Selection gradient

Last updated

A selection gradient describes the relationship between a character trait and a species' relative fitness. [1] A trait may be a physical characteristic, such as height or eye color, or behavioral, such as flying or vocalizing. Changes in a trait, such as the amount of seeds a plant produces or the length of a bird's beak, may improve or reduce their relative fitness. Changes in traits may accumulate in a population under an ongoing process of natural selection. Understanding how changes in a trait affect fitness helps evolutionary biologists understand the nature of evolutionary pressures on a population. [2] [3]

Contents

Relationship between traits and fitness

In a population, heritable traits that increase an organism's ability to survive and reproduce tend to increase in frequency over generations through a process known as natural selection. [4] The selection gradient shows how much an organism's relative fitness (ω) changes in response to a given increase or decrease in the value of a trait. It is defined as the slope of that relationship, which may be linear or more complex. [1] [5] The shape of the selection gradient function also can help identify the type of selection that is acting on a population. [1] When the function is linear, selection is directional. [1] [5] Directional selection favors one extreme of a trait over another. An individual with the favored extreme value of the trait will survive more than others, causing the mean value of that trait in the population to shift in the next generation. [6] When the relationship is quadratic, selection may be stabilizing or disruptive. [7] Stabilizing selection reduces variation in a trait within a population by reducing the frequencies of more extreme values. Individuals with intermediate phenotypes will survive more than others. As a result, the values of the trait in the population in the following generation will cluster more closely around the peak of the population mean. [6] Disruptive selection increases variation by increasing the frequencies of the more extreme values of a trait. [8] Individuals with extreme trait values will survive more than those with intermediate phenotypes, leading to two peaks in frequency at the extreme values of the trait. [9]

Calculation

The first and most common function to estimate fitness of a trait is linear ω =α +βz , which represents directional selection. [1] [10] The slope of the linear regression line (β) is the selection gradient, ω is the fitness of a trait value z, and α is the y-intercept of the fitness function. Here, the function indicates either an increase or decrease in fitness with increases in the value of a trait. The second fitness function is nonlinear ω = α +βz +(γ/2)z2, which represents stabilizing or disruptive selection. [1] [5] The quadratic regression (γ) is the selection gradient, ω is the fitness of a trait value z, and α is the y-intercept of the fitness function. Here, individuals with intermediate trait values may have the highest fitness (stabilizing selection) or those with extreme trait values may have the highest fitness (disruptive selection). When, β = 0 and γ is significantly positive, the selection gradient indicates disruptive selection. [1] When, β= 0 and γ is significantly negative, the selection gradient indicates stabilizing selection. [1] In both the cases γ measures the strength of selection. [1]

Application

Evolutionary biologists use estimates of the selection gradient of traits to identify patterns in the evolutionary pressures on a population and predict changes in species traits. When traits are correlated with one another to some degree, for example beak length (z1) and body size (z2) in a bird, selection on one will affect the distribution of the other. [10] For correlated traits, the effects of natural selection can be separated by estimating the selection gradient for one trait (beak length (z1)) while holding the other trait (body size (z2)) constant. The process enables researchers to determine how greatly variations in one trait (beak length) affect fitness among individuals with the same body size. [10] [1] In 1977 when the Galapagos Islands suffered from severe drought, Peter and Rosemary Grant estimated the selection gradient for Darwin's finches to estimate the strength of the relationship between fitness and each trait while holding other traits constant. They estimated selection gradient for finches’ weight (0.23), bill length (-0.17) and bill depth (0.43). The result showed that selection strongly favored larger birds with deeper bills. [10] Evolutionary biologists also use selection gradients to estimate strength and mode of natural selection. Selection gradients, for example, have provided an explanation for fitness differences among individuals in a population, among different species and strengths of selection. In a study of the fresh-water Eurasian perch, a change in fitness was reported with a change in their density. An estimate of the selection gradient by linear and quadratic regression indicated a shift of the selection regime between stabilizing and directional selection at low density to disruptive selection at higher density. [11]

Criticism

Despite the conceptual simplicity of the selection gradient, there are ongoing debates about its usefulness as an estimator of causes and consequences of natural selection. In 2017, Franklin & Morrissey showed that when performance measures such as body size, biomass, or growth rate are used in place of fitness components in regression-based analysis, accurate estimation of selection gradient is limited, which may lead to under-estimates of selection. [12] Another complication of using selection gradient as an estimator of natural selection is when the phenotype of an individual is itself affected by individuals it interacts with. [13] It complicates the process of separating direct and indirect selection as there are multiple ways selection can work. One alternative to selection gradients is the use of high throughput sequencing to identify targets and agents of selection. [14]

See also

Related Research Articles

<span class="mw-page-title-main">Evolution</span> Change in the heritable characteristics of biological populations

Evolution is the change in the heritable characteristics of biological populations over successive generations. Evolution occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, resulting in certain characteristics becoming more or less common within a population over successive generations. The process of evolution has given rise to biodiversity at every level of biological organisation.

