This is the second of eight posts on evolutionary research to celebrate Darwin’s bicentennial.
When new species arise, they can set off evolutionary chain reactions that cause even more new species to spring forth – fresh buds on the tree of life create conditions that encourage more budding on different branches.
Biologists have long suspected that these “cascades of speciation” exist but have struggled to test them. Enter Andrew Forbes from the University of Notre Dame – his team of has found a stunning new proof of the concept by studying a fruit fly called the apple maggot (Rhagoletis pomonella) and the parasitic wasps that use it as a host.
Contrary to its name, the apple maggot’s natural host is not apples – it’s hawthorn. The fly only developed a taste for apples about 150 years ago, when the fruit was first introduced to North America. This culinary switch has created two races of apple maggot – one that eats hawthorn and another that eats apples. Even though they are often found in the same place, the two races don’t mix and they don’t breed together. They are well on the road to becoming separate, genetically distinct species.
And so are their parasites. A wasp called Diachasma alloeum specialises in attacking apple maggots. It lays its eggs inside the fly larvae, and its grubs eat the victim from the inside out. Forbes found that the wasp has also started to form separate races that don’t crossbreed with one another, even though they have overlapping ranges. By adapting to new host plants, the flies inadvertently set up barriers that separated their respective parasites from one another. Now, the wasp, like its hosts, are also on the way to becoming separate species. It’s a fantastic example of diversity bringing itself about.