This blog is the second piece in a series by Darrel Falk and David Kerk. The previous entry is found here.
A really fun family outing in San Diego is to visit Sea World and see the many fascinating and exciting marine exhibits. But the unquestioned main attraction is Shamu, the killer whale. If you are a real bona-fide thrill-seeker, you sit in the first few rows next to the tank, virtually guaranteeing that when the sleek but massive animal breaches the water and then falls back, you will be inundated by a huge wave and soaked to the skin! How did such marvelous creatures arise in the first place? It has taken many years of patient work by scientists operating in very different specialties, but we are now at the point where we can relate the “Whales’ Tale.” It is a story of evolution over a critical period of about ten million years, which is supported by three main types of evidence. We will consider the first two types of evidence (which are molecular in nature) in this essay, and the third type (which is fossils), in our next essay.
If evolution is true, then modern whales and other mammals should be related to previously living ancestral species, through a process of “descent with modification.” It should therefore be true that the living organisms and ancestral ones (now extinct) should form a sort of “family tree.” If you have taken an interest in your family genealogy, then you know right away what this means. You, your siblings, parents, aunts, uncles, grandparents, and so forth, can be arranged in a diagram that passes from one generation to the next. If we visualize this going deep into the past, we can use the “tree analogy” even further – the most recent generation of members of the family can be said to lie at the tips of the branches, while very early generations of the family would lie deeper in the tree, at branching points.
The metaphor of using a tree to represent ancestry comes in other varieties too—not just families. Consider, for example the growth and diversification of the historic Christian church – from its roots in ancient history to the tips of its branches—the various denominations still in existence today. As shown below, the Christian traditions which are especially closely related to each other are located near one another at the branch tips. The more distant the relationship, the further away they are in the tree of Christian traditions.
So how can one derive the family tree for organisms like whales—how can one determine the tale of the whale? Cetaceans, after all, have such a dramatically different body plan compared to all other mammals; deciphering their family tree presents a fascinating challenge. If evolution is true though, there is one group of organisms to which whales are more closely related than any other. Furthermore, if evolution is true, independent ways of deriving tree structure ought to produce very similar results.
In today’s essay we will show two methods that have enabled biologists to trace the lineage of the whale family: two somewhat independent methods that allow us to explore the structure of the whale’s family tree. In our next post, we will examine a third.
The instructions on how to build an organism are contained within the four letter DNA code: A, G, C, and T. Each gene is a short stretch of this code and the specific order of the 4 letter code is called its “sequence.” The cells of the organism read the code, gene-by-gene, working in concert with one another in constructing the body. Because it is very different than that of other living mammals, understanding the origin of the whale body presents an interesting challenge. Whales are mammals though, so if evolution is true they must have a family tree which shows how they are connected to other groups of mammals.
One useful source of information in whale family tree construction is the sequence of the DNA code-letters (bases) in a particular gene in whales compared to the sequence of that same gene in other mammals. Why would this information help us? Both whales and their related mammalian “sibling and cousin” species will each possess a version of whatever gene we look at that was inherited from their common ancestor. Random mutation will have changed each version of the gene slightly, so that the descendant organisms will generally each have a distinct sequence. More closely related species will have a more recent common ancestor, and will, therefore, have more similar sequences. This means they will tend to lie closer together in our reconstructed family tree.
We can put this DNA gene sequence information from whales and comparison mammals into a tree-building computer program. The living organisms form the tips of the branches and the interior branch points represent extinct predicted ancestral organisms. It turns out that whales sit closest in the tree to a set of hoofed mammals including cows, sheep, pigs, camels, and hippopotamuses.1 This entire group of hoofed mammals is technically called the “Artiodactyla” (Greek for “even toed”). If evolution is true, this means that whales and these even-toed hoofed mammals share a common ancestral species that existed much more recently than the ancient common ancestral species that gave rise to all mammals. Indeed, even before that there would have been a common ancestral species that gave rise to all mammals and all reptiles. All of this can be represented on the metaphorical tree of life.
