Example of natural selection in animals

Through examples across different species, it becomes clear how certain traits provide advantages that are passed on, while others may lead to decline or extinction. Save my name, email, and website in this browser for the next time I comment. This site uses Akismet to reduce spam. Learn how your comment data is processed. On this page. In This Article.

What is Natural Selection? Did this page help you? Helpful Not Helpful. Description Optional. Evolution is also chaotic, meaning that the changes it produces are not always predictable. This is partly because it relies on random mutations to produce the raw material on which natural selection can act. More controversially, it has been argued that natural selection has a kind of memory that allows it to swiftly recreate old solutions when they are needed.

Nowadays there is also a new force in evolution: humans. We are exerting new selection pressures on many specieschanging them in unpredictable ways. Receive a weekly dose of discovery in your inbox! We'll also keep you up to date with New Scientist events and special offers. Close Advertisement. Subscribe now. Natural selection. InCharles 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". As long as there is some variation between them and that variation is heritablethere will be an inevitable selection of individuals with the most advantageous variations. If the variations are heritable, then differential reproductive success leads to the 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 —and by the work of a political economist, Thomas Robert Malthuswho, in An Essay on the Principle of Populationnoted that population if unchecked increases exponentiallywhereas the food supply grows only arithmetically ; thus, inevitable limitations of resources would have demographic implications, leading to a "struggle for existence".

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. If during the 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.

Once he had this hypothesisDarwin was meticulous about gathering and refining evidence of consilience to meet standards of methodology before making his scientific theory public. Lyell and Joseph Dalton Hooker decided to present his essay together with unpublished writings that 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 of London announcing co-discovery of the principle in July In later editions Darwin acknowledged that earlier writers—like William Charles Wells in[ 24 ] and Patrick Matthew in —had proposed similar basic ideas.

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.

For Darwin and his contemporaries, natural selection was in essence synonymous with evolution by natural selection. After the publication of On the Origin of Species[ 28 ] 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 "examples of natural selection in animals" of living organisms, and partly because even supporters of evolution balked at its "unguided" and non- progressive nature, [ 29 ] a response that has been characterised as the single most significant impediment to the idea's acceptance.

Herbert Spencer of the Survival of the Fittest is more accurate, and is sometimes equally convenient. Natural selection relies crucially on the idea of heredity, but developed before the basic concepts of genetics. Although the Moravian monk Gregor Mendelthe father of modern genetics, was a contemporary of Darwin's, his work lay in obscurity, only being rediscovered in Haldane introduced the concept of the "cost" of natural selection.

Ernst Mayr recognised the key importance of reproductive isolation for speciation in his Systematics and the Origin of Species Hamilton conceived of kin selection in A second synthesis was brought about at the end of the 20th century by advances in molecular geneticscreating the field of evolutionary developmental biology "evo-devo"which seeks to explain the evolution of form in terms of the genetic regulatory programs which control the development of the embryo at molecular level.

Natural selection is here understood to act on embryonic development to change the morphology of the adult body. The term natural selection is 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. 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 heritablethat 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. The peppered moth exists in both light and dark colours in Great Britain, but during the Industrial Revolutionmany of the examples of natural selection in animals on which the moths rested became blackened by sootgiving 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 Actand the dark moths became rare again, demonstrating the influence of natural selection on peppered moth evolution.

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.

Example of natural selection in animals: Peacock females pick their mate according

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. A mathematical example of "survival of the fittest" is given by Haldane in his paper "The Cost of Natural Selection". This is correctly described by the differential survival and reproduction of individuals due to differences in phenotype.

On the other hand, "improvement in fitness" is not dependent on the differential survival and reproduction of individuals due to differences in phenotype, it is dependent on the absolute survival of the particular variant. The probability of a beneficial mutation occurring on some member of a population depends on the example of natural selection in animals number of replications of that variant.

The mathematics of "improvement in fitness was described by Kleinman. Fixation or substitution is not required for this "improvement in fitness". On the other hand, "improvement in fitness" can occur in an environment where "survival of the fittest" is also acting. Richard Lenski 's classic E. The variant which is a candidate for a beneficial mutation in this limited carrying capacity environment must first out-compete the "less fit" variants in order to accumulate the requisite number of replications for there to be a reasonable probability of that beneficial mutation occurring.

