Are Honey Bees Evolving?

Selection Pressures as Drivers of Honey Bee Evolution

“Are honey bees still evolving, or is this it for them?” Asked an attendee at my beekeeper’s club. “Bees evolved to live in trees and now we use selective breeding to requeen our hives, is this the end of their evolution?”. I forgot the presenter’s response, as I was thinking about my own answer for this topic. In short, honey bees are still evolving even when under beekeeper management (both Apis mellifera and Apis cerana), however the results are not apparent in the present. Evolution is a slow process, taking many hundreds to tens of thousands of years for a new “species” to emerge.

This article explores the current evidence for evolution of bees from wasps, as well as the drivers of evolution. It compares natural & artificial selection, and examines how hybridisation is a rapid form of evolution. It ends with potential evolutionary directions for A. mellifera.

Honey Bee Evolution via Natural Selection

A textbook definition of evolution via natural selection assumes the following:

1. Within a population, there exists variations – physical, physiological or behavioural.
2. Over time, an environmental event occurs causing a selective pressure – pest, diseases, resource shortages etc…
3. Those populations with variations that favour survival under the selective pressure may live, while others die out.
4. The surviving species reproduces and spreads its favourable genes into the population. Over time, an area may be dominated by a species showing that favourable trait.

One can easily apply this to honey bees, who face stressors like Varroa, pesticides, nectar dearths and many more. I’ve named these items specifically as they’ll be covered later on.

Artificial Selection, or Beekeeper-guided “Evolution”

Unlike natural selection, the artificial selection process does not include a “selective pressure” from the environment, rather from the beekeeper. In this case, criteria may be “high honey yield” or “gentle during inspections”. The beekeeper would note these traits, and when it comes time to raise queens, they would favour the drones and queens from their chosen colony.

Perhaps these economically-favourable traits would advantageous in the wild- for example, bees would need to show some degree of defensiveness to deter predators. However, in the controlled environment of a managed hive, such characteristics would make hive management difficult. Artificial selection can be described by modifying the natural selection process listed above:

1. Within an apiary, there exists variations in colonies, physical or physiological or behavioural.
2. A beekeeper chooses to expand his apiary and raise new queens from economically-desirable hives: low aggression, high honey production, reduced swarming tendencies… and so on.
3. The colonies exhibiting the favourable traits are permitted to swarm in designated areas (to control for the male populations at the drone congregation areas), whilst others undergo swarm management to prevent swarming.
4. The colonies that swarm and return with a mated queen are kept/sold off by the beekeeper, and are monitored to ensure they hold the desiring traits. The beekeeper will continue letting them swarm/form colony splits, until their favourable characteristics become dominant in the apiary.

Hybridisiation as Evolution – Africanised Honeybees

I consider hybridisation to be a form of evolution, especially when the offspring are fertile and can reproduce, as in the case of Africanised honey bees. When A.m scutellata escaped the Brazilian laboratory and mated with the local A.m carnica & ligustica, it produced a new honey bee subspecies, the “Africanised” honey bee. Work by Nelson & colleagues examined the DNA samples of these new hybrids, and found them to be 84% African ancestry (Lineage A), with the remaining 16% being of European origin (lineages C & M; Nelson et al., 2017).

These hybrids have been successful in colonising the western hemisphere as their colonies grow faster, due to increased brood and drone rearing, with faster developing queens (Winston, 1992). Refer to the article on Africanised honey bees for further reading.

Evidence for Ongoing and Previous Evolution Honey Bee Evolution

Genetic diversity, environmental pressures, and the constant threat of pests and disease drive evolutionary adaptions. These areas provide evidence on how A. mellifera is constantly changing in response to the environment, as described below.

Ongoing Evolutionary Processes: Mitochondrial DNA Sequence Data

DNA sequences from the overly-cliche “powerhouse of the cell” are used to assess the history and diversity of the honey bee species alive today. The mitochondrial Cytochrome-c subunits as well as the non-coding regions, can be analysed to see variations between bees and their subspecies; part of the reason why we have the traditional A-M-C-O honey bee lineages. A (African), M (Western European), C (North-Mediterranean/Eastern European), and O (Middle East) (Cornuet & Garnery, 1991 – although they don’t mention lineage O). A fifth lineage, Y, comes out of the Arabian Peninsula and Ethiopian highlands (Franck et al., 2001)

In the aforementioned work, the mitochondrial DNA variations between A. mellifera and A. cerana were 643 base pairs, or 8.5% different. Considering that the two species are very difficult to distinguish with the naked eye (you’d have to look their wing vein patterns, which are small enough as it is), an 8.5% difference is no surprise. However, the authors note that another method of analysis. restriction mapping¹ puts the difference between species as 13.7%. No matter, the data shows that A. mellifera and A. cerana first split around 5.9 million years ago (Cornuet & Garnery, 1991).

