If the predator search image hypothesis is valid for blue jays, perhaps it is also valid for the birds that prey upon happy-face spiders — the Hawaiian Honeycreeper (one type of honeycreeper is shown below).
Gillespie and colleagues currently hypothesize that predators searching for happy-face spiders maintain the 2:1 ratio on the islands. On each island, predators are efficiently searching for the most common morph, the yellow morph, or inefficiently searching for several morphs. This gives an advantage to non-yellow morphs, since they escape predation more often. But anytime other morphs get very common, predators start looking for them instead, which drives their frequencies back down. This mechanism could help explain why each island has evolved a variety of morphs and why we consistently observe a 2:1 ratio of yellow to other spiders.
Gillespie and her colleagues have not yet directly tested the predator search image hypothesis — although it is not contradicted by the evidence they have collected so far. Stay tuned for further updates! Ultimately, additional evidence may reject this hypothesis as well — but that is simply the nature of science. By proposing hypotheses and collecting evidence, scientists learn more about the natural world and are led to even better hypotheses that help us understand how that world works.
Discussion and extension questions:
- What kind of genetic variation in happy-face spiders was Dr. Gillespie studying?
- What surprising observation about that variation was she trying to explain?
- Describe two early hypotheses that Dr. Gillespie proposed to explain the morph ratio in the happy-face spider.
- Why did Dr. Gillespie reject the dispersal hypothesis?
- Describe Dr. Gillespie’s most recent hypotheses to explain her observations.
- Why is genetic variation important to natural selection?