How Many Genes to Make a New Species?

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An interdisciplinary research team at the UW has taken a major step toward solving one of the central mysteries that has plagued evolutionary biology for a century: How much genetic change is needed in order for a new species to evolve?

The researchers have applied, for the first time, the latest gene mapping techniques to the problem and have found that evolution may require far fewer changes than scientists previously thought.

The work was carried out by H. D. (Toby) Bradshaw, research faculty member working in the Center for Urban Horticulture, College of Forest Resources, in collaboration with UW botany professor Douglas Schemske and biochemist Sibylle Wilbert. Their findings were published in August, 1995 in the scientific journal Nature.

Bradshaw and colleagues worked with two species of wildflower, called monkeyflower, or Mimulus. The species Mimulus lewisii appeals to bumblebees; it has forward thrusting petals that provide a landing pad, easy-to-see yellow stripes ("nectarguides"), and highly concentrated nectar. M. cardinalis, on the other hand, has a deep tubular shape that excludes bees but favors hummingbirds. Its nectar is less concentrated but more abundant; and its red-orange color is not even readily seen by bees.

The researchers were able to map eight characteristic differences between the flowers, such as shape, color, and nectar, to a very few genes in the flowers. A pigment affecting flower color was mapped to a single chromosome region; two others, nectar volume and flower width, were controlled by at least one gene of large effect.

The finding that relatively few genetic changes may give rise to a new species throws new light on the theory that Darwin put forward decades ago. He and many other scientists that followed have held the belief that adaptations are almost always based on a gradual accumulation of a large number of mutations, each of small effect. With the genome mapping approach developed by Bradshaw and colleagues, the door is now open to settle an age-old controversy.

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