Monday, August 19, 2013

Evolution For Dummies - Part 1

Welcome to my multi-part series on the Theory of Evolution. I hope you enjoy it and that it helps dispel some widely held misconceptions. In this first part, I'm going to talk about

 "Families, Forks and Fallacies"

Below is a picture of two babies. I'd like you to look carefully at them and take a moment to answer this simple question: Do you think they are related? There are three possibilities to consider:

1. They are actually two photographs of the same baby.
2. They are different babies, but related in some way.
3. They are not related at all.

Awww! Aren't they cute?!

Well, if you went for the third option and thought that they are not related at all, well I'm sorry, but you'd be way off mark. This is a post about evolution and both of these children are quite obviously Homo sapiens - they're human beings, so they are most definitely related! 

If you opted for the first possibility, and thought that they are the same baby, well you’re also wrong, but I’m ok with that because the one on the left is me - taken in about 1960 - and the one on the right is my daughter, taken in 2012. So I am excited that there are similarities and that my daughter doesn't look more like the milkman! She and I are undoubtedly related and you can see a likeness in part because my daughter obviously inherited some genes from me (perhaps the 'cute smile' gene or the 'crazy hair' gene) and she inherited some genes from her mother. 

In fact, if you trace a simple family tree, or if we talk about the transfer of genes, you could trace a direct line - or a direct branch on the tree - between me and my daughter, or between my wife and our daughter. This would be a "line of inheritance" or a "line of gene transfer". 

Tracing the transfer of genes between parents and children                       

And this introduces us nicely to the concept of family trees and inheritance.

 Now, here is that picture of me again, but this time it is in its original context:

Me and my big sister

The little girl sitting next to me - as the image caption says - is my big sister. You can see that we have similarities but that we also have differences. She’s a girl and I’m a boy. She has dark hair, mine is fair. I have big ears, she doesn’t. You can’t tell from this photograph, but her eyes are hazel and mine are green. And these differences are because although we share the same parents, we inherited a different share of genes from each of them. Also, although we are related, you can’t draw a straight line - or a branch - on our family tree to connect us. You have to go back to our parents who are the most recent common ancestors of me and my sister. And this is important. Although you can draw a straight line – this line of gene transfer or line of inheritance - between me and my daughter (or my wife and our daughter), in order to connect anyone to a brother or sister - a sibling - you have to go back one generation to the parents - to the most recent common ancestor.

Brothers and sisters - related, but obviously different - have no direct line of gene transfer between them. You have to go back one generation to find their most recent common ancestors – their parents.

I trust this idea is simple enough but it is worth stressing again. There is no direct line of gene transfer between me and my sister - between siblings. To identify the genes that determine who I am, or that determine who my sister is, you have to trace the lines of gene transfer - or lines of inheritance - back at least one generation. Back to the parents, who represent the most recent common ancestor.

Now, please remember that image (and that concept of gene transfer) while I take you on a little digression.

I'd like to introduce you to a very good friend of mine who once told me that he doesn’t believe in evolution.

This is Max, a very cool dude and a heck of a guitar player.


Now what Max actually said was that he just doesn’t believe in evolution because he doesn't "buy" the idea that we, as human beings, are descended from monkeys.

And what this means is that my friend - along with many other people - simply does not understand that central theme of evolution I was just writing about: the theme of common ancestry.

Max’s strong stance against evolution, then, is based on a misunderstanding. And going back to that photograph of me and my sister, he might as well have stated that he doesn’t “buy” families, because he doesn’t "buy" the idea that I am descended from my sister, or that he is descended from his brother. Clearly he isn’t descended from his brother. But he and his brother do share an easily traceable common ancestor - actually two - their parents.

This misconception can be made to seem even more extreme if he had asked “Why, if families are true, is there no evidence of an intermediate ‘species’: someone that is half me and half my sister”?

Nobody expects this - I hope! Nobody thinks that there should be an "intermediate sibling" - some sort of weird hybrid from which brothers and sisters are derived. And yet this is the common misconception that often manifests itself in questions such as “Why, if evolution is true, do we never see an intermediate species, a creature that is, for example, half crocodile and half duck? A creature such as a 'crocoduck':
The impossible (and actually rather ludicrous) crocoduck.

As I hope to convince you, dear reader, the theory of evolution and the concept of common ancestry don’t suggest this as even a remote possibility, any more than it allows a 'half-me, half-my-sister', or a 'half-my-friend, half-his-brother'. The common ancestor of the crocodile and the duck existed about 10.3 million years ago. This means that to find - if you will - the ancestral parents - the ancestral most recent common ancestor of the crocodile and the duck, you have to go back over 10 million years and you simply cannot draw a straight line between the duck and the crocodile. So no half-crocodile, half-duck type of creature. It is silly! Absolutely, positively, no way in heck is this even remotely possible based on evolutionary theory.

