Natural selection

© Photo: Bence Máté
Natural selection at work: An Eyelash Viper (Bothriechis schlegelii) in eye contact with a kolibri.

In biology, natural selection is the process by which natural environmental changes favor for survival and reproductive success individual organisms who happen to possess traits that allow them to resist the adverse effects of the environmental changes. Those lacking those traits tend to have a lesser chance of surviving and reproducing, a process of 'natural elimination' that obversely accompanies natural selection. Thus natural environmental changes tend to 'select' individuals whose physical and mental traits give them a reproductive advantage over their less so-endowed conspecifics in the particular environment they live in , particularly, for example, when there is not enough food for all to flourish, when only those whose traits give them competitive advantage do so. When those advantageous traits can be inherited, they will tend to become more common in the subsequent generation. If the hostile environment persists, further selection may continue for many generations, increasing what biologists call the 'fit' between the species and environment. Thus biologists characterize natural selection as increasing the 'fitness' of the species, meaning its better 'adaptation' to its environment.

Natural selection depends on the fact that, in any species, individuals will all be slightly different from each other; they all have slightly different features or abilities - we call these different traits. Many of these differences will be accidents of the environment - some will be stronger than others because they were better fed when young for example, but some differences will be heritable - they can be passed on to the offspring. Any offspring that inherit useful characteristics for the prevailing environment will more likely survive and reproduce, and so will tend pass on those traits to their offspring. Thus, over many generations, species can become increasingly adapted to their habitats. Subgroups of a species that live in different habitats will diverge over time, as each becomes adapted to their own niche; such divergence, given enough time, can lead to subpopulations becoming distinct species unable to interbreed.

This process of natural selection is a long term series of events that accounts for the 'fit' between living creatures and their habitat, explaining the presence of drought tolerant plants in the desert and moisture-loving plants in the rain forest, heavily furred mammals in the colder climates and lightly furred mammals in the warmer climes. Across generations, natural selection results in change in the characteristics of species of plants, animals, and other organisms, so that some features become emphasized and others diminished. These changes, called adaptations, lead to particular strains or "natural breeds" within a species and can account for the creation of new species (speciation), and their further evolution over time.

The theory of evolution by natural selection, like all theories, makes certain assumptions. It presumes some variety among the individual entities of any particular kind of living thing. Second, it presumes that offspring can inherit some of those variations. Thirdly, it assumes that in any given situation, some characteristics make reproductive success of individuals more likely, whereas other characteristics make it less likely. Those characteristics often consist simply of the ability to survive. The theory of evolution by natural selection predicts that living things that inherit features that bestow survival advantages for an individual, or otherwise increase the the ability of the individual that has them to successfully reproduce offspring that in turn can mature and reproduce, will tend to multiply in frequency among members of the species over generations.

The theory of evolution by natural selection is one of the cornerstones of modern biology. Charles Darwin, introduced the term "natural selection" in his 1859 book The Origin of Species , through analogy with artificial selection, by which a farmers select breeding stock. Given time, a passive process of natural selection can result in adaptations and speciations (see evolution). Less dramatically, natural selection accounts for the differing strains and breeds of plants and animals of the same species that are found in varying habitats and geographic regions. The independent discoverer of the process of natural selection, Alfred Russel Wallace, preferred the term 'survival of the fittest', in part because he saw soom different consequences, and thus a poor analogy, between artificial and natural selection.

Examples:

With viruses and bacteria one can witness evolution in real time. One of the selective forces that viruses such as influenza face is antibody in the bloodstream of influenza patients that neutralise the virus and prevent further infections, bringing epidemic spread to a halt. Similarly, bacteria that infect and colonise humans and domestic animals have been faced with widespread massive use of antibiotics since the discovery of penicillin and other new medicines in the 1940s. The natural selection of antigenic variants of influenza virus, and shifts in the major components of the virus caused by re-assortment and recombination of influenza virus genes, and the selection of multiple-drug resistant variants of bacteria are examples of natural selection occurring over the last half-century or so that are extensively documented in the medical literature.

