Is the Science Ever Settled? Theories, Hypotheses, and What Science Really Does
The traditional definition is “a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding.” But this definition has some surprises: a jackass and a mare can interbreed and produce a mule. Mules are usually infertile, but sometimes a female, a “molly mule,” can become pregnant. The famous ligers and tigons -- lion-tiger crosses -- also show that sometimes apparently pretty distant species are inter-fertile. Going the other direction, populations that are separated may develop different characteristics but still interbreed quite happily.
So, let’s think of a different hypothesis. We now have much more advanced methods of exploring genetics: we can sequence whole genomes of organisms, reduce these to sequences of symbols, and then compare them using computer methods. When you do this, you get trees of more and more distant common ancestors.
So, my siblings and I have one closest common male ancestor, my father. We also turn out to be the only direct descendants of my grandfather Hobson, but then we share our great-grandfather Jefferson Davis Martin (born in Georgia in 1861 and guess which side his family was on) with a whole mess of other people -- Jeff had 13 or 14 kids. This process of associating organisms by the common ancestors is called phylogenetics, and the descendants of a common ancestor form a group called a clade.
If species arise through natural selection, you would expect all the examined species to form a single phylogenetic tree, or at most a very few trees. So, to falsify the notion of natural selection leading to new species, you would examine the genomes of a lot of organisms. If they don’t form a clean tree, that would seem to falsify this notion of natural selection.
Sure enough, the phylogenetic trees of big groups of organisms have been constructed, and they do form simple “clean” trees. What’s more, organisms that are actually very far apart -- like humans and fruit flies, or humans and broccoli -- still share a significant number of genes and very similar chemical mechanisms that support life, which suggest we have common ancestors, too. So, this attempt to falsify natural selection fails.
Now, let’s go back through this and summarize: Darwin’s idea of evolution, that species arise through natural selection over time, requires us to gain confidence in three inferences Darwin made:
First, that there is a struggle for survival, and some individuals don’t live long enough to reproduce. We can appeal to experience to tell us this is true.
Second, that some individuals are more likely to survive and reproduce in an environment, and so are better suited to that environment, which we call “natural selection.”
Third, over time, this natural selection can change the characteristics of a population. Our penny-flipping thought experiment shows how this might happen, but more importantly, we then looked back at the actual genes of various organisms and saw that they do appear to be related genetically as this would predict.
That means that we are reasonably confident in Darwin’s whole argument.
It also means that if we can perform an experiment that falsifies any of those steps, we lose confidence in Darwin’s argument.
As things stand today, Darwin’s notion of evolution, especially when we extend it with the things we’ve learned in the intervening 160 or so years, has stood up very well to attempts to falsify it.
That’s what science as a process really is: that process of observing, proposing explanations, and then trying to knock those explanations down. Eventually, you have only a few explanations left standing: our best explanations for what we observe in the real world. It’s that collection of best explanations that we call “science.”