Dark Grit, Pale Sand and the D Allele
A population of mice lives on a sand dune where the sand is pale. Dark fur is produced by a dominant allele D and pale fur by the genotype dd. Before 2010 the dune was uniformly pale and dark mice were about 5 percent of the population. In 2012 a storm covered part of the dune with dark volcanic grit, and the dark patch has expanded each year as the grit spreads. Trapping gives the frequency of the D allele as 0.31 in 2012, 0.22 in 2016 and 0.09 in 2020. Owl pellets collected on the dark patch contain 61 percent dark mice, although dark mice are only about 18 percent of the population there by 2020. A visitor says the mice grew darker fur to match the new sand and passed the change to their offspring.
1. Explain whether the data show that evolution has occurred, and describe the mechanism most likely to be responsible.
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Evolution is a change in allele frequency in a population over time, and the D allele falls from 0.31 to 0.09 between 2012 and 2020, so evolution has occurred by definition. The question is what caused it.
The mechanism is natural selection. Variation in coat colour already existed: the D allele was present at low frequency before the storm. On pale sand, dark mice are more visible to owls and are removed disproportionately; on dark grit the same dark phenotype is better camouflaged and survives better. The owl pellet data support this: dark mice make up 61 percent of prey but only about 18 percent of the population on the dark patch, which needs careful interpretation. If owls are hunting mainly on the dark patch, that figure suggests dark mice are being taken more than expected there, which would not fit simple camouflage and should prompt a check of where the pellets were collected.
The overall decline of D across the dune is consistent with selection against dark mice on the large pale area outweighing any local advantage on the grit. The answer should state that selection acts on phenotypes, and therefore on the survival of the alleles those phenotypes carry, not on alleles directly.
2. Evaluate the visitor's explanation that the mice grew darker fur to match the new sand and passed the change to their offspring.
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The explanation is Lamarckian and teleological, and it conflicts with the data. It implies that an individual's need or experience changes its inherited material in a directed way, so that offspring inherit an acquired coat colour. There is no mechanism for this: coat colour is determined by alleles inherited at fertilisation, and a mouse exposed to dark grit does not alter the alleles in its gametes.
The observed pattern is the reverse of the visitor's story. The dark allele was already present before the environment changed, and its frequency fell as the dark patch spread, which is what selection against dark mice on the remaining pale sand would produce. The population did not become darker; the D allele became rarer.
A better statement is that the storm changed the relative fitness of existing phenotypes, and the population responded by a change in allele frequencies across generations. The visitor has confused a change in a population over time with a change in an individual during its lifetime, and has assumed that need can direct mutation. Mutations arise at random with respect to the environment, and selection then acts on them.
3. The D allele has not reached a frequency of zero. Suggest two reasons, and explain how each would slow or halt its loss.
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Several processes can keep a disadvantageous allele in a population:
- Gene flow from pale areas: mice moving in from surrounding pale dune bring d alleles, so the frequency in the grit patch is continually topped up and selection cannot drive D to zero. The effect is strongest if the dark patch is small relative to the whole dune.
- Dominance and hidden alleles: D is dominant, so heterozygous Dd mice are dark and are selected against, but the allele persists in those heterozygotes until they die; selection removes phenotypes, not alleles, so loss is gradual and depends on the frequency of heterozygotes.
- Heterozygote advantage or balancing selection: if Dd mice have some advantage, for example in intermediate or disturbed habitat, selection would maintain both alleles rather than fix either.
- Genetic drift: in a small population, chance fluctuations can keep or remove alleles regardless of their effect, and sampling error is large when few mice are trapped.
A complete answer notes that the dark patch covers only part of the dune, so selection is not uniform across the range; the overall frequency is an average of opposing selective pressures.