How Similar are the Bee Lineages?

A lineage-based breakdown shows that the African and Eastern European lineages are most closely related, whereas the North-Mediterranean/Eastern European & Western European lineages are least related (Cornuet & Garnery, 1991). Odd in my opinion, considering they’re from the same continent – precise figures below.

• Lineage A – lineage C = 2.1% difference,
• lineage A – lineage M = 2.8% difference,
• lineage C – lineage M = 3.3%, difference

Within lineages there is much less diversity, at 0.25, 0.30 and 0.07% for lineages A, C and M, respectively. Most of the African subspecies belong in the branch A, whereas branch M covers intermissa (North West Africa) to mellifera (European Dark bee) to iberica (Iberian penninsula). Branch C encompasses carnica (Austrian Bee) ligusitica (Italian Bee) and cecropia (Greek bee). Lineages O & Y would cover subspecies like syrica (Syrian bee) or meda (around the Iraq region). For more on bee lineages, refer to on the scientific name of bees.

Pathogens and Parasites as Drivers of Evolution

A queen mates with a dozen (or more) drones during her mating flight. Why is that? One theory for this emergence of multiple partners (“polyandry”) by social insect queens is that it increases the genetic diversity within a colony (reviewed in Sherman et al., 1998). The more variation within a colony, the more likely that they’ll have a favourable characteristic when challenged by a pest or disease. Consider that the incidence of deformed wing virus or Varroa did not wipe out the entire bee populations. Another theory is the “sperm-limitation hypothesis”, which is covered in this article on drone biology.

Polyandry-driven genetic variations are part of the reason we still have bees – although some colonies were wiped out, others had characteristics to resist the infection/parasitism. The survivor stock then reproduced, passing on this resistance (not immunity) to their daughter colonies. Pathogens continue driving evolution, and this evolutionary arms race will continue as both host and pathogen attempt to overpower one another.

The Introduction of Varroa & Emergence of Parasitation-resistant Colonies

Following the introduction of Varroa destructor in A. mellifera, there was a major loss of wild colony diversity as a new selective pressure emerged: parasitism (Mikheyev et al., 2015). Mikheyev and colleagues analysed New York museum specimens from 1977 (before Varroa) and 2010 (after Varroa’s arrival and establishment in the 1990s). The authors note that the wild populations of A. mellifera did not go extinct following the introduction of V. destructor, rather they survived in a combination of adaption to selective pressures and swarming colonies from commercial beekeepers’ hives.

232 genes were altered by the introduction of Varroa, half of which are involved in the honey bee development. The authors suggest that these changes could be due to larval movement in the cells or moulting patterns, which can kill the feeding mites. On this basis, they found a reduction in overall body size, leading to a smaller head, abdomen and wingspan. A word of caution, these changes may not be the bees’ evolving resistance to Varroa, rather a stress response to mite infestation or something else entirely.

The mutations involve multiple genes, and can either be introduced from selective pressures like Varroa, or interbreeding with wild colonies that have migrated into previously occupied bee trees. Nonetheless, the New York honey bees became smaller after the arrival of Varroa, which shares similarities with A. cerana or the A. mellifera African subspecies, both of which have mite resistance due to their smaller size (Mikheyev et al., 2015).

Pesticides: a New Selective Pressure?

Diving into hypotheticals here. We know that insects (and bacteria, and fungi and protozoa….) can develop resistance to pesticides over time. Why can’t the same be true for bees? What if there existed honey bee populations with natural resistance to neonicotinoids? In a commercial beekeeping setting, this colony can forage on the pesticide-coated plants like canola with a lesser impacts than other hives, enabling them to survive and swarm, while others die out.

