New Scientist – August 17, 2019

40 | New Scientist | 17 August 2019

measure their aggressiveness by kicking the

hive or throwing a brick at it,” he says. “Some of

them even had zero response. They didn’t want

to sting at all.” These insects looked like hybrid

bees, but they didn’t act like them. Giray was

forced to conclude that, sometime during the

decade since their arrival, the once-vicious

killers had evolved into bees as docile as their

European cousins. Nature had finished what

Kerr had started.

What was the mysterious force driving

this transformation? Giray’s first thought

was that it had something to do with habitat.

Islands tend to have fewer predators than the

mainland, but the challenge of island living

is to get by with scarcer, seasonally available

resources. As a result, species often adapt to

focus on energy efficiency and ensuring they

have calories for leaner times, making them

fat and docile. The turkey-sized dodo, for

example, evolved from a small, airborne

pigeon that arrived on Mauritius. This

“harsh seasonality” could have had a similar

effect on Puerto Rico’s bees, forcing them to

forage well and pick their battles. Hurricanes

would have played a part too, by killing off

“ What was the

mystery force

driving the bees’

remarkable

transformation?”

Social insects, such as ants and many

bee and wasp species, have a remarkable

ability to rapidly adapt to changing

environments. In Puerto Rico, honeybees

evolved from being highly aggressive

killers to gentle foragers in just a decade

(see main story). This rate of change

seems extraordinary, even given the

recent realisation that evolution can

happen surprisingly quickly. It is easy to

see how a bacterium, reproducing every

20 minutes or so, can evolve immunity

to a drug, but the generation time of bees

is far longer. Worker honeybees live for

around a month, drones survive for five

to 10 weeks and the queen can make

it to three years or more. So if rapid

reproduction isn’t the key, what has

allowed Puerto Rico’s bees to evolve

so quickly?

In part, it is the fact that when bees

produce eggs and sperm, their genes

are more thoroughly shuffled than in

other species and the mutation rate is

very high. This creates lots of different

genetic combinations, increasing the

potential to produce highly variable

offspring able to survive in a wide variety

of environments. Although this sounds like

a winning formula, there is a reason most

animals don’t adopt it: mutations are more

often harmful than helpful. When so many

occur during the production of sex cells

there is a risk of losing vital bits of genetic

information and ultimately the entire

species. But bees, and some other colonial

insects, have a trump card to get around

this problem: the haplodiploid sex system.

It works like this. Male drones develop

from unfertilised eggs and have only one

pair of chromosomes: they are haploid.

Female workers develop from fertilised

eggs and have both pairs of chromosomes,

and so are diploid. The queen is the only

fertile female in the colony and mates with

about 12 to 20 males to produce the hive’s

population. Because drones only have one

pair of her chromosomes, any harmful

mutations that arise have no alternative

versions of the genes to counteract the

bad ones. So, if one of the hive’s lines of

descendants mutates too far, the drones

it produces will die or be unable to mate,

removing the line from the gene pool.

This combination of haplodiploidy

and high mutation rates creates enormous

genetic diversity in colonial insects. It is

this that allows the colony to respond

rapidly, and with nuance, to subtle

alterations in the environment – as if it

were a single organism. In Puerto Rico,

the environment seems to have favoured

active foragers over aggressive individuals

and the drive to change was so strong

that it happened in just a few years.

Yet despite such remarkable

adaptability, bees and other colonial

insects are declining in numbers

worldwide. Marla Spivak at the University

of Minnesota sees this as an indictment

of the environmental conditions humans

are imposing upon them. “The fact

they’re struggling right now really shows

that they are being pushed to, or beyond,

their tipping points,” she says.

The killer that changed its stripes

all but the most skilful hoarders.

There is another possible explanation.

Giray’s collaborator, Arian Avalos, now at the

US Department of Agriculture, suggested that

the bee’s rapid evolution might be connected

to the island having the highest human

population density of anywhere in the hybrid

honeybee’s range. This could have led to

unwitting human selection because people

regularly encountered hives and those

containing the most aggressive bees were

systematically destroyed, leaving only the

calmer bees to repopulate the isolated island.

Disease-resistant bees

The likelihood is that all these factors

played a part. What is truly amazing is that

the bees have evolved incredibly rapidly,

yet have retained an enormous amount

of genetic variation even though their

numbers have crashed on several occasions

(see “The killer that changed its stripes”,

below). The result is a remarkable creature

that isn’t just docile, but resistant to disease

and good at producing honey.