General principles

Natural selection acts on the phenotype. The phenotype is the overall result of an individual's genetic make-up (genotype), the environment, and the interactions between genes and between genes and the environment, and an individual's phenotype changes throughout its life.

Some traits are determined by a single gene, but most are affected by many different genes. Variation in most of these genes has only a small effect on the phenotypic value of a trait, and the study of the genetics of these quantitative traits is called quantitative genetics.

The key element in understanding natural selection is the concept of fitness. Natural selection acts on individuals, but its average effect on all individuals with a particular genotype is the fitness of that genotype. Fitness is measured as the proportion of progeny that survives, multiplied by the average fecundity, and it is equivalent to the reproductive success of a genotype. A fitness value of greater than one indicates that the frequency of that genotype in the population increases, while a value of less than one indicates that it decreases. The relative fitness of a genotype is estimated as the proportion of the fitness of a reference genotype. Related to relative fitness is the selection coefficient, which is the difference between the relative fitness of two genotypes. The larger the selection coefficient, the stronger natural selection will act against the genotype with the lowest fitness.

Natural selection occurs at every life stage of an individual (see Figure 2), and selection at any of these stages can affect the likelihood that an individual will survive and reproduce. After an individual is born, it has to survive until adulthood before it can reproduce, and selection of those that reach this stage is called viability selection. In many species, adults must compete with each other for mates (sexual selection), and success in this competition determines who will parent the next generation. When species reproduce more than once, a longer survival in the reproductive phase increases the number of offspring (survival selection). The fecundity of both females (e.g. how many eggs a female bird produces) and males (e.g. giant sperm in certain species of Drosophila) can be limited (fecundity selection).

The viability of produced gametes can differ, while intragenomic conflict (meiotic drive) between the haploid gametes can result in gametic or genic selection. Finally, the union of some combinations of eggs and sperm might be more compatible that others (compatibility selection).

The creative power of natural selection

While it is easy to see how natural selection can act as a force that refines pre-existing attributes, it is less easy to see how it can work to build wholly new functions. Today, molecular biologists can in part reconstruct how new functions might have arisen through evolution by natural selection.

To take just one example, oxytocin and vasopressin are two closely related molecules - they differ by just one amino acid. They come from two separate but very similar genes, genes so similar that we think they must have arisen by an initial step of gene duplication. Oxytocin and vasopressin are peptide hormones that exert their quite different physiological actions by acting on specific receptors, expressed in different target tissues. The genes for the receptors are also very similar to each other, so the receptor genes also arose by gene duplication, probably at the same time as the gene for the peptides was duplicated So the initial mutation was a large scale gene duplication - such changes are generally neutral mutations, with no consequences for the organism. Almost all vertebrates that have been studied have genes for two peptides closely related to oxytocin and vasopressin, and virtually all invertebrates that have been studied have just one such gene. Among vertebrates, only Cyclostomata (lampreys and hagfishes) are known to have only one gene related to vasopressin and oxytocin, so the initial duplication probably occurred about 400 million years ago, before the evolution of the fishes . When a gene is duplicated, one copy is now redundant, and so is under no immediate selection pressure; accordingly it will accumulate further mutations, some of which may have incidental benefits to the organism unrelated to the function of the original gene. Over time, as natural selection works on these initially minor and incidental benefits, the two genes diverge in functionality. Now (400 million years later), in modern mammals, vasopressin mainly controls water loss from the kidneys while oxytocin mainly controls the let down of milk from the mammary gland in lactation. This function of oxytocin is quite clearly new in evolutionary terms (and specific to mammals), yet it arose from slight mutations of elements that had previously existed for a quite different purpose, refined by natural selection over millions of years.

"Ecological selection" and "sexual selection"

It is useful to distinguish between ecological selection and the narrower term, sexual selection. Ecological selection covers any mechanism of selection as a result of the environment (including relatives, e.g. kin selection, and conspecifics, e.g. competition or infanticide). Sexual selection refers specifically to competition between conspecifics for mates .

