Why Can’t Female Worker Bees Reproduce?

Female worker bees lost their ability to reproduce because they share more DNA with their sisters than with their children.

In 1831, Charles Darwin began his legendary journey to the Galapagos Islands on the HMS Beagle to escape his boring life as a failing physician. On that trip, he experienced one of the most important intellectual epiphanies in human history – he began conceptualizing his theory of natural selection. Another man, Alfred Russell Wallace, stumbled upon a similar epiphany in the Amazon rainforest while collecting exotic bird species. These two men laid the first foundations of evolution and natural selection as we know them today.

However, there was one tiny glitch in this nearly perfect tale. Darwin couldn’t figure out why in some animals, such as bees, only one female could reproduce, while all the others were sterile. Darwin failed to solve this problem in his lifetime, and in fact, it took many more years and numerous valiant attempts to finally reach a satisfying answer.

The above picture shows Hamilton’s rule and the mathematical formula in a simplified form.

Hamilton’s rule simply states that a particular social behavior will be favored if b, the benefit to the recipient (in terms of reproductive fitness), is greater than c, the cost born by the donor (in terms of reproductive fitness). Mathematically, this can be expressed as rb > c, where r is the coefficient of relatedness that measures the statistical relationship between the donor’s genes and recipient’s genes.

How Does Hamilton’s Rule Tell Us Anything About Social Insects And Sterile Worker Bees?

Before explaining how Hamilton’s rule explains worker bee sterility, we must take a quick look into bee reproduction and the distribution of genetic material amongst the male and female gametes.

In humans, there is a simple halving of genetic material, 23 chromosomes from the female and 23 chromosomes from the male. This isn’t the case in bees, as they are haplodiploid creatures, meaning that the female has 2n chromosomes, but the male has only n (half) chromosomes. This changes the relatedness that any offspring will have to each other and to their parents.

Daughters are only related to their mother by r = ½, but since males contribute their entire genome to the offspring, the daughters are related to their fathers by r = 1. Therefore, the manner that siblings are related to each other can be expressed as: r = 1+1/2=3/4.

Since the female workers would only be related to their offspring by r = ½, the benefit of rearing their offspring and increasing the reproductive fitness of the queen is greater than the cost. A female worker is actually helping establish her own genome in the gene pool better, as she shares more DNA with her sibling than she would with her offspring.

While Hamilton’s rule seems a bit too simple for the complexity of evolutionary biology, some of the nuances are still under debate. The idea of Hamilton’s rule being applied to more complex situations and animal societies (aside from haplodiploid insects) is complicated and there remains a good deal of debate on its usefulness in the study of social evolution.

Furthermore, as scientists probe into the molecular workings of the evolution of sterile bees (and other such social insects), there is increasing evidence that there may be many more missing pieces in the puzzle. Even so, this doesn’t diminish the importance of Hamilton’s rule, which lies in its mathematical basis, allowing it to serve as a framework for future research within evolutionary science.