Natural selection is a process by which organisms that are most suited to their environment survive and reproduce most successfully; also called survival of the fittest

The struggle for existence is a natural history. It refers to the competition between living things to survive. This, and the struggle for life, were used over 40 times by Charles Darwin in the Origin of Species, and the phrase is the title of chapter 3 of the Origin.

Extinct: describes a species that has completely disappeared from Earth.

Adaptation: is a structure, behaviour, or physiological process that helps an organism survive and reproduce in a particular environment. e.g.: Hibernation is an example of a physiological adaptation

Survival of the fittest: The idea that species adapt and change by natural selection with the best suited mutations becoming dominant. This expression is often attributed to Charles Darwin and, although it appears in the fifth edition of his Origin of Species, 1869, it is there attributed to Herbert Spencer: "The expression often used by Mr. Herbert Spencer of the survival of the fittest is more accurate..." Spencer had published The principles of biology in 1864. In that he referred to 'survival of the fittest' twice: "This survival of the fittest, implies multiplication of the fittest." "This survival of the fittest... is that which Mr. Darwin has called 'natural selection, or the preservation of favoured races in the struggle for life'."

By 'fittest', of course, Spencer and Darwin didn't have in mind the commonly used meaning of the word now, that is, the most highly trained and physically energetic. The 'fittest' referred to here are those animals which are the most suited to their environment, that is, those which are best fitted to survive.

Natural selection is the process whereby organisms better adapted to their environment tend to survive and produce more offspring. The theory of its action was first fully expounded by Charles Darwin and is now believed to be the main process that brings about evolution. For example, some beetles are green and some are brown. Since the environment can't support unlimited population growth, not all individuals get to reproduce to their full potential. In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do. The surviving brown beetles have brown baby beetles because this trait has a genetic basis. The more advantageous trait, brown coloration, which allows the beetle to have more offspring, becomes more common in the population. If this process continues, eventually, all individuals in the population will be brown. If you have variation, differential reproduction, and heredity, you will have evolution by natural selection as an outcome.

Common descent is that all species, living or extinct, are descended from ancient common ancestors.

In evolutionary biology, a group of organisms share common descent if they have a common ancestor. There is strong quantitative support for the theory that all living organisms on Earth are descended from a common ancestor.

Charles Darwin proposed the theory of universal common descent through an evolutionary process in On the Origin of Species, twice stating the hypothesis that there was only one progenitor for all life forms and ending with "There is a grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one". The theory asserts that all currently living organisms on Earth share a common genetic heritage with each being the descendant from a single original species, though the suggestion of substantial horizontal gene transfer during early evolution has led to questions about monophyly of life.