Evolutionary Biology in the Context of Beekeeping
Now my dear reader, it’s time for me take it easy and explain things technically. This article serves as a glossary of terms for evolutionary biology and evolutionary theory. While not explicitly tied to bees or beekeeping (that’s what the “are honey bees evolving?” article is for), examples are provided from the context of Apis mellifera.
A Definition of Evolution
A textbook definition of evolution is “the process by which populations of organisms change over generations, driven by mechanisms such as natural selection, genetic drift, and gene flow”. More recently, the definition expanded to include the cumulative changes in genetic composition (allele frequency) of a population over time. Genetic variations within populations lead to changes in traits over time, resulting in the emergence of new species.
The key is to have a species that, upon mating, produces fertile offspring. For example, drones from the honey bee subspecies Apis mellifera ligustica reproducing with an A. mellifera carnica queen. The resulting queen lays fertile offspring, as shown by subsequent generations of drones and queens being able to reproduce (with bees from other colonies!) and produce queens that lay fertilized eggs.
Common Terms in Evolutionary Theory
Evolution is applicable to all life forms, from the unicelluar bacteria to the multicellular organisms we see every day. Seven common evolution terms are presented below and tied into the context of beekeeping. The bottom line is that the behaviours beekeepers see in their bees today were shaped by selective pressures, from the waggle dance to their precise navigation and bee-balling abilities, amongst others. As honey bee populations face new challenges like pesticides, pathogens, and changing climates, these pressures will continue to shape their responses and adaptations.
Natural Selection
Natural selection is perhaps one of the most used and misused terms in and out of science. It’s defined as the process in which organisms with traits better adapted to their environment are more likely to survive and reproduce. Those with the disadvantageous traits die out, and over time, the advantageous traits become more common in the population. The caveat here is that these traits must be “genetically heritable” – if a generation of bees had better foraging success after cutting off their mid-legs (unlikely), the subsequent generation must manually remove their mid-legs in order to share in that “success”; they will not be born without midlegs as removal of these appendages is not genetically inherited.
An example from honey bees: Colonies that are better at identifying potential predators (say, that hovering Asian Giant Hornet right outside the hive) have an advantage over other colonies, as they will survive and pass on this trait via reproductive swarming. Over time, this defensive attitude becomes more prevalent in the population, as seen by the recent example in the Asian honey bee, Apis cerana, who smears animal faeces over its hive walls in response to attacks by wasp predators (Mattila et al., 2020).
Readers interested in the primary source on natural selection are directed to the foundational work “On the Origin of Species” by Charles Darwin. However if you’re reading this article then you already know this, don’t you?
Genetic Drift
Genetic drift refers to the random changes in the allele frequencies (the form of a gene that’s shown in the organism) in a population due to chance events (reproduction, displacement, even survival). It can lead to the random loss or recovery of alleles, and has the strongest impact in small populations as these chances can change genetic variations from generation to generation.
In Apis mellifera, look to islands like Madagascar or Cyprus. A small subset of colonies were isolated from others, and random events like disease or predation (the oriental hornet Vespa orientalis in Cyprus is one such example) disproportionately affected the gene pool, causing certain alleles to become more or less common purely by chance of being on that island. Over time, this resulted in a separate subspecies of bees evolving, namely Apis mellifera malagasy and Apis mellifera cypria respectively.
Gene Flow
Gene flow (a fancy term for migration of alleles) is the transfer of genetic material from one population to another as a result of movement by individuals between populations. Pretty simple to see in managed honey bee colonies; just consider how many different subspecies of drones are present at a drone congregation area, especially in cities with large numbers of urban beekeepers. You can have multiple subtypes of A. mellifera in one area, and when the virgin queen arrives to mate, she’ll receive a variety of genetic material from the drone semen.
Mutation
Mutations are changes in DNA sequences that can drive new traits, or remove current ones, or nothing at all (“neutral mutations”). The waggle dance and homing abilities mentioned above are examples of mutations that have become dominant as they were deemed beneficial. More recent examples of mutations can be of Varroa-sensitive hygiene in A. mellifera, or the smearing faeces on hive walls by A. cerana, also mentioned above. Mutation are the changes in the DNA sequences of an organism, and are the primary source of genetic variation. If enough mutations occur to the point where the organism becomes a different species, then it becomes biological evolution.
