There are a few people out there who don't accept or understand Darwin's theory of natural selection. That's too bad, because it is extremely powerful, elegant and simple. It is also something that we see every day and in all facets of the world- not just biology but in human interaction and economics and your own learning.
At the common sense level, the theory is this: In any given population of like things (cows or people or ideas or political movements or...) there are differences between the members. In any given environment, some of those differences are advantages. Though the population will maintain differences, the next generation will look more like the successful members than the unsuccessful ones.
It's statistical- if the temperature drops over a period of years, the hairier racoon has a slight advantage, produces more kits ...and in a hundred generations you get predominately shaggy racoons. For all the crap that free-market capitalism gets in certain circles, it's consistantly produced the higest standard of living and all other systems have drifted toward it or collapsed.
The principle goes even to personal behavior: you have many possible responses in any given situation, but over time this variation will drift towards what has worked best in the past.
This system can be manipulated- we have so many varieties of dog because of selective breeding, which can do in a few generations what would take nature eons to accomplish, provided it drifted in the right direction at all.
There's also culling, or "eugenics". If someone had the power and decided to kill all the blue-eyed children within a few generations humans would be brown-eyed.
Rapid change in environment increases mortality and speeds up this process. Some people are lactose intolerant- if there was a disaster such that protein became very hard to come by and the only reliable source was a milk cow, lactose intolerant children would die of malnutrition in much greater numbers, fewer would grow up to have children...poof. Fast change in the gene pool.
We are used to thinking of this variation in terms of genetics, but it applies to much, much more than that. Even adopted children learn much of how to treat people from their parents. My students will teach more like I did than a stranger will. A good idea can grow and spread to the ends of the world.
Last point, for now- the teleological fallacy. Certain processes, like natural selection, are resource and environment based. In other words, things arise from what is there. This means that they can drift or grow in any way that works. Teleology is the belief that there is a reason or an endpoint to the process. That humans were the entire point of evolution, for example. People are very uncomfortable with powerful processes that don't have plans. They want someone to be in charge. The idea that we are just a stage in something that will go on forever makes some people profoundly uncomfortable. Get over it.
This process is not for or about you. The environment decides what is selected for after the fact. Who had the most babies who grew up to have babies? They are the winners. The next generation will be more like them. Someone who has no children has lost this game... except no one is keeping score.
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