So this is a fundamental - or simple - basis of this aspect of evolution. Importantly, when talking about species, very few of the species that exist today on the planet (extant species) has evolved from any other extant species. And returning to my friend Max, we can tell him that humans have not evolved from monkeys any more than the crocodile evolved from the duck, nor any more than I have evolved from my sister, or cousin, or this guy who lives down the street and mows his lawn in the rain. 


However, my sister and I, Max and his brother, and even Bob and I, all share a common ancestor. And in the case of monkeys and humans (actually, more accurately APES and humans), that common ancestor lived an extraordinarily long time ago. Millions of years.

But let's investigate this a little further.

Importantly, this common ancestor of humans and apes was actually neither a human nor an ape, but was a creature that possessed an ancient genetic blueprint that, over millions of years and via hundreds of thousands of small changes in that blueprint, gradually led to the rise of different modern species - humans and apes. So in one population of the descendants of - or the offspring of - this common ancestor, millions of years and hundreds of thousands of genetic changes led to the rise of modern apes. And in a SEPARATE population of the descendants of this common ancestor, millions of years and hundreds of thousands of genetic changes led to the rise of humans. And I will labor this point in a while.

Modern apes and humans are not directly descended from one another. Like siblings, you have to trace back the lines of inheritance to a common ancestor - in this case, going back millions of years to find it.

 But first, I’d like to show you how such processes that normally take millions of years have actually been seen to occur over much smaller time-frames.  

"A wolf in Chihuahua's clothing"

"Your place or mine?"
So, for example, although domesticated dog breeds are not technically considered distinct species, when you look at the image above it is hard to argue against the obvious observation that a Chihuahua dog would not be able to breed naturally with a Great Dane dog, so in the wild, these two animals would effectively be different species. Their distinct morphological features (their different and extreme sizes) are the result of relatively small differences in their genetic blueprint and would prevent them or their offspring from ever breeding. Thus, two distinct populations of dog have arisen through deliberate 'genetic manipulation' undertaken by us clever humans. We have achieved this genetic manipulation simply by breeding dogs whose features we found appealing. We didn't necessarily understand the genetics, but rest assured - it is the genes that confer the differences being selected for. And, note that there is no obvious 'half Great Dane, half Chihuahua' either, because neither animal is directly descended from the other.

The Great Chihuadane....NO!
But also note that the domestication of dogs has occurred over only a few thousand years - not millions of years - just a few thousand. And it is generally accepted that all ‘breeds’ of modern domestic dog share a common ancestor - the wolf, or at least an animal that was extremely similar to modern wolves. It is also probably the case that it was a particular population of wolves that had already become used to humans and was perhaps scavenging from human encampments. Thus, one of the first appealing features selected for - that of being relatively tame and accepting of humans - had occurred without much active human interference at all. It was more due to a change in the environment of that population of wolves and a change in the way it was able to get food - by scavenging. All it took was for some human to see non-ferocious, cute little wolf puppies and 'voila!' the dog was forever doomed and the inevitable path to Chihuahuas, Great Danes (and Cockapoos) was laid!

from wolves to domesticated dogs in a few thousand years

The dog also gives us an interesting insight into how even small mutations in that genetic blueprint I have referred to can lead to dramatic changes. The genetic blueprint of dogs is made up of approximately 2.5 billion ‘letters’ of DNA code, which as you may already know is a specific and extremely long sequence of A’s, T’s, G’s and C’s (each of these letters refers to a single molecule of either adenine, thymine, guanine or cytosine, which are joined together to form the larger DNA molecule). 

But if just two specific letters in the sequence is changed in the whippet dog - that is, just two in 2.5 billion letters - the result is quite stunning and the dog no longer looks like this:

The 'ordinary' whippet - how cute!

 but like this:

The 'bully' whippet - Oh my God, lock up your children!

You shouldn't be surprised to learn that this single change in the DNA sequence affects the way the whippet makes muscle! You may be able to imagine, if these dogs were in the wild, how different environmental conditions might favor one or the other. The ordinary whippet would be able to run from a predator, whereas the bully whippet may be less inclined to run and - actually - might be able to fight off a bear! But still, you can probably imagine scenarios where one form of this dog would fare better and be more likely to survive to produce offspring.

And if you think again of the Chihuahua and the Great Dane being out in the wild, it is easy to imagine how, if similar relatively small genetic mutations occurred in either one or the other of these different populations of dog, even greater changes may arise over time leading to the Chihuahua looking less like a dog and more like, say, a ground squirrel - or the Great Dane looking more like its ancient ancestor, the wolf. A very big wolf!

Imagine how different environmental challenges may 'select' for either of these dogs

And here's an interesting exercise for you. If you accept that all dogs are related and are derived from a common ancestor dog (the wolf) and if you accept a similar story for all cats, including lions, tigers, leopards, and domesticated cats, can you imagine an animal back in ancient history that might have given rise to two populations of animal - both cats and dogs? An animal that wasn't actually a cat or a dog, but had the genetic blueprint that might, through gradual changes in the genes of two separate populations of its offspring, have given rise to both? 

