Natural selection is the process by which
heritable traits
that make it more likely for an
organism to
survive and successfully
reproduce
become more common in a
population over
successive generations. It is a key mechanism of
evolution.
The natural
genetic variation
within a population of organisms means that some individuals will
survive and reproduce more successfully than others in their
current
environment.
For example, 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-colored moths an advantage in hiding from predators. This gave dark-colored moths a
better chance of surviving to produce dark-colored offspring, and
in just a few generations the majority of the moths were dark.
Factors which affect reproductive success are also important, an
issue which
Charles Darwin developed
in his ideas on
sexual
selection.
Natural selection acts on the
phenotype,
or the observable characteristics of an organism, but the
genetic (heritable) basis of any phenotype which
gives a reproductive advantage will become more common in a
population (see
allele frequency).
Over time, this process can result in
adaptations that specialize organisms for
particular
ecological niches and
may eventually result in the
emergence of new
species. 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 is one of the cornerstones of modern
biology. The term was introduced by Darwin in his
groundbreaking 1859 book
On
the Origin of Species, in which natural selection was
described by analogy to
artificial
selection, a process by which animals and plants with traits
considered desirable by human breeders are systematically favored
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 genetics 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 (such
as differences in eye color in humans), but some differences may
improve the chances of survival 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.
Individuals that have better odds for survival also have better
odds for reproduction. 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, due to
exponential growth. 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, which is the study of heredity. In modern times, it is
understood that 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
gene flow), natural selection is still
understood as a fundamental mechanism for evolution.
Nomenclature and usage
The term
natural selection has slightly different
definitions in different contexts. It is most often defined to
operate on heritable traits, because these are the traits that
directly participate in evolution. However, natural selection is
"blind" in the sense that changes in phenotype (physical and
behavioral characteristics) can give a reproductive advantage
regardless of whether or not the trait is heritable (non heritable
traits can be the result of environmental factors or the life
experience of the organism).
Following Darwin's primary usage the term is often used to refer to
both the evolutionary consequence of blind selection and to its
mechanisms.Works employing or describing this usage:
It is sometimes helpful to explicitly distinguish between
selection's mechanisms and its effects; when this distinction is
important, scientists define "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.
This is sometimes referred to as "phenotypic natural
selection".Works employing or describing this usage:
Lande R & Arnold SJ (1983) The measurement of selection on
correlated characters. Evolution 37:1210-26
Futuyma DJ (2005)
Evolution. Sinauer Associates, Inc., Sunderland
, Massachusetts
. ISBN 0-87893-187-2
Haldane, J.B.S. 1953. The measurement of natural selection.
Proceedings of the 9th International Congress of Genetics. 1:
480-487
Traits that cause greater reproductive success of an organism are
said to be selected for, whereas those that reduce success are
selected against. Selection for a trait may also result in the
selection of other correlated traits that do not themselves
directly influence reproductive advantage. This may occur as a
result of
pleiotropy or
gene linkage.
Fitness
The concept of
fitness is central
to natural selection. Broadly, individuals which are more "fit"
have better potential for survival, as in the well-known phrase
"
survival of the fittest".
However, as with natural selection above, the precise meaning of
the term is much more subtle, and
Richard Dawkins manages in his later books
to avoid it entirely. (He devotes a chapter of his book,
The Extended
Phenotype, to discussing the various senses in which the
term is used). 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 will 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. Very low-fitness genotypes cause their bearers to
have few or no offspring on average; examples include many human
genetic disorders like
cystic fibrosis. Since fitness is an
averaged quantity, it is also possible that a favorable mutation
arises in an individual that does not survive to adulthood for
unrelated reasons. Fitness also depends crucially upon the
environment. Conditions like
sickle-cell anemia may have low fitness
in the general human population, but because the
sickle-cell trait confers immunity from
malaria, it has high fitness value in populations which have high
malaria infection rates.
