Why do ants live in colonies? Part 1: Genetics

When Charles Darwin and Alfred Russel Wallace first wrote about evolution, ants confused them. A lot! The whole point of evolution is to get better at spreading your genes and having as many of your genes in the next generation. So, animals evolve and adapt to become better at having children and having their children have children and so on. Basically, adapt to be better at surviving and having children. At first, eusocial animals don’t seem to do this very well. I mean think about it, 99% of them can’t even have children. So why do they live in colonies like this? Why would eveolution favour them losing the ability to reproduce? These questions were so confusing to Darwin that he thought it disproved evolution itself. Of course, this isn’t true and evolution is a pretty good theory as theories go. It’s certainly better than the world being flat that’s for sure. It has tonnes of evidence for starters. Upon closer inspection, eusociality is a genius solution to evolution’s problems. To explain why we must go into a scary place called genetics. I say scary, it sounds scary and it can be but we’re going go one step at a time and I will try my very best to not get off topic. No promises.

All of these ants and only 1 can have babies. Fire ant queen and workers, Solenopsis invicta.

Darwin lived in the 1800s and he didn’t know about genetics. Nobody did. It wasn’t until genetics were more understood that eusocial animals started to fit into the puzzle that is evolution. With mammals like us, we get half of our genes from my mum and half of our genes from our dad. This means we are around about 50% related to each of our parents. So, two halves make a whole: you.

This was the way Darwin and others in the 1800’s approached the problem when it came to animals like ants. We share around about 50% of our genes with our siblings so helping to raise a sibling means that 50% of your genes will survive. This is the same amount as if you raised your own child. According to this, worker ants gain nothing by giving up their ability to have babies. Remember if they have a child that’s 50% of their genes present in the next generation if they raise a sibling in a colony that’s also 50% of their genes in that generation. Now if your brother or sister has children You only share 25% of your genes with them. That’s half of the 50% you would share with your own children. So, if animals evolved to pass on as many of their genes as possible, why would they give up their ability to have children and raise young that are less related to them? Well, as with everything I’ve talked about so far, it’s not that simple. Brace yourselves for some big words.

Darwin and Wallace both died long before genetics were fully understood. Haplodipliody was discovered even later.

Ant genetics include a mechanism called haplodiploidy. This is big scary word number one. Below is a picture that explains it better than I ever could just using words. Although technically I made the picture and wrote the words, so I am using words to explain it to you. Let’s just ignore that and look at the pretty picture shall we.


This shows how members of the colony are related to their parents (Coloured circles) and how much a worker is related to members of the colony (Percentages).

Male ants have 1 set of chromosomes. They inherit these from their mothers through asexual reproduction. This means they are 100% related to their mum and don’t have a dad. Weird. Females, workers and queens, inherit 100% of their dad’s genes and 50% of their mothers, making them 75% related to their sisters. This happens through sexual reproduction. Crazy right?

So, let’s go back to Darwin’s question: Why would worker ants give up their ability to have children? Remember back to our last post where I said that eusocial animals have more than one generation in the same nest. This means that all babies in the nest are the brothers and sisters of all the workers. So the workers are 75% related to the colony’s brood and would be only 50% related to their own children. This means, through some very complicated maths, ants are 25% more related to their brothers and sisters than their own children.

How weird is that? Imagine being more related to your brother or sister than you are to your own dad. To all the guys out there imagine not even having a dad! So for a worker ant to get as many of its own genes as possible to be passed on, they must raise their siblings instead of having children of their own.

In terms of genetics, this is why ants live in colonies. You can breathe now, you made it through the genetics! You’re already doing better than I did in genetics lectures. In the future I will cover the non-genetic reasons why ants live in colonies and there are a few. Anyway, that’s for another day. For now, I hope you understand why ants live in colonies in terms of genetics. There are other reasons ants live in colonies that we will cover later. I hope you learned something and thank you for reading,

Alex.

Glossary

Chromosome: How your DNA is stored inside your cells.

Haplodiploid: A system of sexual determination in which diploid females develop from fertilised eggs and haploid males develop from unfertilised eggs, as in most hymenopterans, such as ants and bees.

References

Darwin, C., 1909. The origin of species (pp. 95-96). New York: PF Collier & son.

Excerpts from Charles Darwin’s Notebook Cp. 166

Excerpts from Charles Darwin’s Notebook Mp. 46, 52-59

Friedman, D.A. and Gordon, D.M., 2016. Ant genetics: reproductive physiology, worker morphology, and behavior. Annual Review of Neuroscience39, pp.41-56.

Hölldobler, B. and Wilson, E.O., 1990. The ants. Harvard University Press.

Normark, B.B., Jordal, B.H. and Farrell, B.D., 1999. Origin of a haplodiploid beetle lineage. Proceedings of the Royal Society of London. Series B: Biological Sciences266(1435), pp.2253-2259.

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