Sep. 6, 2001 COLLEGE PARK, MD - Like a family that splits in a feud, many species share common ancestors, but they never have much to do with their cousins. In fact, many life forms actually develop into entirely new species as they change to adapt to new environments.
Scientists have theorized that how fast one species branches out to become two, a process called speciation, is in the genes. If a couple of key genes are located close to each other on the species' genome, the theory goes, formation of a new species will move along more quickly.
By studying the genes of a common insect that appears to evolving into two separate species adapting to different environments, two University of Maryland researchers have confirmed that theory for the first time.
In a study published in the August 30 edition of the journal "Nature," Sara Via, an evolutionary biologist, and insect geneticist David Hawthorne discovered that genes involved in speciation are indeed located very close to each other.
Via and Hawthorne studied the pea aphid, a common crop pest that stems from a common ancestral aphid but that appears to be in the process of splitting into two species that infest different plants. Where the ancestors all survived on the same food, one of the new species now lives on alfalfa, the other on clover. And while the species look identical, they show little interest in meeting, either to share a meal or to reproduce.
"If one defines species as two groups that are unwilling or unable to successfully reproduce together, then these aphids are nearly separate species," says Via. "The pea aphids that we studied are now highly specialized on either alfalfa or clover, and they appear to interbreed very little.
"We knew that these aphids differed genetically in the their environmental choice and in how well they survived in that environment," says Via, "but we didn't know how many genes were involved in the difference, or the extent to which the genes for the different traits might act in concert."
Using a genetic map they made of the pea aphid genome, Hawthorne and Via found that a single gene or a few genes that increase performance and the tendency to find mates on one plant while decreasing performance on the other plant lie close together within several small chromosomal regions. Until now, no one has seen such a gene arrangement on actual organisms.
"When the genes lie close together like this, they are more likely to continue to change together, than if the genes were scattered around the genome, because they are less likely to be divided during reproduction," says Via. "This gene arrangement could cause very rapid evolution and speciation."
The term "rapid" is relative. It could mean that it may take only hundreds or thousands of generations for a new species to form, instead of the millions of generations that are typically expected.
"Speciation is often driven by natural selection and adaptation to different environments. When the genetics are right, it can happen very fast," says Via.
Hawthorne and Via expect this type of gene arrangement also will be found in other species that are thought to have speciated by adaptation to different habitats. "The aphids are a great model system for studying how genetic divergence and speciation may occur in other kinds of organisms that use different resources or environments," says Hawthorne.
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