In biology, competition is an interaction between organisms in which the fitness of one is lowered by the presence of another. This may be because both rely on a limited supply of a resource such as food, water, or territory. Wilson 's work on island biogeography. Typically, r -selected species exploit empty nichesand produce many offspring, each with a relatively low probability of surviving to adulthood.

In contrast, K -selected species are strong competitors in crowded niches, and invest more heavily in much fewer offspring, each with a relatively high probability of surviving to adulthood. Natural selection can act on any heritable phenotypic trait[ 73 ] and selective pressure can be produced by any aspect of the environment, including sexual selection and competition with members of the same or other species.

Selection can be classified in several different ways, such as by its effect on a trait, on genetic diversity, by the life cycle stage where it acts, by the unit of selection, or by the resource being competed for. Selection has different effects on traits. Stabilizing selection acts to hold a trait at a stable optimum, and in the simplest case all deviations from this optimum are selectively disadvantageous.

Directional selection favours extreme values of a trait. The uncommon disruptive selection also acts during transition periods when the current mode is sub-optimal, but alters the trait in more than one direction. In particular, if the trait is quantitative and univariate then both higher and lower trait levels are favoured. Disruptive selection can be a precursor to speciation.

Alternatively, selection can be divided according to its effect on genetic diversity. Purifying or negative selection acts to remove genetic variation from the population and is opposed by de novo mutationwhich introduces new variation. One mechanism for this is heterozygote advantagewhere individuals with two different alleles have a selective advantage over individuals with just one allele.

The polymorphism at the human ABO blood group locus has been explained in this way. Another option is to classify selection by the life cycle stage at which it acts. Some biologists recognise just two types: viability or survival selectionwhich acts to increase an organism's probability of survival, and fecundity or fertility or reproductive selection, which acts to increase the rate of reproduction, given survival.

Others split the life cycle into further components of selection. Thus viability and survival selection may be defined separately and respectively as acting to improve the probability of survival before and after reproductive age is reached, while fecundity selection may be split into additional sub-components including sexual selection, gametic selection, acting on gamete survival, and compatibility selection, acting on zygote formation.

Selection can also be classified by the level or unit of selection. Individual selection acts on the individual, in the sense that adaptations are "for" the benefit of the individual, and result from selection among individuals.

Example of natural selection in animals: A classic example of

Gene selection acts directly at the level of the gene. In kin selection and intragenomic conflictgene-level selection provides a more apt explanation of the underlying process. Group selectionif it occurs, acts on groups of organisms, on the assumption that groups replicate and mutate in an analogous way to genes and individuals. There is an ongoing debate over the degree to which group selection occurs in nature.

Finally, selection can be classified according to the resource being competed for. Sexual selection results from competition for mates. Sexual selection typically proceeds via fecundity selection, sometimes at the expense of viability. Ecological selection is natural selection via any means other than sexual selection, such as kin selection, competition, and infanticide.

Example of natural selection in animals: Behavior can also be shaped by

Following Darwin, natural selection is sometimes defined as ecological selection, in which case sexual selection is considered a separate mechanism. Sexual selection as first articulated by Darwin using the example of the peacock 's tail [ 81 ] refers specifically to competition for mates, [ 84 ] which can be intrasexualbetween individuals of the same sex, that is male—male competition, or intersexualwhere one gender chooses matesmost often with males displaying and females choosing.

Phenotypic traits can be displayed in one sex and desired in the other sex, causing a positive feedback loop called a Fisherian runawayfor example, the extravagant plumage of some male birds such as the peacock. Aggression between members of the same sex is sometimes associated with very distinctive features, such as the antlers of stagswhich are used in combat with other stags.

More generally, intrasexual selection is often associated with sexual dimorphismincluding differences in body size between males and females of a species. Natural selection is seen in action in the development of antibiotic resistance in microorganisms. Since the discovery of penicillin inantibiotics have been used to fight bacterial diseases.

