Sociality is a very prominent feature of animal groups and even some microbes. Myxococcus xanthus, a one-celled bacterium, collects in numbers and forms a plant-like structure, in the absence of food. They then form spores that can withstand the deprivation and germinate when the conditions are favourable again. Honeybees are the epitome of sociality. Heading the beehive is the mighty queen whose sole responsibility is to reproduce. Males called drones help the queen make young ones. Worker bees, devote their lives to take care of the eggs laid by the queen. Thanks to them, a beehive is the most efficient factory on earth. Many primates also live in groups that have a distinct hierarchy. This way, they are stronger against predators and can take better care of their young. Sometimes you are better off when you are together. Group living offers benefits beyond an individual’s capacity.
One of the most baffling aspects of sociality is the presence of altruistic individuals. In Myxococcus xanthus, only the cells that become spores get to survive, while others die. Worker bees cannot reproduce. Alarm callers in primates have a greater chance of getting attacked. Why should one risk their survival for others? To answer this question, we must understand what survival refers to here. It means whether the individual can pass on their genetic content to the next generation or not. An individual who can pass on more genetic content in their lifetime is said to have greater biological fitness.
Being selfless is pretty straightforward for microbes. They divide asexually and are genetic clones of each other with minor differences. So it doesn’t matter who survives or dies, as long as at least one of them survives the adversity. But, it’s not this simple when it comes to organisms that reproduce sexually. Take humans, for example; you received half your genetic content from your mom and the other from your dad. This makes your relatedness to each of your parents 1/2. What about your grandparents? Your mom received half her genetic content from her mom, of which you received a half. By simple multiplication, you received a quarter of your grandmother’s genetic content, making your relatedness with her 1/4. Since all humans have a common ancestor, we are all related. But, this relatedness ranges from any value between 0 and 1. Dr William Hamilton used this logic to explain why animals help each other, even at a cost. Your relatives are the closest individuals capable of propagating your genetic content. By helping them, you are indirectly improving your fitness. This is the kin-selection theory.
Although this theory was successful in explaining eusociality (an extreme form of sociality seen in honeybees, some wasps and ants), it failed to explain why certain animals help individuals who are not closely related. There is another theory that explains the existence of selfless individuals and sociality. The idea was proposed by the father of evolutionary biology, Charles Darwin himself. According to this theory, the fitness of a single individual is inconsequential; what matters is the fitness of the whole group. A group of selfless individuals who look out for each other can outcompete another group that has selfish individuals. Since altruism favours the survival of the group, the gene that favours it propagates. The trait might not be the best to have at an individual level. This is the group selection theory.
There are countless arguments against this theory, one being the instability of altruistic groups. In an altruistic group, cheaters can quickly override the group. Cheaters are selfish individuals who accept benefits from their altruistic counterparts. Since they pay no costs, they will end up having a greater fitness than selfless individuals. They will multiply in number leading to the extinction of the group.
There has been a long-standing debate in the sociobiology world on the correctness of the above theories. Celebrated sociobiologist Dr Edward Wilson was once a strong advocate of kin-selection theory. Later on, he switched his alliance to favour the long-forgotten group selection theory under a new name-multilevel selection. In the review published in 2008, he and David Wilson said that relatedness in groups promotes between-group selection. Take an inbred group of chickens, for example. The individual members of this group are more related to each other than to another inbred group of chickens. Hence, there is greater genetic variation between the groups, than within each of the groups. This enables selection to act on groups. In 2010, Wilson and colleagues published a controversial paper. Among other things, they described relatedness within a group as an effect of sociality and not its cause as claimed by the kin-selection theory. The paper faced a harsh backlash from many prominent biologists.
Another popular, yet controversial opinion is that kin selection, and multilevel selection theories are two sides of the same coin. Hamilton proposed this in 1975. Both approaches are presumed to be different mathematical interpretations of the Price equation. The equation measures the ability of a trait to evolve. For many scientists, a preference for either theory is a matter of personal choice. Again, some theoretical biologists oppose this conviction with mathematical arguments. Others question the practice of using traditional statistical methods to describe a complex and causal concept.
Kin selection theory has been the favourite child of sociobiologists for quite some time. That explains the considerable empirical evidence backing it up. Multilevel selection theory was fully embraced only until recently, needs more experimental evidence but has a lot to offer. We need to wait and watch if the greatest battle in sociobiology will continue on further or lead to a revolutionary outcome.
I thank Zakhiya PC and Varun Manavazhi for their valuable suggestions on the post.