Darwin's idea of natural selection is simple. Good mutations are passed on, because the animals who have them will survive to reproduce. But how do you pass on those good mutations in honeybee colonies, where most bees are sterile workers who never have babies? A group of researchers decided to find out.
Photo of a perfect worker bee, with nectar in her belly and pollen on her legs, by Alex Wild
York University biologist Amro Zayed worked with a team of Canadian and Saudi Arabian researchers to unravel a genetic mystery that has long intrigued evolutionary biologists. Worker bees are the most important part of any bee colony, gathering food, building the hive, taking care of babies, and maintaining the temperature inside the hive at what Zayed calls a "balmy 33 degrees Celsius." If a worker has a mutation that makes it better at finding food in a new region, evolutionary reason would predict that mutation ought to be passed along to the next generation of workers. But how can it be, if only the queen bee is having babies?
In the 1960s, W.D. Hamilton first proposed a theoretical model that could explain how each generation of sterile bees contributed to the fitness of their species. The theory was called "kin selection," and it suggests that non-reproducing animals still benefit when another member of their group is having babies. As the term "kin selection" suggests, a sterile bee is still helping natural selection along, even if it's only by making it easier for her queen to lay eggs. After all, the queen is either mother or sister to every bee in her colony.
Bee eggs and larvae in the hive. Photo by Alex Wild.
For decades, kin selection was just a theory. Recently, however, we've accumulated enough genetic information about honeybees that we're able to analyze how they're evolving at a DNA level. And a few years ago, Zayed and his colleagues wondered whether they could use genetic evidence to back up Hamilton's fifty-year-old idea. First, they gathered 40 honeybee genomes from subspecies regions all over Africa, Asia, the Middle East and the Americas. "One researcher even risked great danger to get a honeybee in Syria," Zayed said.
After an intensive analysis, the team discovered that there are certain regions of the honeybee genome that are undergoing rapid positive selection. That means they found several relatively new mutations that have spread quickly throughout honeybee subspecies because they are so useful. And nearly all of those mutations were associated with honeybee workers' ability to adapt their behavior to new environments and manage the worker division of labor. Essentially, all the most rapidly-changing parts of the honeybee genomes were selecting for traits that gave advantages to those sterile workers.
"This is the first time anyone has shown workers contributing to adaptation," Zayed told io9 by phone from Ontario, Canada. And he said it makes perfect sense, considering that workers are the front line when it comes to the colony's survival in the environment. But the queen doesn't need to adapt as much, because her environment is always stable. Zayed continued:
If you think about a bee colony, the queen bee is living in an almost lab-like environment. She's fed and groomed by workers. The temperature in the colony is 33 degrees Celsius for most of her life. She's in this benign environment, afforded by the actions of workers who collect food and nurse the brood and thermo-regulate the hive. Through their behavior they bear brunt of changes in environment. They have to cope with changes in environment. And they deal with changes in the environment with changes in worker behavior.
And indeed, as Zayed's team found, the DNA regions that are evolving the most rapidly in honeybees are responsible for worker behavior.
Honeybees using their wings to fan the hive and keep it cool. Photo by Alex Wild.
Again, we have to ask, how are these good traits being passed on from one sterile generation to the next? The answer takes us back to kin selection. The better the workers are, the more likely their queen's hive environment will remain stable and benign. And that means she's more likely to have more children, many of whom will also have the good mutations that made her workers so adaptable in the first place. Among those children will be future queens, who will carry those genes with them when they found new colonies and birth new generations of workers.
This also helps explain why there is such a huge amount of variation in honeybee behavior from subspecies to subspecies — and even from colony to colony. Zayed pointed out that the famously aggressive behavior of African honeybees is a direct result of adapting to an environment full of predators like the honey badger who attack hives. In other parts of the world, such as Yemen, bees have adapted to be excellent at air conditioning their hives in the hot desert air. And in Japan, bees have used the "hot defensive bee ball" to combat deadly wasps. All of these behavior modifications are the result of rapid changes under the pressures of natural selection.
As a result of Zayed and his colleagues' work, published last month in the Proceedings of the National Academy of Sciences, we have solid genetic evidence that even when animals don't reproduce, they still contribute to the evolution of their species and their family lineage. Put another way, altruism pays off.
Though humans are very different from social insects, there is still something poignant in the lifecycles of honeybees for us city-dwelling Homo sapiens. It's hard not to ponder the implications of those queens with their static, unchanging behavior inside the hive. Their evolution is entirely dependent on adaptable sisters and daughters, working in the dangerous world outside. And yet the workers can't pass along the behaviors they've evolved without their queen. It sounds harmonious. Maybe that's because it's nature's perfect system for quelling worker revolt.
Read the full article in PNAS
Annalee Newitz is the editor-in-chief of io9. She's also the author of Scatter, Adapt and Remember: How Humans Will Survive a Mass Extinction.