How Kin Selection Works

A honeybee collects pollen from a sunflower. Why do bees do all this work for the hive when they won’t have the chance to mate themselves? Kin selection has been advanced as the reason.
A honeybee collects pollen from a sunflower. Why do bees do all this work for the hive when they won’t have the chance to mate themselves? Kin selection has been advanced as the reason.
Frank Cezus/Photographer's Choice/Getty Images

When Charles Darwin published "On the Origin of Species" in 1859, he proposed a revolutionary new way of thinking about the natural world. The magnificent beauty and diversity of nature isn't a static creation, Darwin insisted, but the result of hundreds of millions of years of impossibly slow and small changes called evolution.

At the heart of Darwin's theory is the process of natural selection. The rules of natural selection are simple: survive and reproduce. While that might be easy enough for human beings — even the goofy-looking among us — the natural world is far more selective.

There's a reason why plants and animals reproduce in far greater numbers than necessary to maintain their population: Most of those seeds, eggs and babies don't survive. When resources are scarce — food, light, water, mates — the slightest physical advantage may be the difference between life and death, extinction or reproduction [source: Montgomery].

During his exhaustive studies of diverse species from pigeons to barnacles, Darwin realized that variation is the engine of evolution. Even when the same two animals mate, their offspring exhibit noticeable variations in size, color and other physical traits.

If one of those random variations improves a plant or animal's chance of survival — sharper teeth or longer claws — then that trait is more likely to be passed on to the next generation. Likewise, if a random variation improves the chance of successful mating and reproduction — like colorful plumage to attract mates — then the genes for that trait are more likely to be passed on.

In this way, evolution is the sum of billions of "choices." There is logic to these choices, but it's a cruel one — survival of the fittest.

Evolutionary biologists measure an organism's "fitness" by its ability to reproduce. An individual that passes its genetic material to 20 offspring during its lifetime is more reproductively fit than another member of the same species that only produces 13 offspring. By that logic, an individual that has no offspring has a fitness level of zero — congratulations, you've been kicked out of the gene pool!

Or maybe not. What if there's a mechanism within natural selection that not only rewards the strongest, smartest and fastest, but also the most helpful and selfless? Keep reading to explore the fascinating and somewhat controversial theory of kin selection.

The Problem of Altruism

Competition is key to Darwin's theory of natural selection. In nature, members of the same species ruthlessly compete over limited resources. Without competition, the genetically weak would have the same chance of survival and reproduction as the strong, and evolution would stall. For evolution to work — for organisms to becoming increasingly more fit over time — there must be winners and losers.

But there's a problem. In nature, there are some species that refuse to play the competition game. Instead of fighting tooth and nail in order to survive and reproduce, these animals dedicate their lives to helping others survive and reproduce. In evolutionary biology, such behavior is called biological altruism.

What do we make of the female worker bee that spends every moment of her short life collecting nectar to feed the hive without ever mating herself? Or the bachelor bird that volunteers to help build nests and protect other birds' hatchlings, but never has his own family?

Altruism in nature seems to go against the primal tenets of natural selection — how do the genes for altruistic behavior get passed from one generation to the next if those altruistic individuals never produce offspring of their own? The so-called "problem of altruism" puzzled scientists for a century after Darwin.

The leading solution, hatched in the 1960s by a little-known graduate student named William Hamilton, is called kin selection [source: Bourke]. Hamilton proposed that altruistic behavior in the natural world wasn't random. A helper-bird doesn't randomly pick a couple of strangers and offer protection to their young. Instead, altruistic behavior in animals is more likely to be expressed toward kin, organisms related to the do-gooder by blood.

By helping a close blood relative, the altruistic organism ensures that at least some of its shared genetic material will be passed on the next generation. According to Hamilton's Rule, the altruistic cost of not reproducing is more than compensated by the increased reproductive success of the extended family [source: Rausher]. The genes for altruism are passed on because they improve the inclusive fitness of the group.

Here's what Hamilton's Rule looks like mathematically [source: Okasha]:

b > c/r

c is the cost incurred by the altruist

b is the benefit enjoyed by others

r is the "co-efficient of relationship," with higher values signaling closer blood ties

Confused? Maybe it will help if we look at some specific examples of altruistic behavior in animals and how they are driven by kin selection.

Examples of Kin Selection

The Florida scrub jay is one bird species where some members act as helpers during the breeding season. Instead of mating themselves, the helpers assist the breeding pairs with getting food and fighting off predators.
The Florida scrub jay is one bird species where some members act as helpers during the breeding season. Instead of mating themselves, the helpers assist the breeding pairs with getting food and fighting off predators.
Richard Rodvold/iStock/Thinkstock

There are plenty of examples in nature of altruistic behavior, but only a handful that seem to be clear examples of what evolutionary biologists called kin selection.

