The Importance of Development in Evolution 
The Importance of Development in Evolution
by Yale / Stephen C. Stearns
Video Lecture 7 of 36
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Date Added: November 6, 2009

Lecture Description


Development is responsible for the complexity of multicellular organisms. It helps to map the genotype into the phenotype expressed by the organism. Development is responsible for ancient patterns among related organisms, and many structures important to development shared by many life forms have changed little over hundreds of millions of years. Development is expressed combinatorially, allowing a relatively small amount of genetic information to be expressed in many different ways.



Reading assignment:


Stearns, Stephen C. and Rolf Hoekstra. Evolution: An Introduction, chapter 6




Transcript



January 28, 2009



Professor Stephen Stearns: Today we're going to talk about, or we're going to introduce the role of development in evolution, and I would like to start by asking you to make two jumps in your head. In the first case I want you to think of yourself as inside a single-celled bacterium. If you go to a good book on the gene networks and the biochemical networks that can be found inside a single-celled bacterium, you will be stunned by the complexity of it and you will deeply hope that you never have to reproduce it on any examination, because there is so much of it. Okay?



That's inside one cell. Development doesn't arise until we get to multi-cellular organisms. And if you were able to go into the body of a multi-cellular organism, such as yourself, and look at all the signaling pathways, all the way that information is transferred and integrated in a multi-cellular organism, it's just as complex as the whole picture down inside that one single-celled bacterium.



So there are two huge kinds of orders of magnitude, levels of hierarchical complexity of information integration that happen in a multi-cellular organism like yourself. We call the way that the information in the genes maps into the structure of the genotype the genotype-phenotype map. It takes us through all of that complexity to produce something that we can then try to understand, which is a whole organism. Another name for the genotype-phenotype map is development. Okay? And what I'm going to try to show you today is that out of this almost unimaginable complexity, two hierarchical layers of orders of magnitude of information, biologists have been able to extract some interesting simple rules and show that there are some large-scale patterns in evolution.



However, that task is far from done, and the understanding of the genotype-phenotype map, or the understanding in general of developmental biology, remains probably the most pressing issue in basic research in biology for the twenty-first century. It's something that occupies some of the best faculty in this department and some of the best scientists across the globe. So it's a basic issue. And the thing that I hope you take home from this lecture today is that it's important, both for microevolution, and for macroevolution.



Today I will be talking about patterns that are pretty large scale, they are more macroevolutionary; and next time I will be talking about patterns that arise within populations that reflect that macroevolutionary history, but that have immediate consequences for microevolution. So development isn't simple. It's going on both at large scale, over long periods of time, producing patterns, and it's going on at a very, very short scale in every generation, as each individual grows up and turns into an adult.



So what's involved in development? It isn't really just the production of the adult form from an egg. It is the living of the entire lifecycle, from the formation of the gamete, through the adult, through all the changes the adult goes through until it dies and produces the next generation. So development refers to the entire lifecycle, and evolution shapes the entire lifecycle.



So something that we get from a very important discovery in nineteenth century biology is that all of life is made of cells. It could've been organized differently, and in fact there are interesting science fiction novels, like Solaris, by the great Polish science fiction novelist Stanislaw Lem, that conceptualizes what life would be like if it were not cellular. For example, what if the whole ocean were one living thing? But we know that that's not the way life is on our planet.



On our planet life is all built out of cells, and that means that the problem of development is a problem of communicating between cells. And cells are all set up as information signalers and receivers. They have cell adhesion molecules on their surfaces. They produce information molecules, hormones, and other signaling molecules for export. This information is used to change the fate of a cell.



Now every cell in your body has all the information in it that's needed to build you, and that is true of virtually every organism. The only exception in us is our red blood cells because they don't have any nuclei, so they don't have any DNA in them. Okay? But in every other organism that we know of, with minor exceptions like our red blood cells, all the information is in all the cells, and that means that development is a matter of editing, it's a matter of determining which information gets turned on in the right place at the right time. So the evolution of development is about shaping those patterns in space and time, within the framework of an organism, to produce something that works.



Development does a bunch of things in evolution. One of the important ones is that it has a strong role in the course of the production of individual organisms in shaping the kind of variation that is presented to selection. Okay? So the developmental mechanisms that are shared by particular organisms determine that only certain kinds of phenotypes are going to be produced--and there's a lot of variation within those phenotypes--but they are a tiny portion of phenotype space.



