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 heritable traits of a population over time. The term "natural selection" was popularised by Charles Darwin who compared it with artificial selection, now more commonly referred to as selective breeding.
Variation exists within all populations of organisms. This occurs partly because random mutations arise in the genome of an individual organism, and these mutations can be passed to offspring. Throughout the individuals’ lives, their genomes interact with their environments to cause variations in traits. (The environment of a genome includes the molecular biology in the cell, other cells, other individuals, populations, species, as well as the abiotic environment.) Individuals with certain variants
of the trait may survive and reproduce more than individuals with other,
less successful, variants. Therefore, the population evolves. Factors
that affect reproductive success are also important, an issue that
Darwin developed in his ideas on sexual selection, which was redefined as being included in natural selection in the 1930s
when biologists considered it not to be very important, and fecundity selection, for example.
Natural selection acts on the phenotype, or the observable characteristics of an organism, but the genetic (heritable) basis of any phenotype that gives a reproductive advantage may become more common in a population (see allele frequency). Over time, this process can result in populations that specialise for particular ecological niches (microevolution) and may eventually result in the emergence of new species (macroevolution).
In other words, natural selection is an important process (though not
the only process) by which evolution takes place within a population of
organisms. Natural selection can be contrasted with artificial
selection, in which humans intentionally choose specific traits (although they may not always get
what they want). In natural selection there is no intentional choice. In
other words, artificial selection is teleological and natural selection is not teleological.
Natural selection is one of the cornerstones of modern biology. The concept was published by Darwin and Alfred Russel Wallace in a joint presentation of papers in 1858, and set out in Darwin's influential 1859 book On the Origin of Species, in which natural selection was described as analogous to artificial
selection, a process by which animals and plants with traits considered
desirable by human breeders are systematically favoured for
reproduction. The concept of natural selection was originally developed
in the absence of a valid theory of heredity; at the time of Darwin's writing, nothing was known of modern genetics. The union of traditional Darwinian evolution with subsequent discoveries in classical and molecular gentics is termed the modern evolutionary synthesis.Natural selection remains the primary explanation for adaptive evolution.
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 colours 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.
If the traits that give these individuals a reproductive advantage
are also heritable, that is, passed from parent to offspring, 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.
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.
By
Reshma P
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