Sexual selection includes mechanisms such as mate choice and male-male competition although the two forms can act in combination in some species, when females choose the winners of the male-male competition. Mate choice, or intersexual selection, typically involves female choice, as it is usually the females who are most choosy, but in some sex-role reversed species it is the males that choose. Some features that are confined to one sex only of a particular species can be explained by selection exercised by the other sex in the choice of a mate, e.g. the extravagant plumage of some male birds. Aggression between members of the same sex (intrasexual selection) is typically referred to as male-male competition, and is sometimes associated with very distinctive features, such as the antlers of stags, which are used in combat with other stags. More generally, intrasexual selection is often associated with sexual dimorphism, including differences in body size between males and females of a species.

Genetical theory of natural selection

Natural selection by itself is a simple concept, in which fitness differences between phenotypes are crucial. However, its explanatory power comes from understanding the interplay of the selection mechanism with the underlying genetics.

Directionality of selection

When some component of a variable trait is heritable, selection can alter the frequencies of the different alleles (variants of a gene) that are responsible for that variability. Selection can be divided into three classes:

Positive or directional selection occurs when a certain allele is associated with a greater fitness than others, resulting in an increase in frequency of that allele until it is fixed and the entire population expresses the fitter phenotype.

Far more common is purifying or stabilizing selection, which lowers the frequency of alleles which have a deleterious effect on the phenotype until they are eliminated from the population. Purifying selection results in functional genetic features (e.g. protein-coding sequences or regulatory sequences) being conserved over time because of selective pressure against deleterious variants.

Finally, many forms of balancing selection do not result in fixation, but maintain an allele at intermediate frequencies in a population. This can happen in diploid species (with two pair of chromosomes) when individuals with a combination of two different alleles at a single position at the chomosome (heterozygote) have a higher fitness than individuals that have two copies of the same allele (homozygote). This is called heterozygote advantage or overdominance. Allelic variation can also be maintained through disruptive or diversifying selection, which favors genotypes that depart from the average in either direction (that is, the opposite of overdominance), and can result in a bimodal distribution of trait values. Finally, it can occur by frequency-dependent selection, where the fitness of one particular phenotype depends on the prevalence of other phenotypes in the population (see also Game theory).

Selection and genetic variation

Some genetic variation is functionally neutral; i.e., it produces no phenotypic effect or significant differences in fitness. Previously, this was thought to encompass most of the genetic variation in non-coding DNA, but parts of those sequences are highly conserved, indicating that they are under strong purifying selection, and suggesting that mutations in these regions have deleterious consequences. When genetic variation does not result in differences in fitness, selection cannot directly affect the frequency of such variation. As a result, the genetic variation at those sites will be higher than at sites where selection does have a result.

Genetic linkage

Genetic linkage occurs when two alleles are close to each other. During the formation of the gametes, recombination of the genetic material results in a reshuffling of the alleles. However, the chance that such a reshuffle occurs between two alleles depends on the distance between those alleles; the closer the alleles are to each other, the less likely it is that such a reshuffle will occur. Consequently, when selection targets one allele, this automatically results in selection of the other allele as well; through this mechanism, selection can have a strong influence on patterns of variation in the genome.

Mutation-selection balance

Natural selection results in less genetic variation by eliminating maladapted individuals and, through that, the mutations that causes the maladaptation. At the same time, new mutations arise spontaneously, resulting in a mutation-selection balance. The exact outcome depends both on the rate at which new mutations occur and on the strength of the natural selection.

Selective sweep

Selective sweeps occur when an allele becomes more common in a population as a result of positive selection. As the prevalence of one allele increases, linked alleles (those nearby on the chromosome) can also become more common, whether they are neutral or even slightly deleterious. This is called genetic hitchhiking. A strong selective sweep results in a region of the genome where the positively selected haplotype (the allele and its neighbours) are essentially the only ones that exist in the population.

Whether a selective sweep has occurred or not can be investigated by measuring linkage disequilibrium, i.e., whether a given haplotype is overrepresented in the population. Normally, genetic recombination results in a 'reshuffle' of the alleles within a haplotype, and none of the haplotypes will dominate the population. However, during a selective sweep, selection for a specific allele will also result in selection of neighbouring alleles. Therefore, the presence of strong linkage disequilibrium might indicate that there has been a 'recent' selective sweep, and this can be used to identify sites recently under selection.