Adaptive Evolution
Adaptative evolution is related to adaptive radiation, just note quite the same. The latter refers to the rapid evolution of diversely adapted species from a common ancestor when introduced to new environmental opportunities. The former are evolutionary changes driven by natural selection that increase an organism’s fitness. This one is harder to see in honey bees, yet is very common in solitary and native bees. Theoretically, if honey bee colonies were to be transported to new areas with different species of flowering plants than those previously identified, then different colonies can evolve specialized traits suited to each niche, such as longer proboscises or larger corbiculae (pollen baskets on the hind legs of foragers).
Individual Fitness
A measure of an organism’s ability to survive and reproduce. An organism’s fitness is determined by the number of its offspring that survive to reproductive age, or how long it takes to complete a marathon (maybe not the last part). In eusocial insects like bees, the individual does not reproduce as the workers are sterile (only the queen and drones do), so this section would have been better renamed as “colony fitness”, and can be assessed by the rate of swarming and successful establishment and subsequent swarming of said swarms.
Speciation
Speciation can be deemed as one such point where evolution occurs. It is the formation of new and distinct species during the evolutionary processes, and is driven by mechanisms like geographic isolation, reproductive isolation, or adaptive radiation. Though our honey bee heroes are a single species, if two populations were to be isolated long enough and subjected to different selective pressures, they could, in theory, diverge into different species (as opposed to subspecies). This can be simulated using mathematical models, but is much harder to see in practice.
Variation and Inheritance Between Closely Related Species
The factors described above all give rise to variations, which refer to the differences in the physical characteristics and genetic diversity within a population, or one that is closely related to it. The big three drivers are mutations in the DNA sequences, genetic recombination during sexual reproduction and environment-driven gene expressions.
Inheritance on the other hand refers to which genetic information is passed onto the offspring from the parent (i.e. DNA). The genetic makeup of an individual (“genotype”) will determine its physical traits (“phenotype”), based on the laws of inheritance proposed by Gregor Mendel. It’s where we see terms like “homozygous” (two identical alleles) and “Dominant/Recessive” trait values (if the dominant form of an allele is present, it will be expressed over the recessive one).
The 2023 review from Guichard et al., used the example of beekeepers selecting for honey production and then identifying colonies whose queen shows the most favorable honey producing ability. The variation within this hive is said to be above average compared the entire population (say, the apiary). They suggest that all traits are hereditable, and where low heritability is observed, it is due to the low measurement accuracy or unidentified environmental factors that override the genetic effect.
One test for determining if a variation was successfully inherited is to look at the performance of their close relatives. In our example, if the parent hive has high honey production and successfully swarms three times in one year (that poor beekeeper!), then those four colonies should also have a high honey production (one parent and three daughters). Thus, it can be estimated that the genetic material within this breed of queens is of high quality for honey production, and the beekeeper may embark on raising bee queens from this stock.
The Basis and Basics of Natural Selection
Natural selection is a foundational concept in biology that explains how species change over time. The cycle is broken down into four parts: existing variation, a selection pressure, a competition whereby the survivors win and reproduce, and ending with the eventual accumulation of those advantageous, surviving traits.
For simplicity, we shall continue considering the queen as a representative of the all workers in the hive, which is a reasonable analogy considering that she is the one who produces fertile offspring.
Genetic Variation in Living Organisms
Genetic variants are present in the individuals of every population. No two worker bees in a hive are genetically identical (drones are however, as they come from unfertilized eggs and are considered as being identical to their mother, Her Majesty the Queen). Examples of genetic diversity between hives can be colonies that are more disease resistant, colonies with a higher degree of hygienic behaviour or colonies with worker bees that posses a slightly longer proboscises than others.
Enter the Selection Pressure
Suppose the environment changes, such as the introduction of a new pest (Varroa jumping over from Apis cerana is an example), or there is a shortage of flowering plants with shallow nectaries versus deeper ones (the area where nectar is produced on the blossom). When such resources are limited, along with challenges like predators, diseases, and environmental changes, it exerts a selection pressure – which populations will triumph?
Returning to the proboscis scenario, a hive needs to gather enough nectar and pollen to sustain itself. However, the suitable flowers are at varying distances and there is competition with other bees and insects for flowers with short nectaries. If one species of bees had a slightly longer proboscis, it would have access to flowers with slightly deeper nectaries, thus accessing a source of food with lower competition, and providing her hive with sustenance.
