Chapter 13 PHENOTYPIC EVOLUTION
Variation in most phenotypic characters is continuous or quantitative, and it is based on effects of several or many variable gene loci, as well as those of the environment.
Height, finger length, eye color, skin color, etc.
Quantitative genetics studies continuous characters and the mechanisms that produce them.
It includes traits that are determined by several genes: polygenic traits. These traits may have a strong environmental component.
“Quantitative genetics deals with the genetics of continuously varying characters. Rather than considering changes in the frequencies of specific alleles of genotypes, quantitative genetics seeks to "quantify" changes in the frequency distribution of traits that cannot easily be placed in discrete phenotypic classes. The reason for the continuous variation is usually that the traits are polygenic (controlled by many genes) and there are environmental effects that alter the phenotypic state of each individual.” http://biomed.brown.edu/Courses/BIO48/10.Quan.genetics.HTML
GENETIC ARCHITECTURE OF PHENOTYPIC TRAITS
Genetic architecture refers to the genetic basis of a trait and its relationship to other traits.
Genetic architecture has several components:
Number of loci that contribute to its development, variation within species and among species or populations. Magnitude of the effect on the trait of different alleles at each locus – the allelic effect. The additive effect and dominance at the various loci. The synergistic effect among loci – epistasis. The pleiotropic effect of the loci – the extent to which individual genes affect more than one trait.
The field of quantitative genetics analyzes quantitative characters. It studies the evolution of morphology, life history characteristics, behavior, etc.
Quantitative, multifactorial or polygenic inheritance refers to phenotypic characteristics that are the result of two or more genes and their interaction with the environment.
Polygenic traits do not follow Mendelian inheritance, and the phenotypes vary along a gradient depicted by a bell shape curve.
A quantitative trait locus (QTL) is a region of DNA that is associated with a particular phenotypic trait - these QTLs are often found on different chromosomes.
A QTL is not necessarily a single gene, but rather a chromosome interval that may include many genes of which one or more affect the character under study.
QTLs are loci that influence a quantitative trait.
A quantitative trait locus (QTL) is a region of DNA that is associated with a particular phenotypic trait - these QTLs are often found on different chromosomes.
Knowing the number of QTLs that explains variation in the phenotypic trait tells us about the genetic architecture of a trait.
It may tell us that plant height is controlled by many genes of small effect, or by a few genes of large effect.
Some QTLs have large effects while other have small effects.
Many QTLs have both additive effects on the phenotype and strong epistatic interaction.
Strong epistatic effect means that the joint effect of some pairs of loci differed from the sum of their individual effects.
Some QTLs show additive effect on the phenotype.
Phenotypic effects can differ among the sexes other effects depend on the temperature at which the organisms are raised
QTL mapping is the statistical study of the alleles that occur in a locus and the phenotypes (physical forms or traits) that they produce.
QTL mapping is the collective name for a suite of related techniques that employ marker loci to scan chromosomes and identify regions containing genes that contribute to a quantitative trait. (Freeman and Herron, 2007)
A marker is segment of DNA that is significantly more likely to co-occur with the trait than expected by chance that is a marker that has a statistical association with the trait.
Candidate genes are genes thought to be involved in the evolution of a phenotypic trait based on its mutant phenotype of the protein they encode.
COMPONENTS OF PHENOTYPIC VARIATION
Variance: average of the squared deviations of observations from the arithmetic mean of a sample (V).
Standard deviation: the square root of the variance, s = √V. http://www.chssc.salford.ac.uk/healthSci/resmeth2000/resmeth/variance.htm
If a variable has a normal (bell shaped) frequency distribution, about 68% of the observations lie within one standard deviation on either side of the mean, 96% within two standard deviations, and 99.7% within three.
Variation (variance) in a phenotypic trait (VP) may include genetic variance (VG) and variance due to environmental factors (VE).
The phenotype variation among individuals with the same genotype is the environmental variance, VE.