Background selection

Background selection is the opposite of a selective sweep. If a specific site experiences strong and persistent purifying selection (perhaps as a result of mutation-selection balance), linked variation will tend to be weeded out along with it. However, background selection acts as a result of new mutations, which can occur randomly in any haplotype. It therefore produces no linkage disequilibrium, although it reduces the amount of variation in the region.

Evolution by means of natural selection

A prerequisite for natural selection to result in adaptive evolution, novel traits and speciation, is the presence of heritable genetic variation that results in fitness differences. Genetic variation is the result of mutations, recombinations and alterations in the karyotype (the number, shape, size and internal arrangement of the chromosomes). Any of these changes might have an effect that is highly advantageous or highly disadvantageous, but large effects are very rare. In the past, most changes in the genetic material were considered neutral or close to neutral because they occurred in noncoding DNA or resulted in a synonymous substitution. However, recent research suggests that many mutations in non-coding DNA do have slight deleterious effects. Overall, of those mutations that do affect the fitness of the individual, most are slightly deleterious, some reduce the fitness dramatically and some increase the fitness.

By the definition of fitness, individuals with greater fitness are more likely to contribute offspring to the next generation, while individuals with lesser fitness are more likely to die early or they fail to reproduce. As a result, alleles which on average result in greater fitness become more abundant in the next generation, while alleles which generally reduce fitness become rarer. If the selection forces remain the same for many generations, beneficial alleles become more and more abundant, until they dominate the population, while alleles with a lesser fitness disappear. According to evolutionary biologists, in every generation, new mutations and recombinations arise spontaneously, producing a new spectrum of phenotypes (new physical characteristics: eye color, skin color, etc). However, this idea is disputed amongst biologists of different backgrounds, with other biologists saying that each new generation will be preserved by the selection of previously existing traits that were favored by the species, but not brand new characteristics.

Some mutations occur in so-called regulatory genes. Changes in these can have large effects on the phenotype of the individual because they regulate the function of many other genes. Most, but not all, mutations in regulatory genes result in non-viable zygotes. For example, mutations in some HOX genes in humans result in an increase in the number of fingers or toes or a cervical rib. When such mutations result in a higher fitness, natural selection will favor these phenotypes and the novel trait will spread in the population.

Established traits are not immutable: an established trait may lose its fitness if environmental conditions change. In these circumstances, in the absence of natural selection to preserve the trait, the trait will become more variable and will deteriorate over time. The power of natural selection will also inevitably depend upon prevailing environmental factors; in general, the number of offspring is (far) greater than the number of individuals that can survive to the next generation, and there will be intense selection of the best adapted individuals for the next generation.

Speciation

Speciation requires selective mating, which result in a reduced gene flow. Selective mating can be the result of, for example, a change in the physical environment (physical isolation by an extrinsic barrier), or by sexual selection resulting in assortative mating. Over time, these subgroups might diverge radically to become different species, either because of differences in selection pressures on the different subgroups, or because different mutations arise spontaneously in the different populations, or because of founder effects - some potentially beneficial alleles may, by chance, be present in only one or other of two subgroups when they first become separated. When the genetic changes result in increasing incompatibility between the genotypes of the two subgroups, gene flow between the groups will be reduced even more, and will stop altogether as soon as the mutations become fixed in the respective subgroups. As few as two mutations can result in speciation: if each mutation has a neutral or positive effect on fitness when they occur separately, but a negative effect when they occur together, then fixation of these genes in the respective subgroups will lead to two reproductively isolated populations. According to the biological species concept, these will be two different species.

Historical context

Until the early 19th century, the established view in Western societies was that differences between individuals of a species were uninteresting departures from their Platonic ideal (or typus) of created kinds. However, growing awareness of the fossil record led to the recognition that species that lived in the distant past were often very different from those that exist today. In the early 19th century, radical evolutionists such as Jean Baptiste Lamarck had proposed that characteristics (adaptations) acquired by individuals might be inherited by their progeny, causing, in enough time, transmutation of species (see Lamarckism). Between 1842 and 1844, Charles Darwin outlined 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, there will be an inevitable selection of individuals with the most advantageous variations. If the variations are inherited, then differential reproductive success will lead to a progressive evolution of particular populations of a species, and populations that evolve to be sufficiently different might eventually become different species.

