Selection and Fitness

What is Natural Selection?• Natural selection can be defined simply as the differential survival and reproduction of

alternate genetic variants in a population• It’s a process that promotes adaptation and keeps the disorganizing effects of the other

evolutionary processes in check• Natural selection is the most critical evolutionary process, because only selection

accounts for the adaptive and highly organized nature of living things; also explains the diversity of organisms because it promotes their adaptation to different ways of life

• Natural selection is a process that occurs whenever two conditions are met:

(1) variation in a trait between individuals

(2) a relationship between the trait and reproductive success• Natural selection, then, is not caused by differential reproduction, it is differential

reproduction.

Four Postulates of Natural Selection

1. Individuals within a species are variable2. Some of these variations are passed along to offspring3. In every generation, more offspring are produced than can survive4. Survival and reproduction are not random; those with favorable variations survive and go on to reproduce

1. Individuals within a species are variable

2. Some of these variations are passed along to offspring

3. In every generation, more offspring are produced than can survive

4. Survival and reproduction are not random; those with favorable variations survive and go on to reproduce

4. Survival and reproduction are not random; those with favorable variations survive and go on to reproduce

Additional Comments regarding Selection• Heritability is an essential component for natural selection.

• However, it's important to realize that it's phenotypic differences alone that affect reproductive success.

• And, if traits are not heritable, there will be what is called PHENOTYPIC SELECTION.

• Because natural selection occurs through the differential reproduction of phenotypes, it's possible to have natural selection without evolution - differential reproduction but no evolutionary response to selection.

• So, while selection and evolution are related, selection and evolution are not synonymous.

• Selection can occur in the absence of genetic variation, but an evolutionary response to selection requires genetic variation.

Fitness

• Under a given set of environmental conditions, some phenotypes are more successful reproductively than others.

• This difference in reproductive success is referred to as RELATIVE FITNESS (W).

• Fitness is a relative measure in two ways:

(1) Relative to the local environment at the time

(2) Relative to the quality of other phenotypes present• Relative fitness is the parameter that we can use to measure natural selection

• For mathematical convenience, fitness is defined to range from 0-1. • A phenotype with the greatest reproductive success has a fitness of 1; the success of all other phenotypes is measured relative to the success of this most successful phenotype.

Computation of Fitness

Additional Comments about Fitness

• If a phenotype has a fitness of 1, it doesn't mean that this is the best possible phenotype; it's just the best available phenotype.

• Fitness measured locally: a phenotype with a fitness of 1 may be the best available phenotype in its population, but that doesn't mean there aren't phenotypes in other populations that would do better.

• If environmental conditions change, the phenotype that is the most fit may also change.

• In summary: The fitness of a phenotype is measured relative to other phenotypes in the same population under the current set of environmental conditions.

Fitness Components• There are many aspects in the life of an individual that may

contribute to fitness

• These various aspects are collectively called fitness components

• The main fitness components are: survival and fertility; other components can be treated separately or incorporated into these two

Selection against Recessive Defects

• Multi-legged frog trait assumed to be due to a defective recessive allele: A – dominant, normal legs; a – recessive, multi-legged

• Possible bullfrog genotypes: AA (normal), Aa (normal but a carrier), or aa (multi-legged)

• Given genotypes of the initial generation: 24 AA, 48 Aa, and 24 aa

• Assume that aa individuals are unable to mate; thus, parents of the next generation will have either AA or Aa genotypes

• Q. Will the incidence of the multi-legged trait decline and eventually disappear from the population?

Selection against Recessive Defects cont.

Heterozygotes are twice as numerous as homozygous dominants:• 2/3 (48/72) of the total breeding members are Aa and 1/3 (24/72) are AA

• The different types of matings and their genotypes are as follows:

Selection against Recessive Defects cont.

• Given at total of 36 matings, we can express the frequencies as whole numbers:

AA male x AA female – 1/9 x 36 = 4AA male x Aa female – 2/9 x 36 = 8AA female x Aa male – 2/9 x 36 = 8

Aa male x Aa female – 4/9 x 36 = 16

Q. What is the outcome of each type of cross?Assume that each mated pair contributes 4 progeny to the next generationAssume that the actual number of offspring is a reflection of the different types of mating

Example: A single AA female x Aa male mating yield 4 offspring in the Mendelian ratio of 2AA:2AaHowever, there are 8 matings of this kind so that the number of offspring would be 16AA:16 Aa

Selection against Recessive Defects cont.

• To calculate the frequency of a recessive allele after n generations of selection:

• Thus, with the initial value q0 = 0.5, the frequency of the recessive allele after two generations (n=2) will be:

q2 = 0.5 = 0.5 = 0.25 1 + 2(0.5) 2.0

• If the frequency of the recessive a allele is q, then the frequency of the recessive individual (aa) is q2; the frequency of the recessive heterozygote is (0.25)2 or 0.0625 (6.25%)• Therefore, in the second generation, the incidence of the multi-legged condition (aa) decreases to 6.25%

where q0 is the original frequency of the recessive allele, qn is the frequency after n generations

qn = q0

1 + nq0

Outcome of Selection against a Recessive Trait over Numerous Generations

The Interplay of Mutation and Selection

• In theory, if the process of selection against a homozygous recessive trait were to continue over 100s of generations, the detrimental recessive allele would be present at a very low frequency in the population • But, deleterious recessive alleles are continually be replenished by mutation (e.g., from A -----> a)• If a certain portion of A alleles are converted to a alleles, then a population will carry a certain proportion of a recessive mutant allele (a) regardless of selection against it• An equilibrium state will be reached when the rate at which recessive alleles are lost by selection equals the rate at which variant recessive alleles are produced by mutation

Selection Against Dominant Defects

• Imagine a situation in which AA or Aa individuals do not leave any offspring• You might expect the A allele to be quickly eliminated, and for the population to be comprised only of aa individuals• However, it is important to realize that detrimental dominant alleles are likely to be maintained in a population due other factors, including mutation and the effects of partial selection

Example • Begin with a population of 500,000 individuals, all of which are homozygous recessive (aa); 1,000,000 recessive a alleles• 10 dominant mutant alleles occur in the first generation; but, assume the dominant mutant gene is semi-lethal - only 5 of the newly arisen dominant alleles are transmitted to the second generation• The second generation would contain at total of 15 dominant alleles – 5 carried over from the first generation and 10 new ones added by mutation

Selection Against Dominant Defects cont.

