Phenotypic Evolution

Quantitative Traits

• Quantitative traits vary continuously (e.g., size) or meristically (e.g., number of bristles).

• They are often distributed normally, forming a bell-shaped frequency distribution.

Genetic Basis of Quantitative Traits

• The genetic basis of quantitative traits is often complex and not fully understood.

• Variation often involves many genes.

• Alleles often interact in an additive fashion, resembling multiple co-dominant or incompletely dominant alleles.

• Variation is also influenced by the environment.

Example: Finch Beak Depth

• Consider a simplified example with three genes, each with two alleles.

• Each '+' allele contributes one unit to beak depth, while each '-' allele does not.

Frequency of Genotypes

• Only one genotype results in a very small beak or a very large beak.

• Multiple genotypes can result in a medium-sized beak.

Bell-Shaped Frequency Distribution

• The basis of the bell-shaped frequency distribution is the various combinations of alleles that give intermediate phenotypes. Examples include: 3 + 3 -, 2 + 4 -, 1 + 5 -, 4 + 2 -, 5 + 1 -.

Environmental Variation

• Environmental variation exists around each genotype.

• This environmental variation smoothes the overall phenotypic variation.

Impact of Loci and Environmental Variance

• As the number of loci increases, the distribution becomes smoother.

• More environmental variance also smooths the variation.

Selection and Quantitative Traits

• Selection alone can cause significant changes in quantitative traits, even without new mutations.

• Recombination plays a crucial role in this process.

Allele Frequencies

• With 50 loci, the frequency of '+' alleles (p) might be 0.75, while the frequency of '-' alleles might be 0.25.

Selection and Average Trait Value

• Selection alone can move the average trait value well beyond the original range.

• For example, if only the tallest people reproduced, the average height could become taller than the tallest people alive today over generations.

Variance

• Variance (V) is the statistical term for measuring variation and is a critical mathematical concept in science.

• $V = \sum (deviations \, from \, the \, mean)^2 / (n – 1)$

• If every individual is the same, with no deviations from the average, V = 0.

• If many individuals are far from the average, V is very large.

Variance and Distribution

• A tall, narrow curve indicates small variance.

• A short, wide curve indicates large variance.

Phenotypic Variance

• Phenotypic variance (VP) is the variance of a trait.

• $V_P$ is usually the result of both genetic and environmental factors.

• Mathematically, it is the sum of genetic and environmental variances: Vp=Vg+Ve

Heritability The fraction of phenotypic variation that is genetic is called heritability ($h^2$).

• $h^2 = V<em>G / V</em>P$

• Heritability is a population-specific parameter.

• If there is no genetic variance in the population, then $h^2 = 0$.

Dependence on Population

• $h^2$ depends on the population; if $V_G$ is 0, then $h^2$ is 0.

• Imagine uniform genetic variation (constant $V_G$).

• If the population lives in a variable environment:

  • High $V_E$ would result in a low $h^2$.t

Environment and Genotype Interaction  

• /Relative amounts of $V<em>G$ and $V</em>E$ can vary with the environment, leading to environment x genotype interaction.

• Environment can affect different genotypes differently.

Estimating Heritability

• Heritability can be estimated through response to artificial, directional selection (e.g., truncation selection).

• Illustrates the importance of $h^2$ to natural selection.

Truncation Selection

• Truncation selection involves selecting individuals above a certain threshold for breeding.

Selection Differential

• The selection differential (S) is the difference between the population mean and the selected subpopulation's mean.

Response to Selection

• Response to selection (R) is the difference between the original population mean and the offspring’s mean.

Expected Outcomes

• If all the variance is genetic ($h^2 = 1$), R = S.

• If all the variance is environmental ($h^2 = 0$), R = 0.

• If a small fraction of the variance is genetic, R is small.

• If a large fraction of the variance is genetic, R is large.

Response and Genetic Variance

• Response depends on how much variance is genetic ($h^2$).

• If all the variance is genetic, R will equal S.

• If none of the variance is genetic, R will be 0.

• If half the variance is genetic, R will be half of S.

• Therefore, $h^2 = R / S$

Selection Response

• A population can respond to selection only if $h^2 ≠ 0$ and $V_G ≠ 0$.

• Assuming $V<em>E$ stays the same, as $V</em>G$ decreases, so does $h^2$.

• With $h^2 = 0$, the response to selection goes to 0 until new variation is introduced.

Human Races and Heritability

• The relationship between $h^2$ and differences between groups has been misapplied to humans (e.g., racial differences in IQ).

Scenario

• Two populations significantly differ in average trait value (e.g., corn height).

• Imagine all variation within each population is heritable ($h^2 = 1.0$).

• What explains the difference between the populations?