The success of an organism in surviving and reproducing is a measure of its fitness.

Here's why:

* Fitness in evolutionary biology refers to an organism's ability to survive, reproduce, and pass on its genes to the next generation.

* Survival is crucial because an organism needs to live long enough to reproduce.

* Reproduction is essential for passing on genes and ensuring the continuation of the species.

So, the more offspring an organism produces that survive and reproduce themselves, the higher its fitness.