Over time, this theoretical resistant colony may come to dominate an area, and a science-minded beekeeper may realise this, capture the colony and begin selective breeding of the queens as they show pesticide-resistant traits. Such a claim is not too far fetched when you consider that Varroa-sensitive hygienic bees orChalkbrood-sensitive bees started out in a similar manner: beekeepers noted that these hives were less prone to the disease, so they took the brood and used it to rear successive generations of disease/parasite resistant bees.

Does Modern Beekeeping Affect Honey Bee Evolution?

The scientific literature does not address this question directly. Many of the topics deal with a range of studies related to beekeeping history, impacts of beekeeping on honey production and biodiversity, as well as the various challenges faced by the European honey bee. That being said, modern beekeeping could be impacting the evolution of bees- selective breeding and hybridisation were covered above, but what about interfering with natural selection?

If a colony was infected with European Foulbrood, there is a high chance of it succumbing to the infection and dying. With beekeeper intervention, oxytetracycline may be administered to slow disease progression, enabling the colony to gain strength and overcome the bacteria itself. So the beekeeper has interfered with this episode of natural selection, and this (weaker) colony survives and possibly produces daughter swarms with EFB-vulnerable characteristics.

There is also the case of robber bees – in the wild, this is a rare, high-risk behaviour as colonies are often kilometers apart. In a bee yard, they are often spaced a few meters apart, if not less. Over time, this could lead to the emergence of opportunistic colonies who spend more time engaging in inter-hive warfare, robbing honey stores rather than focusing on foraging or hive hygiene.

Impacts of Modern Beekeeping on Local Floral & Fauna

Consider that placing a large number of hives within a single area can alter the floral ecosystem. Torne-Noguera and colleagues discussed how high densities of honey bees affect pollen and nectar availability, and impacting native pollinators (Torné-Noguera et al. 2016). In the rosemary and thyme flowers studied, honey bees were shown to consume most of the nectar and pollen produced, leaving less for other pollinators like the bumble bee Bombus terrestris.

Sure, these flowering plants still had nectar and pollen available at the end of the day, however the quantities were lower, which may not be sufficient for a larger-sized pollinator. Smaller native bees (including the social stingless bees) often have no issues coexisting with a colony of A. mellifera as their resource needs are lower on account of a smaller body size. What about larger native bees, or social species like B. terrestris?

These larger bees may be forced to nest elsewhere, widen their foraging ranges, or die out altogether. One study has shown that bumble bee colonies produce smaller workers in areas with managed A. mellifera hives (Elbgami et al., 2014). In solitary bees, these low levels of floral resources may also the rate of parasitism by cuckoo bees and wasps. For example, parasitism of Mason bee Osima pumila nests is increased when the female spends longer periods away from her nest (Goodell, 2003). The same could be true in other solitary bee species², if females leave their nests unguarded for extended periods of time.

What Will Honey Bees Evolve Into?

I conclude with some future directions for the A. mellifera species. Will the combination of wild and beekeeper-mediated breeding lead to two distinct species, a “wild” honey bee and a “commercial” bee? Not likely, considering how many swarms escape an apiary and establish themselves in a natural beehive– this is reinforced by the study from Mikheyev and colleagues (Mikheyev et al., 2015). The rate of interbreeding between managed and feral colonies also ensures that the genetic variation remain varied rather than confined to certain traits.

Globally, A. mellifera is likely to stay the same. How about regionally? If an entire population in an island like Australia or New Zealand was to develop a certain characteristic, then over time, this would be passed on to the offspring and will differ to the rest of the world. Of course, this is unlikely in practice due to trade, travel and biosecurity quarantine measures- Humans. Cheeky little buggers.

Perhaps the Africanised bees will takeover the temperate regions of the world, since they are far more successful than the other subspecies? No matter the outcome, any evolutionary changes will occur slowly in the absence of a high-impact event, like the introduction of Varroa mite parasitism on A. mellifera.

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Footnotes

1. Restriction mapping works by breaking the target segments of DNA it into pieces and then identifying the locations of the break points. This creates a map that shows the locations of these break points along the molecule, identifying changes in the genetic sequences. ↩︎
2. Bumble bees are also parasitised – if workers are out of foraging for extended periods, it increases the chances of a parasitic intruder entering the nest and wrecking havoc ↩︎