What is Genetic Drift?

In reading some of the arguments Dr. Sanford gives for creation, I noticed that one of his main points is that most mutations are harmful (which is not true) and that even those mutations that aren’t harmful usually aren’t beneficial enough to spread through a population. But is this true?

First, we have to understand a bit about how mutation works. Traits are coded for by genes, which are made up of DNA. All DNA is composed of four base pairs: adenine, guanine, cytosine, and thiamine, commonly abbreviated A, G, C, and T. A mutation occurs when base pairs are added, deleted, or changed. These mutations can be positive, negative, or neutral (they produce no effect).

Another important piece of information is that every organism (technically every eucharyotic organism, but we won’t go into that) possesses two copies of each gene. Because most organisms are diploid (their chromosomes come in pairs) they have a copy of every gene on each chromosome. Further, these copies can be different from each other. We call these copies alleles.

Our alleles are completely separate from each other, so if a mutation happens on one allele, it doesn’t affect the other. And the way that genetic expression works is that if the two alleles of a gene are different from each other, only one gets expressed as an actual trait. The expressed allele is called dominant, and the other is called recessive. If both alleles are the same, then of course they both code for the same trait, so they both get expressed. This is the only way for recessive alleles to get expressed.

Many mutations are recessive, which means that even if an organism gets an extremely negative mutation, it probably won’t affect them at all, because it won’t be expressed. This means that mutations can be passed down from generation to generation, and as long as only one of the alleles is mutated, everyone will be fine.

Now, let’s see what happens if an organism acquires a recessive beneficial mutation. If they reproduce, their offspring each have a 25% chance of also inheriting that mutation. Let’s say this organism is a rabbit, and this rabbit has a hundred children over her lifetime (not an excessive estimate at all, by the way). This means that she’ll have 25 children with a mutated gene.

Each of these 25 offspring can each have 100 offspring of their own, which can then have 100 offspring, and so on. We see that very quickly, mutations can accumulate in the rabbit population. Also notice there is no selection pressure. This mutation doesn’t do anything. It spreads through the population entirely through statistics.

This is genetic drift. Genetic drift is a powerful force that doesn’t even require natural selection to spread mutations. It’s also completely ignored by Dr. Sanford, to his detriment. Genetic drift allows mutations that may only be slightly beneficial to spread through the population, even when natural selection has little to no impact on that spread.

One of Dr. Sanford’s major points is that those rare beneficial mutations don’t give enough of a genetic advantage to take hold in the population. As we’ve seen, that’s completely untrue. As a geneticist, Dr. Sanford should know better.

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Transitional Forms – The Case of the Fish-ibian Part Two

This is a guest post by Dr. Pennilyn (Penny) Higgins. Dr. Higgins is a Research Associate in the Department of Earth and Environmental Sciences at the University of Rochester. This post originally appeared on Paleopix. Part one originally appeared on September 13, 2006, at the Skeptical Inquirer.

Maybe it’s just that I’m paying more attention to it these days, but it sure feels like there’s a lot of talks, and legislation, and debates, regarding the concept of “Creationism” in contrast to “Science.” Creationism is this idea that the Earth and everything on it were created by some supernatural means. There are lots of religions what include some manner of creation story. After all, what is more human than wanting to know where you came from?

But when the discussion of Creationism come up, it’s not just any creation story that’s being discussed. Instead, it’s the version of creation that is in the Old Testament of the bible. The first chapter. Genesis. This is the one where God created the Earth and everything on it (including us) in seven days, then did something stupid and God sent us off. Then we did something stupid again, and God wiped us out with a flood. Creationists claim that the origin of the Earth and the universe happened literally as is described in the Bible. Everything was created in seven 24-hour days, and all this kicked off some 6000 years ago. The great flood happened a bit later, maybe 4000 years later.

One of the major arguments put forward by creationists includes the concept of the ‘kind.’ Two each of each ‘kind’ of organism, one male and one female, were sheltered on Noah’s Ark during the great flood. These pairs then went off and formed the basis of the diversity of life as we know it today. Obviously, it would be impossible for two each of every species on Earth to fit on the Ark, hence the concept of ‘kind.’ You only need one pair from the dog ‘kind’ to reproduce to get dogs, wolves, foxes, coyotes, etc. You have the horse kind, and the pig kind. There’s the falcon kind and the owl kind. The bee kind, perhaps.

The argument goes like this: The different kinds are distinct. You can’t have pigs and horses reproducing. Creationists contend that the kinds remain distinct all the way back until the time of creation, thus there is no evidence for the scientific theory of biological Evolution. You not see horse-rhinos, nor pig-deer, and this is thinking about animals that are seemingly closely related to each other. And, according to creationism, you most certainly would not see intermediate forms between larger groups of organisms, like between fish and amphibians, or between reptiles and birds. And in modern times, we don’t really see obvious transitional forms. There aren’t any repti-birds. You might argue that a lungfish is sort of land dwelling, since it will ‘walk’ across land from pond to pond, but it’s still pretty clearly a fish.

