Evolution is not a theory. It is not a science, or a study. Evolution is a fact, one that is evident in the biology of every organism, from algae to humans, on earth. Put simply, evolution = descent with modification. The mechanisms that cause evolution to occur are another story and those mechanisms are what people tend to debate (i.e. the “evolutionary theory”).
There are two levels to this story, the emotional and the scientific. The emotional consists of arguments about how we feel about evolution and whether we believe it occurs. In my opinion this is less interesting than the scientific aspect, which leads me to ask one simple question: are humans still evolving? The short answer is, yes. The long answer is, but not in the same way as before. Our cultures, our industries, our science have all altered how evolution happens. Let me lay it out for you.
For most of the history of life on earth, several billion years, in fact, evolution has operated via four basic mechanisms, so far as we know. Those mechanisms are the well-known natural selection, or “Darwinian” evolution, as some people like to call it, gene flow, genetic drift, and mutation. Each mechanism has its idiosyncrasies, but here are the quick and dirty definitions.
Natural selection is a matter of who has more kids. Environment and local ecology determine which phenotype (the expression of an individual’s genotype or genetic make-up) is most likely to survive, live well, and reproduce a lot. Those with the most kids and grandchildren pass on their genes, including, maybe, those traits that enabled them to survive. So if one frog can leap just a millimeter or two farther than his buddies he might get more food, more mates, and more kids. It’s a simplistic explanation, but there you go. Gene flow is essentially migration. Frog #1 moves into a neighboring community of frogs, mates and has kids. The new community benefits genetically from having the shiny new genes. It also alters the gene pool in the community that he left. Genetic drift is random chance. Sometimes there are catastrophic events like earthquakes and hurricanes when lots of frogs die. These can have major effects on the gene pool of the survivors especially if the population was small to begin with. In fact it can be detrimental. All species need a certain amount of variability in their gene pool to survive as a group. Without it, they can go extinct. Not all genetic drift is so dramatic, but it illustrates the point. Mutation is the only mechanism that creates entirely new genes. All the other mechanisms just play around with existing genetic variability. Mutations can be good, they can be bad, or they can be neutral. Sometimes they stick and get passed on and sometimes they don’t. Genetic drift plays a role in which ones stick.
So now that you’ve had the primer, let’s get back to how evolution affects us as humans. Well, we still have mutations. Mutations happen all the time when parental DNA is combined. So we can check off one of the mechanisms on our list. Genetic drift certainly happens. Chance occurrences, be they at the level of genes or at the level of earthquakes and hurricanes, happen. Every day people die, individually or in large groups, and regardless of the explanation for why it occurred (religious, scientific or otherwise), it alters the gene pool. Of course we humans are quite numerous so for the most part genetic drift does not have a significant effect on the gene pool. However, there are exceptions. There populations of humans that are isolated either culturally or geographically, from the rest of the human species. For example, the Amish mostly marry and have kids with other Amish. This is true for several small religious groups. In fact some groups, like Ashkenazi Jews, have distinct gene pools in which a disease that fatally affects children, such as Tay-Sachs, which would ordinarily die out in a population, continues to occur at a low frequency. Therefore, drift occurs but has the greatest effects in these relatively small, insular populations.
We have now checked off two out of the four mechanisms. Gene flow is next and most definitely occurs. In fact gene flow is probably at its height in human populations. We can easily travel to the other side of the globe, get a job, marry locally and have kids with wonderfully diverse mixes of DNA. This is a good thing for our species as a whole. It boosts immune systems and in turn cuts down on disease susceptibility. That’s three out of four mechanisms that affect humans. The most contentious is the only one left: natural selection.
Modern medicine extends our lives far beyond what was ever possible in human history. The people having the most kids are not necessarily the healthiest or the most capable of surviving in their respective environments. But this is not straightforward. Yes, it is true that medicine, culture, religion, and our very ability to make choices and changes in our lives, have all made it so that natural selection has less of an impact. However, natural selection does not only operate at the level of our phenotypes. In other words, natural selection can also happen in the womb. Remember when I said that some mutations are bad? Well, some mutations are so bad that they cause the fetus to miscarry. This is a form of natural selection. And it is not the only way in which natural selection is occurring today.
Many people have a natural immunity to malaria, or to other diseases such as certain strains of HIV, or they seem incapable of getting cancer despite years of smoking, or live to 100 years old on a diet of steak and eggs every day. All of those traits are the result of natural selection. Yes, perhaps medicine artificially extends the lives of those who otherwise might not have survived to have lots of kids. But there are also those people who just happen to have a great genetic make-up and pass that on to their kids. Lifestyle can always be said to have an important impact on survival, but at the end of the day, some people do actually just have “good genes” that enable them to survive longer and have more kids. That’s our fourth mechanism. Yes, evolution affects us. It affects us on many levels and it is constantly changing. This is only one small piece of the picture.
If you want to know more, UC Berkeley has a fantastic, easy-to-understand website all about evolution including why it is relevant to our every day lives.