The main characteristic that helps organisms survive is adaptation. In order for an organism to survive, it must struggle. According to Darwin's theory of evolution, those that possess characteristics that allow them to survive are more likely to reproduce and pass them on to the next generation.
Depending on the type of organism, adaptation can be rapid, or it may take hundreds of years. Microorganisms reproduce rapidly, which allows them to adapt quickly. Mammals, on the other hand, are much slower due to their longer lifespan. Organisms may adapt their physiology, behavior or structure to adapt to their environment. For example, worms living in deep sea environments alter their bacterial covering to survive, animals compete against each other to hunt for food that allows them to survive and cacti with more thorns are less likely to experience attacks.
How an organism adapts depends on their environment. For example, if brown rabbits and white rabbits are living in a snowy environment, the white rabbit is more likely to survive as it is less noticeable against the similarly colored backdrop. When fewer white rabbits die before reproducing than brown rabbits, more white rabbits are born into the area. In order for organisms to adapt, they must face struggles. For example, when there is not enough food in a population, those that are fast enough to catch prey are more likely to survive through the struggle, leading to a faster generation.