A highway bisects a deer population into two groups. Camera traps confirm that some deer occasionally cross the highway. What evidence would most directly establish whether gene flow is actually occurring between the two subpopulations?
AThe frequency with which deer are observed crossing the highway
BWhether the crossing deer are male or female
CWhether there are more deer per square kilometer on one side
DWhether crossing individuals are successfully mating and producing surviving offspring on the other side
Gene flow requires *effective* migration — movement followed by successful reproduction. A deer that crosses the highway but fails to mate, or whose offspring do not survive, contributes nothing to gene flow. Physical movement alone is not gene flow. This distinction matters because many barriers to gene flow are not geographic: behavioral differences, timing mismatches, and post-migration mortality can all block gene flow even when organisms move between populations.
Question 2 Multiple Choice
Genetic analysis of two isolated plant populations shows their allele frequencies are diverging over generations. What does this suggest about gene flow between them?
AGene flow is high, driving the populations apart through competition
BGene flow is occurring but introducing locally maladapted alleles
CGene flow is minimal or absent, allowing genetic drift and local selection to differentiate the populations
DBoth populations have identical mutation rates, causing parallel divergence
Gene flow is a homogenizing force — it pulls allele frequencies in different populations toward each other. When populations diverge genetically, it means gene flow is too low to counteract the differentiating forces of drift and local selection. Population genetic theory predicts that even one effective migrant per generation can substantially limit divergence through drift; populations showing clear divergence therefore must be experiencing very little or no gene flow.
Question 3 True / False
Even a single migrant per generation between two populations is generally sufficient to prevent genetic drift from causing significant allele frequency divergence.
TTrue
FFalse
Answer: True
This is one of the classic results of population genetics. The degree of differentiation between populations (measured by Fst) depends on the product of population size and migration rate (Nm). When Nm is as low as 1 (one effective migrant per generation), genetic drift is substantially countered and populations remain relatively undifferentiated. This means that even rare connectivity between populations can have large evolutionary consequences.
Question 4 True / False
Gene flow accelerates local adaptation by constantly introducing new beneficial alleles from populations in different environments.
TTrue
FFalse
Answer: False
Gene flow typically counteracts local adaptation rather than accelerating it. When alleles flow from an environment with different selective pressures, they are usually maladapted to the recipient population's conditions. This 'migration load' can actually reduce the average fitness of locally adapted populations. Gene flow homogenizes allele frequencies across different environments, eroding the genetic distinctiveness that local selection has built up — this is why reduced gene flow is a necessary precursor to speciation and sustained local adaptation.
Question 5 Short Answer
Why is the distinction between 'migration' and 'gene flow' biologically important? Give a specific example of how migration can occur without gene flow.
Think about your answer, then reveal below.
Model answer: Migration is simply physical movement between populations; gene flow is movement that results in successful reproduction and genetic contribution to the recipient population. Only effective migrants — those who mate and leave surviving offspring — contribute to gene flow. Migration without gene flow occurs when migrants do not reproduce: for example, a salmon that enters a different river system but is eaten by a bear before spawning, or a bird that disperses to a new territory but fails to attract a mate. Behavioral incompatibilities, mismatched breeding seasons, or post-mating hybrid incompatibilities can also block gene flow even when movement occurs.
This distinction is critical for understanding population structure and speciation. If migration automatically equaled gene flow, any physical connectivity would prevent speciation. But speciation can occur even in regions with physical movement of individuals, as long as barriers prevent those individuals from contributing alleles to the other population. Recognizing the gap between physical dispersal and genetic contribution allows biologists to correctly identify what is actually limiting divergence.