Genetic variance may include both additive genetic variance (VA) due to the additive effects of alleles and non-additive genetic variance due to dominance and epistasis.
Additive effects of alleles are responsible for the degree of similarity between parents and offspring and are the basis for response to selection within population.
Only the additive variance creates a correlation between parents and offspring.
Only VA is affected by selection.
Phenotypic variance (VP) is variance of the phenotype (physical and biochemical characteristics) that results from the sum of the genotypic (VG) and environmental (VE) variances.
VP = VG + VE
Additive genetic variance: When the heterozygote lies at midpoint between two homozygous, inheritance is the additive. A1A1 A1A2 A2A2 -1 0 +1
The additive genetic variance depends both on the magnitude of the additive effects of alleles on the phenotype and on the genotype frequencies.
If a genotype is too frequent, most individuals are close to the average phenotype and the variance is lower. If several genotypes have similar frequencies, the variance will be greater.
When several loci contribute additively to the phenotype, the average phenotype of any particular genotype is the sum of the phenotypic values of each of the loci.
The additive genetic variance plays a key role in evolutionary theory because the additive effects of alleles are responsible for the degree of similarity between parents and offspring and, therefore, are the basis for response to selection within populations.
Response to selection is the change in the average genotype after selection in the previous generation.
Selection among phenotypically different parents is reflected in the mean phenotype of the next generation.
The proportion of the genetic variance to the total variance is heritability.
Heritability does not indicate the degree to which a trait is genetic; it measures the proportion of the phenotypic variance that is the result of genetic factors.
The heritability is determined by the additive genetic variance (VA), which depends on allele frequencies; and by the environmental variance (VE), which depends in part on how variable the environmental factors are that affect the development or expression of the character.
Two specific types of heritability can be estimated.
The broad-sense heritability is the ratio of total genetic variance to total phenotypic variance. 2 H = VG/VP
The narrow-sense heritability is the ratio of additive genetic variance to the total phenotypic variance. 2 h N = VA/VP where VP = VG+VE and VG is the sum of VA and non-additive genetic components.
The estimate heritability is valid only for the population in which it was measured, and only in the environment in which it was measured. This is because frequencies vary between populations and so do environmental conditions.
Narrow sense heritability quantifies only the portion of the phenotypic variation that is additive (allelic) by nature… When interested in improving livestock via artificial selection, for example, knowing the narrow-sense heritability of the trait of interest will allow predicting how much the mean of the trait will increase in the next generation as a function of how much the mean of the selected parents differs from the mean of the population from which the selected parents were chosen. The observed response to selection leads to an estimate of the narrow-sense heritability (called realized heritability). http://en.wikipedia.org/wiki/Heritability
LINKAGE DISEQUILIBRIUM
Linkage disequilibrium is a term used in population genetics.
When alleles at two loci are randomly associated with each other, they are said to in a state of linkage equilibrium; when they are non-randomly associated, they are in linkage disequilibrium
Linkage disequilibrium: The association of two alleles at two or more loci more or less frequently than predicted by their individual frequencies
The loci in linkage disequilibrium are usually located in different chromosomes.
Linkage disequilibrium is generally caused by selection in the form of epistatic fitness interactions between genes, genetic linkage and the rate of recombination, random drift or non-random mating, and population structure.
Epistasis occurs when the effect of a gene is affected by the activity of other genes located in different loci. A gene produces a protein that prevents transcription of another gene.
The coefficient of linkage disequilibrium is the measure of the deviation from the two-locus equilibrium.
Positive linkage disequilibrium increases the frequency of phenotypic variance.
The level of linkage disequilibrium declines due to recombination in the double heterozygotes with the passage of generations.
Linkage disequilibrium is caused by:
1. Natural selection. If selection favors individuals with particular combinations of alleles, then it produces linkage disequilibrium. If two or more gene combinations are much fitter than recombinant genotypes linkage disequilibrium will be favored.
