A plant breeder selects the tallest plants from a population each generation and uses them as parents for the next. After five generations, the average height has barely changed. What is the most likely explanation?
AThe population does not have enough individuals to show a detectable evolutionary response
BMost of the height variation in the population has an environmental rather than genetic basis, so selection cannot produce a heritable response
CThe plants adapted to the selection pressure by downregulating growth genes
DFive generations is not enough time for natural selection to produce a measurable shift in mean height
If tall plants owe their height to favorable growing conditions rather than 'tall' alleles, their offspring will have no more growth-promoting alleles than average. Selection can only shift population means when the selected trait is heritable — when parents pass relevant alleles to offspring. Environmental variation in phenotype contributes to what you see but is invisible to natural selection. A failed selection response is one of the clearest signals that the variation being selected is mostly environmental rather than genetic.
Question 2 Multiple Choice
Twin studies show that IQ scores are substantially heritable. Does this mean that educational interventions cannot raise IQ?
AYes — high heritability means the trait is primarily genetically determined, so environmental interventions will have minimal effect
BNo — heritability describes the proportion of variance within a population that is genetic, not whether the trait can be changed by altering the environment
CYes — if heritability exceeds 0.5, genetic factors dominate and environmental effects are negligible
DNo — twin studies systematically underestimate environmental effects and cannot be used to make inferences about individual plasticity
This is the most common and consequential misinterpretation of heritability. Heritability is a population statistic: it describes how much of the variation *among individuals in a specific population under current conditions* is attributable to genetic differences. It says nothing about whether a different environment would change all individuals' phenotypes. Height is highly heritable in well-nourished populations, yet average height has increased dramatically over the 20th century due to improved nutrition — a purely environmental change. High heritability and high environmental sensitivity are fully compatible.
Question 3 True / False
A population in which all phenotypic variation is caused by environmental differences cannot evolve by natural selection, even if the trait under selection is strongly correlated with survival and reproduction.
TTrue
FFalse
Answer: True
Natural selection requires heritable variation — differences among individuals that are transmitted from parent to offspring through genes. If all variation is environmental, selecting individuals with favorable phenotypes produces no change in allele frequencies, because those individuals do not carry special alleles to pass on. The selected trait will correlate with survival and reproduction, but the population's genetic composition — and therefore its mean phenotype under standard conditions — will not change. This is why heritability is the crucial quantity for predicting evolutionary response.
Question 4 True / False
Observing high phenotypic variation in a population is reliable evidence that substantial genetic variation exists for natural selection to act upon.
TTrue
FFalse
Answer: False
Phenotypic variation conflates genetic and environmental sources. A population planted across a range of soil types and rainfall regimes may show enormous variation in growth even if every individual is genetically identical. Conversely, a population with substantial genetic variation may show little phenotypic variation if the environment is highly uniform and genotype-by-environment interactions are small. Only partitioning the variance — through breeding experiments, parent-offspring regressions, or controlled common-garden studies — reveals how much variation is heritable.
Question 5 Short Answer
Why can natural selection only act on the heritable portion of phenotypic variation, and what does this imply for predicting evolutionary responses in a population?
Think about your answer, then reveal below.
Model answer: Selection acts by differentially transmitting alleles to the next generation: individuals with favorable phenotypes leave more offspring, and if those phenotypes reflect genetic differences, the relevant alleles become more common over time. If the favorable phenotype is instead caused by environmental luck, the individual's offspring will not inherit the advantage — there are no 'favorable environment' alleles. To predict evolutionary response, you must know heritability: the breeder's equation states that response to selection equals heritability times selection differential. Without heritability, even strong selection produces no evolutionary change.
This principle is practically critical. A conservation biologist asking whether a species can adapt to climate change needs to know whether the relevant traits (thermal tolerance, flowering time, drought resistance) have heritable variation. A crop breeder choosing which plants to cross needs to know whether yield differences between lines are genetic or due to field variation. Measuring phenotypic variation alone gives an upper bound on what selection could achieve; the heritable fraction tells you what it actually will achieve.