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In four o'clock plants, there are three phenotypes for flower color, but only two alleles. The table shows both the expected and observed numbers of plants of each flower color in a population of 1,000.




Phenotype		Number expectedNumber observed

Red flowers		105173

Pink flowers		484343

White flowers		411484






Which statement best explains the results in the table?

AThe population is in equilibrium because the total number of plants expected and observed was about the same.
BThe population evolved because the frequency of the alleles and the genotypes changed.
CThe frequency of the alleles and genotypes in the population changed because of gene flow.
DThe observed frequency of the phenotypes is different from the expected frequency because some alleles mutated.

Verified Answer :

  • The Hardy-Weinberg equilibrium acts as a null hypothesis where allele and genotype frequencies remain constant over time.
  • The significant variance between the expected counts and the observed counts of phenotypes indicates that the population is not in equilibrium.
  • A shift in phenotype distribution indicates a corresponding shift in genotype and allele frequencies.
  • Consequently, the best explanation is that the population has evolved.

Final Answer: \(\boxed{\text{The population evolved because the frequency of the alleles and the genotypes changed.}}\)

Explanation

  1. Understanding Hardy-Weinberg equilibrium

To understand why the observed phenotype counts differ from the expected counts, we first look at the principle of Hardy-Weinberg equilibrium. A population is in Hardy-Weinberg equilibrium if the allele and genotype frequencies remain constant from generation to generation. This theoretical equilibrium serves as a baseline, where expected frequencies of phenotypes are predicted based on the initial allele frequencies within the population under specific conditions (such as random mating, no migration, no mutations, no natural selection, and a large population size).

  1. Analyzing the discrepancy between expected and observed counts

We compare the provided data for the \(1{,}000\) four o'clock plants:

  • Red flowers: Expected \(105\), Observed \(173\) (a significant increase).
  • Pink flowers: Expected \(484\), Observed \(343\) (a significant decrease).
  • White flowers: Expected \(411\), Observed \(484\) (an increase).

Although the total number of plants is consistent (\(1{,}000\)), the distribution across the three phenotypes has shifted considerably from what was expected. Because the observed phenotype distribution deviates from the expected theoretical distribution, we conclude that the population is not in Hardy-Weinberg equilibrium.

  1. Evaluating the options and drawing a conclusion

When a population deviates from Hardy-Weinberg equilibrium, it implies that the frequencies of alleles and genotypes in the population have changed over time. This shift in genetic composition is the fundamental definition of evolution at the population level. While mechanisms like gene flow or mutation (mentioned in other options) are potential causes of evolutionary change, the observation that the frequencies have changed itself is sufficient evidence to state that the population is evolving. Therefore, the statement "The population evolved because the frequency of the alleles and the genotypes changed" is the most comprehensive and accurate explanation for the results shown in the table.

Examples

This problem demonstrates how biologists use statistical expectations of genotype frequencies to detect evolutionary change in real-world populations. For example, if a conservationist observes that the current distribution of traits in an endangered species significantly differs from the expected distribution, they can infer that external pressures—like habitat loss or selective breeding—are driving the population to evolve, allowing them to take informed conservation actions.

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