All Draw Curiosity videos are fully subtitled in English and Spanish. The blog post builds on the concepts touched upon in the video.
Inclusive fitness remains as one of my favourite topics from when I read Biological Sciences, and I’d say the Zoology Department in Oxford is amongst the strongest in evolutionary biology. As such, I couldn’t resist paying special homage to a topic I believe everyone should know about. Both the video and the blog post should give you a more than basic understanding in the subject – and I have two more videos on the topic on the way too!
What is inclusive fitness and Hamilton’s Rule?
Inclusive fitness is the sum of the individual’s personal fitness as well as the fitness of relatives which carry their genes.
What is their impact on evolution?
Selection operates at the lowest level of transmissible information, which we consider to be the gene. Genes are contained within their ‘vehicle’, the individual, which is the smallest unit that can transfer the genes to other individuals. Transmission is often vertical, via sexual reproduction or parthenogenesis, although in some special cases, such as bacterial conjugation, horizontal gene transfer may occur.
Natural selection dictates that genes encoding traits that increase their fitness will spread, whilst genes that decrease the individual’s fitness will be wiped out because they don’t generate more copies of the gene in subsequent generations.
Intuitively, maximising your personal fitness by having as many offspring as possible may seem like the best strategy to get as many copies of your genes into the next generation. However, because they can also increase the number of copies of genes into future populations by helping related individuals that share genes to also increase their fitness, altruistic behaviour may be performed accordingly.
What is Hamilton’s Rule?
From this concept, Hamilton devised the following equation that describes the conditions under which altruism will be performed. In evolutionary biology, altruism is the act of helping other individuals despite incurring a cost themselves.
rb – c > 0
You may also find it rearranged as:
rb > c
r > c/b
r stands for relatedness, b for benefits and c for costs.
It’s important to note that when Hamilton’s rule and inclusive fitness is taken into account, altruistic behaviour is ultimately ‘selfish’ at the level of the gene, as it benefits its evolutionary success.
Likewise, it’s important to note the lack of intent and insight in natural selection – evolution selects for what works best in the present moment and without predicting the distant future.
No conscious decisions are necessarily made, genes aren’t truly selfish – the behaviours and traits they encode shouldn’t be anthropomorphised or interpreted as moral decisions.
What are the costs and benefits?
Essentially, if the fitness benefits outweigh the costs, an individual will help another individual. The units for costs and benefits depend on the situation and the researcher’s idea of fitness – some people measure it in terms of offspring produced, others in terms of goods such as territory, food etc. Although the equation is simple, its precise calculation can be complex, as it is hard to demonstrate how much apportioned amounts of help contribute towards raising a relative’s offspring.
Relatedness however is not measured in units, as it is the proportion of shared alleles between two individuals as compared to the population average. Allele is the technical term for each possible variation of a gene. For instance, the gene coding for coloration in peas has two alleles: yellow and green. The distinction “as compared to the population average” is important, as certain alleles may be common in a population, but we share many more variants with individuals closely related to us.
What does relatedness in diploid organisms look like?
We are diploid, which means that we have two sets of chromosomes, one from our mother and another from our father. The only exception to this is our mitochondrial DNA, which is of maternal inheritance.
The relatedness between us and another family member is quite simple – take the degree of separation between you and your relative (n), and the relatedness can be calculated as 1/2n – the relatedness is also symmetric, if my relatedness to a relative is 0.5, their relatedness to me is also 0.5.
- Relatedness to ourselves and identical twins is 1/20 = 1
- Relatedness to siblings, parents and children is 1/21 = 0.5
- To grandparents, grandchildren, nephews and nieces is 1/22 = 0.25
- It’s easy to extrapolate from here
Another interesting point is that in theory, our relatedness to identical twins, parents and children is 1, 0.5 and 0.5 respectively, but due to recombination, our relatedness towards other relatives is no longer an exact figure but an approximated average.
How does inclusive fitness work in bees?
