Science News vol. 156
Monkeyflowers hint at evolutionary leaps

Parent species. Mimulus lewisii (left) attracts bumblebees, but close kin Mimulus cardinalis (right) interests hummingbirds.
A study of the allure of monkeyflowers to pollinators challenges the long-held wisdom that evolution minces along in baby steps, according to Seattle botanists.

A small bit of the genome, possibly one gene, pumps up nectar flow enough to double hummingbird visits, report Douglas W. Schemske and H.D. Bradshaw Jr. of the University of Washington. Another small bit changes pigments so much that bee visits drop 80 percent, the researchers say in the Oct. 12 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES.

Genetic tweaks causing big changes buttress recent proposals that evolution can take giant steps, Schemske argues. "Adaptations are not simple, but they may not be nearly as complex as we previously thought," he says.

The earlier view held that a mutation causing a huge change typically proved too disruptive to last, so evolution more likely proceeded by many small genetic changes. However, researchers have rarely tested the idea observes.

The alternative giant-step scenario would apply when a species faces catastrophe, such as destruction of its pollinator, Schemske explains. A gene causing a big enough difference to attract substitutes would indeed be a boon.

To test the impact of genetic changes, Schemske's team created two generations of hybrids, jumbling the traits of the parents: red Mimulus cardinalis, a hummingbird favorite, and pink Mimulus lewisii, which is pollinated by bees.

For a field test, researchers filled two 24-foot trucks with more than 200 plants and drove from Washington to California's Yosemite National Park. The trucks weren't air-conditioned, so the caravan traveled at night. After placing plants in a giant grid, researchers raced after pollinators and muttered into a tape recorder the ID number of each plant visited. "It was something else," sighs Schemske.

By linking pollinator interest with plant traits—pigments, nectar volume, and floral-display size—and with genetic markers, the researchers found sections of chromosomes with big effects.

The most impressive finding, says Barbara Anna Schaal of Washington University in St. Louis, is that "relatively few genetic changes are necessary to bring about reproductive isolation and, potentially, speciation. —S. Milius