2. Non-random mating.
3. When a new mutation arises, the single copy is necessarily associated with specific alleles at other loci on the chromosome, and therefore is in linkage disequilibrium with those alleles. The copies of this mutation in subsequent generations will retain this association until it is broken down by recombination.
4. The linkage equilibrium has not yet been reached. It takes a number of generations for recombination to do its randomizing work and, particularly for tightly linked genes; linkage disequilibrium can persist for some time.
5. Recombination is very slow or non-existent. Recombination tends to randomize genotypes at one locus with respect to genotypes at another. It decreases the frequency of overrepresented haplotypes and increases the frequencies of underrepresented haplotypes.
6. Random drift processes can cause persistent linkage disequilibrium. If random sampling produces by chance an excess of a haplotype in a generation, linkage disequilibrium will have arisen. Any haplotype could be 'favored' by chance, so the disequilibrium is equally likely to have D > 0 or D < 0.
EVOLUTION OF QUANTITATIVE CHARACTERS
GENETIC VARIANCE IN NATURAL POPULATIONS
Heritability is determined by the additive genetic variance (VA), which depends on allele frequencies; and by the environmental variance (VE), which depends in part on how variable the environmental factors are that affect the development or expression of the character. Vp is the phenotypic variance.
2 h = VA/VP
Where VP = VG + VE; and VG is the sum of VA and non-additive genetic components.
The causes of genetic variation in natural populations are uncertain, but input by mutation may balance losses due to selection and genetic drift.
The paucity of genetic variation would be a genetic constraint that could affect the direction of evolution or prevent adaptation altogether.
Example: Some grasses growing near mines have developed tolerance to high copper and zinc concentration in the soil; other grass species have not and disappeared from the contaminated areas. Experiments showed that seeds from plants growing in normal soil produced a small number of tolerant seedlings in the species that evolved tolerance in contaminated soils; no tolerant seedlings were found in those species that had not formed tolerant populations in contaminated soil. The genetic variation that allowed for tolerance was apparently missing in the intolerant species.
GENETIC DRIFT OR NATURAL SELECTION
A NEUTRAL MODEL OF THE EVOLUTION OF QUANTITATIVE CHARACTERS
Quantitative characters will evolve by genetic drift if alleles at the underlying loci are selectively neutral.
Selection may be inferred if the data do not fit a model of neutral evolution.
Neutral traits may have a pleiotropic effect on other traits and, therefore, affect fitness.
At equilibrium, mutation is balanced by genetic drift, the genetic variance and heritability should theoretically reach a stable value, which should be quite high if the effective population size is large.
As mutations are fixed by genetic drift, the mean will fluctuate randomly.
Mutation and genetic drift can cause divergence between two isolated populations that originated from a uniform ancestral population.
Allele frequency changes by genetic drift transpire more rapidly in smaller populations and will be noticeable in relatively short time.
If alleles that contribute to variation in a polygenic trait are selectively neutral, variation and evolution of the trait are affected only by mutation (which increases variation) and genetic drift (which erodes it).
Over a long time, if a trait has diverged in populations that originated from a common ancestral population much faster than the expected neutral rate, it is likely to have evolved by directional selection.
If it has changed much more slowly than the expected neutral rate, it is likely that stabilizing selection has operated either on the trait itself or on other characteristics that are pleitropically affected by the same genes.
Studies on the rate of evolution show that many features fluctuate rather rapidly, but show little net change, so that the rate of evolution over the long term is much less than it is over shorter intervals of time.
The rate of evolution over the long term is much less than it is over short periods of time suggesting that stabilizing or fluctuating selection has been in operation.
In stabilizing selection, the intermediate phenotypes are preferred instead of the most extremes and variation decreases.
NATURAL SELECTION ON QUANTITATIVE TRAITS
1. RESPONSE TO DIRECTIONAL SELECTION
The response of a quantitative character to selection depends on the heritability of the character and the selection differential.
