In a new research, tiny leaf beetles that flit among the maple and willow trees have just provided some of the clearest evidence yet, that environmental factors play a major role in the formation of new species.

At Vanderbilt University, graduate student Scott Egan and his adviser Daniel Funk, Associate Professor of Biological Sciences, obtained this new evidence from an experimental study.

Funk has investigated this odd group of leaf beetles for 15 years. In recent years, he and Egan have used Vermont populations that associate with red maples and Bebbs willows, to investigate how divergent ecological habits promote speciation.

Their past observations have suggested that such "maple leaf beetles" and "willow leaf beetles" may be in the process of dividing into two new species: Each prefers to feed and lay eggs on their own "host plant," where they grow and survive best.

Although there is some intermixing, the beetles show a decided preference for mates from the same host.

While maple and willow beetles are visually indistinguishable, the current degree of their divergence is highlighted by the willow beetles' willingness to starve to death rather than feed on the maple leaves readily consumed by their maple beetle cousins.

This research has established these inconspicuous beetles as an important example of how ecological factors contribute to evolutionary divergence and the origin of Earth's millions of species.

"Without this process, life would not have the incredible number and variety of species that we take for granted," said Funk.

The new study goes beyond past investigations by showing that the hybrid offspring produced by mating these beetle types with each other in the lab grow faster than their willow parent but slower than their maple parent when raised on maple leaves, and faster than their maple parent but slower than their willow parent when raised on willow leaves.

Evolutionary biologists consider growth rate as a measure of survivability or fitness.

So, the hybrids' lower fitness reduces opportunities for genetic exchange between beetle types by way of mating between surviving hybrids and the maple and willow beetles themselves.

As a result, it promotes the "reproductive isolation" that pushes the two groups toward becoming distinct species.

This reflects the true power of natural selection, which has driven these beetles to become so intimately adapted to their host plant environment, promoting their speciation along the way.

"It's not a coincidence that plant-feeding insects are among the most ecologically specialized and the most species-rich groups on the planet," said Funk.

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