Types of selection
Natural selection can act on any phenotypic trait, and selective
pressure can be produced by any aspect of the environment,
including
sexual selection and
competition with
members of the same species. However, this does not imply that
natural selection is always directional and results in adaptive
evolution; natural selection often results in the maintenance of
the status quo by eliminating less fit variants.
The
unit of selection can be the
individual or it can be another level within the hierarchy of
biological organisation, such as genes,
cells, and
kin
groups. There is still debate about whether natural selection
acts at the level of
groups or
species to produce adaptations that benefit a larger, non-kin
group. Selection at a different level such as the gene can result
in an increase in fitness for that gene, while at the same time
reducing the fitness of the individuals carrying that gene, in a
process called
intragenomic
conflict. Overall, the combined effect of all selection
pressures at various levels determines the overall fitness of an
individual, and hence the outcome of natural selection.

The life cycle of a sexually
reproducing organism.
Various components of natural selection are indicated for each
life stage.
Natural selection occurs at every life stage of an individual. An
individual organism must 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 via sexual selection, and success in this
competition determines who will parent the next generation. When
individuals can reproduce more than once, a longer survival in the
reproductive phase increases the number of offspring, called
survival selection. The
fecundity
of both females and males (for example, giant
sperm in certain species of
Drosophila) can be limited via "fecundity
selection". The viability of produced
gametes
can differ, while intragenomic conflicts such as 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 than
others; this is termed
compatibility selection.
Sexual selection
It is useful to distinguish between "
ecological selection" and "sexual
selection". Ecological selection covers any mechanism of selection
as a result of the environment (including relatives, e.g.
kin selection,
competition, and
infanticide), while "sexual selection"
refers specifically to competition for mates. Sexual selection can
be
intrasexual, as in cases of competition among
individuals of the same sex in a population, or
intersexual, as in cases where one sex controls
reproductive access by choosing among a population of available
mates. Most commonly, intrasexual selection involves male–male
competition and intersexual selection involves female choice of
suitable males, due to the generally greater investment of
resources for a female than a male in a single offspring. However,
some species exhibit sex-role reversed behavior in which it is
males that are most selective in mate choice; the best-known
examples of this pattern occur in some fishes of the family
Syngnathidae, though likely
examples have also been found in amphibian and bird species. 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, for example, the extravagant plumage of some male
birds. Similarly, aggression between members of the same sex 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.
Examples of natural selection

Resistance to antibiotics is increased
though the survival of individuals which are immune to the effects
of the antibiotic, whose offspring then inherit the resistance,
creating a new population of resistant bacteria.
A well-known example of natural selection in action is the
development of
antibiotic
resistance in
microorganisms.
Since the discovery of
penicillin in 1928
by
Alexander Fleming,
antibiotics have been used to fight
bacterial diseases. Natural populations of bacteria
contain, among their vast numbers of individual members,
considerable variation in their genetic material, primarily as the
result of mutations. When exposed to antibiotics, most bacteria die
quickly, but some may have mutations that make them slightly less
susceptible. If the exposure to antibiotics is short, these
individuals will survive the treatment. This selective elimination
of maladapted individuals from a population is natural
selection.
These surviving bacteria will then reproduce again, producing the
next generation. Due to the elimination of the maladapted
individuals in the past generation, this population contains more
bacteria that have some resistance against the antibiotic. At the
same time, new mutations occur, contributing new genetic variation
to the existing genetic variation. Spontaneous mutations are very
rare, and advantageous mutations are even rarer. However,
populations of bacteria are large enough that a few individuals
will have beneficial mutations. If a new mutation reduces their
susceptibility to an antibiotic, these individuals are more likely
to survive when next confronted with that antibiotic. Given enough
time, and repeated exposure to the antibiotic, a population of
antibiotic-resistant bacteria will emerge. This new changed
population of antibiotic-resistant bacteria is optimally adapted to
the context it evolved in. At the same time, it is not necessarily
optimally adapted any more to the old antibiotic free environment.
The end result of natural selection is two populations that are
both optimally adapted to their specific environment, while both
perform substandard in the other environment.