<span class="mw-page-title-main">Natural selection</span> Mechanism of evolution by differential survival and reproduction of individuals

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

Population genetics is a subfield of genetics that deals with genetic differences within and among populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as adaptation, speciation, and population structure.

<span class="mw-page-title-main">Group selection</span> Proposed mechanism of evolution

Group selection is a proposed mechanism of evolution in which natural selection acts at the level of the group, instead of at the level of the individual or gene.

<span class="mw-page-title-main">Polymorphism (biology)</span> Occurrence of two or more clearly different morphs or forms in the population of a species

In biology, polymorphism is the occurrence of two or more clearly different morphs or forms, also referred to as alternative phenotypes, in the population of a species. To be classified as such, morphs must occupy the same habitat at the same time and belong to a panmictic population.

<span class="mw-page-title-main">Fisherian runaway</span> Sexual selection mechanism

Fisherian runaway or runaway selection is a sexual selection mechanism proposed by the mathematical biologist Ronald Fisher in the early 20th century, to account for the evolution of ostentatious male ornamentation by persistent, directional female choice. An example is the colourful and elaborate peacock plumage compared to the relatively subdued peahen plumage; the costly ornaments, notably the bird's extremely long tail, appear to be incompatible with natural selection. Fisherian runaway can be postulated to include sexually dimorphic phenotypic traits such as behavior expressed by a particular sex.

<span class="mw-page-title-main">Directional selection</span> Type of genetic selection favoring one extreme phenotype

In population genetics, directional selection is a mode of negative natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype. Under directional selection, the advantageous allele increases as a consequence of differences in survival and reproduction among different phenotypes. The increases are independent of the dominance of the allele, and if the allele is recessive, it will eventually become fixed.

<span class="mw-page-title-main">Stabilizing selection</span> Type of selection in evolution where a trait stabilizes around the average value

Stabilizing selection is a type of natural selection in which the population mean stabilizes on a particular non-extreme trait value. This is thought to be the most common mechanism of action for natural selection because most traits do not appear to change drastically over time. Stabilizing selection commonly uses negative selection to select against extreme values of the character. Stabilizing selection is the opposite of disruptive selection. Instead of favoring individuals with extreme phenotypes, it favors the intermediate variants. Stabilizing selection tends to remove the more severe phenotypes, resulting in the reproductive success of the norm or average phenotypes. This means that most common phenotype in the population is selected for and continues to dominate in future generations.

In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the population during that process. Thirdly, it is a phenotypic trait or adaptive trait, with a functional role in each individual organism, that is maintained and has evolved through natural selection.

<span class="mw-page-title-main">Unit of selection</span> Biological entity within the hierarchy of biological organization

A unit of selection is a biological entity within the hierarchy of biological organization that is subject to natural selection. There is debate among evolutionary biologists about the extent to which evolution has been shaped by selective pressures acting at these different levels.

<span class="mw-page-title-main">Disruptive selection</span> Natural selection for extreme trait values over intermediate ones

In evolutionary biology, disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups. In this more individuals acquire peripheral character value at both ends of the distribution curve.

<span class="mw-page-title-main">Functional ecology</span>

Functional ecology is a branch of ecology that focuses on the roles, or functions, that species play in the community or ecosystem in which they occur. In this approach, physiological, anatomical, and life history characteristics of the species are emphasized. The term "function" is used to emphasize certain physiological processes rather than discrete properties, describe an organism's role in a trophic system, or illustrate the effects of natural selective processes on an organism. This sub-discipline of ecology represents the crossroads between ecological patterns and the processes and mechanisms that underlie them. It focuses on traits represented in large number of species and can be measured in two ways – the first being screening, which involves measuring a trait across a number of species, and the second being empiricism, which provides quantitative relationships for the traits measured in screening. Functional ecology often emphasizes an integrative approach, using organism traits and activities to understand community dynamics and ecosystem processes, particularly in response to the rapid global changes occurring in earth's environment.

