A population of bacteria has reached a local fitness peak. Mutations that would allow exploitation of a new substrate require passing through an intermediate genotype with lower fitness. Which scenario most favors crossing this fitness valley?
AA large, well-mixed population under strong selection pressure toward the current peak
BA small population experiencing a temporary bottleneck, allowing drift to push it into the valley against the selection gradient
CEliminating the low-fitness intermediate genotype through targeted mutagenesis
DIncreasing the mutation rate to generate more genetic variation at the current peak
Valley crossing requires drift strong enough to overcome selection against the valley genotypes. Drift is strongest in small populations, where random fluctuations in allele frequency can push a population 'downhill' — against the gradient of natural selection. A large, well-mixed population maintains strong purifying selection that efficiently eliminates below-average valley genotypes, keeping the population pinned to its current peak. A bottleneck creates exactly the small-population condition needed for drift-assisted crossing. Increasing mutation rate (option D) only generates more variation at the peak; it does not help if every path to the higher peak traverses a fitness valley.
Question 2 Multiple Choice
Two biologists disagree. One says 'large populations are evolutionarily superior because selection is more efficient.' The other says 'small populations can access evolutionary innovations that large populations cannot.' Which position is most complete?
AThe first: large populations consistently outperform small ones in all evolutionary contexts
BThe second: small populations are better because drift overrides the inefficiency of selection
CBoth capture something real: large populations excel at exploiting known fitness peaks while small populations can explore the landscape by drifting across valleys
DNeither: population size is irrelevant because mutation rate determines evolutionary potential
The tradeoff is genuine and represents the core of Sewall Wright's shifting balance theory. Large populations are efficient peak-climbers: selection is strong relative to drift, beneficial mutations spread quickly, deleterious ones are purged. But they are trapped on current peaks because drift is too weak to push them into valleys. Small populations are landscape-explorers: drift can move them off peaks and into valleys, and if they find a higher peak, migration can spread that genotype. Neither extreme is universally superior — the advantage depends on whether the immediate challenge is climbing efficiently or exploring the landscape for better peaks.
Question 3 True / False
Environmental change can help a population escape a local fitness peak by reshaping the fitness landscape so that previously suboptimal genotypes become favored.
TTrue
FFalse
Answer: True
A fitness landscape is defined relative to a specific environment. A genotype that occupies a valley under current conditions may be advantaged under different conditions. When the environment shifts — through climate change, ecological upheaval, or altered selection pressures — the landscape topology changes: current peaks can become valleys and valleys can become ridges. This provides a mechanism for escaping local optima that does not require drift; the population stays put while the landscape reshapes around it. Mass extinctions are the extreme case: they flatten existing peaks broadly, releasing many lineages simultaneously into newly available evolutionary space.
Question 4 True / False
Because natural selection usually favors higher fitness, a population will inevitably reach the global fitness maximum given sufficient time.
TTrue
FFalse
Answer: False
Selection only moves populations uphill locally — it cannot cross valleys, and it has no global 'view' of the landscape. A population can become permanently trapped at a local peak that is lower than other accessible peaks, indefinitely, if fitness valleys lie between it and the higher peaks and no mechanism provides a crossing route. Evolutionary stasis — lineages persisting for millions of years with little change despite the theoretical existence of superior forms — is evidence that local optima are genuine evolutionary traps. Selection is necessary but not sufficient for reaching global optima; it is a hill-climber, not a landscape navigator.
Question 5 Short Answer
Why does Wright's shifting balance theory propose that a subdivided population — many small semi-isolated subpopulations connected by occasional migration — may be better at long-run evolutionary exploration than either a single large population or complete isolation?
Think about your answer, then reveal below.
Model answer: A single large population cannot cross valleys because drift is too weak relative to selection. Complete isolation means each small subpopulation can drift and potentially cross valleys, but any innovation stays trapped locally. Population structure combines both advantages: drift within small subpopulations enables valley crossing and landscape exploration; occasional migration between subpopulations allows superior genotypes discovered locally to spread to other subpopulations and ultimately replace inferior ones. The metapopulation simultaneously exploits current peaks (via selection within subpopulations) and explores for higher ones (via drift-assisted valley crossings in small demes).
This framework is directly relevant to how complex traits requiring multiple co-adapted mutations can evolve. If each mutation alone is slightly deleterious, no single large population will assemble the full combination. But in a subdivided population, one small deme may drift to a genotype where the combination becomes advantageous, and migration can then spread this breakthrough. The practical implication is that population fragmentation — often viewed as purely negative in conservation biology — may in some contexts provide evolutionary benefits by enabling the valley-crossing that produces major innovations.