This is where people like me come in. The fossil record provides an abundance of transitional forms, if we care to recognize them. I’m a paleontologist. I study these things. It’s really fascinating.

The first thing to realize when looking for transitional forms between modern groups is that what we’re looking at in the world today is the end-product of millions of years of Evolution. When the transitions were happening, organisms didn’t necessarily look like they do now.

Let’s consider the fossil record for the fish-amphibian transition. This happened some 375 million years ago. I’ll point you first to an article I wrote some time ago. There’s a lot more information in there that I don’t feel the need to reiterate. What’s important is this: The fish lineage that led to amphibians weren’t anything like the fish you’re probably thinking of. They didn’t look like salmon, or like sharks. They were clearly fish, of course, with fins and gills and all the requisite fishy-parts. But the fins were a little different. There was a pair of fins toward the front of the fish, just behind the gills, and a pair much further back. These fins had an internal skeleton quite different from trout or sharks. They had some sturdy bones within them that are known to be homologous (or sharing the same developmental origin) as the femur and humerus of your legs and arms. Yet, there were no hands or feet. The fins ended in the typical fin rays just like a trout might. But because there’s a bony skeleton, the fins looked like they were out on lobes. Hence, these fish are called ‘lobe-finned’ fish. There are some lobe-finned fish alive today, the lungfish and the coelacanth, though none of these are quite like the 375 million-year-old fish we’re talking about.

The earliest amphibian was actually quite fish like. It’s skeleton was not strikingly different than that of the fish it came from. It didn’t look at all like what we call an amphibian today. What distinguishes the first amphibian from the fish it came from are 1) the presence of hands and feet, which are startlingly easy to develop when one considers how hands and feet develop in modern vertebrates (that’s a whole other article), 2) the loss of the opercular bones, which are specialized bones in fish that protect the gills, 3) the ability to look up, 4) the disconnection of the fore limb from the head (odd as that sounds), and 5) the connection of the hind limb to the vertebral column. These are all adaptations necessary to get around on land effectively. None of these require the lobe-finned fish to suddenly look just like a salamander or a frog. No, it can just look like a fish with feet, that lacks gills and can look up.

We have fossil examples of both the ancient, ancestral lobe-finned fish, and of the earliest amphibian. However, Creationists still cry out saying that we have not yet identified a proper transitional form. Well, we hadn’t until we found Tiktaalik.

Tiktaalik is a fabulous little fish discovered on Ellesmere Island in the Canadian High Arctic. I lay out the details of what makes it such a great transition in this older article I wrote for the Center for Inquiry, but I can outline the important things here.

It’s fish-like (in fact it’s called a fish) because it lacks hands and feet. It still clearly has fins. It was a swimming animal.

It’s amphibian-like because it lacks the opercular bones and could look up. It lacked the bony covering that fish have to protect their gills. This also means that its front legs were free of its head. It was capable of getting around pretty well on land.

When all this was first described (and when I wrote that earlier article), all that was known of Tiktaalik was its front end. Well in the last year, its hip bones have been discovered and described. What researchers found was a solid hip joint for supporting the weight of the animal, and that the hip itself was attached to the vertebral column.

Tiktaalik is exactly what one would predict for a transitional form between fish and amphibians. It’s a mosaic of clearly fish-like features with clearly amphibian-like features. That it’s classified as a fish merely reflects the fact that its life was probably most dominantly one of swimming. The classification schemes that we use to parse out modern organisms into convenient groups (like amphibians, fish, mammals, etc.) fail miserably when dealing with transitional forms way back in the fossil record.

When you ignore the artificial classification scheme that we as humans apply to organisms, and just consider the traits present in the various organisms, Tiktaalik stands out as an intermediate form between an animal adapted for life in the water and one adapted for life on land.

Tiktaalik is one of many examples of transitional forms known from the fossil record. We have shown the relationship between dinosaurs (reptiles) and birds (and Archaeopteryx is only part of that story). We understand the transition from a four-legged mammalian land-carnivore to what we see now as whales and dolphins. Transitions between the various ‘kinds’ of the bible (which, incidentally, aren’t very clearly defined, though Ken Ham was kind enough to try to define them as ‘about the same level as Family in taxonomy’ when he debated Bill Nye last month.) can be predicted, and then found in the rock record.

Transitions between kinds are not predicted by a literal interpretation of the New Testament of the Bible, upon which the idea of Creationism is based. Since we do have at least one clear transitional form (Tiktaalik), proponents of Creationism must provide an explanation.