The selection differential (S) is the difference between the mean of a population and the mean of the individuals selected to be parents of the next generation.
The response to selection (R) is the change in the mean phenotype after selection.
Heritability describes how a trait will respond to selection.
Example: The trait to be selected is the length of the tail in a population of rats; the trait has a normal frequency distribution z (bell-shaped curve). See Fig. 13.4.
A normal distribution is expected if a large number of loci, all with relatively small effects on the character, freely recombine.
The experimenter imposes a selection for tail length by breeding only those rats with a tail longer than a certain value. The mean tail length of the selected parents differs from the mean of the entire population; this is the selection differential, S. The average tail length among the offspring of the selected parents differs from that on the parental generation as a whole by an amount R, the response to selection. The magnitude of R is proportionate to the heritability of the trait.
As selection proceeds, it increases the frequencies of those alleles that produce phenotypes closer to the optimum value.
As those frequencies increase, combination of alleles at different loci (multilocus genotypes) that were extremely rare, become more common.
In this way, phenotypes arise that were absent before.
The mean of a polygenic character shifts beyond the original range of variation as directional selection proceeds, even if no further mutations occur.
As selection proceeds, “low” alleles are removed and additive genetic variation is reduced.
2 As the VA decreases, the heritability decreases: h = VA/VP
Prolonged directional selection should ultimately fix all favored alleles, eliminating genetic variation. Alleles with the most favorable effects on the phenotype are likely to be fixed first. Will selection come to a halt? Probably not because mutation is constantly introducing a trickle of new alleles each with different additive effects. Mutational variance, the infusion of new additive genetic variance by mutation, is on the order of -3 10 x VE per generation. A fully homozygous population could attain a heritability of 0.5 in one thousand generations. If mutations have harmful pleiotropic effects and have a net selective disadvantage, the “usable” (advantageous) mutational variance would be much less. Under this view a continual mutation-selection balance might explain the gradual changes in phenotype seen over long evolutionary times.
2. RESPONSES TO ARTIFICIAL SELECTION
Artificial selection is the intentional selection of trait or combination of traits by humans.
The criterion for survival and reproduction is the trait chosen, rather than fitness as determined by the entire genotype.
This is in contrast with natural selection, which focuses on the organism ability to survive and reproduce in its natural habitat, the overall fitness.
An experiment by Yoo (1980) in which he selected for an increased number of bristles in Drosophila melanogaster showed that artificial selection has accomplished an enormous evolutionary change, a rate far higher than is usually observed in the fossil record. See the description of the experiment on page 346.
Several populations eventually stopped responding; they reached a selection plateau, in which response to selection stopped.
Selection plateau is caused by natural selection and not by loss of genetic variation.
Extreme genotypes have low fitness.
Low fitness is due to hitchhiking of deleterious genes and pleiotropy.
After the experiment was stopped, the mean bristle declined showing that genetic variation was still present and that genotypes with fewer bristles had higher fitness.
The response to selection is based on both genetic variation in the original population and new mutations that occur.
Natural selection often opposes artificial selection.
3. DIRECTIONAL SELECTION IN NATURAL POPULATIONS
An attempt has been made to measure the strength of natural selection on quantitative traits.
One index uses the mean and variance of a trait measured within a single generation before and after selection has occurred.
E. g. the measurements are made on juveniles and then on those individuals that successfully survive to adulthood and reproduce. z = mean phonotype; V = variance zb = mean phenotype before selection za = mean phenotype after selection Vb = variance before selection Va = variance after selection VP = phenotypic variance za-zb The index of the intensity of directional selection is i = ------√VP
EXAMPLES OF SELECTION ON QUANTITATIVE CHARACTERS, pages 348 -350.
