Q.95 Consider a large population of a finch species, where both small and big beak sizes
are advantageous, and an intermediate beak size is maladaptive. Over a period of
10 years, which one of the following evolutionary processes is most likely to operate
on the beak size of this finch population?
(A) directional selection
(B) disruptive selection
(C) genetic drift
(D) stabilizing selection

Disruptive selection is the most likely evolutionary process acting on this finch population’s beak size, as both small and large beaks are favored while intermediates are selected against. This matches classic observations in Darwin’s finches where bimodal resource distributions drive such patterns over short timescales like 10 years.​

Option Analysis

(A) Directional selection shifts the population trait distribution toward one extreme, such as larger beaks during droughts when only large seeds remain available. It does not favor both extremes simultaneously, so it fails to explain advantages for both small and big beaks here.​

(B) Disruptive selection favors individuals at both trait extremes (small and large beaks) over intermediates, creating a bimodal distribution as variance increases. Studies on Darwin’s finches confirm this during periods of resource scarcity with small and large seeds but few mediums, matching the scenario exactly.​

(C) Genetic drift causes random allele frequency changes prominent in small populations, but effects weaken substantially in large populations where selection dominates. Over 10 years, drift would not predictably favor specific beak extremes in this large population.​

(D) Stabilizing selection reduces variation by favoring intermediate traits, eliminating extremes, as seen in human birth weights. This directly opposes the given condition where intermediates are maladaptive.​

Correct Answer

(B) disruptive selection​

Introduction to Disruptive Selection Finch Beak Size

In evolutionary biology, disruptive selection on finch beak size occurs when extreme phenotypes—small and large beaks—gain survival advantages over maladaptive intermediates, often in large populations facing bimodal resources. This process, observed in Darwin’s finches, drives bimodal trait distributions over generations, as documented in Galápagos studies spanning droughts. For CSIR NET aspirants, recognizing this distinguishes it from other selection types in competitive exams.​

Defining Disruptive Selection

Disruptive selection favors both trait extremes, increasing population variance and potentially leading to speciation. Unlike other modes, it produces a two-peaked fitness curve where intermediate beak sizes suffer higher mortality due to poor resource access. Field data from medium ground finches (Geospiza fortis) show lowest survival for birds between small and large beak modes.​

Finch Beak Size as Classic Example

Darwin’s finches exhibit disruptive selection when seeds cluster as small and large sizes, disadvantaging intermediate beaks unable to crack either effectively. Over 10 years, as in the query scenario, this shifts populations toward bimodality, confirmed by recapture studies during droughts. Such patterns contribute to adaptive radiation in sympatric populations.​

Comparing Selection Types

CSIR NET Exam Relevance

This question tests comprehension of selection modes in large populations, where drift is negligible and stabilizing opposes the description. Disruptive selection aligns perfectly with the 10-year timescale and maladaptive intermediates. Practice similar PYQs to master evolutionary biology for CSIR NET Life Sciences.​