Darwin thought of natural selection by analogy to how farmers select crops or livestock for breeding (artificial selection); in his early manuscripts he referred to a 'Nature' which would do the selection. In the next twenty years, he shared these theories with just a few friends, while gathering evidence and trying to address all possible objections. In 1858, Alfred Russel Wallace, a young naturalist, independently conceived the principle and described it in a letter to Darwin. Not wanting to be scooped, Darwin contacted scientific friends to find an honorable way to handle this potentially embarrassing situation, and two short papers by the two were read at the Linnean Society announcing co-discovery of the principle. The following year, Darwin published The Origin of Species, along with his evidence and detailed discussion. This became a topic of great dispute; evolutionary theories became the primary way of talking about speciation, but natural selection did not predominate as the mechanism by which it happened. What made natural selection controversial was doubt about whether it was powerful enough to result in speciation, and that it was 'unguided' rather than 'progressive', something that even Darwin's supporters balked at.

Darwin's ideas were inspired by the observations that he had made on the Voyage of the Beagle, and by the economic theories of Thomas Malthus, who noted that population (if unchecked) increases exponentially whereas the food supply grows only arithmetically; thus limitations of resources would inevitably lead to a "struggle for existence", in which only the fittest would survive. Similar ideas go back to ancient times; the Ionian physician Empedocles said that many races "must have been unable to beget and continue their kind. For in the case of every species that exists, either craft or courage or speed has from the beginning of its existence protected and preserved it". Several 18th-century thinkers wrote about similar theories, including Pierre Louis Moreau de Maupertuis in 1745, Lord Monboddo in his theories of species alteration, and Darwin's grandfather Erasmus Darwin in 1794–1796. In the 6th edition of The Origin of Species Darwin acknowledged that others — notably William Charles Wells in 1813, and Patrick Matthew in 1831 — had proposed similar theories, but had not presented them fully or in notable scientific publications. Wells presented his hypothesis to explain the origin of human races in person at the Royal Society, and Matthew published his as an appendix to his book on arboriculture. Edward Blyth had also proposed a method of natural selection as a mechanism of keeping species constant. However, these 'precursors' had little influence on evolutionary thought.

Concurrent with the publication of The Origin of Species, many of Darwin's contemporaries advanced hypotheses regarding evolution. However, of the many ideas of evolution that emerged, only August Weismann's saw natural selection as the main evolutionary force. T.H. Huxley, for example, believed that there was more "purpose" in evolution than natural selection afforded. A revised version of Lamarckism also enjoyed some popularity.

Modern evolutionary synthesis

Only after the integration of a theory of evolution with a complex statistical appreciation of Mendel's 're-discovered' laws of inheritance did natural selection become generally accepted by scientists. The work of Ronald Fisher, who first tried to explain natural selection by the underlying genetic processes); J. B. S. Haldane, who introduced the concept of the 'cost' of natural selection ; Sewall Wright, one of the founders of population genetics ; Theodosius Dobzhansky, who established the idea that mutation, by creating genetic diversity, supplied the raw material for natural selection), William Hamilton, who conceived of kin selection; Ernst Mayr, who recognised the importance of reproductive isolation for speciation and many others formed the modern evolutionary synthesis. This propelled natural selection to the forefront of evolutionary theories, where it remains today. The modern evolutionary synthesis continues to undergo extension and revision

Impact of the idea

Darwin's ideas, along with those of Adam Smith and Karl Marx, had a profound influence on 19th-century thought. 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" evolved from the simplest forms of life by a few simple principles expressed in natural processes. This claim inspired some of Darwin's most ardent supporters —and provoked the most profound opposition. The radicalism of natural selection, according to Stephen Jay Gould , lay in its power to "dethrone some of the deepest and most traditional comforts of Western thought". In particular, it challenged beliefs in nature's benevolence, order, and good design, the belief that humans occupy a summit of power and excellence, belief in an omnipotent, benevolent creator, and belief that nature has any meaningful direction, or that humans fit into any sensible pattern.

