After a whole-genome duplication, one copy of a gene continues expressing in the liver as before. The other copy gains expression exclusively in the brain and acquires mutations that give it a new molecular function there. This outcome is best described as:
APseudogenization — the brain copy has diverged from the original function
BSubfunctionalization — the two copies have divided the original gene's expression domains
CNeofunctionalization — the brain copy has acquired a new function not present in the ancestral gene
DConcerted evolution — both copies are converging toward a shared new function
This is neofunctionalization: one copy retains the original function (liver expression), while the other acquires a genuinely *new* function (novel molecular activity in the brain) not present in the ancestor. Subfunctionalization would apply if the ancestral gene was expressed in *both* liver and brain, and the copies simply partitioned those pre-existing domains — neither copy would have a new function. Here the brain expression is a new capability, not a partitioned one. Pseudogenization would involve loss of function, not gain. Neofunctionalization is the rarer but evolutionarily most significant outcome.
Question 2 Multiple Choice
What is the most common evolutionary fate of duplicated genes over time?
ANeofunctionalization — most duplicates acquire new beneficial functions
BSubfunctionalization — most duplicates partition the original gene's roles between them
CPseudogenization — most duplicates accumulate mutations and become nonfunctional
DConservation — most duplicates are maintained as redundant backup copies indefinitely
The most common fate is pseudogenization: without selection pressure to maintain both copies, one tends to accumulate neutral or deleterious mutations over time, losing function and becoming a nonfunctional pseudogene. The genome is littered with pseudogenes — nonfunctional remnants of once-functional genes. Neofunctionalization is rare but evolutionarily important when it occurs; subfunctionalization is intermediate. Pure redundancy (option D) is evolutionarily unstable because neutral theory predicts one copy will drift to nonfunctionality absent selection pressure maintaining it.
Question 3 True / False
In subfunctionalization, both daughter copies of a duplicated gene are retained by natural selection because neither copy alone can perform the full function of the ancestral gene.
TTrue
FFalse
Answer: True
True. Subfunctionalization divides the ancestral gene's functions between the two copies — by regulatory divergence (different expression patterns across tissues or developmental stages) or by functional divergence (each copy handles a portion of the ancestral protein's activity). Since neither copy alone recapitulates the complete ancestral function, both are selectively maintained: losing either one would leave a functional gap. This selectively locks in both duplicates through purifying selection, making subfunctionalization a more stable retention mechanism than neofunctionalization.
Question 4 True / False
Neofunctionalization is the most common outcome following gene duplication, because evolution exploits available raw material (extra gene copies) to generate new functions.
TTrue
FFalse
Answer: False
False. Pseudogenization — loss of function — is by far the most common outcome after gene duplication. Most duplicate genes accumulate neutral mutations over time and become nonfunctional pseudogenes. Neofunctionalization requires that beneficial mutations arise by chance in the redundant copy, which is rare. While evolutionarily important (it drives protein family diversification), neofunctionalization represents a minority outcome relative to the overwhelming tendency for duplicates to degrade. The genome contains far more pseudogenes than novel gene functions arising from recent duplicates.
Question 5 Short Answer
Why does gene duplication enable evolutionary 'exploration' of new protein functions in a way that single-copy genes encoding essential functions cannot easily achieve?
Think about your answer, then reveal below.
Model answer: A single-copy essential gene cannot easily accumulate mutations to explore new functions because any mutation disrupting the essential function is strongly selected against. Duplication creates redundancy: one copy continues performing the essential function under purifying selection, while the other is freed from this constraint. The redundant copy can accumulate mutations — including changes that would be deleterious if they were the only copy — without penalty to fitness. Occasionally, one mutation confers a beneficial new function (neofunctionalization), producing novelty that would have been impossible without the redundancy safety net.
This redundancy mechanism is why genome duplication events are associated with major evolutionary transitions. The 2R whole-genome duplications in early vertebrate evolution provided quadruplicate copies of signaling and developmental genes, enabling the elaborate developmental programs underlying vertebrate body plans. The concept of evolutionary buffering through redundancy also applies beyond gene duplicates — many regulatory pathways have multiple partially overlapping genes, providing robustness while enabling gradual divergence.