What is Evo Devo?

Fruit Fly Embryo

"Evo Devo". You might have come across this phrase in various articles and comments concerning evolution, but what does it mean? The term is short for evolutionary developmental biology and it is radically changing our views on almost every facet of evolution. The main idea behind this new field is that changes in certain areas of the genome during the development of an organism can explain the amazing diversity of life we see today. Before we get in to how this works, we need to cover a few basics.

A gene is a segment of DNA that codes for a protein. You have about 25000 genes and they control every aspect of your body. It seems like a simple definition doesn't it? However, this basic concept of a gene ignores a very important point, which are that genes have DNA segments around them (usually upstream) that regulate their expression. These areas, which we will call "switches", turn a gene on or off in response to certain types of proteins binding to the DNA. For an easy example, lets look at the lac operon. This group of genes regulate lactose metabolism in bacteria by sensing if lactose is available in the environment. It does this through the use of a repressor, which binds to a switch located just before the gene sequence and prevents the DNA from being "read" and eventually producing proteins. However, when in the environment, lactose binds to the repressor and causes it to fall off of the DNA allowing proteins to be made. This is important because the proteins in the lac operon are used to metabolise lactose, and it would be useless to express the proteins if there was no lactose in the area. The opposite type of regulation also exists which uses activators to bind to a switch and initiate transcription (the process of making a protein). So this one switch controls when the lac genes are on or off depending on a signal it gets. This is the basic premise of gene regulation.

Now, most genes have many switches that are mostly independent of each other and respond to different things. Also, instead of just making one copy of a protein, the switches can stay on and make many many copies, creating certain concentrations. Also, proteins do not just go make something, they can also go and turn other switches for other genes on and off. Bottom line is that there are large networks of genes that turn on (or turn off) other genes in order to get a wide range of end products each at the needed concentrations. This is always an important feature, but it is especially important for evolution when it takes place in embryos, which is the play ground of evo devo.

The development of an organism from a single cell to the complex organisms we see today are largely a result of what occurs in the embryo. Genes are turned on and off at different times and in different places throughout the development of the embryo in order to build the body. So gene switches are used to regulate the expression of genes that in turn produce the proteins that build the parts that constitute an organism. (Make sure you understand this before moving on)

Given the difference in appearance between a mouse, fruit fly, and human we would expect that the genes responsible for building the respective bodies would be radically different from each other. However, once genomes began to be sequenced, an amazing discovery was made: the genes are incredibly similar across species (sometimes differing by only a few DNA base pairs). This means that differences in the actual gene sequences are not enough to account for all of the diversity of life. Evo Devo has shown us the way out of this problem. Differences arise when the gene switches are changed (or mutated) during development causing the same genes to be expressed in radically new ways. To illustrate this point, we take a look at Hox genes.

Hox Genes

Hox genes were a group of genes that were discovered to be the main organizers of the fruit fly body during development. The 10 genes that make up the group (or cluster) are each used in certain places at certain times in a fruit fly embryo to lay out the general body design. The picture at the top of the page is of a fruit fly embryo that has had different hox genes labeled with various colors. What you see is that certain Hox genes (like Hox1, Hox3, etc) are turned on in certain places (segments). These places will later develop into distinct regions of the fruit fly. The Hox regions are also important for limb development, with certain arms, wings, etc only being "made" in the specific region of a specific hox gene. So the Hox cluster is a key group for body formation in an embryo. And you know what? They are found in virtually every plant, fungi, and animal in some form. The major difference is in what hox genes are present and how many of them (grouped in clusters) there are. We have 4 Hox clusters that lay out out body plan in the womb. (see here for the comparison of a fruit fly and a mouse). This means that nature uses the same genes to build very different organisms. And it does this through gene switches. The Hox genes are turned on and off by these switches so that only certain ones are expressed in certain places. Now because the genes themselves are so similar across species, that means that differences in switches uses the same "instructions" to build different bodies. And some of these instructions are ancient.

The Cambrian Explosion

The Cambrian explosion is the name of an era 543 million years ago that shows a large increase in diversity of life (as judged by the fossil record) over a 15-20 million year span. There were two main groups of animals during this era that spit off to form much of the life we know. Now the original organisms themselves are long gone, but we know that they both had and used Hox genes because each branch has them and thus their common ancestor must have as well. Think about this. Nature has used the same basic genes to build organisms for millions and millions of years by altering the combinations and sequences of the switches.

Putting it all Together

The great thing about switches is that they can alter the expression of a certain protein in regards to a certain task. In other words, a change in one switch can alter one protein's function while the protein is expressed and used normally by all the other switches. It can also change where in the embryo certain genes are expressed, altering zones of expression that can effect body structure. This creates a concept of modularity, which creates an enormous number of combinations for genes to be used via gene switches. The variations of color on animals, the different structure of wings, the subtle differences in organs, and everything in between can (possibly) be chalked up to alterations of gene switches.

Look at you limbs, especially your hands. Now take a look at this picture of various types of limbs from different species. They are pretty similar. Because limbs only develop in certain areas of an embryo, minor changes can be made and "experimented" with (through natural selection of course) to produce wings, hands, hooves, etc. All that is needed are changes in the expression of certain proteins at certain concentrations at certain times during development using switches. The same concept is true for colors on animals and insects. Butterflies can change the color on their wings by small alterations in the expression of certain genes. The genes themselves rarely change so the variation is caused by altering the gene switches.

So hopefully you have a better idea of what evo devo is all about and how it is changing the traditional views on evolution. Nature uses the same set of genes to produce enormous diversity by small alterations in the switches that control gene expression. Most of these changes are gradual and accumulate over time. Instead of inventing features from scratch, nature uses pre-existing genes in new ways to produce innovation.

Finally, an interesting figure. Only 1.5% of your genome codes for actual genes. The amount of your genome that is involved in the regulation of those genes constitutes over 3%.

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