And this is why we actually see, morphologically, the Tree of Life. This is why dogs look like wolves. This is why humans look like chimpanzees. This is why birds look like birds and we can call them birds. It's because they share developmental pathways that have been inherited from ancestors and that have constrained the range of phenotypes that can be presented to selection.



Now there's another important thing about development. You might think that you could conceive of the body as produced by an engineer, but it's not really constructed that way in evolution. What goes on is that genes can only build organisms out of the materials that are available at a certain time, and then there is an evolutionary memory, of which materials are selected, and of the control systems that are used to shape the phenotype with them. Now I'll give you a couple of examples.



What is the cell membrane? The cell membrane is a lipid bilayer, and it's been--it's actually a marvelous organ now; it has all sorts of special channels in it, things that are filters to let particular stuff in and keep other stuff out. It's been heavily modified by evolution. But there is no way that you can take simply the DNA sequence in the genome and get a reaction system that's going to construct cell membranes. All known cell membranes are actually constructed biologically by using pre-existing cell membranes as templates. In other words, the cell membrane itself is an information transfer molecule. So that's one.



There's another, and that's bones. Your bones are made out of a material called hydroxyapatite, which is a calcium phosphate material. And hydroxyapatite has the following extremely convenient feature. If you take hydroxyapatite and you put it under stress, it will strengthen itself in the direction of the stress. That means that the genes don't have to have sensor systems to detect stress, and they don't have to worry about how they're going to make a bone strong in the direction of stress. All they have to do is say, "Hey, I'm going to use hydroxyapatite to make bones, and then when that baby first starts to toddle around and walk on its legs, its hips start getting strengthened in the direction of stress."



Now, in fact, there are modifier genes that then take this and use it to their own advantage, by building in protein molecules that strengthen the bone in the direction of the stress. Okay? But the initial signal, which direction is the stress coming from, is a freebie. It's given by the biochemical properties of a hydroxyapatite. So that's one, a second one.



Here's a third one, gastrulation. When vertebrate embryos, and many other embryos, grow, they grow up as a ball of cells which then becomes a hollow sphere of cells, and that hollow sphere of cells, which by the time it gastrulates has thousands of cells in it. You can think of it as a little pulsing basketball. Okay? It's a little sphere that's pulsing, and if you simply let this thing grow to a certain size, the tension in the actin tubules in the cells will cause it to spontaneously invaginate. So it'll get a dimple in it, like you pushed your thumbs into it.



This happens spontaneously. It is not as though the genes have to say, "I am going to construct a mechanism that's going to make my gastrula form. When my blastula turns into a gastrula, when my hollow ball of cells turns into a thing that's got a dimple in it, and then forms three cell layers, out of which I can make muscles and bones and skin and gut and all of that kind of stuff, when that happens that's a freebie." Okay? That is just in the tension of the actin filaments in an expanding ball of cells.



So these are some of the properties of the biological materials that organisms are constructed out of, and it means that on the one hand the genes don't have complete control over the phenotype, but on the other hand they are given certain things by the materials that don't have to be specified in the DNA sequence.



So where does development fit? I'm giving you real large-scale messages now. Okay? The biological disciplines that we call Ecology and Behavior actually deal with the processes that reduce the cohort of newborn organisms to the ones that survive to reproduce. Okay? Ecology and Behavior actually study the mechanics of natural selection, while they study a lot of other stuff as well. But that's the level at which that happens.



Genetics takes the genotypes of the parents and it transforms the genotypes of the parents through Hardy-Weinberg equations, through the selection that's operating on them, through all of that stuff that we just looked at quickly. It takes the genotypes of the parents and transforms them into the genotypes of the offspring. Okay? So genetics is all about information transfer. Lots of people that like computers do pretty well at genetics.



What development does is it takes that information in the genotype and it maps it into the material of the phenotype. You can think of development as being a big transduction mechanism that takes material--takes information and turns it into material. Okay? And, in the process, it places limits on what the phenotypes can look like, so that not every conceivable phenotype is going to arise out of the DNA sequence in a genome; only a certain range. Flies are going to look like flies, sheep are going to look like sheep, and daffodils are going to look like daffodils.



