Darwin's Theory Of Natural Selection

Darwin’s theory is based on the notion of variation. It argues that the numerous traits and adaptations that differentiate species from each other also explain how species evolved over time and gradually diverged. Variations in organisms are apparent both within domesticated species and within species throughout the natural world. Variations in colors, structures, organs, and physical traits differentiate a multitude of species from one another. Heredity is the mechanism that perpetuates variations, Darwin argues, as traits are passed from parents to offspring. What is important about these variations to Darwin, though, is the way they allow species to adapt and survive in the natural world. He gives numerous examples of variations that illustrate
Borrowing from Thomas Malthus’s principle of exponential population growth, Darwin argues that the possibility of infinite growth of population sizes is checked by the limits of geography and natural resources, which will not allow an infinite number of beings to survive. As a result of limited food, water, shelter, and so on, species must engage in a “struggle for existence,” creating competition for survival. What decides, then, which species will survive and which will become extinct? Here is where “natural selection” comes in. Darwin argues that organisms exhibiting “advantageous variations”—variations that will allow them to adapt to their environment better than other organisms do—will be more likely to survive. Through heredity, these advantageous variations will be passed on to the organisms’ offspring. Eventually, natural selection will allow those species best adapted to their environments to survive and prosper, while species without these advantageous adaptations will lose the struggle for existence and become
Organisms will continually give birth to offspring that carry variations, some of which are advantageous and some of which are not. As advantageous variations are naturally selected and become perpetuated through successive generations, organisms carrying these advantageous variations will diverge from the original species, eventually becoming a species of their own. Continual modification and divergence, then, create a branching scheme of evolution, in which new species continually branch off from old ones. The “branches” help biologists link later species back to an original parent species, identifying the point at which different species are related to one another. Darwin notes that existing classification systems developed by naturalists already show these relationships between species. Darwin’s theory of descent with modification, then, simply provides an explanation for why many species seem so similar: Either they evolved from one another, or they both evolved from a common parent