The widespread use and misuse of antibiotics has resulted in
increased microbial resistance to antibiotics in clinical use, to
the point that the
methicillin-resistant
Staphylococcus aureus (MRSA) has been described as a
"superbug" because of the threat it poses to health and its
relative invulnerability to existing drugs. Response strategies
typically include the use of different, stronger antibiotics;
however, new
strains of MRSA have
recently emerged that are resistant even to these drugs.
[3326] This is an example of what is known as
an
evolutionary arms race, in
which bacteria continue to develop strains that are less
susceptible to antibiotics, while medical researchers continue to
develop new antibiotics that can kill them. A similar situation
occurs with
pesticide
resistance in plants and insects. Arms races are not
necessarily induced by man; a well-documented example involves the
elaboration of the
RNA interference
pathway in plants as means of
innate
immunity against
viruses.
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,
recombination 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.
Although both mutation rates and average fitness effects of
mutations are dependent on the organism, estimates from data in
humans have found that a majority of mutations
are slightly deleterious.
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
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. In every
generation, new mutations and recombinations arise spontaneously,
producing a new spectrum of phenotypes. Therefore, each new
generation will be enriched by the increasing abundance of alleles
that contribute to those traits that were favored by selection,
enhancing these traits over successive generations.
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. Examples of nonlethal regulatory mutations
occur in
HOX genes in humans, which can
result in a
cervical rib or
polydactyly, an increase in the number of
fingers or toes. 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; traits that have high fitness
in one environmental context may be much less fit if environmental
conditions change. In the absence of natural selection to preserve
such a trait, it will become more variable and deteriorate over
time, possibly resulting in a
vestigial manifestation of the trait. In
many circumstances, the apparently vestigial structure may retain a
limited functionality, or may be co-opted for other advantageous
traits in a phenomenon known as
preadaptation. A famous example of a vestigial
structure, the
eye of the
blind mole rat, is believed to retain
function in
photoperiod
perception.
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. A lesser-known
mechanism of speciation occurs via
hybrid, well-documented in plants and
occasionally observed in species-rich groups of animals such as
cichlid fishes. Such mechanisms of rapid
speciation can reflect a mechanism of evolutionary change known as
punctuated equilibrium, which
suggests that evolutionary change and particularly speciation
typically happens quickly after interrupting long periods of
stasis.
Genetic changes within groups result in increasing incompatibility
between the genomes of the two subgroups, thus reducing gene flow
between the groups. Gene flow will effectively cease when the
distinctive mutations characterizing each subgroup become fixed. 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 development
Pre-Darwinian theories
Several ancient philosophers expressed the idea that nature
produces a huge variety of creatures, apparently randomly, and that
only those creatures survive that manage to provide for themselves
and reproduce successfully; well-known examples include
Empedocles and his intellectual successor,
Lucretius, while related ideas were later
refined by
Aristotle. The
struggle for existence was later described by
al-Jahiz in the 9th century. Such classical
arguments were reintroduced in the 18th century by
Pierre Louis Maupertuis and others,
including Charles Darwin's grandfather
Erasmus Darwin. While these forerunners had
an influence on Darwinism, they later had little influence on the
trajectory of evolutionary thought after Charles Darwin.
Until the early 19th century, the
prevailing view
in Western societies was that differences between individuals of a
species were uninteresting departures from their
Platonic idealism (or typus) of
created kinds. However, the theory of
uniformitarianism in
geology promoted the idea that simple, weak forces
could act continuously over long periods of time to produce radical
changes in the Earth's landscape. The success of this theory raised
awareness of the vast scale of
geological time and made plausible the idea
that tiny, virtually imperceptible changes in successive
generations could produce consequences on the scale of differences
between species. Early 19th century evolutionists such as
Jean Baptiste Lamarck suggested the
inheritance of
acquired characteristics as a mechanism for evolutionary
change; adaptive traits acquired by an organism during its lifetime
could be inherited by that organism's progeny, eventually causing
transmutation of species.