If we look at what development has been able to produce, well. in a large-scale it has produced some very basic things, and we can see that by a comparison of the body plans of the major groups of organisms. I'd just like to take a moment here and see if I can elicit from you some sense of what it is that we're looking at. Okay? I assume that you're all pretty comfortable with chordates, because that's what you are. Okay?



Can anybody tell me what a bryozoan is? A bryozoan is a moss animalcule; it is a moss animal. Bryozoans produce beautiful exoskeletons, and you can find them on tropical reefs. Anybody comfortable with what a priapulate is? A priapulid is a deep-sea worm that forages with a tentacle and when you pull it out of its hole, it looks like a penis; which is why it's called a priapulid. Okay? What about a tardigrade? Tardigrades are little water bears. Okay? They look a little bit like insects or crustaceans, and they're very tiny, and they're extremely cute. Okay? So tardigrades are water bears. Arthropods, who's in the arthropods? Insects are arthropods. What are the other big groups of arthropods?



Student: [Inaudible]



Professor Stephen Stearns: Crustaceans; yes spiders, spiders and their relatives. Arthropods are anything with jointed legs--that's just Greek for jointed leg, arthro-pod. Pogonophorans? A tiny phylum of worms that live in the sand and are living fossils and haven't really changed their morphology for about 400 million years. So there's a lot of big-scale stuff here.



This is basically the animal kingdom. Okay? And it's formed into these groups. And just to give you a little bit of timeline, it's about 600 million years, 700--Bilateria, yeah, six or seven-hundred million years ago. A lot of stuff happens really quickly, because from this point to this point here is only about a hundred million years. At this point we're still 500 million years ago. Okay? So this is big-scale stuff. The cnidarians and the ctenophores formed trace fossils in the pre-Cambrian ooze. So they may go back a billion years; not sure about how far back.



So if you look at what happened when multi-cellular organisms formed and one branch of them went off to become animals, this is what development was able to produce. It could produce body axes--front and back, left/right; produce a skeleton, organ system, symmetry and cell layers. And figuring out the shared general mechanisms by which development can produce those things, and then how evolution can tweak them to make them different in these different groups, is evolutionary developmental biology, or evo-devo.



There are some big and striking differences among these groups. For example, this group up here has an exoskeleton, and this group down here has an endoskeleton; and that places very fundamental constraints on growth, size, all kinds of things.



What can it do in plants? Well here is an extremely sketchy view of the plant world. And, by the way, I would've used a much more complex view of the plant world than this one, but this is the one which is publicly okay without copyright protection. Okay? So for you plant biologists that are worried about the view of the plant world, this is pretty simple.



Basically what this does is it takes you from ferns and their relatives through cycads, ginkgoes, pine trees and fir trees; the gnetophyta, which have some really cool plants that live in Namibia; Wellitschia and other things like that are gnetophytes. And then the magnoliophyta are all flowering plants, going out here. So this is a huge group down here. And if you look at that, what has development and the evolution of development been able to produce? Well these guys are all variations on the following themes: a meristem-root axis; where xylem and phloem appear; where wood appears; the kinds of branching patterns they have; whether they have naked or covered seeds; whether they have leaves; and whether they have flowers.



So you can see that the image that I'm trying to create here, in both plants and in animals, is that there's certain shared general features, and that what evolution has done is that it has made many, many different combinations of those features, to create the diversity that we see, and that this is done through the evolution of development.



Mechanistically a lot of this is going on at the level of gene regulation; and this is a recap of the structure of the eukaryotic gene. And the parts that I want you to focus on right now are the promoters and the enhancers. So these are parts of the DNA molecule that receive a signal, which says turn this gene on or turn it off. And I want to show you just exactly what kind of a network of information that results in.



You can think of there being regulators--and we will talk a little bit about what kinds of regulators are used early in development--and that these then feed into a signaling cascade where a signal goes out, there's a receiver, there's a transducer that turns that signal into a transcription factor. A transcription factor then is going to go out and it's going to bind to the enhancer region of a gene, or a promoter region of a gene.



Okay, so it's bringing in a signal. The signal might be either turn it on or turn it off, but it's bringing in a signal; that's what a transcription factor will do. You can think of there being other pathways and other signals going through them, and they can turn on transcription factors, and some of those will come down and sit down on the same gene. It isn't all only one transcription factor that can sit down on one gene.