The concept of natural selection has had an influence on philosophical and ethical reflection. Francis Galton, Thomas Malthus and other Social Darwinists used the principle of natural selection to justify social policies that gave no help or assistance to the poor and needy on the basis that one is then preventing the advancement of the species. Rather the poor and needy should die off to promote genetic fitness. This went much further in eugenics movements which moved to simply omitting to help those in need to actively sterilising people who fail to meet the relevant criteria for genetic health. Even today, some groups like Project Prevention – a U.S. charity that pays drug addicts $300 to be sterilised voluntarily – are considered by many to be advocating a form of eugenics. In contemporary Western societies, mainstream opinion generally regards the practice of eugenics and social Darwinism as barbaric and scientists condemn it as not just being ethically problematic but actually scientifically implausible too.

Eugenic and social Darwinist theories have had an especially horrendous role regarding race with nineteenth century scientists and social Darwinists producing elaborate descriptions of racial difference and grounding said differences in claims of biological difference, sometimes wrapped with natural selection: often suggesting that white Europeans were selected by evolution to be more intelligent – Peter Bowker lists a number of these including Petrus Camper's physiological study of skulls, where he argued that the facial angle of black races was between that of ape ancestors and European races as well as pre-genetic classifications by Carl Linnaeus and others. The development of genetics as a science has allowed many of these to be dismissed as myths. Some creationists use these unfortunate developments as a reason to dismiss evolutionary theory although such claims tend to get short shrift from mainstream historians.

The linking of natural selection as an account of how things are to how things ought to be is dismissed early on in Richard Dawkins' The Selfish Gene, one of the more popular books in the English-speaking world on evolutionary theory for a general audience. While he is a Darwinian in terms of believing that the relevant facts in biology are best explained through evolution, he argues he is strongly anti-Darwinian in a normative sense:

I am not advocating a morality of evolution. I am saying how things have evolved. I am not saying how we humans morally ought to behave. I stress this, because I know I am in danger of being misunderstood by those people, all too numerous, who cannot distingush a statement of belief in what is the case from an advocacy of what ought to be the case. My own feeling is that a human society based simply on the gene's law of universal ruthless selfishness would be a very nasty society in which to live. But unfortunately, however much we may deplore something, it does not stop it being true. This book is mainly intended to be interesting, but if you would extract a moral from it, read it as a warning. Be warned that if you wish, as I do, to build a society in which individuals cooperate generously and unselfishly towards a common good, you can expect little help from biological nature. Let us try to teach generosity and altruism, because we are born selfish. Let us understand what our own selfish genes are up to, because we may then at least have the chance to upset their designs, something that no other species has ever aspired to.

Darwinian thinking and the idea of natural selection has had an effect on other areas outside of the biological sciences. Philosophers including Daniel Dennett and Michael Ruse have argued that evolution and natural selection provide a new lens by which one can approach a variety of philosophical problems, with Dennett going so far as to argue that evolution is a "universal acid".

Debates in epistemology have shifted to take on board that cognitive functions are evolved. Much epistemological work in justification is itself often based on a rejection of the Cartesian picture of the mind, and the purported compatibility of externalist accounts of justification (that is, accounts of justification that are based on the state of the world rather than the beliefs, mental states and conscious accessibility of ideas) with the actual history of our cognitive functions as produced by natural selection is argued to be a major point in their favour.

Some 'causal history' accounts of various philosophical concepts now make reference to evolutionary history: for instance, to explain proper function (and to defend it from the competing supernaturalist account provided by the Christian philosopher Alvin Plantinga), so-called etiological accounts of proper function have been proposed by Ruth Millikan and Karen Neander. To say that a particular biological system is functioning properly in some instance is to say that it is functioning in the same way as it has in previous generations, and that function has been a result of natural selection.

In political philosophy, Peter Singer had advocated that left-wing political movements need to become "Darwinian" in the sense that they ought to accept the fact of natural selection and evolution and craft political views around the reality of an evolved human nature.