This
theory has come to be known as Lamarckism
and was an influence on the anti-genetic ideas of the Stalinist Soviet
biologist
Trofim Lysenko.
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, 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
eventually become different species.
Darwin's ideas were inspired by the observations that he had made
on the
Beagle
voyage, and by the work of two political economists. The first
was the Reverend
Thomas Malthus, who
in
An Essay
on the Principle of Population, 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" as a divinely ordained law
"in order to rouse man into action, and form his mind to reason"
for the greater good despite the "partial evil" limiting
population. The second was
Adam Smith
who, in
The Wealth of
Nations, identified a regulating mechanism in free
markets, which he referred to as the "
invisible hand", which suggests that prices
self-adjust according to supplies and demand. When Darwin read
Malthus in 1838 he was already primed by his work as a naturalist
to appreciate the "struggle for existence" in nature and 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."
Once he had his theory "by which to work", Darwin was meticulous
about gathering and refining evidence as his "prime hobby" before
making his idea public. He was in the process of writing his "big
book" to present his researches when the naturalist
Alfred Russel Wallace independently
conceived of the principle and described it in an essay he sent to
Darwin to forward to
Charles Lyell.
Lyell and
Joseph Dalton Hooker
decided (without Wallace's knowledge) to present his essay together
with unpublished writings which Darwin had sent to fellow
naturalists, and
On the Tendency of Species to form Varieties; and on the
Perpetuation of Varieties and Species by Natural Means of
Selection was read to the
Linnean Society announcing co-discovery of
the principle in July 1858. Darwin published a detailed account of
his evidence and conclusions in
On the Origin of Species
in 1859. In the 3rd edition of 1861 Darwin acknowledged that others
— notably
William Charles
Wells in 1813, and
Patrick
Matthew in 1831 — had proposed similar ideas, but had neither
developed them nor presented them in notable scientific
publications.
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 [it] 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". For Darwin and his contemporaries, natural selection
was essentially synonymous with evolution by natural selection.
After the publication of
The Origin of Species, educated
people generally accepted that evolution had occurred in some form.
However, natural selection remained controversial as a mechanism,
partly because it was perceived to be too weak to explain the range
of observed characteristics of living organisms, and partly because
even supporters of evolution balked at its "unguided" and
non-progressive nature, a response that has been characterized as
the single most significant impediment to the idea's acceptance.
However, some thinkers enthusiastically embraced natural selection;
after reading Darwin,
Herbert
Spencer introduced the term
survival of the fittest,
which became a popular summary of the theory.
The fifth edition of
On the Origin of Species
published in 1869 included Spencer's phrase as an alternative to
natural selection, with credit given: "But the expression often
used by Mr. Herbert Spencer, of the Survival of the Fittest, is
more accurate, and is sometimes equally convenient." Although the
phrase is still often used by non-biologists, modern biologists
avoid it because it is
tautological if "fittest" is read to
mean "functionally superior" and is applied to individuals rather
than considered as an averaged quantity over populations.
Modern evolutionary synthesis
Natural selection relies crucially on the idea of heredity, but it
was developed long before the basic concepts of genetics. Although
Austrian monk
Gregor Mendel, the
father of modern genetics, was a contemporary of Darwin's, his work
would lie in obscurity until the early 20th century. Only after the
integration of Darwin's theory of evolution with a complex
statistical appreciation of Gregor Mendel's 're-discovered' laws of
inheritance did natural selection become generally accepted by
scientists. The work of
Ronald Fisher
(who developed the required mathematical language and
the genetical theory
of natural selection),
J.B.S.
Haldane (who introduced the concept
of the "cost" of natural selection),
Sewall Wright (who elucidated the nature of
selection and adaptation),
Theodosius Dobzhansky (who established
the idea that mutation, by creating genetic diversity, supplied the
raw material for natural selection: see
Genetics and the Origin of
Species),
William Hamilton (who
conceived of kin selection),
Ernst Mayr
(who recognised the key importance of reproductive isolation for
speciation: see
Systematics and the Origin
of Species) and many others formed the modern evolutionary
synthesis. This synthesis cemented natural selection as the
foundation of evolutionary theory, where it remains today.