See the examples of:
Stabilizing selection: The Darwin’s finch Geospiza fortis on Daphne Island. The conflicting selection pressures create stabilizing selection that, on average, favors an intermediate bill size. Stabilizing selection: Birth weight in humans. See page 348. Stabilizing selection: Antagonistic agents of selection. The larvae of the goldenrod gall fly, its parasitoids and predators. Taken together, these enemies imposed rather strong stabilizing selection. See page 348.
Evolution observed:
The codling moth Cydia pomonella, diapause timing, page 348. Blackcap songbird Sylvia atricapilla, new migratory behavior, page 349. Soapberry bug Jadera haematoloma, new food sources, page 349. Heavy metal and plant adaptation, page 350. Cod and bighorn sheep, page 350.
STRENGTH OF NATURAL SELECTION
The strength of natural selection appears to be quite modest, although strong selection has often been recorded.
Stabilizing selection and diversifying selection appear to be about equally common.
There is a tendency for the strength of selection due to variation in mating success and female fecundity to be greater, on average, than that of selection due to differences in survival.
WHAT MAINTAINS GENETIC VARIATION IN QUANTITATIVE CHARACTERS?
There are high levels of variation in quantitative traits in both large and small populations.
Alleles that contribute to quantitative traits are probably seldom selectively neutral.
Many quantitative characters are subject to fairly intense selection.
Genes contributing to quantitative traits have pleiotropic effects of survival and other fitness components.
Genes that affect quantitative characters may have other primary functions and only incidentally affect quantitative trait.
Characters that in themselves may be neutral can be subject to indirect selection because of the pleiotropic effects of the underlying genes.
Gene flow between populations with optimal phenotypes can maintain variation in both populations, but studies of captive populations in which gene flow does not take place show that these populations do not differ substantially from wild populations.
MUTATION-SELECTION BALANCE HYPOTHESIS
Gene flow between populations with optimal phenotypes can maintain variation in each population.
Levels of polygenic variation reflect a balance between the erosion of variation by stabilizing selection and the input of new variation by mutation.
There is some doubt that variation by mutation can counter balance the strong stabilizing selection that acts on many traits.
Many data are consistent with a model of mutation/selection balance in which stabilizing selection on a character acts on mutations of large effect at a few loci, while much of the variation consists of small pleiotropic effects of slightly deleterious mutations at many other loci that have a variety of other primary functions.
CORRELATED EVOLUTION OF QUANTITATIVE TRAITS
Evolutionary change in one character is often correlated with change in other features. If changes by selection occur in one trait, other traits will also change.
The overall phenotype must be maintained in order to avoid deleterious changes.
CORRELATED SELECTION
Characters that are functionally related are usually selected together.
GENETIC CORRELATION
The correlation coefficient (r) expresses the magnitude or degree to which two traits are correlated.
The correlation coefficient ranges from +1 for perfect correlation, when two features increase or decrease together, to -1 when one feature increases exactly in the same proportion as the other increases; 0 means that the characters are not correlated at all.
The evolution of a trait is governed both by selection on that trait and by selection on other traits with which it is genetically correlated.
Phenotypic correlation (rP) may have a genetic and an environmental component.
E. g difference in nutrition may affect two phenotypic traits of individuals with the same genotype. This is referred as environmental correlation, rE. Phenotypic correlation: rP = rG + rE
Correlated variation may be caused by genetic differences that affect two characters, causing genetic correlation, rG.
Causes of genetic correlation:
Linkage disequilibrium among the genes that independently affect each character. Pleiotropy, the influence of the same gene in two characters.
In pleiotropy:
Perfect genetic correlation occurs when one gene affects all characters in the same direction: all increase or decrease: rG= 1.0 or -1.0 Imperfect genetic correlation occurs when one gene affects some characters in one direction (+,+ or -,-) while others have opposite effect (+,- or -,+), or affect only one of the characters.