• In the third generation, there would be 17.5 dominant alleles, etc.• Overall, the total number of dominant alleles would increase slightly with each subsequent generation• Equilibrium is achieved (about 12 generations) when the rate of elimination of abnormal dominant alleles balances the mutation rate

Balanced Polymorphisms • When a genes(s) and their alleles are balanced or approximate an equilibrium state, e.g., due to interplay of mutation and selection

The Role of Heterozygote Advantage

• Selection in favor of heterozygotes over homozygotes

Sickle-cell Anemia• Due to homozygosis for a recessive HbS allele that produces abnormal hemoglobin instead of the normal hemoglobin due to a HbA allele • Genetics of sickle cell anemia are as follows:

HbS HbS - sickle cell anemiaHbA HbS - normal RBCs (codominance) HbA HbA - normal RBCs

Distribution of the Sickle-cell Allele

• Heterozygotes are more fit than either of the homozygotes• Heterozygotes are resistant to malarial infections, whereas the HbA HbA are not• HbA HbS red blood cells do not normally sickle.

Why is there are relatively high frequency of the HbS allele?

• However, when malaria attacks an HbA HbS red blood cell, oxygen concentration of the cell drops, inducing red blood cells to sickle • This destroys the red blood cell, but it also destroys the parasite.

• So, in areas where malaria is rife, the heterozygotes have a selective advantage over both the homozygotes, which have a fairly high probability of dying from either anemia (HbS HbS) or malaria (HbA HbA)

Heterozygote Advantage and Relative Fitness

Balanced Polymorphism and Frequency-Dependent Selection

• Imagine a scenario in which allele frequencies in a population remain near an equilibrium, but the reason is because the direction of selection fluctuates

• First selection favors one allele, then it favors another

• This scenario is called frequency dependent selection - the fitness of an allele depends on its frequency

Example

• The scale eating cichlid fish Perissodus microlepis from Lake Tanganyika in Africa• This fish bites the scales off of other fish – attacks from behind, grabbing the scale off the victim’s flank• Within the species are right-handed (mouth twisted to the right) and left-handed (mouth twisted to the left) fish• Also, there is evidence to suggest that handedness is determined by a single locus with 2 alleles, with right handedness is dominant over left handedness• Behavior observations and an examination of scales recovered from the fish’s stomach indicate that right—handed fish always attack their victim’s left flank; left-handed fish attack the right flank• The prey species are usually wary and alert, with the scale-eating predators only successful in 20% of their attacks

Hypothesis:

• If right-handed scale eaters were more abundant than left-handed individuals, then the prey species would be more vigilant against attacks from the left• This would then give left-handed scale eaters and advantage in catching prey• Left-handed scale eaters would secure more food, leave more offspring, and pass on more of their left-handed genes• This would ultimately increase the frequency of left-handed scale eaters in the population

• After left-handed scale eaters had become more abundant, the prey fish would start to be more vigilant for attacks from the right• This would, in turn, give right-handed scale eaters and advantage• As a result of this, left-handed and right-handed fish should be just about equally abundant in the population at any given time

Experiment

• Hori (1993) sampled fish from Lake Tanganyika every year for 11 years and found that the frequency of the 2 phenotypes oscillated around 0.5 for each• In any given year, one of the two phenotypes may have a slightly higher frequency, but invariable the pendulum would swing back to the other direction

Multiple Niche Polymorphism and Environmental Heterogeneity

• Jones and Probert (1980) experiments with normal and white-eyed (mutant) Drosophila simulans

• Set up cages with a mixture of the two genotypes, in either red or white light; mutants were at a disadvantage and selection reduced their frequency

• Polymorphisms can occur as a result of heterogeneous environments, an idea some times referred to as multiple niche polymorphisms

Multiple Niche Polymorphism cont.

• They also placed the two groups of flies together in a cage that was illuminated with red light in one half and white light in the other half of the cage; both genotypes were maintained

• The white flies concentrated in the red light half of the cage and the normal red-eyed flies in the half with white light – the flies showed “habitat selection”

Types of Selection

Selection type mean trait value trait variance stabilizing no change decreases directional change usually no change disruptive no change increases

Stabilizing Selection

• This form of selection occurs when intermediate phenotypes have a higher fitness that extreme phenotypes.

• In fact, uncommon phenotypes may be eliminated

• Under stabilizing selection, the mean value of a trait is unaffected but trait

variance is reduced

Stabilizing Selection and New Born Birth Weights

Directional Selection

• Directional selection occurs when an extreme phenotype has higher fitness than other phenotypes. • It is usually in response to new environmental conditions• Under directional selection, the mean value of the trait is affected and trait variance is often not affected.

Directional Selection and Cheetahs

Disruptive Selection

• Disruptive selection occurs when intermediate phenotypes are of lower fitness than extreme phenotypes. • Under disruptive selection, the mean value of the trait remains the same, but trait variance increases.

Disruptive Selection and Cacti