Questions: Evolvability: Capacity for Evolutionary Change

Question 1

Two island populations of lizards face the same novel pathogen outbreak. Population A's immune response is polygenic (many genes, small effects, high modularity); Population B's immune response is controlled by a single highly pleiotropic gene affecting both immunity and coloration. Which population is more likely to adapt rapidly, and why?

• A. Population B — a single gene can be changed by one mutation, making adaptation faster
• B. Population A — polygenic, modular architecture harbors more standing variation and allows immune traits to change without disrupting other functions
• C. Population B — pleiotropic genes have higher mutation rates due to their biological importance
• D. Both equally — natural selection acts on phenotype regardless of genetic architecture

Answer: (B)

Population A has higher evolvability. A polygenic system harbors abundant standing variation that selection can act on immediately. Modularity means changes in immune function don't cascade into coloration or other traits — there's no constraint from pleiotropy. Population B's single pleiotropic gene means any beneficial immune mutation likely simultaneously disrupts coloration (possibly increasing predation risk), creating antagonistic selection that constrains adaptation. The genetic architecture of a trait shapes whether beneficial variation is accessible.

Question 2

Genome duplications enhance evolvability primarily because they:

• A. Double the mutation rate across the entire genome
• B. Free duplicate gene copies from purifying selection, allowing them to accumulate mutations and potentially evolve new functions
• C. Immediately produce new phenotypes that natural selection can act on
• D. Increase recombination frequency, generating new allele combinations faster

Answer: (B)

When a gene is duplicated, one copy can continue performing the original function while the other accumulates mutations that would otherwise be eliminated by purifying selection. The duplicate can eventually evolve a novel function (neofunctionalization) or divide the original function with the parent copy (subfunctionalization). This is why genome duplications are associated with major evolutionary innovations — they provide 'spare' genetic material that can be repurposed without fitness cost to existing functions.

Question 3

High genetic integration — where many traits share genetic determinants — increases evolvability by allowing coordinated adaptation of multiple traits simultaneously.

• T. True
• F. False

Answer: False

High integration (low modularity) typically *reduces* evolvability. When many traits share genetic determinants (high pleiotropy), a mutation that would benefit one trait is likely to simultaneously disrupt others. This creates opposing selective pressures that constrain evolutionary change — any step toward adaptation in one direction is penalized in another. Modularity — semi-independent genetic modules — enhances evolvability precisely because beneficial changes in one module don't cascade destructively through others.

Question 4

Evolvability can be shaped by lineage-level selection over long evolutionary timescales, even though no individual organism is selected for its capacity to produce adaptive variation.

• T. True
• F. False

Answer: True

This is one of the more counterintuitive aspects of the evolvability concept. Within any generation, natural selection acts on individual fitness now — not on the ability to adapt in the future. But over millions of years, lineages with higher evolvability are more likely to survive environmental changes and diversify into new niches. Lineages with rigid, low-evolvability architectures are more likely to go extinct when conditions change. The result is a form of selection at the lineage level that can explain why evolvability-enhancing features like modularity and recombination are widespread.

Question 5

Explain why a high mutation rate does not simply equal high evolvability.

Model answer: Most mutations are neutral or deleterious — only a tiny fraction are beneficial. A very high mutation rate generates abundant variation, but much of it is harmful, reducing average fitness and potentially overwhelming the population's ability to maintain its current adaptations. Evolvability depends not just on generating variation but on generating *accessible* adaptive variation — which is shaped by genetic architecture (modularity, pleiotropy), effective population size (which determines how efficiently selection can act), and recombination. A population with moderate mutation rates but highly modular genetic architecture may be more evolvable than one with high mutation rates and tight genetic integration.

The concept of error threshold illustrates the risk: if mutation rates exceed a critical threshold, populations accumulate too many deleterious mutations to maintain fitness ('mutational meltdown'). Some organisms have evolved stress-induced hypermutation as a bet-hedging strategy — temporarily increasing mutation rates under stress to gamble on producing rare beneficial variants — but this is a carefully regulated response, not a simple 'more mutation = more adaptation' rule.