Q.86 Which one of the following provides the strongest support for the theory of “descent with
modification”?
(A) Early embryonic forms of diverse organisms (examples: fishes, birds and mammals) appear
similar
(B) Ability of fishes and whales to swim
(C) Variation in flower colour in a given species
(D) Skin colour variation among individuals in a human population
Correct Answer: (A) Early embryonic forms of diverse organisms appear similar, providing the strongest evidence for “descent with modification” by showing shared developmental pathways from a common ancestor, despite later divergent adult forms.
Option Analysis
Option (A)
Correct. Embryonic similarities (e.g., pharyngeal arches in fish/bird/mammal embryos, gill slits in human embryos) indicate conserved developmental genes from common ancestry, modified later—classic Darwinian evidence.
Option (B)
Incorrect. Swimming ability in fishes/whales is convergent evolution (analogous structures for similar environments), not descent from shared ancestor with that trait.
Option (C)
Incorrect. Flower color variation within one species reflects genetic polymorphism/microevolution, not deep phylogenetic “descent with modification” across lineages.
Option (D)
Incorrect. Human skin color variation results from recent adaptation (melanin for UV protection), representing microevolution within modern Homo sapiens, not ancient descent patterns.
Introduction to Descent with Modification
Descent with modification, Darwin’s core evolutionary principle, explains biodiversity through common ancestry plus adaptive changes. GATE Q.86 tests this: embryonic similarities across vertebrates (A) best support shared ancestry, unlike convergent traits or within-species variation.
Embryonic Evidence: Strongest Support
Early embryos of fish, birds, mammals, reptiles show striking similarities—pharyngeal pouches (gill arches in fish, jaw/ear bones in mammals), tails, segmented somites—reflecting conserved Hox genes from ~550 MYA chordate ancestor. These modify into species-specific adult structures, direct proof of descent with modification (von Baer’s laws).
Why Other Options Fail
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Fishes/whales swimming (B): Hydrodynamic bodies evolved independently (convergent evolution); bones differ (ray fins vs. mammalian limbs).
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Flower color variation (C): Microevolution via alleles (e.g., anthocyanin genes); same species, no cross-lineage descent.
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Skin color in humans (D): Recent SLC24A5/MC1R mutations (~10-50k years ago); within-population variation, not ancestral phylogeny.
GATE Exam Context
Descent with modification questions span Evolution/Biochemistry (XL-H/I). Key: Distinguish homology (shared ancestry: embryos, homologous bones) from analogy (convergence: wings of bats/insects). Memorize: Embryos > vestigials > fossils for evidence strength.
