Natural selection by  Charles Darwin


Natural selection is the differential survival and reproduction  
of individuals due to differences in phenotype.
 It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charles Darwin popularised the term "natural selection", contrasting it with artificial selection, which is intentional, whereas natural selection is not.

Darwin's theory

In 1859, Charles Darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. He
defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved". The concept was
simple but powerful: individuals best adapted to their environments are more likely to survive and reproduce. As long as there is
some variation between them and that variation is heritable, there will be an inevitable selection of individuals with the most
advantageous variations. If the variations are heritable, then differential reproductive success leads to a progressive evolution of
particular populations of a species, and populations that evolve to be sufficiently different eventually become different species.
Darwin's ideas were inspired by the observations that he had made on the second voyage of HMS Beagle (1831–1836), and by the

work of a political economist, Thomas Robert Malthus, who, in An Essay on the Principle of Population (1798), noted that

population (if unchecked) increases exponentially, whereas the food supply grows only arithmetically; thus, inevitable limitations of
resources would have demographic implications, leading to a "struggle for existence".  When Darwin read Malthus in 1838 he was already primed by his work as a naturalist to appreciate the "struggle for existence" in nature. It struck him that as population outgrew resources,       "favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species."


Darwin wrote:-  If during  long course of ages and under varying  
conditions of life, organic beings vary at all in the several parts of 
their organisation, and I think this cannot be disputed; if there be, 
owing to the high geometrical powers of increase of each species, 
at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed;
 then, considering the infinite complexity of the
relations of all organic beings to each other and to their conditions 
of existence, causing an infinite diversity in structure, constitution, 
and habits, to be advantageous to them, I think it would be a most
extraordinary fact if no variation ever had occurred useful to each
being's own welfare, in the same way as so many variations have
occurred useful to man. But if variations useful to any organic 
being do occur, assuredly individuals thus characterised will have 
the best chance of being preserved in the struggle for life; and from 
the strong principle of inheritance they will tend to produce 
offspring similarly characterised. This principle of preservation, I 
have called,for the sake of brevity, Natural Selection.


Darwin thought of natural selection by analogy to how farmers select crops or livestock for breeding, which he called "artificial

selection"; in his early manuscripts he referred to a "Nature" which would do the selection. At the time, other mechanisms of

evolution such as evolution by genetic drift were not yet explicitly formulated, and Darwin believed that selection was likely only
part of the story: "I am convinced that Natural Selection has been the main but not exclusive means of modification." In a letter to
Charles Lyell in September 1860, Darwin regretted the use of the term "Natural Selection", preferring the term "Natural
Preservation"


The modern synthesis


Natural selection relies crucially on the idea of heredity, but developed before the basic concepts of genetics. Although the Moravian monk Gregor Mendel, the father of modern
genetics, was a contemporary of Darwin's, his work lay in obscurity, only being rediscovered in 1900. With the early 20th century integration of evolution with Mendel's laws of
inheritance, the so-called modern synthesis, scientists generally came to accept natural selection. The synthesis grew from advances in different fields. Ronald Fisher developed the required mathematical language and wrote The Genetical Theory of Natural
Selection 1930. J. B. S. Haldane introduced the concept of the "cost" of natural selection. Sewall Wright elucidated the nature of selection and adaptation. In his book Genetics and the Origin of Species 1937, Theodosius Dobzhanskyestablished the idea
that mutation, once seen as a rival to selection, actually supplied the raw material for natural selection by creating genetic diversity.


Terminology

The term natural selectionis most often defined to operate on heritable traits, because these directly participate in evolution. However, natural selection is "blind" in the sense that
changes in phenotype can give a reproductive advantage regardless of whether or not the trait is heritable. Following Darwin's primary usage, the term is used to refer both to the evolutionary consequence of blind selection and to its mechanisms. It is
sometimes helpful to explicitly distinguish between selection's mechanisms and its effects; when this distinction is important, scientists define "(phenotypic) natural selection" specifically as "those mechanisms that contribute to the selection of individuals that reproduce",  without regard to whether the basis of the selection is heritable. Traits that cause greater reproductive success
 of an organism are said to be selected for, while those that reduce success are selected against.


Heritable variation, differential reproduction


Natural variation occurs among the individuals of any population of organisms. Some differences may improve an individual's
chances of surviving and reproducing such that its lifetime reproductive rate is increased, which means that it leaves more offspring.

If the traits that give these individuals a reproductive advantage are also heritable, that is, passed from parent to offspring, then there
will be differential reproduction, that is, a slightly higher proportion of fast rabbits or efficient algae in the next generation. Even if
the reproductive advantage is very slight, over many generations any advantageous heritable trait becomes dominant in the
population. In this way the natural environment of an organism "selects for" traits that confer a reproductive advantage, causing evolutionary change, as Darwin described. This gives the appearance of purpose, but in natural selection there is no intentional choice. Artificial selection is purposive where natural selection is not, though biologists often use teleological language to describe it.

The peppered moth exists in both light and dark colours in Great Britain, 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. A recent study, using image analysis and avian vision models, shows that pale individuals more closely
match lichen backgrounds than dark morphs and for the first time quantifies the camouflage of moths to predation risk.

Fitness

The concept of fitness is central to natural selection. In broad terms, individuals that are more "fit" have better potential for survival,
as in the well-known phrase "survival of the fittest", but the precise meaning of the term is much more subtle. Modern evolutionary
theory defines fitness not by how long an organism lives, but by how successful it is at reproducing. If an organism lives half as long as others of its species, but has twice as many offspring surviving to adulthood, its genes become more common in the adult
population of the next generation. Though natural selection acts on individuals, the effects of chance mean that fitness can only really
be defined "on average" for the individuals within a population. The fitness of a particular genotype corresponds to the average 
effect on all individuals with that genotype.

Origin of life

How life originated from inorganic matter remains an unresolved problem in biology. One prominent hypothesis is that life first
appeared in the form of short self-replicating RNA polymers. On this view, life may have come into existence when RNA chains
first experienced the basic conditions, as conceived by Charles Darwin, for natural selection to operate. These conditions are:
heritability, variation of type, and competition for limited resources. The fitness of an early RNA replicatorwould likely have been a
function of adaptive capacities that were intrinsic (i.e., determined by the nucleotide sequence) and the availability of
resources. The three primary adaptive capacities could logically have been: (1) the capacity to replicate with moderate
fidelity (giving rise to both heritability and variation of type), (2) the capacity to avoid decay, and (3) the capacity to acquire and
process resources. These capacities would have been determined initially by the folded configurations (including those
configurations with ribozyme activity) of the RNA replicators that, in turn, would have been encoded in their individual nucleotide
sequences.

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