The ability of an organism to persist and reproduce in its environment.Survival, in biology, means staying alive long enough to reproduce — or, more precisely, leaving descendants in a population over time. It is not simply about avoiding death; an organism that lives a century but produces no offspring contributes nothing to the next generation, while one that dies young after reproducing successfully may contribute a great deal. This is why biologists usually speak of survival in terms of fitness, the relative success of an individual's traits in leaving copies of its genes behind. Survival depends on a wide range of abilities: finding food and water, avoiding predators and disease, tolerating the local climate, and competing with others of the same or different species. Each of these pressures shapes the traits that persist in a population, from camouflage and immune defences to behaviours such as migration or cooperation.
The modern understanding of survival as differential reproductive success was set out by Charles Darwin in On the Origin of Species in 1859, building on earlier ideas about populations and inheritance. The term fitness in its technical, quantitative sense was developed in the twentieth century as genetics and evolutionary theory were combined.
Individuals in a population vary in their traits, and some of that variation is inherited. When a trait makes its bearer better at coping with its surroundings, that individual tends to live longer or reproduce more, so the trait becomes more common in later generations. This process, natural selection, is not goal-directed: it simply reflects which variants leave more descendants under the conditions actually present. Conditions change, so a trait that aids survival in one environment may be neutral or harmful in another, and survival is always relative to a particular place and time rather than an absolute standard.
Survival analysis is central to conservation biology, where it guides efforts to protect endangered species, and to medicine, where it informs treatment decisions and public-health priorities.
Some organisms survive conditions that would kill most life: tardigrades can endure years of complete dehydration and have survived exposure to open space.