A concept in evolution linking survival of the fittest to natural selection. Traits or behavior benefiting the survival of a organism or species will thrive by having an advantage in being passed on to future generations.
Maybe some examples would help:
- Primate A is afraid of snakes.
Primate B is afraid of armadillos.
Primate A has a better chance of living longer and is therefore more likely to produce offspring and pass on his phobia.
- Members of animal pack A develop a ritual that ensures only the strongest individuals mate.
Members of animal pack B reproduce randomly.
Pack A will become more resilient and more likely to survive the elements and pass on their ritual.
- Tree A produces seeds wrapped in a tasty filling.
Tree B does not.
Hungry animals will give tree A an advantage in producing more offspring over a broader area.
In the examples above the behavior or trait in case A has a higher survival value than case B. This is partially why more people are afraid of snakes than armadillos, male rams headbut each other for mating rights, and peach trees grow more than just the pits. Survival value can explain a range of behavior, from many animals' compulsion to run away to die (to save others from infection) to the Coolidge effect to the maternal instinct.
Survival value cannot explain everything throughout the history of evolution. Genetic anomolies such as albinoism occur despite having a low survival value. And thanks to contact lenses 20/20 vision no longer has a big survival value. Evolution is far more complex than such a simple rule, especially within a civilized society (ex. the survival value of the ability to feel pain is easily understood, but when it comes to guilt, however, it's more complicated). Those who would say homicidal maniacs are acting on a triggered evolutionary safeguard gainst overpopulation, for example, are stretching the idea rather far. | 
