|DNA image by Sunagatov Dmitry|
Evolution is not a process whereby creatures morph into more splendid versions of themselves, in a nice straight line, over immense periods of time. That's the cute, slap-happy version of evolution. The thing people don't grasp is that evolution is equally about creatures dying because they cannot adapt to changing conditions in their local environment -- a climatic shift, the arrival of a new predator, a change in oxygen levels, an upgrading in the armament of their favorite prey, etc. Evolution is a bloody, painful story of survival and death -- lots of death.
Let's consider a specific case: primitive horse precursors that existed long ago. These creatures were happily trotting around for hundreds of thousands of years in a beneficent climate, eating food that was easy to chew and swallow -- soft, moist vegetation, in other words. But then climate change hit their world and as a result, the vegetation changed dramatically. Suddenly these horse progenitors had to rely on harsher, drier, more abrasive vegetation for sustenance. Now, here's the thing -- it's not that the horses magically developed new teeth that were perfectly suited to their new food source. They didn't morph into a happy new shape. That's not how evolution rolls.
On the contrary -- all the horses that had soft dentition died because they couldn't eat the new food. And of their young, only those few that happened to have harder chomping surfaces in their mouths, survived. They could obtain nourishment from the vegetation and were able to thrive and reproduce, passing harder dentition along to their young.
Evolution requires that massive numbers of creatures die when they cannot survive the changing environment of their world. Only those few creatures that develop a beneficial mutation (harder chomping surfaces, in this case) survive into the future. And in the case of our horse-like creatures, they survived all the way up to the current day. As a result, we see horses with nice, big, hard teeth.
Now let's take a simpler, more visual example. Consider a huge population of happy, fat, brown mice who are living far north at a time when even the Arctic is warm and inviting. This idyllic happy-brown-mouse period goes on for ages, and in that time the population doesn't change much. But then one day, climate change arrives and the north is suddenly snow-covered all year round. In this new environment, the brown mice are very easy for predators, especially avian predators, to see and catch -- so all the brown mice are eaten. And only their young that happen to be born white because of a mutation, can survive in the new, snow-covered landscape.
It's not that the mice suddenly morphed their fur from brown to white to survive -- it's that the brown mice died, and only their white or near-white descendants survived. In the end, all the mice in the area were white. If an occasional brown one was born, it would be eaten before it could reproduce. (And if conditions changed and it became warm again, the white mice would stick out and be killed -- and the group would soon be brown again.)
See how this works? Species that are ill-equipped to survive a change in their environment, die. Many times an entire species will go extinct because of environmental factors. In fact, most species do become extinct. Evolution eats up a lot of creatures. It is stunningly successful, but there's a sea of blood and suffering in its wake.
This process is what scientists mean by "selection pressure". Creatures with beneficial characteristics are selected by evolution to survive into the next generation and pass their genes on to their young. All this means is: the creatures are a good match for current conditions, so they survive while others don't. And as for the selection pressure, it's many things: competition for food, quality of air, strength of predators, climate, etc. This is the process of natural selection, which we refer to in our simple way, as "survival of the fittest." Yup, and the deaths of trillions of creatures who weren't as lucky.
Evolution isn't hard to understand but you do have to pay attention. It's not grasped in a minute. You have to actually look into it, read about it and then think about it. Most people don't do this and never come to understand how evolution works. It's not a morphing contest; it's not quick; and there is a ton of death and suffering that goes into it. (Which also tells us there is no god; if there was, and he allowed this bloody process to be the way life proceeds within "his" creation, he would have to be a sadistic monster, not a god.)
This post barely touches on the topic of evolution. It's a fascinating field. If you pick up a book on evolution, I don't think you'll be sorry. And it's a principle that, once learned, deepens your understanding of the universe. It's not just critters that evolve -- but that's a post for another day.