The Florida scrub jay is one of the best-studied examples of an organism that clearly favors its close blood relatives when exhibiting altruistic behavior. The Florida scrub jay is one of several bird species in which some members of the social group act as helpers during the breeding season. Instead of pairing up with their own mates, the helpers forgo reproduction and assist other breeding pairs with gathering food and protecting the nest from predators.

When a flock of Florida scrub jays was tagged and monitored over several generations, researchers found a significant preference for helper relationships between close family members. Of the 74 helper relationships observed, 48 helpers were assisting both of their biological parents, 16 helped a biological father, seven assisted a brother, two assisted a mother, and only one helped an unrelated stranger [source: Rausher].

Large colonies of certain ants, bees and wasps are other popular examples of kin selection at work. In many of these colonies, the queen is the only female that reproduces. Throngs of sterile female workers handle nearly every other task in the colony, from scouting and collecting food, to building the nest or hive, and raising the young. Since successive generations of these insects are born from the same mother, they are, in fact, sisters. This may explain the single-minded drive to feed and protect the young at the cost of their own reproduction.

Alarm calls are another popular example of altruistic behavior motivated by kin selection. In certain groups of closely related animals, such as squirrels and apes, members of the extended family will call out an alarm signal when a predator is within striking range. This warning call allows family members to flee from danger, while potentially drawing dangerous attention to the caller itself, therefore qualifying as altruistic behavior [source: Rausher].

Despite its logical and emotional appeal, kin selection does have its critics. Next we'll look at some of the strongest critiques of kin selection, including one from a pioneering evolutionary biologist.

Arguments Against Kin Selection

On the surface, the theory of kin selection makes a heck of a lot of sense. If natural selection is designed to weed out traits that lower reproductive success, then how do we explain the existence of altruistic traits that reduce reproductive fitness to zero? Kin selection explains that those traits are passed on through close relatives who share much of the same genetic material and have more babies thanks to the help of the altruists.

But what if we're focusing too much on the individual? Sure, altruistic behavior like guarding another bird's nest or collecting food for the queen may reduce the reproductive fitness of the individual bird or bee to zero, but what if it raises the net fitness of the whole group?

Darwin himself, in 1871's "The Descent of Man," first postulated the idea of group selection. Under this theory, altruistic traits are passed on if they increase the reproductive capacity of the group. Blood relations had nothing to do with it. Darwin explained it using the notion of self-sacrifice. Tribes of apes that exhibited unselfish and self-sacrificing behavior as a whole would defeat rival groups that looked out only for themselves, thus passing on those "nobler" genes [source: Okasha].

In fact, group selection was the prevailing solution to the problem of altruism before kin selection came along. The strength of the kin selection argument, it turns out, isn't theoretical but mathematical. Hamilton and his colleague Maynard Smith proved — using complex mathematical simulations — that simple group selection didn't have the evolutionary strength to result in the continued persistence of altruistic behavior generation after generation.

Complicating the argument is the fact that in nature, many tight-knit social groups are also composed of close relatives. Where does kin selection end and group selection begin? The debate rages on today. The distinguished Harvard biologist Edward O. Wilson, one of the earliest proponents of kin selection, co-authored a 2011 article in Nature claiming that kin selection was bunk and he had the math to prove it. He went on to say that group selection happens in humans [source: Neyfakh].

The 85-year-old Wilson was blasted by his peers, but his adamant stance is proof alone that Darwin's revolutionary ideas are still making waves more than 150 years later.

Author's Note: How Kin Selection Works

When talking about the "rules" of natural selection and evolution, we tend to hold ourselves separate from the rest of nature. Altruism in animals might be blindly motivated by group preservation, but self-sacrificing behavior in humans is different, right? It's moral, ethical, noble or just plain nice. But what if we're more driven by our genes than we'd like to think? What if we only act kind and selfless in order to raise our attractiveness as a mate? Evolutionary biologists have a name for that, too — reciprocal selection. I'll scratch your back if you'll scratch mine. I'll protect your nest this year, if you protect mine next year. There's no doubt that much of our behavior is an expression of our deep biological urges to survive and reproduce, but I like to think that we also have the capacity to sacrifice and serve without thought of reward, other than it feels really good.

Related Articles


  • Bourke, Andrew F. G. "Kin Selection." Oxford Bibliographies. March 39, 2015 (April 26, 2015)
  • Montgomery, Stephen. "Natural Selection." Christ's College, Cambridge. 2009 (April 26, 2015)
  • Neyfakh, Leon. "Where does good come from?" The Boston Globe. April 17, 2011 (April 26, 2015)
  • Okasha, Samir. "Biological Altruism." Stanford Encyclopedia of Philosophy. June 3, 2003 (April 26, 2015)
  • Rausher, Mark D. "Principles of Evolution, Lectures 11 & 12: Altruism and Kin Selection." Duke University. (April 26, 2015)