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 the 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. 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 long-standing beliefs in such concepts as a special and
exalted place for humans in the natural world and a benevolent
creator whose intentions were reflected in nature's order and
design.
Social and psychological theory
The social implications of the theory of evolution by natural
selection also became the source of continuing controversy.
Friedrich Engels, a German
political philosopher and
co-originator of the ideology of
communism, wrote in 1872 that "Darwin did not know
what a bitter satire he wrote on mankind when he showed that free
competition, the struggle for existence, which the economists
celebrate as the highest historical achievement, is the normal
state of the animal kingdom". Interpretation of natural selection
as necessarily 'progressive', leading to increasing 'advances' in
intelligence and civilisation, was used as a justification for
colonialism and policies of
eugenics, as well as broader sociopolitical
positions now described as
Social
Darwinism.
Konrad Lorenz won the
Nobel Prize in
Physiology or Medicine in 1973 for his analysis of animal
behavior in terms of the role of natural selection (particularly
group selection). However, in Germany in 1940, in writings that he
subsequently disowned, he used the theory as a justification for
policies of the
Nazi state. He wrote "...
selection for toughness, heroism, and social utility...must be
accomplished by some human institution, if mankind, in default of
selective factors, is not to be ruined by domestication-induced
degeneracy. The racial idea as the basis of our state has already
accomplished much in this respect." Others have developed ideas
that human societies and culture
evolve by mechanisms that are
analogous to those that apply to evolution of species.
More recently, work among anthropologists and psychologists has led
to the development of
sociobiology and
later
evolutionary
psychology, a field that attempts to explain features of
human psychology in terms of
adaptation to the ancestral environment. The most prominent such
example, notably advanced in the early work of
Noam Chomsky and later by
Steven Pinker, is the hypothesis that the
human brain is adapted to
acquire the
grammatical rules of
natural language. Other aspects of human
behavior and social structures, from specific cultural norms such
as
incest
avoidance to broader patterns such as
gender roles, have been hypothesized to have
similar origins as adaptations to the early environment in which
modern humans evolved. By analogy to the action of natural
selection on genes, the concept of
memes -
"units of cultural transmission", or culture's equivalents of genes
undergoing selection and recombination - has arisen, first
described in this form by
Richard
Dawkins and subsequently expanded upon by philosophers such as
Daniel Dennett as explanations for
complex cultural activities, including human
consciousness. Extensions of the theory of
natural selection to such a wide range of cultural phenomena have
been distinctly controversial and are not widely accepted.
Information and systems theory
In 1922,
Alfred Lotka proposed that
natural selection might be understood as a physical principle which
could be described in terms of the use of
energy by a system,Lotka AJ (1922a)
Contribution to the energetics of evolution
[PDF]
Proc Natl Acad Sci USA 8:147–51
Lotka AJ (1922b)
Natural selection as a physical principle [PDF]
Proc Natl Acad Sci USA 8:151–4 a concept that was later
developed by
Howard Odum as the
maximum power principle
whereby evolutionary systems with selective advantage maximise the
rate of useful energy transformation. Such concepts are sometimes
relevant in the study of applied
thermodynamics.
The principles of natural selection have inspired a variety of
computational techniques, such as "soft"
artificial life, that simulate selective
processes and can be highly efficient in 'adapting' entities to an
environment defined by a specified
fitness function. For example, a class of
heuristic optimization algorithms known as
genetic algorithms, pioneered by
John Holland in the 1970s and expanded
upon by
David E. Goldberg, identify optimal solutions by
simulated reproduction and mutation of a population of solutions
defined by an initial
probability distribution. Such
algorithms are particularly useful when applied to problems whose
solution landscape is very rough or
has many local minima.
Genetic basis of natural selection
The idea of natural selection predates the understanding of
genetics. We now have a much better idea of the biology underlying
heritability, which is the basis of natural selection.
Genotype and Phenotype
- See also: Genotype-phenotype
distinction.