Genetic divergence is the process in which two or more populations of an ancestral species accumulate independent genetic changes (mutations) through time, often leading to reproductive isolation and continued mutation even after the populations have become reproductively isolated for some period of time, as there is not any genetic exchange anymore. In some cases, subpopulations cover living in ecologically distinct peripheral environments can exhibit genetic divergence from the remainder of a population, especially where the range of a population is very large. The genetic differences among divergent populations can involve silent mutations or give rise to significant morphological and/or physiological changes. Genetic divergence will always accompany reproductive isolation, either due to novel adaptations via selection and/or due to genetic drift, and is the principal mechanism underlying speciation.

Phylogenetic comparative methods (PCMs) use information on the historical relationships of lineages (phylogenies) to test evolutionary hypotheses. The comparative method has a long history in evolutionary biology; indeed, Charles Darwin used differences and similarities between species as a major source of evidence in The Origin of Species. However, the fact that closely related lineages share many traits and trait combinations as a result of the process of descent with modification means that lineages are not independent. This realization inspired the development of explicitly phylogenetic comparative methods. Initially, these methods were primarily developed to control for phylogenetic history when testing for adaptation; however, in recent years the use of the term has broadened to include any use of phylogenies in statistical tests. Although most studies that employ PCMs focus on extant organisms, many methods can also be applied to extinct taxa and can incorporate information from the fossil record.

Evolutionary invasion analysis, also known as adaptive dynamics, is a set of mathematical modeling techniques that use differential equations to study the long-term evolution of traits in asexually and sexually reproducing populations. It rests on the following three assumptions about mutation and population dynamics:

  1. Mutations are infrequent. The population can be assumed to be at equilibrium when a new mutant arises.
  2. The number of individuals with the mutant trait is initially negligible in the large, established resident population.
  3. Mutant phenotypes are only slightly different from the resident phenotype.

Genic capture is a hypothesis explaining the maintenance of genetic variance in traits under sexual selection. A classic problem in sexual selection is the fixation of alleles that are beneficial under strong selection, thereby eliminating the benefits of mate choice. Genic capture resolves this paradox by suggesting that additive genetic variance of sexually selected traits reflects the genetic variance in total condition. A deleterious mutation anywhere in the genome will adversely affect condition, and thereby adversely affect a condition-dependent sexually selected trait. Genic capture therefore resolves the lek paradox by proposing that recurrent deleterious mutation maintains additive genetic variance in fitness by incorporating the entire mutation load of an individual. Thus any condition-dependent trait "captures" the overall genetic variance in condition. Rowe and Houle argued that genic capture ensures that good genes will become a central feature of the evolution of any sexually selected trait.

Evolutionary psychiatry, also known as Darwinian psychiatry, is a theoretical approach to psychiatry that aims to explain psychiatric disorders in evolutionary terms. As a branch of the field of evolutionary medicine, it is distinct from the medical practice of psychiatry in its emphasis on providing scientific explanations rather than treatments for mental disorder. This often concerns questions of ultimate causation. For example, psychiatric genetics may discover genes associated with mental disorders, but evolutionary psychiatry asks why those genes persist in the population. Other core questions in evolutionary psychiatry are why heritable mental disorders are so common how to distinguish mental function and dysfunction, and whether certain forms of suffering conveyed an adaptive advantage. Disorders commonly considered are depression, anxiety, schizophrenia, autism, eating disorders, and others. Key explanatory concepts are of evolutionary mismatch and the fact that evolution is guided by reproductive success rather than health or wellbeing. Rather than providing an alternative account of the cause of mental disorder, evolutionary psychiatry seeks to integrate findings from traditional schools of psychology and psychiatry such as social psychology, behaviourism, biological psychiatry and psychoanalysis into a holistic account related to evolutionary biology. In this sense, it aims to meet the criteria of a Kuhnian paradigm shift.

<span class="mw-page-title-main">Genetic variance</span> Biological concept

Genetic variance is a concept outlined by the English biologist and statistician Ronald Fisher in his fundamental theorem of natural selection. In his 1930 book The Genetical Theory of Natural Selection, Fisher postulates that the rate of change of biological fitness can be calculated by the genetic variance of the fitness itself. Fisher tried to give a statistical formula about how the change of fitness in a population can be attributed to changes in the allele frequency. Fisher made no restrictive assumptions in his formula concerning fitness parameters, mate choices or the number of alleles and loci involved.

This glossary of genetics and evolutionary biology is a list of definitions of terms and concepts used in the study of genetics and evolutionary biology, as well as sub-disciplines and related fields, with an emphasis on classical genetics, quantitative genetics, population biology, phylogenetics, speciation, and systematics. Overlapping and related terms can be found in Glossary of cellular and molecular biology, Glossary of ecology, and Glossary of biology.

