Fitness facts for kids

Biological fitness is a way to measure how well an organism can survive and pass on its genes to the next generation. It's a super important idea in the study of evolution. Simply put, fitness shows how many of an individual's genes end up in the genes of the next generation.

In evolutionary biology, fitness is looked at within a group of animals or plants that can breed together. This group is called a population. If some individuals in a population have genes that help them survive and reproduce better, those genes will become more common over time. This natural process is known as natural selection.

An individual's fitness is linked to its physical traits, called its phenotype. These traits come from its genotype, which is its unique set of genes. However, even individuals with the same genes might have different fitness levels. This depends on their environment and chance events. But overall, the fitness of a specific set of genes (genotype) is an average of how well all individuals with those genes reproduce.

How Genes Spread: Relatedness and Kin Selection

Fitness isn't just about an individual having babies. It's also about how many copies of its genes make it to the next generation, no matter how they get there. For example, it can be just as good for an individual to help its relatives have babies, because relatives share many of the same genes. This idea is called kin selection. It means helping family can also help your own genes survive.

Our closest family members, like parents, brothers, sisters, and our own children, share about half (50%) of our genes. Grandparents share about a quarter (25%) of our genes. First cousins (the children of our parents' siblings) share about 12.5% (1/8) of our genes. This shows how closely we are related to them.

Hamilton's Rule: When Helping Pays Off

A scientist named William Hamilton came up with a rule to explain when helping relatives makes sense in terms of fitness. His rule suggests that an animal might help a relative, even if it costs the helper something, if the benefit to the relative is big enough, considering how closely they are related.

The rule looks like this:

• C is the cost to the helper (like using energy or risking danger).
• B is the benefit to the one being helped (like having more babies).
• R is how closely related the helper and the helped individual are.

Costs and benefits are measured by how many offspring are produced.

Inclusive Fitness: The Bigger Picture

Inclusive fitness is a term that's very similar to regular fitness. But it really highlights that fitness is about the success of a group of genes, not just one individual.

So, while biological fitness tells us how well an organism reproduces, inclusive fitness adds another layer. It says that an organism's fitness also goes up if its close relatives reproduce successfully. This is because relatives share many of the same genes.

Another way to think about it is that inclusive fitness isn't just about one organism. It's about how well its entire set of genes spreads. It's figured out by looking at how many offspring an individual has, plus how many offspring its relatives have, with each relative's success weighted by how closely they are related.

History of "Survival of the Fittest"

The famous phrase "survival of the fittest" was first used by a British thinker named Herbert Spencer in 1864. He used it in his book Principles of Biology. Spencer meant the same thing that Charles Darwin called natural selection. The original phrase Spencer used was "survival of the best fitted."