32. Darwinian finches of three different size of beaks-large, intermediate and small feeds on seeds size with bimodal distribution. Under such condition natural selection will favor finches having beak size
    (1) Intermediate (2) Large
    (3) Large and Small both (4) All types of beak size

Correct answer:
(3) Large and Small both

Introduction

Darwin’s finches are a textbook example of natural selection and adaptive evolution. Their beak size and shape have evolved in response to the types of food available on the Galápagos Islands. When the environment presents a bimodal seed distribution—mostly large and small seeds, with few intermediates—finches with either large or small beaks have a distinct advantage over those with intermediate-sized beaks.

The Science Behind Disruptive Selection

Disruptive selection occurs when environmental conditions favor individuals at both extremes of a trait, while those with intermediate traits are less successful. In the case of Darwin’s finches:

• Large-beaked finches can efficiently crack and eat large, hard seeds.

• Small-beaked finches are better at handling small, soft seeds.

• Intermediate-beaked finches struggle with both seed types, making them less competitive and less likely to survive or reproduce.

Over time, this leads to an increase in the proportion of birds with either large or small beaks, and a decrease in those with intermediate beak sizes.

Real-World Evidence

Field studies and long-term observations of finch populations have shown that when the seed supply is bimodal, disruptive selection can rapidly shift the population away from intermediate forms. This process can even contribute to speciation, as the two extreme forms may eventually become reproductively isolated.

Why Not Other Types of Selection?

• Stabilizing selection would favor intermediate beak sizes if only medium seeds were available.

• Directional selection would favor one extreme (large or small) if only one seed type dominated.

• Disruptive selection is unique in favoring both extremes, as seen with bimodal seed availability.

Genetic Basis

Research has identified genes such as HMGA2 and ALX1 that influence beak size and shape in Darwin’s finches, enabling rapid adaptation to environmental changes and resource availability.

Conclusion

When seeds are distributed bimodally, natural selection favors finches with either large or small beak sizes, leading to disruptive selection. This evolutionary process demonstrates how environmental factors shape the diversity of life and drive the formation of new species.

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Darwin’s finches continue to provide powerful evidence of how natural selection operates in real time, illustrating the dynamic relationship between organisms and their environment