Bees are eusocial organisms, and most live in hives surrounded by other individuals. Different individuals carry out different tasks within the nests, and the three key labour divisions we observe are:
- The queen – whose sole purpose is to found the hive, lay eggs and populate the hive
- The drone – whose sole purpose is to provide sperm to fertilise the female and generally dies shortly after
- The workers – who look after the queen, forage for food, maintain the hive and raise other workers and queens.
Bees are also haplodiploid organisms, which is a form of sex-determination in which the males are haploid, carrying only one set of chromosomes, whereas the females are diploid, carrying two sets of chromosomes.
If you ever need a full table of the relatedness between different haplodiploid organisms – you’ve come to the right place! The calculations are simple, but they can be somewhat confusing and tedious.
The key points about haplodiploid genetics cover:
- Unfertilised eggs develop into males and derive from 50% of the female’s genetic material. Males develop into drones, who fertilise future females.
- Fertilised eggs develop into females, and contain 100% of the drone’s genetic material and 50% of the female’s genetic material. Females can develop into workers or queens depending on the diet they receive.
From here, it is simple to start calculating relative relatedness values, but it is important to realise that being haplodiploid means that relatedness is not symmetrical between individuals of the opposite sex due to the difference in amount of genetic material carried. Here is a chart displaying some of the main relatedness relationships between different individuals in the hive:
How do we calculate relatedness in bees?
The relatedness between the queen bee (or a laying worker) and her offspring is 0.5, because whether producing a fertilised or unfertilised egg, she contributes half of her genetic material to it.
However, the relatedness between a drone and his mother, as well as any daughters, will be 1 because she carries his entire genetic material, whereas between a worker and her mother it will be 0.5, as the other 0.5 relatedness will come from the drone.
When it comes to relatedness between siblings, the relatedness between workers is unusually high at 0.75, but this is because they share 0.25 (0.5 x 0.5) from the queen and 0.5 from the drone. On the other hand, the relatedness between workers and drones (siblings) is 0.25, because they share 0.5 x 0.5 of the queen’s genetic material, whereas the relatedness between drones and workers is 0.5, because the workers carry half of the queen’s genetic material, which on average will correspond to half that the drones have inherited too.
The relatedness between workers and their offspring is the same as that between a queen bee and her offspring (0.5). However, the relatedness between to the offspring laid by other workers, that is, her nephews and nieces will be 0.375 (0.5 x 0.75), assuming the queen was fertilised by a single drone. Because both of these values are very low, workers are therefore more inclined to favour raising the queen’s eggs to which they have a relatedness of 0.75 than their own offspring, and will often engage in policing the other worker’s eggs if they lay any. However, this is under the assumption that there is a unique queen and drone. As I hope to convey in a future video, the hive politics can be greatly affected based on Hamilton’s rules and the number of partners the queen bee is fertilised by.
How do we know who our relatives are?
This is another fascinating question in this field of Biology – successful genes maximise their inclusive fitness through selective altruism towards relatives in addition to maximising their personal fitness, but identifying kin accurately is an important part of the process. It is also a vast question and the perfect topic for a future video!
However, for a brief summary, different individuals identify kin in different matters, some may simply rely on proximity if they tend to only interact and travel with related individuals, others may rely on pheromones and particular odours which are formed from highly specific and variant alleles, so relatives are more likely to share those alleles.
This question also prompted the development of the Green Beard Hypothesis, which proposes a manner in which a single gene could ‘selfishly’ increase its propagation throughout a population by only helping individuals that carry it. In order for this effect to occur, the allele of the gene in question must have the three characteristics:
- A detectable trait
- The ability for others to identify the trait
- Selective altruism on behalf of others who share the trait
Essentially, it provides a detectable signal for others to detect and help those who share the trait. Unfortunately, it is also a system that lends itself to cheating, as certain individuals could evolve the ability to express the trait and receive the help from others, whilst refusing to return it.
I hope you enjoyed and learned something new today! Let me know what you think in the comments – I would love to know! If you enjoyed this blog and would like to be notified of new entries, consider signing up to the mailing list here and subscribing to the YouTube channel!