Natural selection acts on an organism's phenotype, or physical
characteristics. Phenotype is determined by an organism's genetic
make-up (genotype) and the environment in which the organism lives.
Often, natural selection acts on specific traits of an individual,
and the terms phenotype and genotype are used narrowly to indicate
these specific traits.
When different organisms in a population possess different versions
of a gene for a certain trait, each of these versions is known as
an allele. It is this genetic variation that underlies phenotypic
traits. A typical example is that certain combinations of genes for
eye color in humans which, for instance,
give rise to the phenotype of blue eyes. (On the other hand, when
all the organisms in a population share the same allele for a
particular trait, and this state is stable over time, the allele is
said to be
fixed in that
population.)
Some traits are governed by only a single gene, but most traits are
influenced by the interactions of many genes. A variation in one of
the many genes that contributes to a trait may have only a small
effect on the phenotype; together, these genes can produce a
continuum of possible phenotypic values.
Directionality of selection
When some component of a trait is heritable, selection will alter
the frequencies of the different alleles, or variants of the gene
that produces the variants of the trait. Selection can be divided
into three classes, on the basis of its effect on allele
frequencies.
Directional selection occurs
when a certain allele has a greater fitness than others, resulting
in an increase of its frequency. This process can continue until
the allele is
fixed
and the entire population shares the fitter phenotype. It is
directional selection that is illustrated in the antibiotic
resistance example
above.
Far more common is
stabilizing
selection (also known as
purifying selection), which
lowers the frequency of alleles that have a deleterious effect on
the phenotype - that is, produce organisms of lower fitness. This
process can continue until the allele is eliminated from the
population. Purifying selection results in functional genetic
features, such as
protein-coding
genes or
regulatory
sequences, being
conserved over time due to selective
pressure against deleterious variants.
Finally, a number of forms of
balancing selection exist, which do not
result in fixation, but maintain an allele at intermediate
frequencies in a population. This can occur in
diploid species (that is, those that have two pairs
of
chromosomes) when
heterozygote individuals, who have different
alleles on each chromosome at a single
genetic locus, have a higher fitness than
homozygote individuals that have two of
the same alleles. This is called
heterozygote advantage or
overdominance, of which the best-known example is the
malarial resistance observed in heterozygous humans
who carry only one copy of the gene for
sickle cell anemia. Maintenance of
allelic variation can also occur 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, balancing selection can occur through
frequency-dependent
selection, where the fitness of one particular phenotype
depends on the distribution of other phenotypes in the population.
The principles of
game theory have been
applied to understand the fitness distributions in these
situations, particularly in the study of
kin selection and the evolution of
reciprocal altruism.
Selection and genetic variation
A portion of all
genetic variation
is functionally neutral in that it produces no phenotypic effect or
significant difference in fitness; the hypothesis that this
variation accounts for a large fraction of observed
genetic diversity is known as the
neutral theory of
molecular evolution and was originated by
Motoo Kimura. 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 variation does influence fitness.
Mutation selection balance
Natural selection results in the reduction of genetic variation
through the elimination of maladapted individuals and consequently
of the mutations that caused the maladaptation. At the same time,
new mutations occur, resulting in a
mutation-selection balance. The
exact outcome of the two processes depends both on the rate at
which new mutations occur and on the strength of the natural
selection, which is a function of how unfavorable the mutation
proves to be. Consequently, changes in the mutation rate or the
selection pressure will result in a different mutation-selection
balance.
Genetic linkage
Genetic linkage occurs when the
loci of two alleles are
linked, or in close proximity to each other on the
chromosome. During the formation of
gametes,
recombination of the genetic
material results in 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.
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 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, or whether a given haplotype is overrepresented
in the population. Normally,
genetic recombination results in a
reshuffling of the different 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 is the
opposite of a selective sweep. If a specific site experiences
strong and persistent purifying selection, linked variation will
tend to be weeded out along with it, producing a region in the
genome of low overall variability. Because background selection is
a result of deleterious new mutations, which can occur randomly in
any haplotype, it produces no linkage disequilibrium.
