Questions: Ecological Speciation and Sympatric Divergence Mechanisms

Question 1

In a lake, a fish population feeds on two food sources: benthic invertebrates (requiring robust jaws) and zooplankton (requiring streamlined bodies). If intermediate generalist fish have lower fitness than either specialist type, what process does this represent and how does it contribute to speciation?

• A. Directional selection — the entire population is pushed toward one extreme body form, which eventually splits the species
• B. Stabilizing selection — intermediate forms are maintained, preventing divergence into two distinct types
• C. Disruptive selection — fitness favors both extremes over intermediates, potentially leading to reproductive isolation if assortative mating follows
• D. Genetic drift — alleles for extreme body forms randomly reach fixation in separate sub-populations

Answer: (C)

Disruptive selection is the engine of ecological speciation: both specialist phenotypes outcompete generalists at their respective resources, pushing the population's distribution toward two peaks. For this to lead to speciation, disruptive selection must be paired with assortative mating — if specialists preferentially mate with others like themselves, genetic divergence can accumulate. Without the mating barrier, gene flow between specialists would continually produce less-fit intermediates.

Question 2

Two apple maggot fly (Rhagoletis) populations feed on different hosts — hawthorn vs. apple. Adults emerge at different times of year, reducing interbreeding. For this difference in timing to eventually produce complete reproductive isolation, what is most critical?

• A. Geographic barriers must eventually separate hawthorn and apple orchards
• B. The two races must develop polyploidy to achieve chromosomal incompatibility
• C. The timing difference must maintain reduced gene flow long enough for divergent selection to accumulate additional reproductive barriers
• D. A third, intermediate host plant must appear to absorb hybrids and remove them from both populations

Answer: (C)

Ecological speciation is a gradual process. The timing difference creates partial reproductive isolation by reducing when the two races encounter each other. This reduced gene flow allows divergent selection on host-adapted traits to accumulate further genetic differences — including potentially additional behavioral or physiological barriers to interbreeding. Complete isolation doesn't require a single dramatic event; it requires that the initial partial barrier be reinforced by accumulating divergence over generations.

Question 3

Ecological speciation requires geographic isolation to prevent gene flow long enough for reproductive isolation to evolve between diverging populations.

• T. True
• F. False

Answer: False

This is the key distinction from allopatric speciation. Ecological speciation is specifically the process by which reproductive isolation evolves within a geographically overlapping population — without any physical barrier. Gene flow is reduced not by geography but by disruptive selection (generalists are less fit) and assortative mating (specialists encounter and mate with similar individuals). If ecological divergence is strong enough and mate choice sufficiently non-random, genetic divergence accumulates despite ongoing gene flow.

Question 4

Assortative mating — where individuals preferentially mate with others sharing their ecological specialization — can reduce gene flow between sympatric populations without any geographic barrier.

• T. True
• F. False

Answer: True

Assortative mating is the mechanism that translates ecological divergence into genetic divergence. If benthic-feeding fish spend their time in benthic habitats and mate with other fish encountered there, mating is non-random with respect to ecology even though no geographic barrier exists. This non-random mating acts like a partial genetic barrier, allowing divergent selection to accumulate differences between ecotypes over time — eventually leading to reproductive isolation.

Question 5

Why would intermediate 'hybrid' individuals have lower fitness in an ecological speciation scenario, and why does this matter for the speciation process?

Model answer: In a disruptive selection scenario, intermediate phenotypes are less efficient specialists than either extreme. A fish with intermediate jaw morphology is outcompeted by robust-jawed specialists on benthic prey and by streamlined specialists on zooplankton — it is worse at both resources. This fitness disadvantage of hybrids is called reinforcement: it creates selection pressure against mating across the two specialist types, because interbreeding produces less-fit offspring. Reinforcement strengthens reproductive isolation beyond what ecological divergence alone would generate, accelerating speciation by adding a direct cost to hybridization.

The hybrid fitness disadvantage converts what might otherwise be a continuously distributed polymorphism into a genuine speciation event — the fitness valley between ecotypes becomes wide enough that crossing it is increasingly rare and costly.