Wednesday, August 21, 2013

Evolution For Dummies - Part 3

Welcome to Evolution For Dummies - a multiple part post that will hopefully dispel a few misunderstandings about the sometimes thorny topic of Evolution. 

In Part 1 and Part 2, I introduced the idea of family trees and inheritance. I talked about how these trees can be simplified and expanded to demonstrate the interrelationships between the members of enormous families. I used the example of my friend Max, to illustrate the possibility that a single individual could have thousands of  contemporary relatives, just by looking at 5 generations - maybe 150 years - and how some of those relatives may look entirely different, speak different languages and have very strange customs (such as bog-snorkelling)! This, if you recall, was even in a "reproductively inefficient" family (only 2 offspring per generation and only 2 siblings for everyone who married into the family). At this stage, I should mention that I didn't include the concept of "removed cousins" either - Max's 1st cousins once removed (the children of his first cousins), for example. Family trees really can get complicated!

Just as a reminder, here is the family tree I first introduced, showing the relationships between my friend, Max, his sibling, and his 1st, 2nd, 3rd and 4th cousins. The various forks in the tree represent relatives such as mother, grandmothers, aunts, great aunts etc:

At this point it may be worth pointing out what may quite possibly have been obvious all along - that in these family trees, time is represented along the horizontal plane. As you move right to left, you are moving from my friend and his cousins, through the generations, backwards in time.

In this tree we were able to show that the most recent common ancestor of Max and his 4th cousins is "great-great-great grandmother" (which is exactly what the term "4th cousin" defines - someone who shares a great-great-great grandmother, but not a great-great grandmother nor anyone else higher on the family tree). We also determined that Max's 3rd cousins are more closely related to him than they are to his 4th cousins, because Max's 3rd cousins share a more recent common ancestor with Max than they do with his 4th cousins.

And this is the more simplified family tree I used to show the relationships between Max and his 5th cousins, but now leaving out a lot of detail about aunts (the mothers of 1st cousins), great aunts (mothers of 2nd cousins) and lot's of other distant family members.

I explained that we must look at the common ancestors of individuals, or individual groups, on the tree to infer relationships, but to NOT count forks - or nodes - because many of the forks may be missing. For example, a fork representing Max's parents is missing, as are the forks that would represent parents, grandparents and great-grandparents of his 3rd cousins.

I then used the "primate" family to show how, if you trace a family tree back far enough in time - millions of years - you can find the common ancestors of all primates and, more specifically, the common ancestor of humans and all other primates. I used the tree below to show the relationships between Max (a human being - Homo sapiens) and several other members of the primate family (remember that Homo sapiens is actually a member of the primate family): 

And, before we move on, I'll reiterate a very important point about these relationships. Using the same rules we applied to Max's more recent family, where we saw that his 3rd cousins are more closely related to him than they are to his 4th cousins (they share a more recent common ancestor), we can see that chimpanzees are more closely related to humans than they are to gorillas, because the common ancestor of gorillas and chimpanzees lived much further back in time than did the common ancestor of humans and chimpanzees. 

Aawwlrighty this is all about common ancestry. And, actually, we are already getting used to the next concept of evolution - that of phylogeny.

Phylogeny can be defined as "the evolutionary development and history of a species or higher taxonomic grouping of organisms". To put that much more simply, it is the study of family trees - which is what we've already been doing. Aren't we awesome?!

So that we can delve a little deeper into the interrelationships between larger groups of animals, I'm going to introduce a slightly different graphical representation of these relationships. When talking about my friend’s family, I used these handy little forks and lines depicting that transfer of genes between parents and offspring:

Where you see the labels "parent" or "grandparent" or "aunts/uncles", you will see a fork – what we call a “node” in the family tree. These are family members who produced offspring. At the tip of the tree are the offspring of the same generation, or contemporaries - my friend, his sibling and his cousins. And the lines are those lines of inheritance or gene transfer I talked about right at the beginning of Evolution For Dummies - Part 1

In the next few examples, I’ll be showing you similar family trees, but now the fork-like diagrams are replaced with much more square diagrams. But the principle is the same. The nodes are now where two lines meet perpendicular to each other to form a T-shape. So, for example, here is my friend’s simple family tree - as above - shown over two generations. The red arrows show where the nodes are. 

And here is the primate family - similar to how I showed it before - but now using this more square phylogenetic tree:

The principle is the same. Looking right to left along the horizontal plane takes you backwards in time, and where the lines meet perpendicular to each other to form a T-shape is a node that represents a common ancestor. So you can see that the two types (species) of chimpanzee (Pan paniscus and Pan troglodytes) share a more recent common ancestor with each other than they do with any other primate. You can also see that chimpanzees share a more recent common ancestor with humans (Homo sapiens) than they do with gorillas. Remember, also, that the two species Homo erectus and Australopithecus afarensis became extinct and so their lines are shorter to show that they are not contemporary - not "of the same generation".