Genetic correlation can change over time:
Linkage disequilibrium will decline with time due to recombination unless selection for the adaptive gene combinations maintains it. Some alleles may become fixed so they contribute no variation and no correlation, while other loci remain variable, and changes in frequency in these loci will affect rG. Natural selection may affect modifier alleles that alter the pleiotropic effects of other loci. See the example of Australian blowflies on page 353.
Genetic correlation among character and environmental correlation give rise to phenotypic correlation.
HOW GENETIC CORRELATION AFFECTS EVOLUTION
The evolution of a trait is governed both by selection on that trait and by selection on other traits with which it is genetically correlated.
Genetic correlation may be positive or negative.
Genetic correlations among characters can cause them to evolve in concert.
Multiple characters may evolve as an integrated ensemble more rapidly if they are genetically correlated, as when they are subject to the same developmental controls.
E. g. characters that are functionally related like the organs of the body and the body size. Body size would evolve much more slowly in response to selection if every organ had to undergo independent genetic change than if there were coordinated increases or decreases in the sizes of the various organs. During development the various organs grow in concert, and alleles that change body size have correlated effects on most body parts.
Genetic correlation among characters may retard the rate of adaptive evolution, depending on circumstances.
A conflict may exist between the genetic correlation of characters and directional selection on those characters.
When such a conflict exists, the two characters may evolve to their optimal states only slowly, and may evolve temporarily in a maladaptive direction.
E. g. selection for a deeper bill in the Galapagos finch Geospiza fortis caused average bill length to increase, even though selection favored a shorter bill.
E.g. An example from beef cattle breeding. Selection for fast growth is a common goal of selection. By doing so it is hard to avoid a change in the final adult weight. At the same time, the selection will also result in larger calves being born, which can cause difficulties during calving if the ratio between the weight of the calf and the cow is changed unfavorably. http://www.kursus.kvl.dk/shares/vetgen/_Popgen/genetics/8/4.htm
CAN GENETICS PREDICT LONG-TERM EVOLUTION?
Several traits may be affected by the same set of genes, and selection on one of the traits would result in changes in the others. These traits would be genetically correlated with one another. The G-matrix describes these genetic correlations.
The G matrix is created by plotting the measurements of two traits, one in the x-axis and the other on the y-axis.
If large values for the trait in the y axis was favored by natural selection, the values for the trait in the x axis will also increase as well over time, and individuals with large values for one trait will also show large values for the other trait. These two traits are correlated and the dots will form an oval in general outline. See Fig. 13.17. If the plotted values are more or less evenly scattered and do not form an oval in general outline, the traits are not correlated and evolved independently of one another. http://www.bio.tamu.edu/users/ajones/gmatrixonline/whatisg.html
Some characters may have very little variation and may constraint or determine the initial direction of evolution.
Studies have shown that genetic correlation between characters may remain consistent for a long time and can influence the direction of selection. Some investigators have found that the genetic correlations among certain characters are fairly similar among geographic populations of a species or among closely related species.
Other studies have found lower similarities showing that the strength of genetic correlations can evolve rather rapidly.
NORMS OF REACTION
The norm of reaction of a genotype is the set of phenotypes it expresses in different environments.
“For every genotype, phenotypic trait, and environmental variable, a different norm of reaction can exist; in other words, an enormous complexity can exist in the interrelationships between genetic and environmental factors in determining traits.” http://en.wikipedia.org/wiki/Norms_of_reaction
PHENOTYPIC PLASTICITY
Phenotypic plasticity: Organisms with a given genotype may change its phenotype in order to adapt to environmental changes.
A single genotype may produce radically different phenotypes in response to environmental stimuli.
Adaptive phenotypic plasticity has evolved by natural selection for those genotypes with norms of reaction that most nearly yield the optimal phenotype for the various environments the organism commonly encounters.
E.g. the leaf form of the aquatic plant Ranunculus aquatilis depends on whether it is submerged, aerial or situated at the air-water interface during development.