There are other independent ways in which DNA analysis can be used to test whether we have correctly positioned whales on the tree of life. Scientists are always eager to obtain different sorts of data. If all independent methods lead to the same conclusion, if “all roads lead to Rome” to use the analogy introduced in an earlier essay, then we can become increasingly convinced that our model is correct. So what is another DNA feature that can be used to determine the whale family history? There are certain chunks of DNA which, on rare occasions in the history of life, move to a new location in a chromosome. These mobile chunks of DNA are sometimes called “jumping genes” although it should be emphasized that they don’t “jump” very often. The location at which a jumping gene inserts itself into a chromosome is quite random. When such an element inserts itself into a particular place in the chromosome, it will reside at that location for many generations. Indeed since “jumping” is so rare, it generally stays at the same location for millions of years.2 Since the insertion process is almost random, and the element almost never moves out once it is in a chromosome at a particular position, the chance that a “jumping gene” will be in precisely the same place in the chromosome of unrelated organisms is vanishingly small—(essentially zero). In other words, the “jumping gene” makes an ideal “marker” to trace the ancestry of living species. If you examine a set of such “jumping genes,” each inserted into a particular place in the chromosome, only related organisms will share a particular insertion, since they inherited it from their common ancestor. If one of a pair of organisms lacks this insertion at this site, it supports the conclusion that those two organisms do not share a recent common ancestor.
The figure below shows a set of chromosomes, and then enlarges one part of one chromosome to show the DNA molecule. Imagine a “jumping gene” moving in precisely between two of the millions of units of DNA in a chromosome. Since DNA replicates each generation, the chromosome with its inserted “jumping gene” gets passed on faithfully through millions of years. Once a piece of DNA has moved into a chromosome between two bases, it is a great marker to identify species that descend from a common ancestor.
One of the very nice things about this type of DNA information is that it can be tabulated, and is simple enough that you can do a little head scratching and puzzle out the relationships of the organisms involved. The data either consists of a particular “jumping gene” being present (call that a “1”), or if it is absent (call that a “0”). In practice we need a third category, and that is “we don’t know if the “jumping gene” was there or not” (call that a “?”). This third category is necessary because sometimes a random genetic event will result in the loss (deletion) of the entire region which might have contained the jumping gene insertion. Now with this background, take a look at the following figure.3 For this somewhat simplified example, we show 20 “jumping genes.” If two species share a “jumping gene” at exactly the same position, this means those species are derived from the same ancestral species. This tree confirms the prediction made based on DNA sequence data previously, that is, that whales should be closely related to the group of even-toed hoofed mammals. For example, whales share “jumping genes” 10,12, and 18 with a broad assortment these animals. This means that they all share a common ancestor with insertions in these exact same positions. No other living organisms will share this group of common insertions, or this common ancestor. In addition, these data show that whales are most closely related to hippos (note that they each share “jumping genes” 4, 5, 6, and 7). (In fact, DNA gene sequence studies also support such a relationship, so this is not an aspect of using “jumping gene” data alone).4
Now we come to the bottom line: so far we have two roads (DNA sequence data and “jumping gene” data), both of which lead to “Rome.” Both point to exactly the same conclusion. Whales, despite their highly specialized body form, can now be confidently predicted to lie within the group of even-toed hoofed mammals. Furthermore, of that group of living mammals, hippos are predicted to be the most closely related to whales. There is agreement between two types of DNA data, and more confidence in our result.
Editor's Note: For a correction to the data in this chart, please see David Kerk's comment below.
Therefore, if evolution is true, we would expect that living whales and living hoofed mammals should share extinct common ancestors, from which they descended with modification. Or, put another way, we should be able to find “transitional fossil forms” which we can identify by their structural features as being ancestral to both living hoofed mammals and also whales. But about how long ago would we expect such extinct forms to have been alive? It turns out that application of DNA data once again can give us a time estimate with which to start.
We mentioned above that random mutational changes to DNA in an ancestor are passed on to descendant organisms. It turns out that for a particular gene, this sort of change acts as a sort of “molecular clock.” That is, for a particular gene, the rate of change over time is approximately constant. If we can “calibrate” how fast a particular molecular clock for a particular gene is ticking, then we can use it to determine how long ago in the past two species last shared a common ancestor. For example, we know from the fossil record (which has been dated by radioactive isotope clocks, as discussed in a previous essay), that cows and pigs last shared a common ancestor about 55-60 million years ago. We can measure the total number of changes in the DNA of a particular gene in cows and pigs, divide that by the age of a fossil from an ancient species believed to be ancestral to both of them, and determine an average rate of DNA change. Our molecular clock for this gene is now calibrated. If we want to determine when whales last shared a common ancestor with cows, and then pigs, we can measure the total DNA change in our clock gene between whales and cows, and between whales and pigs. We can then divide by the rate of “ticking” of the clock, and determine when in the past these ancestors should have lived. When we do this, it turns out that such common ancestors should have lived about 45 to 50 million years ago.1 So if evolution is true, we should expect to find fossil “transitional forms” showing evidence of common ancestry of hoofed mammals and whales, dating from about this period. We will see in our next essay that this prediction is borne out.
The next blog in this series can be found here.