In an average gene in Drosophila there are about ten to twenty binding sites in its control region. That means that ten to twenty different transcription factors can sit down on a single gene, and one transcription factor can bind to the control regions of anywhere from one gene to several hundred genes. Okay? There are about 13,000 genes in a drosophila genome.



So that gives you some idea of the array of possibilities of turning all those things on or off, as you go through. That is a huge array of possibilities, and I think you'll see, when I show you later, how you take an onychophoran worm and turn it into a Drosophila, that you could imagine evolution might have had to use a lot of them, in order to turn something like a worm into something like a fruit fly.



There is another way to think about this. Think about the control region of a gene as the keys on a piano, and think about the transcription factors as the fingers on the hand of the person who's playing the piano, and think about evolution as the composer who wrote the score. You know perfectly well that you can play all kinds of tunes on the keys of a piano, and you know perfectly well that there are traditions in music of different kinds of related music. So anyone who has listened to Bach is probably going to recognize Telemann, and people who have listened to Schoenberg are going to recognize more modernist composers.



So you can think of those as being clades, and you can think of the cultural tradition as being inherited, and there's constraining variation within the range of scores that are composed on the piano. Of course, what we're dealing with in a genome is like the biggest symphony orchestra you ever saw in your life. Okay? So it's much, much bigger than that.



So a few points about the control of development. At the beginning of development, when the first cell is getting set up to divide, and in the formation of the very early multi-cellular stages, there are concentration gradients that are produced. So before the first cell divides, there will be like a front end and a back end of the cell, and chemistry will get set up to produce molecules that then form a concentration gradient across the cell, and the concentration of those molecules is positional information on what's the front and what's the back. And as the cell divides, it retains that information on where it is in the front or the back.



That's how the Drosophila embryo is set up. And we will also see that this kind of concentration gradient is used in the construction of the vertebrate limb. By the way, the signaling center on the vertebrate limb, when it's just a little paddle of cells, is basically in the armpit. Okay? So if you want to think of smelly molecules being produced, think armpits.



What then happens is that transcription factors are used to define specific areas where only a precise subset of genes is expressed. So remember, all the info in the whole genome is in every cell. You only want a certain subset for this part of the organism that you're making. So the gradient, the chemical gradient then sets up gene expression, and the transcription factors appropriate to that position get turned on.



Genes are regulated by combinations of activators and repressors, and this combinatorial control is what gives you the huge diversity of cell specific gene expressions. When I say combinatorial control, think of composers writing notes and people playing pianos; that's combinatorial control too. All the music that's ever been written, that can be played on a piano, is simply a variation on all the combinations of those keys in space and time.



So, the control can get complex. It can be a cascade of information, and genes that produce transcription factors can be regulated by genes that produce transcription factors. And this sets up situations where genes can switch their roles. It is not correct to think that there are some genes that are early development genes, and then there are other genes that are adult genes.



Genes are used flexibly, in many different contexts, depending upon the information that's coming in to regulate them. Certainly there are some genes that are quite important in the embryo, but it turns out they also play a role in the adult. We'll see a case of this a little bit later.



Not surprisingly it's the genes that determine the general pattern that switch on first, and the ones that are controlling detail switch on later. So when I say 'general pattern' I mean--for example, in the vertebrate embryo, the front and the back, the top and the bottom, the left and the right: that gets laid down first.



Then the embryo gets chopped up into a sequence of segments. Some of them turn into the head, some of them turn into leg segments; some of them have extremities, some of them don't--that kind of thing. Okay? So this sequence of how the early general pattern gets set up, and then how the details are developed, that's all developmental genetics, and that's produced in evolution a whole lot of stuff.



Now, a little bit of vocabulary. You're going to hear, in this area, about boxes. Okay? You're going to hear about homeoboxes, MADS boxes, stuff like that. I want to tell you what boxes are. They are very highly conserved sequence motifs, and they are found in the DNA that codes for a particular family of transcription factors.



The reason that this sequence--by the way, it's about I think--I think it's about--I'm not sure whether its seventy to eighty codons, or seventy to eighty nucleotides long. But these boxes are not too long. They're conserved because they have a very important function, and that is to bind to the DNA. So they have a helix twist, helix structure, and that means that if the DNA molecule is here, this part of that protein, this part of that transcription factor, is going to fit right into it.



And it's because they are transcription factors--the boxes are found in transcription factors--that this is a very conserved interaction. Because DNA hasn't changed its structure in three billion years. So if they're going to bind to it, they have to have that structure, and so selection has made sure that that sequence is preserved.



