The organisms which are provided with favourable variations would survive, because they are the fittest organisms to face their surroundings while the unfit are destroyed. Darwin explained the fitness in terms of reproductive fitness, which states that population evolving maximum reproductive fitness will survive. For example, During the evolution of reptiles, giant reptiles, the dinosaurs, etc., appeared. Majority of them were herbivores, but due to certain climatic changes, the vegetation disappeared and therefore, most of them became extinct. However, small animals who can change their feeding habits from herbivorous to carnivorous diet survived, because they could easily get adapted to changed environment. These, therefore survived more and hence are selected by nature.
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