chapter 9 Flashcards
What is the main focus of Chapter 9: Evolution at Multiple Loci?
Understanding how multiple genes interact to influence phenotypes and using quantitative genetics to predict evolutionary change.
What is the difference between discrete and continuous traits?
Discrete traits have distinct categories (e.g., Mendel’s pea colors), while continuous traits show a range of variation (e.g., human height) and are influenced by multiple loci and the environment.
Why did Darwin and Mendel appear to have conflicting views on inheritance?
Darwin focused on continuous variation and natural selection, while Mendel studied discrete traits with clear inheritance patterns. The modern understanding combines both perspectives.
How does the number of loci influence trait variation?
The more loci involved in a trait, the more continuous the variation, with multiple genotypes blending to create a smooth distribution of phenotypes.
What experiment did Edward East (1916) conduct, and what did it demonstrate?
He crossbred tobacco plants with long and short corollas and showed that F2 generations had a reduced range of variation but could recover parental phenotypes through selective breeding.
How can new phenotypic extremes evolve beyond the original population range?
With multiple loci, recombination and selection can reshuffle alleles to produce new combinations that exceed the original variation.
How does the environment influence quantitative traits?
Environmental factors can modify phenotypic expression, as shown in genetically identical yarrow plants that grow to different heights at different elevations.
What is broad-sense heritability (H2), and how is it calculated?
It measures the proportion of phenotypic variation due to genetic variation, given by: H2 = VG / VP, where VG is genetic variance and VP is total phenotypic variance.
What is narrow-sense heritability (h2), and why is it more useful than broad-sense heritability?
h2 measures the proportion of variance due to additive genetic effects, which directly predicts a trait’s response to selection: h2 = VA / VP. It is more useful because it determines how much of the trait variation is passed from parent to offspring.
How can heritability be estimated without complex equations?
By measuring the correlation between parent and offspring traits, where the slope of the regression line approximates narrow-sense heritability.
What is an example of a real-world study estimating heritability?
Studies on migratory timing in birds have shown strong correlations between parent and offspring migration dates, indicating a genetic component to the trait.
What is the selection differential (S), and how is it calculated?
The difference between the mean trait value of selected parents (P1) and the original population mean (P0): S = P1 - P0.
What is the response to selection (R), and how is it calculated?
The difference between the mean trait value of offspring (O0) and the original population mean (P0): R = O0 - P0.
What is the Breeder’s Equation, and what does it predict?
R = h2 S. It predicts the evolutionary response to selection based on heritability and selection strength.
If heritability (h2) is 0.5 and the selection differential (S) is 2 grams, what is the predicted response to selection (R)?
R = 0.5 × 2 = 1. Offspring will be 1 gram heavier than pre-selection parents.
In a soybean breeding experiment, if P0 = 3.08, P1 = 3.66, and h2 = 0.8866, what is the predicted response to selection?
S = 3.66 - 3.08 = 0.58; R = 0.8866 × 0.58 = 0.514. Offspring will have a 0.514 kg higher yield.
What is the longest-running artificial selection experiment in crop plants?
The soybean oil experiment (since 1896), selecting for high- and low-oil lines.
What does the soybean oil experiment reveal about latent variation?
Strong selection can uncover hidden genetic variation, pushing traits beyond the original population’s observed range.
What happens to heritability (h2) over long periods of strong selection?
It often declines as genetic variation for the selected trait is depleted, but it can remain >0 if new mutations introduce variability.
What is realized heritability, and how is it estimated?
Estimated by reshuffling the Breeder’s Equation (R = h2 S) to solve for h2 using observed response to selection.
What is the key takeaway about heritability from long-term selection experiments?
Heritability is a statistical property of a population, not a fixed genetic fact, and can change with selection and environmental shifts.
Why is heritability important for understanding evolutionary processes?
It helps predict how populations will respond to natural or artificial selection, shaping future generations’ traits.
Why does a trait with high heritability not always show strong selection response?
Selection response depends on both heritability and the strength of selection (S). If selection is weak, even a highly heritable trait may not change much.
How do environmental effects influence heritability estimates?
If environmental variation is high, heritability estimates can be lower, even if genetic variation is present.