See also
References
- Darwin C (1859) On the Origin of Species by Means of
Natural Selection, or the Preservation of Favoured Races in the
Struggle for Life John Murray, London; modern reprint
Published online at The complete work of Charles Darwin online: On the origin of species by means of natural
selection, or the preservation of favoured races in the struggle
for life.
- Fisher RA (1930) The Genetical Theory
of Natural Selection Clarendon Press, Oxford
- Sober E
(1984; 1993) The Nature of Selection: Evolutionary Theory in
Philosophical Focus University of Chicago Press
ISBN 0-226-76748-5
- Modified from Christiansen FB (1984) The definition and
measurement of fitness. In: Evolutionary ecology (ed.
Shorrocks B) pp65-79. Blackwell Scientific, Oxford by adding survival selection in
the reproductive phase
- Pitnick S & Markow TA (1994) Large-male advantage
associated with the costs of sperm production in Drosophila
hydei, a species with giant sperm. Proc Natl Acad Sci USA 91:9277-81; Pitnick S
(1996) Investment in testes and the cost of making long sperm in
Drosophila. Am Nat 148:57-80
- Eens M, Pinxten R. (2000). Sex-role reversal in vertebrates:
behavioural and endocrinological accounts. Behav Processes
51(1-3):135-147. PMID 11074317
- Barlow GW. (2005). How Do We Decide that a Species is Sex-Role
Reversed? The Quarterly Review of
Biology 80(1):28–35. PMID 15884733
- Lucy A, Guo H, Li W, Ding S (2000). "Suppression of
post-transcriptional gene silencing by a plant viral protein
localized in the nucleus". EMBO J 19 (7): 1672–80.
PMID 10747034.
- Kryukov GV, Schmidt S & Sunyaev S (2005) Small fitness
effect of mutations in highly conserved non-coding regions.
Human Molecular Genetics 14:2221-9
- Bejerano G, Pheasant M, Makunin I, Stephen S, Kent WJ, Mattick
JS & Haussler D (2004) Ultraconserved elements in the human
genome. Science 304:1321-5
- Eyre-Walker A, Woolfit M, Phelps T. (2006). The distribution of
fitness effects of new deleterious amino acid mutations in humans.
Genetics 173(2):891-900. PMID 16547091
- Galis F (1999) Why do almost all mammals have seven cervical
vertebrae?
developmental constraints, Hox genes, and cancer. J Exp
Zool 285:19-26
- Zakany J, FromentalRamain C, Warot X & Duboule D (1997)
Regulation of number and size of digits by posterior Hox genes: a
dose-dependent mechanism with potential evolutionary implications.
Proc Natl Acad Sci
USA 94:13695-700
- Sanyal S, Jansen HG, de Grip WJ, Nevo E, de Jong WW. (1990).
The eye of the blind mole rat, Spalax ehrenbergi. Rudiment with
hidden function? Invest Ophthalmol Vis Sci. 1990
31(7):1398-404. PMID 2142147
- Salzburger W, Baric S, Sturmbauer C. (2002). Speciation via
introgressive hybridization in East African cichlids? Mol
Ecol 11(3): 619–625. PMID 11918795
- Conway Zirkle (1941). Natural Selection before the "Origin of
Species", Proceedings of the American Philosophical
Society 84 (1), p. 71-123.
- Mehmet Bayrakdar (Third Quarter, 1983). "Al-Jahiz And the Rise
of Biological Evolutionism", The Islamic Quarterly.
London.
- Chevalier de Lamarck J-B, de Monet PA
(1809) Philosophie Zoologique
- Joravsky D. (1959). Soviet Marxism and Biology before Lysenko.
Journal of the History of Ideas 20(1):85-104.
- Orrell,
David (2007) Apollo's Arrow Toronto: HarperCollins
Publishers Ltd. [1]
- Wallace, Alfred Russel (1870)
Contributions to the Theory of Natural Selection New York:
Macmillan & Co. [2]
- Eisley L. (1958). Darwin's Century: Evolution and the Men
Who Discovered It. Doubleday & Co: New York, USA.