The following examples are from: http://en.wikipedia.org/wiki/Phenotypic_plasticity
E. g. In…social insects, colonies of which depend on the division of their members into distinct castes, such as workers and guards. These two castes differ dramatically in appearance and behavior. However, these differences are not genetic; they arise during development and depend on the manner of treatment of the eggs by the queen and the workers, who manipulate such factors as embryonic diet and incubation temperature. The genome of each individual contains all the instructions needed to develop into any one of several 'morphs', but only the genes that form part of one developmental program are activated.
E. g. In epidemiology, a popular theory is that rising incidences of coronary heart disease and Type II diabetes in human populations undergoing industrialization is due to a mismatch between a metabolic phenotype determined in development and the nutritional environment an individual is subsequently exposed to. This is known as the 'Thrifty phenotype' hypothesis.
CANALIZATION
Canalization is the buffering of developmental pathways that tend to produce a standard phenotype despite environmental fluctuations and underlying genetic variability: developmental homeostasis.
Uniform expression of traits or patterns of development despite slight changes in environment or genotype. Maintenance of a single adaptive phenotype.
A trait is canalized when the same phenotype is produced in a wide range of genotypic and environmental backgrounds.
Canalization occurs when selection allows a larger number of different genotypes to produce the same phenotype.
Selection for genotypes that can produce the same phenotype despite their variability.
Genetic assimilation has taken place when a character state that initially developed in response to the environment can become genetically determined and inherited.
Canalized characters include threshold characters, in which underlying polygenic variation is not phenotypically expressed unless a drastic mutation or environmental perturbation breaks down canalization.
The following quotation refers to the experiment performed by Waddington on the effect of heat shock and the development of crossvainless trait in Drosophila on page 358,
“As explained by WADDINGTON, the development of crossveins and other apparently very stable morphological traits can be influenced by environmental disturbances above a certain threshold of intensity, but individuals from wild-type populations have a threshold so high that only an unusually strong stimulus, such as a heat shock, can effectively induce a modified expression. According to WADDINGTON's explanation, the phenotypic uniformity generally observed in these traits can easily coexist with the abundant genetic variability demonstrated by artificial selection in assimilation experiments. Although different genotypes available in a population are sensitive to different threshold values of external stimuli, phenotypic variation does not arise if all of them have too high a threshold to be affected by the disturbances prevailing in the usual environment. However, when an exceptionally severe disturbance occurs, the subpopulation of individuals in which a phenotypic change is induced is necessarily enriched for the most sensitive genotypes, which provide the material for artificial selection.” Canalization, Genetic Assimilation and Preadaptation: A Quantitative Genetic Model. Ilan Eshel and Carlo Matessi. Genetics, Vol. 149, 2119-2133, August 1998
Genotypes of flies differ in their degree of susceptibility to the influence of the environment (heat shock) – they differ in their degree of canalization so that some are less easily deflected into an aberrant developmental pattern.
Selection for this pattern favors alleles that canalize development into the newly favored pathway.
As such alleles accumulate, less environmental stimulus is required to produce the new phenotype.
Waddington explained that genetic assimilation provided a potentially beneficial mechanism for populations under stress
The phenotype produced under stress becomes the phenotype for every condition.
