What do population geneticists mean when they refer to the fitness of an allele?
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Selection represents systematic differences in the chance that individuals will contribute genes to later generations. It can represent differences in survival or in reproduction. Show
The fitness of a genotype is measured relative to the fitness of an arbitrary reference genotype, normally the most fit. For example, the following shows a case in which dominant homozygotes and heterozygotes are most fit, and recessive homozygotes are much less fit, such as is seen for many recessive genetic diseases: genotypeAAAaaafitness1.01.00.2The fitness of an allele depends on what genotypes it finds itself in, so we can't determine it unless we know the genotype frequencies, or can estimate them using Hardy-Weinberg. We determine the total population fitness W as: W = pAAwAA+ pAawAa+ p aawaa and can then find the fitness of each allele: wA= (pAAwAA+ 1/2 pAawAa) / W wa= (1/2 pAawAa+ p aawaa) / W Note that the allele fitnesses will change if the allele frequencies change. A classic example is that a rare recessive has very little fitness effect even if it is lethal in the homozygote, because when it is very rare, it is almost never in homozygotes. A population fitness less than 1 does not mean a dead population, just a population that is not reaching its maximum possible fitness. An alternative way to write these is in terms of a selection coefficient-the proportion of fitness lost due to a particular genotype. Usually wAAis taken as a reference, waais assigned the selection coefficient s, and the fitness of the heterozygote is represented by a multiplier h. genotypeAAAaaafitness1.01-hs1-sThe multiplier h can then be thought of as a measure of dominance. h=1 means that a is dominant, h=0 means that it is recessive, h=0.5 means that it is perfectly additive. Directional / Purifying selectionAdditive or Co-dominant s = 0.5, h = 0.5. In this case p awill drop smoothly toward 0. Example: melanin in sunny climates.
s = 0.5, h = 0. In this case p awill drop rapidly when a is common and then slow down, approaching 0 very slowly. The a alleles are hidden in heterozygotes and wabecomes very close to 1. Example: phenylketonuria. Selection against Dominant genotypeAAAaaafitness1.00.50.5s = 0.5, h = 1. In this case p awill drop slowly when a is common (since there are few AA individuals competing with it) and speed up as a approaches 0. Example: Huntington's disease. Overdominant and underdominant cases are usually written in terms, not of s and h, but of s1 and s2, selection against or for the two heterozygotes. Balancing Selection / Overdominance / Heterozygote AdvantagegenotypeAAAaaafitness0.91.00.2In this case p awill approach a value that maximizes W and stay there. Both A and a will persist in the population. The equilibrium point depends on wAAand waa(in this case, it's p A= 0.89, p a= 0.11). Examples: Sickle-cell anemia. (If you're interested, the formula is: p A= s2/(s1 + s2) which basically says that the frequency of A depends on the proportion of the homozygous fitness loss that is due to a .) Disruptive Selection / Underdominance / Heterozygote DisadvantagegenotypeAAAaaafitness1.00.81.0In this case we cannot know what will happen unless we know where we started. If p Astarted out higher than 0.5 we will move to p A= 1.0. If it started out lower than 0.5 we will move to p A= 0.0. There is an equilibrium point at exactly 0.5 but it is unstable. Any least change will start the population moving to 1.0 or 0.0. An interesting case is the following: genotypeAAAaaafitness1.00.40.8Here the population will move to all A or all a depending on starting frequency, which means that in some cases it will move to all a even though that is not the maximum possible fitness. Populations move to a local maximum, not the overall maximum. Example: butterfly mimics. Overall NotesDirectional and disruptive selection remove variation from the population, while balancing selection maintains it. A population may evolve into a suboptimal state. With heterozygote advantage, the optimal state is unreachable for genetic reasons-there is no way to have only heterozygotes, short of a change in the genetic system. With heterozygote disadvantage, the optimal state is reachable but may not be reached if the starting conditions are wrong. "As many more individuals of each species are born than can possibly survive, and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected." Some Definitions
What factors might contribute to fitness?
The frequency of A individuals among the offspring is therefore:
Point 1: It is often easier to measure relative fitness rather than absolute fitness. Point 2: Population genetics models generally require only relative fitnesses, eg
These may be ordered in a number of ways:
More terms to remember: If WAA=WAa, allele A is said to be "dominant" and allele a is said to be "recessive". [Note: geneticists usually name alleles that are recessive with lower case letters (eg ubx) and those that are dominant with upper case letters (eg Ubx).] If WAa=Waa, allele a is said to be "dominant" and allele A is said to be "recessive". Even more terms to remember: "Additive" "Partially dominant" "Partially recessive" Relative fitnesses of AA, Aa, and aa all equal one. Let
x+y+z=1. Why? Case 1: Individuals produce haploid gametes that form a gamete pool. The frequency of allele A in the gamete pool will be? p = The frequency of allele a in the gamete pool will be? q = Gametes unite at random in the gamete pool to produce diploid offspring. To calculate offspring frequencies we use mating tables. Point 1: Populations not at Hardy-Weinberg reach Hardy-Weinberg equilibrium after only one generation of random mating (as in the above example). Caveat: Generations must be discrete. The frequency of allele A in the next gamete pool will be? p' = The frequency of allele a in the next gamete pool will be? q' = Point 2: In the absence of selection and mutation, allele frequencies stay constant. Segregation does not change allele frequencies. Will Hardy-Weinberg frequencies still obtain? Again, to calculate offspring frequencies we use mating tables. This shows that x'=p2: Hardy-Weinberg equilibrium is reached after only one generation of random mating. Since the allele frequency of A in the parents is x+y/2=p (by definition), and since the allele frequency of A in the offspring equals x'+y'/2 = p2 + 2 p q/2 = p (p+q) = p , the allele frequencies again remain constant. What is the fitness of an allele?It can represent differences in survival or in reproduction. The fitness of an allele depends on what genotypes it finds itself in, so we can't determine it unless we know the genotype frequencies, or can estimate them using Hardy-Weinberg. Note that the allele fitnesses will change if the allele frequencies change.
What would population geneticists refer to as fitness?In the crudest terms, fitness involves the ability of organisms— or, more rarely, populations or species— to survive and reproduce in the environment in which they find themselves 6–9. The consequence of this survival and reproduction is that organisms contribute genes to the next generation.
What does fitness measure in genetics?To an evolutionary biologist, fitness simply means reproductive success and reflects how well an organism is adapted to its environment.
How does genetic variation increase the fitness of a population?Genetic variations that alter gene activity or protein function can introduce different traits in an organism. If a trait is advantageous and helps the individual survive and reproduce, the genetic variation is more likely to be passed to the next generation (a process known as natural selection).
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