They're called boxes simply because if you lay these DNA sequences out, if you sequence a lot of DNA, and you are looking for one of these things, what you find is that wherever there's a transcription factor, there is a stereotypic sequence. And the people who were analyzing this, first on computer printouts, or now with imaging on computer screens, drew boxes around them, to locate them. That's why the word 'box.' Okay? So when you see one of these in a DNA sequence, you know you probably have a gene for a transcription factor.



Here's the homeobox family. Okay? There are thirteen homeobox genes that have been identified. And this is from a number of years ago; this has probably been filled out now. And they have two--well there's more than two striking things about them. But the first thing that's striking about them is that they're deeply conserved. That means that they have retained so much of their sequence identity that you can recognize homeobox gene 1 in a human, and you can see that the same gene is there in flatworms and in earthworms, in priapulids and so forth. All the way through the animal kingdom this gene has been conserved, and you can pick up something like it in cnidarian.



And you wonder, well how did there get to be thirteen in this family? Well here you can see a gene duplication event right here. Homeobox gene 1 in the jellyfish and corals was duplicated right here--this was the expansion of the central HOX genes--and it happened here as well, so that now there were two copies. And that meant that this developmental control switch, which was an extremely clever piece of machinery to have around, now existed in two copies. You could use the first one to do whatever it used to be doing, and you can now evolve a new function for the second one.



This went on up until the time that the vertebrates started to evolve. And in our closest relatives there's one copy of the HOX gene, and it was duplicated twice. It was duplicated at the level of the Agnatha; so the ancestors of the sharks. And that means that all of the vertebrates, the higher vertebrates, have four sets of developmental control genes. Interestingly, the first set is still used to lay down the major body axis, and the fourth set is used to make a limb; that's the new function. Okay?



Now they have deeply conserved sequences, but they are also collinear. By collinear, I mean this. Look at the sequence on the genome, and look at what part of the body is controlled. The parts that are on one end control the head area, the parts on the other end control the tail area, and the parts in the middle control the stuff in the middle of the body.



There isn't any logical reason why it had to be this way. It's probably simply that when vertebrates first started--well when animals, prior to vertebrates, first started to get formed as multi-cellular things, this happened to be one convenient way to control development. But logically speaking, giving the signaling apparatus that's available in the genes, there's no reason logically that the genes have to be collinear; but it's a fact that they are, and it's a marvelous fact that they are. Okay?



This is just to show you that you can take homeobox genes and look at their DNA sequences and say, "Oh, they have similar DNA sequences." And then look at what parts of the bodies they control and see that a homeobox gene in a fly, that is homologous to a homeobox gene in a mouse, is controlling a similar part of the body.



So the things that are controlling the tail end, the green genes here, are expressed in this part of the fly and in this part of the mouse, and the things that are controlling the head end, that are expressed here in the fly, are actually expressed here in the mouse. That's kind of interesting, because it suggests that the mouse has added on some stuff that is in front of the hind brain.



So here's the vertebrate limb. Okay? And this is controlled by the fourth copy of the HOX genes, the D copy. And it shows you that if you have, for example, D9 being the only one that's turned on, you get that. If you have D9 and D10, you get that. You have D9, D10, D11, you get that. You have D9 through D12, you get that. And you get all five of these on and you get that. Okay?



So basically what this means is the following. If you just have D9 on, make a shoulder; D9 and D10, make a humerus; D9 through 11, make a radius and ulna; D9 through D12, make a wrist; and all five, make fingers. How simple, how logical. Remember when I said at the beginning we have these orders of magnitude of complexity within cells, and these orders of magnitude of complexity between cells. Out of all of that complexity, this simple pattern emerges.



Oh, and I forgot, notice that the genes are collinear in the limb. Okay? The ones that are on one end of the gene are controlling the shoulder, and the ones that are on the other end of the gene are controlling the fingers. So it's still collinear. It's the like the body axis, it's just been translated into a limb.



Well what about flowers? The MADS genes also have a sequence in them which shows that they're a transcription factor; there's a MADS box. M-A-D-S is an acronym for the original names that these genes had. Okay? Some of them started with an m, some with an a, some with a d, some with an s. Then after all that had happened, it was noticed that they were related. So people started calling them MADS genes.