“Canalization implies that a genotype's phenotype remains relatively invariant when individuals of a particular genotype are exposed to different environments (environmental canalization) or when individuals of the same single- or multilocus genotype differ in their genetic background (genetic canalization). Consequently, genetic canalization can be viewed as a particular kind of epistasis, and environmental canalization and phenotypic plasticity are two aspects of the same phenomenon. Canalization results in the accumulation of phenotypically cryptic genetic variation, which can be released after a "decanalizing" event. Thus, canalized genotypes maintain a cryptic potential for expressing particular phenotypes, which are only uncovered under particular decanalizing environmental or genetic conditions. Selection may then act on this newly released genetic variation. The accumulation of cryptic genetic variation by canalization may therefore increase evolvability at the population level by leading to phenotypic diversification under decanalizing conditions. On the other hand, under canalizing conditions, a major part of the segregating genetic variation may remain phenotypically cryptic; canalization may therefore, at least temporarily, constrain phenotypic evolution. Mechanistically, canalization can be understood in terms of transmission patterns, such as epistasis, pleiotropy, and genotype by environment interactions, and in terms of genetic redundancy, modularity, and emergent properties of gene networks and biochemical pathways. While different forms of selection can favor canalization, the requirements for its evolution are typically rather restrictive. Although there are several methods to detect canalization, there are still serious problems with unambiguously demonstrating canalization, particularly its adaptive value.” The evolutionary genetics of canalization. Q Rev Biol. 2005 Sep; 80(3):287-316.
PHENOTYPIC PLASTICITY
Adaptive phenotypic plasticity has evolved when a genotype has the capacity to produce different phenotypes suitable for different environmental conditions.
Phenotypic plasticity includes rapidly reversible changes in morphology, physiology, and behavior, as well as developmental switches that cannot be reversed during the organism’s lifetime.
The genotypes produce the optimal phenotype for the kind of environment most commonly encountered by the species.
West-Eberhard (2003) has proposed that phenotypic changes induced by a new environment may sometimes prove adaptive, and may later be genetically assimilated.
Few cases of adaptation initiated by phenotypic plasticity have been well documented so far.
EVOLUTION OF VARIABILITY
Variation refers to the differences actually present in a sample or species.
Variability refers to the potential to vary.
The variability of individual characters is affected by the evolution of canalization.
A character that is insensitive to alteration by environmental factors in environmentally canalized.
Genetic canalization describes insensitivity of a character to mutations.
In genetic canalization, the phenotype remains unchanged when the genes underlying its development vary.
“…genetic canalization can be viewed as a particular kind of epistasis, and environmental canalization and phenotypic plasticity are two aspects of the same phenomenon. Canalization results in the accumulation of phenotypically cryptic genetic variation, which can be released after a "decanalizing" event. Thus, canalized genotypes maintain a cryptic potential for expressing particular phenotypes, which are only uncovered under particular decanalizing environmental or genetic conditions. Selection may then act on this newly released genetic variation.” The evolutionary genetics of canalization. Q Rev Biol. 80(3):287-316; 2005 Sep.
Threshold traits are controlled by polygenic variation rather than by single loci.
The polygenic variation is not expressed phenotypically unless development is disturbed by substantially beyond the threshold by a large enough genetic or environmental change.
The evolution of canalization may explain the constancy of some characters over vast periods of evolutionary time.
E. g. Early Devonian amphibians had a variable number of eight or nine toes; then five-toed limbs evolved and became canalized, so vertebrate tetrapods since then have had more than five digits.
The hypothesis of morphological or phenotypic integration holds that functionally related characteristics should be genetically correlated with one another.
Wagner and Altenberg (1996) have shown theoretically that prolonged directional selection favors modifier alleles that enhance a pleiotropic correlation between functionally related traits.
E.g. if it were functionally important for the upper and lower mandibles of a bird’s bill to be the same length, then selection for a longer bill would include selection for alleles that coordinate the development of the two mandibles, creating a pleiotropic correlation between them.
For additional information see: http://eebweb.arizona.edu/faculty/badyaev/papers/73.pdf
GENETIC CONSTRAINTS ON EVOLUTION
Many species have become extinct because of their failure to adapt. Why didn’t they adapt?
Evolution may be constrained by the amount of genetic variation (VA) in individual characters and genetic correlation (rG) among characters
Some adaptations are the result of a single mutation rather than great genetic variation, e.g. resistance of Culex pipiens to organophosphate insecticides is due to a single mutation that spread around the world by gene flow.
Recent adaptive evolution has been based on genetic variation or new mutations.