natural selection can happen
without the formation of new species. Natural selection is only
the process of adaptation within species, and we see many
examples of that. Under some circumstances natural selection
does play a role in the origin of new species, by which I mean
a splitting of one species lineage into two different lineages
that do not interbreed with one another — for example, the
splitting of one ancestral primate lineage into one that became
today’s chimpanzee and the other that became the hominid
line resulting in our own species. The process of splitting and
becoming reproductively isolated, that is, incapable of breeding
with one another, can often involve natural selection but
perhaps not always.

9. Natural selection acts solely through the preservation of variations in
some way advantageous, which consequently endure. Owing to the high
geometrical rate of increase of all organic beings, each area is already
fully stocked with inhabitants; and it follows from this, that as the favored
forms increase in number, so, generally, will the less favored decrease and
become rare. Rarity, as geology tells us, is the precursor to extinction. We
can see that any form which is represented by few individuals will run a
good chance of utter extinction, during great fluctuations in the nature of
the seasons, or from a temporary increase in the number of its enemies.
But we may go further than this; for, as new forms are produced, unless
we admit that specific forms can go on indefinitely increasing in number,
many old forms must become extinct. That the number of specific forms
has not indefinitely increased, geology plainly tells us; and we shall
presently attempt to show why it is that the number of species
throughout the world has not become immeasurably great.
Extinction Caused by Natural
selection

10. We have seen that the species which are most numerous in
individuals have the best chance of producing favorable
variations within any given period. We have evidence of this,
in the facts stated in the second chapter showing that it is
the common and diffused or dominant species which offer
the greatest number of recorded varieties. Hence, rare
species will be less quickly modified or improved within any
given period; they will consequently be beaten in the race
for life by the modified and improved descendants of the
commoner species.

11. From these several considerations I think it inevitably follows, that as new species
in the course of time are formed through natural selection, others will become rarer
and rarer, and finally extinct. The forms which stand in closest competition with
those undergoing modification and improvement will naturally suffer most. And we
have seen in the chapter on the Struggle for Existence that it is the most closely-
allied forms,—varieties of the same species, and species of the same genus or of
related genera,—which, from having nearly the same structure, constitution, and
habits, generally come into the severest competition with each other; consequently,
each new variety or species, during the progress of its formation, will generally
press hardest on its nearest kindred, and tend to exterminate them. We see the
same process of extermination amongst our domesticated productions, through the
selection of improved forms by man. Many curious instances could be given
showing how quickly new breeds of cattle, sheep, and other animals, and varieties
of flowers, take the place of older and inferior kinds. In Yorkshire, it is historically
known that the ancient black cattle were displaced by the long-horns, and that
these “were swept away by the shorthorns” (I quote the words of an agricultural
writer) “as if by some murderous pestilence.”