The widespread misuse of antibiotics has selected for 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. 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 spread of a gene in the butterfly Hypolimnas bolina suppressing male-killing activity by Wolbachia bacteria parasites on the island of Samoawhere the spread of the gene is known to have occurred over a period of just five years. 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, genetic 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 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, many mutations in non-coding DNA have deleterious effects. Some mutations occur in "toolkit" or regulatory genes. Changes in these often 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 embryos.

Some nonlethal regulatory mutations occur in HOX genes in humans, which can result in a cervical rib [ 95 ] or polydactylyan increase in the number of fingers or toes. 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 becomes more variable and deteriorate over time, possibly resulting in a vestigial manifestation of the trait, also called evolutionary baggage.

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-ratis believed to retain function in photoperiod perception. Speciation requires a degree of reproductive isolation —that is, a reduction in gene flow.

However, it is intrinsic to the concept of a species that hybrids are selected against, opposing the evolution of reproductive isolation, a problem that was recognised by Darwin. The problem does not occur in allopatric speciation with geographically separated populations, which can diverge with different sets of mutations. Poulton realized in that reproductive isolation could evolve through divergence, if each lineage acquired a different, incompatible allele of the same gene.

Selection against the heterozygote would then directly create reproductive isolation, leading to the Bateson—Dobzhansky—Muller modelfurther elaborated by H. Allen Orr [ 98 ] and Sergey Gavrilets. 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.

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 differences in phenotype. An example is the ABO blood type antigens in humans, where three alleles govern the phenotype. Some traits are governed by only a single gene, but most traits are influenced by the interactions of many genes.

A example of natural selection in animals 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. When some component of a trait is heritable, selection alters 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: directionalstabilizingand disruptive selection. This process can continue until the allele is fixed and the entire population shares the fitter phenotype. This process can continue until the allele is eliminated from the example of natural selection in animals.

Stabilizing selection conserves functional genetic features, such as protein-coding genes or regulatory sequencesover time by selective pressure against deleterious variants. Disruptive selection may cause sympatric speciation through niche partitioning. Some forms of balancing selection do not result in fixation, but maintain an allele at intermediate frequencies in a population.

Only plants that produced flowers early in the season produced seeds. Thus, field mustard quickly evolved to have a short growing season. There are two species of Encelia: Asteraceae in Baja, Californiawith one type living in the desert region and the other living in the coastal dunes that border the desert. Even though these two species could interbreed, they do not.

This is likely due to maintaining selection that has to take place to prepare each of the species to thrive in its distinct environment. Key factors related to plant appearance and characteristics impact natural selection in flowering plants. For example, flower height, stem length and other traits will vary based on factors like what pollinators are present while flowers are growing and environmental factors like snowmelt and other things that impact temperature and access to water.

A biological ecosystem may be home to giraffes with different length necks. If something caused low-lying shrubs to die out, the giraffes with shorter necks would not get enough food. They would not survive to produce offspring. After a few generations, the surviving giraffes would have longer necks, because that body type is more suited to survive in the environment.

If a hypothetical species of rats live in a certain type of tree with the branches evenly spaced, natural selection would lead the rats to be right-sized for that type of tree. Smaller rats could not reach from branch to branch and larger rats would break the branches and fall. Right-sized rats would survive and reproduce. Soon, most rats would be just the right size for the tree branches.

If there are red bugs and green bugs in a hypothetical environment where predators such as birds prefer the taste of the red bugs, the green ones are more likely to survive. Soon there will be many green bugs and few red bugs. The green bugs will reproduce and make more green bugs, leading to a reality in which nearly all of the bugs born into this area will be green.

In a hypothetical ecosystem that is prone to flooding, lizards with long legs could climb better to avoid floods and reach food. As a result, eventually, most of the lizards in that type of ecosystem would have long legs. This trait would be passed to them from their parents, who survived because they had long legs.

Example of natural selection in animals: Rather, natural selection occurs as species

Insects can become resistant to pesticides very quickly, sometimes in one generation. If an insect has a mutation that makes it resistant to a certain chemical, then some of its offspring will also be resistant. Insect generations can be just a matter of weeks, so insects in an area can become immune to a chemical very quickly.