Evolution occurs when random mutations result in offspring that are slightly or profoundly different from their parental ancestors. If the genetic change renders the mutant better able to get along in its ancestral environment than its unchanged siblings and other contemporaries, the mutant has a better chance to survive the life hazards present in its habitat than the unchanged members of its species.

In breeding with the unaltered individuals, the successful mutant's genetic change is more apt to be passed along to the next generation, and to spread throughout the entire species population in following generations. This creates a new species that supplants the old one and eventually takes over the entire habitat. The old displaced species either goes extinct, or migrates to a similar habitat, or adapts to a different one. This type of evolution takes place slowly, over thousands of years, in long-lasting stable environments. Thus, evolution of a new species may cause extinction of an older one.

However, in unstable suddenly changing environments (such as emerging sea floors, submerging land surfaces, and global warming or cooling), relatively rapid mass extinctions occur that open up myriads of ecological niches that are quickly filled by multitudes of new mutated species which are able to survive because they don't face the competition of the former occupants of those habitats. In this case, extinction of an old species enables evolution of a new one. Mutations that would have died out in a former stable environment are able to flourish in the newly formed one.

This happened 65 million years ago, when a large asteroid collided with Earth in Yucatan and blasted out millions of tons of exploded rock dust fragments that encircled the earth and remained in the atmosphere for years. This blocked sunlight and killed off many plants and herbivorous (as well as carnivorous) dinosaurs and other reptiles. These had previously dominated most of the former land, sea, and air environments that had existed for more than 200 million years prior to the asteroid's Yucatan collision and explosion. Had that mass extinction not taken place 65 million years ago, mammals might not have evolved beyond the early primitive species that existed then; and humans might not ever have appeared on the earth. Sic transit Hominidae.

Charles Darwin was the genius who first discovered that all life on Earth evolved from earlier forms that lived millions and billions of years ago in environments vastly different from what we have today. He recognized that Mutation and Natural Selection were the driving forces that enabled evolution to occur. Natural Selection, also known as Survival of the Fittest, refers to the competition between existing species and their mutant descendants (over food, sex, desirable habitat living space, etc.) that determines which forms lose out and go extinct, and which ones win out and evolve.

Darwin had explained Natural Selection thusly: "Natural selection is daily and hourly scrutinizing, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers."

Today, we humans are able to influence evolution. We can alter DNA sequences, we can selectively breed dogs, cats, sheep, cattle, horses, fishes, food crops, flowers, trees, shrubs, and even people -- to derive the types we want. Some call this Artificial Selection, but if humans are part of Nature, isn't our dabbling in the realm of deities also part of Natural Selection?