They're scattered throughout the genome. They are not collinear. Okay? In Arabidopsis, they're on all five chromosomes. There's nothing resembling the HOX genes in the way they're genetically organized. They fall into three groups: the A group, the B group and the C group. So the A is not a single gene, it's a group of related MADS genes. B is another group of related MADS genes.



And within each of these groups the genes are sharing phylogenetic relationship. That means that the members of the A group are probably all duplicates of an ancestral gene; the members of the B group are probably duplicates of an ancestral gene for the B group; and so forth.



Now the neat thing about the MADS genes is the way they control flowers. And we're going to see that evolutionary developmental biology has a lot to do with the production of beauty. [Chuckles] Two of the best understood examples in evolutionary developmental biology are flowers and butterfly wings. Okay? So this is an area where researchers who go out to give talks get to use a lot of neat slides.



The ABC model of flower development goes like this. If only a gene from Group A is turned on, make a sepal; if only A and B are turned on, make a petal; if only B and C are turned on, make an anther; if only C is turned on make a pistol and an ovary. So the regulation of B and C is controlling male and female organ development. This is combinatorial. Okay? It's the same general logical principle.



However, it's with a completely different set of genes, and plants evolved multi-cellularity independently of animals, which means that plants invented development in evolution independently of animals. They both hit upon combinatorial control as a simple, logical way to control development. That probably means it's a very good idea. Okay? It's a very simple and economical way of expressing information.



Of course every gene has a history, and these MADS genes were doing something before they made flowers. In fact, if you go back and you look at the homologous genes in the plants that do not have flowers, you discover that in ferns they are controlling leaf development, in conifers they are controlling cone development; things like that. It's not as though these genes were invented in order to make flowers. They were pre-existing, and they were co-opted by evolution at the point where flowers started to evolve; and gene duplications probably helped in that process.



So if we go back to the Burgess Shale, back to the Cambrian, we find lots of onychophorans running around. And if you go to an Australian rainforest today, you will find onychophorans still running around, and they look the same. So 500 million years of evolution hasn't changed onychophorans. onychophorans are these neat, velvet worms. They are, by the way, viviparous. If you pick them up they squirt glue on you. They have their own neat little biology.



They are the ancestors of the arthropods. So what evolution basically did was it took an onychophoran and, among other things, it turned them into fruit flies, and butterflies, and horseshoe crabs and king crabs and lobsters and shrimp.



How do you do it? Well it was done basically by changing the range of segments in which particular things were expressed. If we go back here, you can see that the onychophoran has a lot of segments. It's got one leg on each segment. Okay? The fruit fly has many fewer segments and only six legs; the onychophoran has fifty legs or so.



So basically what was going on is that the HOX genes were used to say, "Oh, okay, now we're going to say that these segments become a head, these segments become a thorax, these segments become an abdomen. We're going to make antennae on the head, and we're going to make wings on the thorax, and we're going to make legs on the thorax, and the abdomen isn't going to have any wings or legs." Okay?



The way this initially happened--and you can find fossils that replicate some of these stages--you first make a generalized segment, it's got both legs and wings on it. Okay? You make it many times. So you've got both legs and wings on lots of segments. Then you restrict the range of expression so that only certain segments have wings, only certain segments have legs.



You do that by altering the domain of expression of control genes, and you use that kind of combinatorial specificity to say, "Okay, this is an antenna and not a leg. Okay, it's an appendage growing out of the body wall, but I'm going to make it into an antenna that's on this segment and I'll make it into a leg if it's on that segment."



So, of course, this goes on in seconds; evolution took hundreds of millions of years. So it's not the same process. Some of these HOX genes have retained incredibly conserved functions. In a famous experiment that was done in the 1990s, Walter Gehring's group in Switzerland, took Pax6, which is a gene that's shared by all bilateral organisms, and they genetically engineered fruit flies with an extra copy of Pax6--it is a gene that induces the development of eyes--and by turning this gene on, they were able to make that fruit fly grow eyes in unexpected places. Okay? So it could grow one on its--this is the regular eye; this is an eye growing on an antenna; this is an eye growing on a haltier; and so forth.



The interesting thing was that they could do this with Pax6 from a human or a mouse; in other words, the DNA sequence in the gene was so similar that it could be used to control the developmental pathway in an organism in which that gene had not been sitting for 600 million years. That's pretty remarkable.



