William Donald Hamilton’s name may not be as widely recognized as Charles Darwin’s, but Hamilton's work in evolutionary biology has managed to explain nature’s oddities in a way that Darwin couldn’t.

This year, scientists are marking the 50th anniversary of Hamilton’s discoveries - ideas that are the foundation for a range of cutting-edge science.

Hamilton's work has caused the field of evolutionary biology to explode in surprising directions - from discovering why queen bees alone lay eggs in a hive to understanding what triggers social conflict to why cancer cells replicate uncontrollably.

As well, his legacy has changed "the way [that] doctors are thinking about diseases" and how illnesses can change over time, says Joan Herbers, professor of evolution and ecology at Ohio State University.

Herbers says that Hamiltonian thinking guides the work of scientists searching for everything from cures for types of cancer to the causes of antibiotic resistance in bacteria and what therapies we can use to conquer it.

Learning about bees

Despite their deep influence on an array of scientific disciplines, Hamilton’s theories had a comparatively humble origin.

Hamilton was born in 1936 in Cairo, Egypt, but his family moved to Europe and settled in Kent, England. Although he published fewer than 60 papers during his career, he was arguably the most influential evolutionary biologist since Darwin.

Darwin’s theories of natural selection explained the form, function and behaviours of many different animals, but failed to account for altruism in bees and humans.

Altruism is basically selflessness. It’s helping another individual for their benefit while suffering a personal cost. In nature, this often manifests as a trait that prevents the helper from having their own babies.

More than 100 years after Darwin’s theory was published, Hamilton found the key to "Darwin’s special problem" of altruism that explained why this seemingly counter-intuitive lifestyle came to be, says Herbers.

Hamilton’s theory is, 'the best all-in-one, Swiss-army-knife theory for evolutionary biology.'- Graham Thompson, University of Western Ontario

At a time when the basic principles of genetics were emerging, and the idea of a gene as a single unit of "inheritance" had just been defined, Hamilton came up with the inclusive fitness theory. It encompasses the idea that from an evolutionary point of view, there are benefits to helping family members.

Thanks to Hamilton’s work, we know that natural selection "doesn’t actually act on the individual at all … but selection acts at the level of the gene," says Graham Thompson, assistant professor in Biology at the University of Western Ontario in London, Ont.

Since genes are shared between siblings and between parent and child, then genes are also shared between aunts or uncles and their nieces or nephews.

So helping a sister raise more kids than she could on her own will actually raise the evolutionary "fitness" of both the mother and the helper sibling, says Thompson.

Graham Thompson

Graham Thompson, assistant professor in Biology at the University of Western Ontario, works with bees as part of his research. If evolution acts on ‘the survival of the fittest,’ why would worker bees evolve to be completely sterile? The answer lies in the fact that they are all related to the queen. (Courtesy Graham Thompson)

Thompson adds that after Hamilton’s work was first published, it took a while to catch on in the scientific community. It wasn’t until the 1980s that he got real scientific fame for explaining the behaviour of social insects, such as bees.

Hamilton died in 2000 at the age of 63 after contracting malaria in the Congo during a research trip to investigate the evolutionary origins of HIV.

Differences between Darwin, Hamilton

The difference between Darwin’s theory of natural selection and Hamilton’s theory of inclusive fitness can be explained by using the analogy of a factory. Think of the number of products sold like the number of babies born.

"You can think of the factory’s profits as the [evolutionary]

fitness," or the number of genes that are passed on to the next generation, says Thompson.

If every worker had to design, assemble, stock, sell and ship a product from beginning to end on their own, they would keep all the profits but produce at a slow rate. Productivity could also be sabotaged by competition from others. This scenario is like Darwin’s theory of natural selection, where every individual has their own babies, explained Thompson.

In Hamilton’s theory, "different workers are specialized in different stages of the process," says Thompson, which increases the production rate and therefore the company profits. Each worker will make less money per sale, but the sale frequency through cooperation far surpasses the output of individuals working alone, so everyone in the factory benefits.

The company itself is the only entity that sells the product to the customer, just as a queen bee is the only one to lay eggs. But with co-operation, the profits are shared, making the situation worthwhile for the workers.

Link between bees, cancer

This theory can be applied to more than insect colonies.

"We call ourselves single organisms, but really, biologically, we’re societies of cells in the same way that a termite or a bee colony is a society of insects," says Andrew Bourke, professor of evolutionary biology at the University of East Anglia in England.

If you think of the cells in the body as individual workers, or task specialists – brain, liver, blood, bone cells and so on, with the egg or sperm being "the queen" - Thompson says it becomes clear that the human body is, in a conceptual way, similar to a beehive with cancer cells acting as selfish rogues.

When a random mutation causes a cell to no longer be genetically related to the cells around it, "there’s nothing in evolution to stop it from replicating uncontrollably," says Bourke. "From its point of view, it no longer cares about the reproduction of the body it’s sitting in, it just cares about its own reproduction."

Thompson says Hamilton’s theory is “the best all-in-one, Swiss-army-knife theory for evolutionary biology.”

This is largely what makes Hamilton such a towering figure, says Herbers.

“Hamilton is one of those giants whose work is going to continue to inspire people for a long time to come."