The Winner Takes it All…
…The loser has to fall…Because of the inherent variations, the colonies possessing traits that provide an advantage to these challenges are deemed more “fit” in the evolutionary context. They are more likely to survive and reproduce (swarm), passing on these advantageous traits to successive generations of daughter colonies. Those swarms will also have an advantage over other organisms in their ecological niche, and will have a better chance of surviving should flowers with short nectaries die out.
Future Generations and Evolutionary Change
Over successive generations these beneficial traits become more common in the population, while less advantageous traits diminish. In our case, the longer proboscises become more common, while those with shorter proboscises die out. Over time, the physical characteristics of one generation will no longer resemble those of the previous generation. At this point, these organisms can be deemed to have undergone evolutionary changes, as they are now better adapted to their environment than other species.
If we were to use mathematical models to have the trends in selection pressures continue, we can expect the bees’ proboscis to get longer and longer in order to keep up with natural selection. Or perhaps a meteor appears and wipes out the species, anything is possible over a long enough timescale.
An Additional Glossary of Terms for Those Not Mentioned Enough Above
As I come to update this article I realise I’m lacking in many, many (many!) terms, so here are some rapidfire definitions:
- Alleles are alternative versions of a gene found in the same region within a chromosome. When allele frequency (“expression”) change in a population over generations, it can be deemed evolution.
- In bees, hygienic behaviour (uncapping-recapping-removal of diseased brood) and defensive behaviour (the sting response) are two such examples of alleles – the higher expression of genes associated with either trait, the more they’ll perform these actions.
- An “adaptation” a heritable trait that increases an organism’s survival or reproductive success in its environment. An adaptation is the trait, and “Adaptive evolution” is the process by which the trait increases its frequency.
- In bees, it could be going from scopae in solitary bees (dense hairs for pollen gathering) into pollen baskets in honey & bumble bees, which are more advanced ways of transporting large amounts of pollen.
- Homologous structures are traits shared by different species because they were inherited from a common ancestor, even if their function differs. In highschool biology, we’re taught about the “pentadactyl limb” and how bats, whales and humans share this trait, even if we use our “arms” for different functions.
- In bees, it’s the mouthparts – all bees have the same ancestral mouthparts, which over time diverged to serve different floral resources (as in long vs short tongued bees).
- On the other hand are “analogous structures”, those that perform similar functions but evolved independently away from a shared ancestral trait.
- It’s harder to say in bees, however there are examples in the form of buzz pollination. How bumble bees and some solitary bees (e.g. blue banded bees) came to perform buzz pollination was evolved separately, not from a common ancestor.
- I’d also say that sociality in different bee species evolved separately, but there is no “one gene” that controls social behaviour
- It’s harder to say in bees, however there are examples in the form of buzz pollination. How bumble bees and some solitary bees (e.g. blue banded bees) came to perform buzz pollination was evolved separately, not from a common ancestor.
- Homoplasy is similar traits arise independently in different evolutionary lineages due to similar selective pressures- a step below the “convergent evolution” of highschool biology.
- For bees, it is the case of kelptoparasitism of cuckoo bees – this evolved in multiple distinct species in bees due to similar ecological opportunities: I invade and takeover your nest without your knowledge and you do all the broodrearing for me.
- Phylogeny is the study of the evolutionary relationships among species- those “branching trees” you see in academic texts. Pretty simple example: how bees diverged from ancestral wasps to become their own species.
Do you want me to list more? Comment below
Natural Selection and the Evolution of Populations
This article attempted to explain how honey bee traits evolved via natural selection. The environment is full of variations and selection pressures that constantly act on bees. If a new disease was to emerge tomorrow, the sheer amount of genetic variation1 within Apis mellifera will mean that some colonies randomly have an advantageous edge over others (eg, predisposed resistance, or more suitable hygienic behaviour), and will survive and pass these traits onto the next swarm.
So my dear reader, it is the entire colony, not the individual workers that evolve over time. Evolutionary examples are seen in genes which favour more hygienic bees (beekeeper-mediated selection), or those with longer proboscis (Mother Nature-mediated selection).
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Footnotes
- Caveat: recent research from Alburaki & colleagues (YouTube interview link) showed that due to beekeeper-mediated artificial selection, there is far less diversity in managed and wild honey bee colonies in the U.S. than what was initially thought to be. Mating of wild colonies isn’t as effective if they all come from the same (great-great-great….) grandmother bee. ↩︎