Development is not easy to evolve, and I think this gets across one of the reasons that it's not easy to evolve changes in development. Every organism has to function and reproduce in order for a gene to get transmitted, and you can't tweak its development around too much or you'll make it fall apart. It's like you're driving down the road and you want to turn your Volkswagen into a Mercedes Benz, and so you get out your tools, and you're going sixty miles an hour, and you want to modify it, but you can't crash. Okay? So that causes constraints; there's only certain things that you can do while you're moving down the road.



These developmental constraints are not permanent. The genetic control of development does change more slowly than many other things, but I would submit to you that if we wiped out everything on the planet--let's first duplicate earth ten million times, and then let's go through and wipe out everything on all of those ten million planets, except for one species, and we leave it some food. Okay? It's the only thing that's there.



But on some planets all you've got is fruit flies. On other planets all you've got is redwood trees. On other planets all you've got is butterflies, and on other planets you've got albatrosses; but they have a food supply, they can live. Give them long enough, go away in your spaceship, come back, five billion, ten billion years later. I would submit to you that every one of those planets is going to have highly diverse life on it, and that many of the things that we see on this planet you will see on each one of those other planets.



They will contain a signature, probably a very interesting signature, of this huge disturbance that has been created on them. But I think that it's possible for redwood trees to evolve into squid. I just think it takes them a very long time. [Laughter]



The things that change slowly constrain things that change rapidly, and genes don't cause development by themselves. They're steering the dynamics of gene products that interact with environmental inputs. So the genes actually are a fair distance away, biochemically, physiologically, from the things they're controlling. They are working through complicated interaction systems.



So, a few take-home points. Development maps the information in genotypes into the material of phenotypes. It's like the process that occurs when a blueprint that an architect has drawn is turned into a building by the construction company.



Developmental control genes use combinatorial logic. There's a lot of other stuff that's really important in evolutionary developmental biology, besides combinatorial logic. It happens to be a pet topic of mine. You will find that if you talk to people who do this for their profession, that they regard combinatorial logic just as so natural, so much a part of the landscape, that they hardly feel the need to mention it anymore. But it is striking, when you compare plants and animals, that they both hit upon this method of controlling gene regulation.



The ancestors of currently existing organisms often had an awful lot of the genes that are now involved in controlling development. Remember that phylogenetic tree I showed you for the HOX genes. Many of them are present back in jellyfish, and certainly many more of them are present in worms and crustaceans and things like that. So a lot of what has gone on in evolution is changing the specificity of expression in time and space, and the specificity of receptors in time and space. Rather than necessarily evolving new genes that make new kinds of proteins, a lot of evolution has been concerned with making combinations of existing genes.



Interestingly, there is a very good evolutionary developmental biologist at Wisconsin. His name is Sean Carroll. He is a charismatic guy. And Sean has gone so far as to say that most of evolution, in the last 500 million years, has consisted of the evolution of gene regulation, rather than the evolution of new structural proteins. And, of course, by taking an extreme stance, he has managed to generate a controversy in which people are saying, "Oh no, it's not just that. There are all of these other new structural proteins that are going on." And this is always very good for one's scientific career, because both sides are increasing their publication rates.



Now, in fact, both things have gone on. Okay? But by having a controversy, people get motivated to pin down the details. So I am making a little bit of fun of controversies, but I also recognize that they are very strong motivating forces.



So next time, we're going to talk about the expression of genetic variation and reaction norms. And I want you to remember today's lecture, because today I have talked mostly about the big macro picture; the impact and the patterns of developmental mechanisms in the Tree of Life, in all plants and animals. And next time we're going to see what difference they make within single populations and in the course of the lifetime of single individuals. This is one way that microevolution connects to macroevolution; it connects through development. Lunch is possible if you would like it.



[end of transcript]

Course Index

Course Description


In this course, Stephen C. Stearns gives 36 video lectures on Evolution, Ecology and Behavior. This course presents the principles of evolution, ecology, and behavior for students beginning their study of biology and of the environment. It discusses major ideas and results in a manner accessible to all Yale College undergraduates. Recent advances have energized these fields with results that have implications well beyond their boundaries: ideas, mechanisms, and processes that should form part of the toolkit of all biologists and educated citizens.



Course Structure:

This Yale College course, taught on campus three times per week for 50 minutes, was recorded for Open Yale Courses in Spring 2009.

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