Now, as promised in Evolution For Dummies Part 1, we can use this sort of chart to depict the common ancestor of bears and humans, dogs and cats and humans and, ultimately the common ancestor of all mammals and humans. In fact, we can trace the common ancestor of all life on Earth with humans. Yes, folks, the Tree Of Life.

This is how we can depict a phylogenetic relationship between humans (shown in red) and all of the mammals (separated into distinct groups):

Now, this tree raises an interesting point. Look at how the relationship between the apes is depicted. Above, I showed you this relationship:

And if I remove the extinct groups for simplicity, and use the same common names used in the larger tree it would look like this:

But in the larger tree, the chimpanzees are at the bottom, and the gorilla and the human are next to each other, whereas in the smaller tree the gorilla is at the bottom and the human and gorilla are separated by the chimpanzees. WTF, man!  Here's how they look next to each other:

Have I made a mistake somewhere?

Well, actually, no! The important thing is that in both of these trees, the relationships between the animals are identical. In both trees, the two species of chimpanzee (Bonobo and Common chimpanzee) share a more recent common ancestor with each other than they do with any other primate. Also, the chimpanzees share a more recent common ancestor with humans than they do with gorillas. And, extending the tree backwards, you will see that gorillas and humans share a more recent common ancestor with each other than either of them share with orangutans, gibbons, old world monkeys and so on backwards in time down the tree. This is what is important - the RELATIONSHIPS between animals. The "tines" of the forks can be rotated, so the position of any animal next to another at the tip of the tree doesn't matter at all. It is simply a matter of preference for whomever is drawing the tree, as long as the relationships don't change. As I said earlier, you have to follow those lines of inheritance backwards in time to find the common ancestors and make inferences about relationships from there.

I want to labor this point a little by using my friend's family tree again. If I rotate the branches at the fork that represents my friend's parents, I can place his sister at the top of the image, where he used to be, like this.

Rotating the branches around the fork of a tree doesn't change the relationships between any family member

Although my friend is now placed more closely to his cousin at the tip of the tree, his relationship to his cousin is unchanged. His cousin is still his cousin and they still share a grandmother.

Similarly, if I rotate the branches at two forks - those that represent my friend's grandmother and great-grandmother, I can bring my friend "closer" to his 3rd cousins at the tip of the tree. Like this:

Rotation of branches around two forks - no change in relationships.

Now, although my friend may be within handshaking distance of his 3rd cousins at the top of the tree, his relationships with them are still unchanged. When you trace back the lines of inheritance - the lines of gene transfer - he still shares a great-great-grandmother with his 3rd cousins, and all of the other relationships are completely intact. And this is why you should never try to infer relationships between animals in a phylogenetic tree from their positions at the tips of the branches. It is only by tracing the branches backwards to find the most recent common ancestor that such relationships can be determined.

Phylogenetic trees such as these are derived using various methods and various measurements. These can include things such as genetic similarities or differences, or morphological similarities or differences. We can make the trees as simple - or as complex - as we need, to highlight the relationships about which we might be interested. So for example, approximately half way up the big mammal tree I showed (above), you can see some animals we discussed in Part 1 – the cat and the dog. In this tree, they have been placed in a group of mammals along with the bear, the weasel and the seal. Here is that section of the tree:

As I said, we can make these trees as simple - or as complex - as we need, to highlight the relationships about which we might be interested. The author of the "mammal family tree" above was clearly not interested in the complexities of the relationships between cats, dogs, weasels and seals, because he/she just lumped them all together into a single group. But, perhaps somebody else might be interested in those complex relationships. So let's expand the "Cat dog bear weasel seal" branch to show the relationships between all of these animals:

Here we can see, as I mentioned in Part 1, how the hyena is more closely related to the cat than the dog, but also – maybe surprisingly - how the dogs, the wolves and foxes (canids) are more closely related to the seal and walrus than to the cat!

Going back further in time, we can put together a phylogenetic tree showing the relationship between all vertebrates (animals with a backbone), now including, for example, birds, reptiles, amphibians and fish:

The vertebrates

And the further we go back in time, the more relationships we can infer to more and more species on the planet until we eventually construct an entire tree of life which can even include extinct species – neanderthal man, dinosaurs, the dodo etc:

It's life, Jim, and exactly as we know it!

 And it really is a beautiful thing to behold:

The Great Tree of Life. Click on the image for high resolution (from

Ok. Deep breath. Shake it out. 

What I’ve told you about so far in this series on evolution is that you can use family trees to follow lines of inheritance - or lines of gene transfer - from offspring, back through time to the common ancestors that gave rise to groups of relatives, or groups of animals. I've told you that populations are immensely important. That they arise quite quickly even in species such as the human, which is not particularly fecund – meaning that the individuals don’t produce large numbers of offspring very quickly. And that it is in populations that the genetic changes occur that provide for diversity in characteristics such as morphology – shape, size, skin color, etc. I’ve also told you how we can construct - and interpret - phylogenetic trees to show the interrelationships between animals and to trace the common ancestors of entire groups of animals. 

In Evolution For Dummies Part 4, I'll try to answer the big question. Why is this important? Why do we care? 

Watch this space!

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