- Kuhn TS. [1962] (1996). The Structure of Scientific
Revolution 3rd ed. University of Chicago Press: Chicago,
Illinois, USA. ISBN 0-226-45808-3
- Mills SK, Beatty JH. [1979] (1994). The Propensity
Interpretation of Fitness. Originally in Philosophy of
Science (1979) 46: 263-286; republished in Conceptual
Issues in Evolutionary Biology 2nd ed. Elliott Sober, ed. MIT
Press: Cambridge, Massachusetts, USA. pp3-23. ISBN
0-262-69162-0.
- Haldane JBS (1932) The Causes of Evolution;
Haldane JBS (1957) The cost of natural selection. J Genet
55:511-24([3].
- Wright S (1932) The roles of mutation, inbreeding, crossbreeding
and selection in evolution Proc 6th Int Cong Genet
1:356–66
- Dobzhansky Th (1937) Genetics and the Origin of
Species Columbia University Press, New York. (2nd ed.,
1941; 3rd edn., 1951)
- Mayr E (1942) Systematics and the Origin
of Species Columbia University Press, New York. ISBN
0-674-86250-3
- The New York Review of Books: Darwinian Fundamentalism
(accessed May 6, 2006)
- Engels F (1873-86)
Dialectics of Nature 3d ed. Moscow: Progress, 1964
[4]
- Quoted in translation in Eisenberg L (2005) Which image for
Lorenz? Am J Psychiatry 162:1760 [5]
- e.g. Wilson, DS (2002) Darwin's Cathedral: Evolution,
Religion, and the Nature of Society. University of Chicago
Press, ISBN 0-226-90134-3
- Pinker S. [1994] (1995). The Language Instinct: How the
Mind Creates Language. HarperCollins: New York, NY, USA. ISBN
0-06-097651-9
- Dawkins R. [1976] (1989). The Selfish Gene. Oxford
University Press: New York, NY, USA, p.192. ISBN 0-19-286092-5
- Dennett DC. (1991). Consciousness Explained. Little,
Brown, and Co: New York, NY, USA. ISBN 0-316-18066-1
- For example, see Rose H, Rose SPR, Jencks C. (2000). Alas,
Poor Darwin: Arguments Against Evolutionary Psychology.
Harmony Books. ISBN 0609605135
- Kauffman
SA (1993) The Origin of order. Self-organization and
selection in evolution. New York: Oxford University Press ISBN
0-19-507951-5
- Goldberg DE. (1989). Genetic Algorithms in Search, Optimization
and Machine Learning. Addison-Wesley: Boston, MA, USA
- Mitchell, Melanie, (1996), An Introduction to Genetic
Algorithms, MIT Press, Cambridge, MA.
- Falconer DS & Mackay TFC (1996) Introduction to
Quantitative Genetics Addison Wesley Longman, Harlow, Essex,
UK ISBN 0-582-24302-5
- Rice SH. (2004). Evolutionary Theory: Mathematical and
Conceptual Foundations. Sinauer Associates: Sunderland,
Massachusetts, USA. ISBN 0-87893-702-1 See esp. ch. 5 and 6 for a
quantitative treatment.
- Hamilton WD. (1964). The genetical evolution of social
behaviour I and II. Journal of Theoretical Biology 7: 1-16
and 17-52. PMID 5875341 PMID 5875340
- Trivers RL. (1971). The evolution of reciprocal altruism. Q
Rev Biol 46: 35-57.
Further reading
- Historical
- Zirkle C (1941). Natural Selection before the "Origin of
Species", Proceedings of the American Philosophical
Society 84 (1), p. 71-123.
- Kohm M (2004) A Reason for Everything: Natural Selection
and the English Imagination. London: Faber and Faber. ISBN
0-571-22392-3. For review, see [3328] van Wyhe J (2005) Human Nature Review
5:1-4
External links