Natural selection is the process by which different traits and, ultimately, alleles spread or shrink throughout a population over time. It is a key mechanism in evolution and an understanding of it is necessary to knowing anything important about life itself. It demonstrates how we can have such a long history with so many seemingly lucky ancestors, culminating in such a massive variety across the planet. But perhaps most importantly, it sheds light on some of our most fundamental questions about existence.
I’ve spoken in the past about Richard Lenksi’s E. coli experiment. He has been tracking a dozen different lineages of the bacteria for the past 25 years, carefully cataloging the genetic changes that take place and when they happen. What he has found is that mutations are often contingent, meaning that for Mutation #2 to happen, Mutation #1 is first needed. However, in his most interesting finding, it turns out that neither of these mutations will necessarily be useful at first. That is, they are neither advantageous nor deleterious, instead merely being neutral, as is so often the case. Yet they persist. It is only by chance, so it is a lucky persistence, but it happens. And, with time, Mutation #3 (or 4 or 5, etc) happens, and it is that mutation which is advantageous. Here natural selection goes to work, causing the new allele to near or reach fixation. But this does not happen in every lineage. Indeed, when Lenski re-runs his experiment using generations he had frozen earlier, he has found that sometimes the mutations all happen again, but more often they fail to occur. The reason is clear: there is nothing but chance that can maintain a neutral mutation. (This is why microsatellites are great for studying short-term generational changes, but not deep evolutionary time.)
I bring this up because Lenksi’s work crystallizes in experiment the words of Stephen Jay Gould:
Replay the tape a million times from a Burgess beginning, and I doubt that anything like Homo sapiens would ever evolve again.
Gould went further, however, than us. No single trait, and certainly no entire species, is inevitable in evolution, he rightly said. Perhaps some features are more likely than others – the eye is said to have evolved independently 40 different times – but no specific feature (such as wings or bipedalism) can be firmly predicted. Indeed, Gould spent much of his time arguing that evolution drives not towards complexity or specificity, but rather diversity; life finds itself at many forks, but it never ‘knows’ which road it will take.
This presents a significant problem for theists. For the young Earth variety, the issues are glaring. That group of people is nothing but woefully ignorant, denying even the most obvious and established science. They don’t deserve any more of my time here. For the older Earth variety that opts for theistic evolution, however, the problems they face are merely buried just far enough away from mild, easy thinking to be ignorable for most. That is, they’ve admitted to the fact of evolution, but they are necessarily ignoring that if humans evolved just like everything else, there was a point where our ancestors weren’t human; we were no different from any other mammal in the distant past. (It is only time and space which allows us to define a species.) That destroys any argument that says, in the eyes of some deity, we might be more special than, say, a giraffe.
The counter to this, as per the Catholic Church and others, is to simply declare that we were infused with souls at some unknown point in our evolutionary history. This isn’t much of an answer: No member of a species gives birth to a different species; evolution is continuous. So to believe the theist’s argument, we must conclude that at some point in our ancestry, a mother with no soul gave birth to offspring that did have a soul. That is, one was not essentially human while, magically, the other was.
The one version of theism that gets around some of this is where it is declared that all of life is equally special, so it doesn’t matter that no particular species or even traits are inevitable. This, however, has its own problem. Namely, it makes the given deity (or deities) entirely superfluous. It’s no more a viable position than seeing a door blown inward from the wind and declaring that there was also a ghost pushing from the outside.
For me, I find it far easier to simply accept natural selection and its clear implications. I have no need to make seemingly comfortable lies comport with contradictory facts.