Today, I want to address a very common claim by Creation Scientists (sic!). It is said that the geologic column is incomplete, and that there are huge gaps where there ought to be gradual change.
What these poor scientific illiterates do not understand is that according to the theory of natural selection, we should not expect to find a complete record. In fact, the very things which cause speciation to occur would necessarily cause gaps in the fossil record.
To begin with, we must understand that speciation is a very, very long process in most cases. In a single population, it is extraordinarily rare for a new species to just appear. This is because of interbreeding. Any new beneficial adaptation is overwhelmingly likely to be quite small, and it is also overwhelmingly likely to spread through the population relatively quickly, in evolutionary terms.
We must also confront a popular notion about species. Species are not defined by their ability to mate. Lions and tigers are different species, and yet, they can breed. Breeding is necessary, but not sufficient, to label a species. In other words, for something to be considered a species, it must be able to breed with itself, but just saying that two animals can interbreed does not make them the same species. In determining species, one of the biggest factors involving breeding is whether they do so without outside interference.
With these two things in mind, we can examine how speciation does occur, and what the evidence would look like after millions of years. In order for a particular population to become different enough from another population of the same species, there has to be isolation. If there is no isolation, mutations will propagate through the whole population, and there will be no speciation event. Let’s do a thought experiment with a rodent, some variety of rat. Let’s suppose that this is a prairie rat, and it lives on a very large island. Now, let’s suppose that a major geologic event occurs (what it is hardly matters) and the island is literally split in two, with a large body of water in between. Some of the rats on both sides survived, and now we have two separate populations.
Since mutations occur essentially randomly, we should not expect both populations to continue down the same evolutionary path. In fact, we’d be amazed if they did. Instead, we will start to notice slight differences, even if the habitat of both populations is still quite similar. Eventually, just from time and mutations, we will have two groups of animals that are different enough from each other that they will no longer interbreed if reintroduced.
The effect of culture upon speciation also cannot be overlooked. The tiniest change in the expression of a gene can cause quite different social behavior. Since we’re talking about rats, it’s worth mentioning that by introducing a single chemical into the brains of polygamous varieties of rodents, we can make them monogamous. Since we know that this occurs, let’s suppose that one island of rats becomes monogamous while the other remains polygamous. This, in itself, would probably be enough for scientists to make a case for a new species, since the two populations would not normally be socially compatible.
However, we’re trying to get at the fossil record, so let’s extend our thought experiment much farther into the future. We need to imagine that enough time passes so that there are two species that are clearly recognizable by the fossil evidence. This is only a matter of geologic time, so let’s suppose now that in one population, the rats grow quite large while in the other, they stay very small.
Here’s where the trick comes in, and evolution predicts gaps in the fossil record. Suppose now that the two bodies of land are reunited. Perhaps sea level drops, and there is now a land bridge. Again, the method doesn’t really matter. In any case, if our new giant rats come back across the land bridge, we can easily imagine that they might directly compete with the small rats for resources, and might drive them extinct within only a few generations. (We see this kind of competition all the time when we inadvertantly introduce species into new environments.)
Now, jump another few million years into the future, and let’s imagine that sea level has risen again, and an archaeologist is digging about on the original site. What will he find? He will see small rat fossils through one strata, and suddenly, all the small rats will disappear, and very large rats will take their place — with no intermediate fossils.
As you can see, it is precisely because some kind of separation is necessary for speciation, and because the geology of the earth is not constant, that we should not expect to find, anywhere in the world, a complete geologic column. All we will ever find is bits and pieces, and we must also reconcile ourselves to the fact that parts of the geologic record are gone forever, reconstituted into the earth’s core, or buried under the sea, or pushed upwards into mountain peaks, only to be eroded away by wind and rain.
Luckily, we are not dependent on the geologic column for our proof of evolution. The phyolgenetic record can be put together without any need for it.* That, of course, is a topic for another blog, lest this turn into several chapters of a book. Before I leave the subject of the geologic column, however, I should point out that we do have complete evolutionary records of several species as they evolve. When I say complete, I do mean absolutely complete in all ways. We have the specimens themselves, and we have their complete genetic data.
How can this be? Quite simply, they’re all still alive. These are called “ring species.” The best known of these is the Herring Gull/Lesser Black-backed Gull ring. If you look at each of these birds, they are quite distinctly different species. Even non-scientists can easily tell them apart. The thing is, if you start in England with a population of Herring Gulls and follow them around, population to population, you will journey around the North Pole to North America, across to Alaska, across Siberia, and back to England again. All along the way, you will find interbreeding groups, but as you travel westward from England, the Herring Gulls begin to look more and more like Lesser Black-backed gulls until you get to the middle of Europe where they are undoubtably not Herring gulls anymore. If you take one member of each “end group” and put them together, they will not breed. However, if you follow the chain either forwards or backwards from Herring Gull to Black-backed gull, you get a continuous chain of interbreeding populations. We literally can see a complete, living example of speciation through separation, with absolutely no gaps in the record at all.
- Herring Gull
* This is not to say that the geologic column is useless. Far from it. Together with the DNA record, the two help us form a very parsimonious explanation of specific events in evolutionary history. The geologic record helps us explain what we find in the DNA record, and vice versa. That’s how science works.
- Evolution: Watching Speciation Occur (scienceblogs.com)