Why is variation beneficial to the species but not necessarily for the individual?

Variations can be beneficial or harmful depending upon the ecological niche of the organism.Nature of evolution selects the variation suitable for the particular niche. Thus variations are necessary for the survival of the species. But sometimes variation in an individual may result in death of that individual or the individuals who are not evolved also die due to non-favorable conditions of the change in surroundings.So collectively we can say that variation are beneficial to the species but not necessarily for the individual.For e.g. If the temperature of water increases by global warming the population of heat resistant bacteria in water will survive but most other bacteria would die.