To determine if a single mutation at the YUP locus causing the color of the flower to change would be sufficient to recruit a different set of pollinators to the plants, the researchers crossbred and then backcrossed the flowers until each flower had the YUP of the other.
"What you are trying to do is get rid of all the genes from the donor species except for the one gene controlling whether or not yellow pigment is in the flowers," said Bradshaw.
After successfully achieving this step, the researchers took the flowers to a field site in the Sierra Nevada mountains of California where both M. lewisii and M. cardinalis are normally found. The pair observed pollinators recruited by the mutant flowers.
According to calculations presented by Bradshaw and Schemske in Nature, if this mutation were to occur in the wild, the mutants would likely die off because they are visited much less than their non-mutant sisters.
"That is sort of to be expected," said Bradshaw. "We expect the normal, non-mutant flower to be optimally adapted."
However, in their paper, the researchers calculated the circumstances under which the mutant could take hold and evolve to a new species of monkeyflower by fully adapting traits specific to their pollinators.
For example, if a bunch of the bumblebees died off owing to a climate change or parasite infestation and the hummingbirds remained stable, suddenly the yellow-orange mutant M. lewisii could recruit a sufficient number of hummingbirds to survive alongside the wild-type pink-flowered ancestor.
"There are circumstances under which the mutant could be favored by natural selection," said Bradshaw.
Over time, the mutant M. lewisii would adapt other traits specific to hummingbird pollinators, such as flower shape and the quantity of nectar they store, becoming a distinct species of monkeyflower, said Bradshaw.
According to Coyne, it makes sense that mutations of large effect are important for monkeyflower speciation, the evolutionary changes that lead to new species. Changes in pollinators are likely to occur when a population becomes isolated from others in a new environment and are suddenly exposed to a new regime of pollinators.
So, if a monkeyflower normally pollinated by bees was suddenly stranded in an area dominated by hummingbirds, it must either adapt to hummingbird pollination or die. "When you change pollinators, it is hard to do that without having a mutation of large effect," said Coyne.
However, there are certain circumstances when a change of large effect would be of no benefit to a species.
For example, males of various duck species differ in color from females, an important cue in attracting mates, said Coyne. If a male duck suddenly changes color, it won't find a mate and will die without leaving offspring. So large color mutations will not be passed on. In the case of duck speciation, the process is likely to involve a gradual accumulation of small changes.
"Like evolution itself, there are no general rules that apply to the formation of species," he said.
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