I cannot tell you how many times I have read or heard people say ridiculous things when talking about evolution like “So a monkey gave birth to a human” or “We evolved from apes” or “There is no evidence for evolution” or “It can’t prove how life started so it’s worthless” or one of the classic arguments from the ignoramus “Evolution is just a theory so it could be wrong”. Then I have to correct them by explaining that species don’t just give birth to new species because evolution happens slowly and gradually, and that we didn’t evolve from apes rather we shared a common ancestor with the apes, and that the evidence for evolution is overwhelming, and that Darwin’s theory explains the origins of species, not the origin of life, and that evolution is just a theory in the same sense that the germ theory of disease and the theory of gravity are just theories.
Evolution is the cumulative adaptation to environmental pressures. Evolution is cumulative because it builds upon the current features and traits of the organism through successive additions. Adaptations are any traits that contribute to the probability of the organism surviving and multiplying. The environment is where the organism lives, and environmental pressures are any factors that affect an organism’s ability to survive and reproduce.
A species living on flat plains could be adapted to avoiding predators and catching prey by being spindly and speedy. On wide open, flat land, speed would be a survival advantage. If some members of its species wander onto the nearby mountainside, maybe due to overpopulation on the plains or scarcity of food or introduction of a new predator on the plain environment or because they got lost in a storm – the reason itself doesn’t matter; a new trait other than speed might be selected for, like strength for climbing over hills and rocks. Over many successive generations the stronger and bulkier –but slower – members of the group will have more reproductive success, passing on their traits of strength and bulk to their offspring. The fast spindly members of the group will eventually fade from existence as their traits are not suited to this new mountain environment. Their spindly and speedy cousins on the nearby plain will still be selected for being spindly and speedy. If the two groups stay separated long enough, eventually the stronger and bulkier group will grow further and further apart from their spindly and speedy cousins and the two groups will have evolved into two separate species. But remember, they shared the same distant common ancestors. Something like this probably happened to donkeys and horses in their evolutionary history.
That’s it. That’s what evolution is in a nutshell. A good way to visualize how it works is by imagining a branch on a tree splitting into two different twigs. The tips of the two twigs are the new species, but they connect back to the one branch on the tree. With that, it’s time to take a look at the process by which evolution works.
Further reading: Climbing Mount Improbable by Richard Dawkins; The Blind Watchmaker by Richard Dawkins