BOBBIE MIXON: Typically, people learning of natural selection think of plants and animals evolving into other forms, as in a mole evolving to become an elephant, but there's another problem new learners rarely consider--that of serial blending. If one combines all the paints at a paint store by pairs over many iterations, everything eventually becomes the same color. In the case of human evolution, everyone would look the same. But, today, after thousands of generations, plants, animals and people are vastly different. But why? Mohamed Noor, professor and associate chair of Duke University's Biology Department, gives us his answer. Dr. Noor, welcome.
DR. MOHAMED NOOR: Thank you. It's a pleasure to speak with you.
BOBBIE MIXON: Tell us, what's serial blending?
DR. MOHAMED NOOR: So, serial blending in coming into it, Darwin's concept of gemmules, which is related to that, Darwin was a genius at working with the body of scientific knowledge that was available to him but he lacked in understanding heredity, how are traits passed on. As I note in my essay, Darwin clearly knew that parents transmit various traits to offspring. This is obvious when one looks at most families. My kids, heaven help them, look a lot like I do. But how this happened was unknown at that time. So, in his 1868 book, Darwin suggested that cells, and this is a direct quote, "throw off minute granules which are dispersed throughout the whole system and are developed into units like those from which they were originally derived." Darwin said, "These granules are then collected from all parts of the body and transmitted into the germ line in mating and passed on to the offspring and used to form the traits of this new offspring. If every offspring becomes effectively an intermediate between mother and father then, over time, interbreeding groups should all congeal into a single form." So, in the example you used with humans, eventually everybody would look the same and that would be just like combining pairs of paint in a store over time.
BOBBIE MIXON: But how would combing gemmules be different from combining paint? Would we run out of gemmules the same we would run out of paint?
DR. MOHAMED NOOR: Well, essentially what would happen with gemmules, unlike with paint, is you're actually introducing new variations from the environment directly. So, you know, for example, if you're somebody who--like a carpenter who hammers a lot, you're building up big muscles and your gemmules, you believe, would actually pass on this, you know, predisposition to having big muscles. So, you're having this introduction of new variation and that's why you don't run out of variation as in the paint example.
BOBBIE MIXON: How did Mendel's idea of allelic inheritance help us overcome the problem of serial blending?
DR. MOHAMED NOOR: That's a great question. With Mendel's concept, each offspring has two gene copies called alleles. One is derived from the mother and one is derived from the father, and the important distinction here is that the individual gene copies are transmitted without modification or merging to their offspring. So, for example, I have alleles that come from my mother and from my father and I will transmit one of those alleles, as is, to my offspring. So, I have a son and a daughter. My son might have a particular allele that is derived from my mother that was not in any way influenced by the copy that I got from my father. Do you see what I mean with that?
BOBBIE MIXON: I do.
DR. MOHAMED NOOR: Yeah. So, that's a very important distinction because with gemmules or with paint, you're actually having this sort of merging in the offspring and then what the offspring passes on is this kind of hybrid or intermediate form, whereas with Mendel's concept, you're actually passing on one distinct type of allele.
BOBBIE MIXON: You also write about hybrid forms. Let's talk about that for a moment.
DR. MOHAMED NOOR: Sure.
BOBBIE MIXON: Male donkey plus female horse equals a mule.
DR. MOHAMED NOOR: That's correct.
BOBBIE MIXON: Horses and donkeys are different species. How do they reproduce offspring?
DR. MOHAMED NOOR: Well, people often talk about species formation as though it's a single point in time, but it's really not. It's not typically that a single gene changes by mutation and--boom--you're a new species. Instead, species formation is typically an iterative process that extends over some time. Now, isolated groups can diverge in, for example, mating preferences or physiology. So, to make it more specific, imagine an isolated group of birds, they change at their gene copies to cause them to prefer the color orange while another isolated group may actually prefer the color red. Now, these changes reduce the potential for interbreeding if, you know, those preferences match their actual body colors, and the different colors will become favored in the two populations. Eventually, after you have a combination of many such changes, the two types will no longer interbreed or if it's a physiological change rather than a mating preference, then maybe they do interbreed but they form a sterile hybrid because they can't make viable gametes anymore.
BOBBIE MIXON: Well, if hybrid animals can't reproduce, how are new species created?
DR. MOHAMED NOOR: How do new species form? Well, evolution, if you think about it, is comprised of two distinct processes. You can have change within a lineage and you can have formation of new lineages. So, let me elaborate on both of those briefly. So, in change within a lineage, if you go back, for example, 60 million years ago, horses were about the size of dogs and over time, from that ancestor 60 million years ago until today, you had these iterative changes that said horses grew to their current height which is, you know, about a meter, meter and a half tall, something like that. Now, those horses are probably not reproductively compatible with the historical horses, the ones from 60 million years ago. Now, we never think of that in the context of new species formation because they don't encounter--they don't encounter each other. They don't use time machines, for example, and go back. Now, you also have--over evolutionary time, you have divergences where different lineages will form from a single lineage. So, horses, zebras and donkeys all shared a common ancestor some time ago but, over time, one of these lineages became the modern zebra, one became the modern donkey, one of them became the modern horse and we had these changes like the types I was talking about with respect to, you know, what happens over time with gene changes and those, ultimately, compounded to the point that you have these distinct species that you see today.
BOBBIE MIXON: That's Dr. Mohamed Noor, Duke University. Thank you for joining us.
DR. MOHAMED NOOR: Thank you for having me. I appreciate it.