In a population of finches on an island, beak sizes vary from small to large. The island experiences alternating periods of drought and abundant rainfall. During droughts, only large seeds are available, while during rainy periods, both small and large seeds are plentiful. Over many generations, the population of finches tends to have more individuals with either very small or very large beaks, with fewer medium-sized beaks. Which type of selection best describes this scenario?

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Let's begin by restating the core scenario and the available choice.
Question: In a population of finches on an island, beak sizes vary from small to large. The island experiences alternating periods of drought and abundant rainfall. During droughts, only large seeds are available, while during rainy periods, both small and large seeds are plentiful. Over many generations, the population tends to have more individuals with either very small or very large beaks, with fewer medium-sized beaks. Which type of selection best describes this scenario?
Answer option presented: 'Disruptive selection'.

Now, evaluate the general patterns of natural selection types to see which fits best.

- Disruptive selection: This form of selection favors individuals at both extremes of a trait distribution (very small and very large beaks in this case) while disfavouring individuals with intermediate traits (medium beaks). It often occurs when environmental conditions change in a way that alternately rewards the extremes, or when different subgroups benefit from different extremes in different contexts. The described island scenario—droughts favoring large beaks to crack large seeds and rainy periods allowing both small and large beaks—creates a situation where extremes have higher fitness overall across cycles, reducing the middle ground over successive generations. This is a textbook illustration of disruptive selection.

- Directional selection: This mode pushes the population mean toward one extreme (either all larger beaks or all smaller beaks over time). In the present description, there is not a single directional shift; instead, both extremes are favored at different times, with intermediates suppressed, which argues against pure directional selection.

- Stabilizing selection: This pattern reduces variation around an optimal middle phenotype, increasing the frequency of intermediates and decreasing extremes. The finch example shows an increase in extremes and a decrease in medium beaks, which is incompatible with stabilizing selection.

- Balancing selection or fluctuating selection: While these concepts involve maintaining diversity, they often refer to genetic mechanisms (like heterozygote advantage) or changing frequencies of alleles due to varying environments. Disruptive selection is a more direct description here since the environment repeatedly rewards the two ends of the beak-size distribution over time.

In summary, the described longitudinal shift toward more individuals at the beak-size extremes, with fewer intermediates, aligns with the pattern of disruptive selection. The mechanism hinges on the alternating environmental pressures that favor different extremes across generations, thereby promoting bimodality in trait distribution.

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