Far from random, evolution follows a predictable pattern

Large milkweed bugs

Large milkweed bugs (above) feed on plants that produce a class of steroid-like cardiotoxins called cardenolides as a natural defense. The ability to eat these plants has evolved separately but in a predictable manner in several different orders of insects, including butterflies and moths (Lepidoptera); beetles and weevils (Coleoptera); and aphids, bed bugs, milkweed bugs and other sucking insects (Hemiptera). (Photo courtesy of Peter Andolfatto)

Evolution, often perceived as a series of random changes, might in fact be driven by a simple and repeated genetic solution to an environmental pressure, according to new research.

“Is evolution predictable? To a surprising extent the answer is yes,” according to Peter Andolfatto, an assistant professor in Princeton’s Department of Ecology and Evolutionary Biology and the Lewis-Sigler Institute for Integrative Genomics.

Andolfatto’s team has found that knowing how external conditions affect the proteins encoded by a species’ genes could allow researchers to determine a predictable evolutionary pattern driven by outside factors. Scientists could then pinpoint how the diversity of adaptations seen in the natural world developed even in distantly related animals.

The researchers carried out a survey of DNA sequences from 29 distantly related insect species, the largest sample of organisms yet examined for a single evolutionary trait. Fourteen of these species have evolved a nearly identical characteristic due to one external influence — they feed on plants that produce cardenolides, a class of steroid-like cardiotoxins that are a natural defense for plants such as milkweed and dogbane.

Though separated by 300 million years of evolution, these diverse insects — which include beetles, butterflies and aphids — experienced changes to a key protein called sodium-potassium adenosine triphosphatase, or the sodium-potassium pump, which regulates a cell’s crucial sodium-to-potassium ratio.

The protein in these insects eventually evolved a resistance to cardenolides, which usually cripple the protein’s ability to “pump” potassium into cells and excess sodium out.

To make this discovery, Andolfatto and his co-authors first sequenced and assembled all the expressed genes in the studied species. They used these sequences to predict how the sodium-potassium pump would be encoded in each of the species’ genes based on cardenolide exposure.

The researchers found that the genes of cardenolide-resistant insects incorporated various mutations that allowed them to resist the toxin. During the evolutionary timeframe examined, the sodium-potassium pump of insects feeding on dogbane and milkweed underwent 33 mutations at sites known to affect sensitivity to cardenolides. These mutations often involved similar or identical amino-acid changes that reduced susceptibility to the toxin. On the other hand, the sodium-potassium pump mutated just once in insects that do not feed on these plants.

Jianzhi Zhang, a University of Michigan professor of ecology and evolutionary biology, said that the Princeton-based study shows that certain traits have a limited number of molecular mechanisms, and that numerous, distinct species can share the few mechanisms there are. “The finding of parallel evolution in not two, but numerous herbivorous insects increases the significance of the study because such frequent parallelism is extremely unlikely to have happened simply by chance,” said Zhang, who is familiar with the study but had no role in it.

Andolfatto worked with lead author and Postdoctoral Research Associate Ying Zhen, and graduate students Matthew Aardema and Molly Schumer, all from Princeton’s ecology and evolutionary biology department, as well as Edgar Medina, a biological sciences graduate student at the University of the Andes in Colombia. The research was supported by grants from the Centre for Genetic Engineering and Biotechnology, the National Science Foundation and the National Institutes of Health and was published in the Sept. 28, 2012, issue of Science.
–By Morgan Kelly