<span class="mw-page-title-main">Phylogenetic signal</span>

Phylogenetic signal is an evolutionary and ecological term, that describes the tendency or the pattern of related biological species to resemble each other more than any other species that is randomly picked from the same phylogenetic tree.

References

  1. 1 2 3 4 5 6 7 8 9 10 Lande, Russell; Arnold, Stevan J. (1983). "The Measurement of Selection on Correlated Characters". Evolution. 37 (6): 1210–1226. doi:10.2307/2408842. ISSN   0014-3820. JSTOR   2408842. PMID   28556011.
  2. ↑ Lenski, Richard E. (2017-04-20). "What is adaptation by natural selection? Perspectives of an experimental microbiologist". PLOS Genetics. 13 (4): e1006668. doi: 10.1371/journal.pgen.1006668 . ISSN   1553-7404. PMC   5398481 . PMID   28426692.
  3. ↑ Gregory, T. Ryan (2009-06-01). "Understanding Natural Selection: Essential Concepts and Common Misconceptions". Evolution: Education and Outreach. 2 (2): 156–175. doi: 10.1007/s12052-009-0128-1 . ISSN   1936-6434.
  4. ↑ Darwin, Charles (1859). On the Origin of Species by Means of Natural Selection, Or, The Preservation of Favoured Races in the Struggle for Life. J. Murray.
  5. 1 2 3 Kingsolver, Joel G.; Pfennig, David W. (2007-07-01). "Patterns and Power of Phenotypic Selection in Nature". BioScience. 57 (7): 561–572. doi: 10.1641/B570706 . ISSN   0006-3568.
  6. 1 2 Sanjak, Jaleal S.; Sidorenko, Julia; Robinson, Matthew R.; Thornton, Kevin R.; Visscher, Peter M. (2018-01-02). "Evidence of directional and stabilizing selection in contemporary humans". Proceedings of the National Academy of Sciences. 115 (1): 151–156. Bibcode:2018PNAS..115..151S. doi: 10.1073/pnas.1707227114 . ISSN   0027-8424. PMC   5776788 . PMID   29255044.
  7. ↑ "Stabilizing selection", Wikipedia, 2019-12-06, retrieved 2019-12-18
  8. ↑ Freeman, Scott; C.Herron, Jon (2007). Evolutionary Analysis. 1301 Sansome St.,San Francisco,CA 94111: Pearson Benjamin Cummings. p. 347. ISBN   978-0132275842.{{cite book}}: CS1 maint: location (link)
  9. ↑ Rueffler, Claus; Van Dooren, Tom J. M.; Leimar, Olof; Abrams, Peter A. (2006-05-01). "Disruptive selection and then what?". Trends in Ecology & Evolution. 21 (5): 238–245. doi:10.1016/j.tree.2006.03.003. ISSN   0169-5347. PMID   16697909. S2CID   30747937.
  10. 1 2 3 4 J.Futuyma, Douglas (2013). Evolution. 23 Plumtree Road, Sunderland,MA 01375 USA: Sinauer Associates, Inc. p. 358. ISBN   9781605351155.{{cite book}}: CS1 maint: location (link)
  11. ↑ Svanbäck, Richard; Persson, Lennart (2009). "Population density fluctuations change the selection gradient in Eurasian perch". The American Naturalist. 173 (4): 507–516. doi:10.1086/597223. ISSN   1537-5323. PMID   19226234. S2CID   5075971.
  12. ↑ Franklin, Oliver D.; Morrissey, Michael B. (2017). "Inference of selection gradients using performance measures as fitness proxies". Methods in Ecology and Evolution. 8 (6): 663–677. doi: 10.1111/2041-210X.12737 . hdl: 10023/13864 . ISSN   2041-210X.
  13. ↑ Thomson, Caroline E.; Hadfield, Jarrod D. (2017). "Measuring selection when parents and offspring interact". Methods in Ecology and Evolution. 8 (6): 678–687. doi: 10.1111/2041-210X.12795 . hdl: 20.500.11820/62aaebd3-c545-4e42-9054-e9f58ffa4ce6 . ISSN   2041-210X.
  14. ↑ Villanueva‐Cañas, José Luis; Rech, Gabriel E.; Cara, Maria Angeles Rodriguez de; González, Josefa (2017). "Beyond SNPs: how to detect selection on transposable element insertions". Methods in Ecology and Evolution. 8 (6): 728–737. doi: 10.1111/2041-210X.12781 . ISSN   2041-210X.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.