The common ancestor of both cats and dogs?

How about this animal - what do you think - is this animal, the brown hyena, a cat, a dog or neither?

The brown hyena. Cat or dog?

It sure looks like a dog, doesn't it?

Well, the answer is that this animal is neither a cat nor a dog, but it is actually more closely related to cats than to dogs. But - as I said - it really looks like a dog! The hyena, like the bully whippet, shows how genetic differences can produce surprising characteristics in an animal and hints at how speciation - or the way new species are evolved - can occur.

Okay,  time for a quick recap....

I’ve told you that relatively small changes in genes can have dramatic effects on an animal, and I’ve explained that given strong enough selection processes, the morphology - or the shape of individuals within a population of animals - can be dramatically changed.

Now I’d like to go back to my friend Max: 

"Populations and Time-Frames"

If Max imagines tracing his family tree back a single generation, back to his parents, he comes to a ‘fork’ in that backwards-in-time path. If he takes that fork and then moves forward again, he might arrive at his sister (or perhaps another sibling, depending on which fork he chose). Like this:

Tracing back a family tree one generation

From this simple drawing, you can immediately see the silliness of the 'intermediate species' idea again. Family trees are called trees because they have branches (or forks) like this and do not take the form of straight lines that might lead to a 'half-my-friend, half-his sister'-type animal (or a Great Chihuahdane, or a crocoduck). Again, evolution does not propose that as being even a remote possibility.

Going back two generations to his grandparents and the forks in the road placed there, he could move forward in time again to see his aunts and uncles within one generation, and his cousins in the next. Like this:

Tracing back a family tree, now through two generations

To labor the point, we see various branches of the family tree here and no straight connections between members of the same generation, so no possibility of their being a 'half-my-friend, half-his-cousin' type animal.

Now, very few of us know our extended families beyond second and third cousins (although my father in-law has 52 first cousins!), but imagine how many potential relatives my friend could find if he followed that path back five generations (through the maternal line to his great-great-great grandmother) and then moved forward again down all the various forks to, for example, a 4th cousin. It might look something like this:
Tracing back a family tree, now through five generations, and moving forward to find a 4th cousin

And note that this is a very simple family tree, because none of the women in this family had any more than two children. However, in spite of that fact, my friend could have at least 31 relatives in this generation (siblings, cousins, 2nd, 3rd and 4th cousins) - just through the maternal line. But actually, in going back just 5 generations, he could identify many more distant relatives by looking at the siblings of the person at the top of the family tree. If my friend's great-great-great grandmother had three siblings (not an entirely unusual prospect five generations ago), this chart could be at least four times bigger, giving my friend more than 120 single-generation relatives, now including 5th cousins (who all share a great-great-great-great grandmother with my friend):

My friend's possible family tree in just 5 generations, now looking at siblings of just one distant relative - his great-great-great grandmother. He could have more than 120 same-generation relatives.
But assuming each person who is represented by a fork in the family tree shown above had a husband or wife or partner (how else did they produce offspring?), an additional layer of complexity can be introduced by identifying the siblings of each individual who married into (or procreated with) the family. Again, if the partner of each ‘fork' in the chart had just two siblings, and each of these siblings had just two children, my friend could identify more than 1500 distant same-generation relatives.  And that's in just 5 generations - probably less than 150 years. Moreover, imagine how complex a family tree would be if my friend came from a much more - shall we say "reproductively efficient" - family and, like my father in-law, had 52 first cousins, rather than just two! Five generations could populate a major university or a small city!

Well, so what? What does this mean, other than that my friend probably has a bunch of distant relatives?

What this demonstrates is that fairly large populations can arise quite quickly, and this is important because it is in populations that genetic variation can arise. Apart from the obvious changes that arise through the mixing of genes when a man and a woman procreate, a single mutation in the genes of, for example, a great-great-great aunt, could mean that many of my friend’s distant relatives may look entirely different. They may be taller or shorter, fatter or thinner, be more or less muscular and have different colored eyes or hair. An interracial marriage three generations back, may mean that my friend has distant relatives with Asian or African American features, for example. A geographic migration instigated by a great-great grandfather may mean that my friend has relatives who look different, have different customs and speak an entirely different language. My friend's family originally hails from Ireland. This means that he probably has distant relatives who still indulge in bog snorkeling or leprechaun hunting! 
One of my friend's distant relatives?

And how many in the United States can trace their ancestry back to Italy, or Gemany, or Scandinavia or Britain - all kinds of countries? In fact, over a few thousand generations, this is all that really separates all of humanity and all of its wonderful diversity. And so, by tracing the same family tree back all of these generations and by taking the right forks, my friend could trace his relationships with every single person alive on the planet today, because if you go far enough backwards, you will find everybody’s human "common ancestor".

But now imagine my friend tracing back several hundred thousand generations - moving backwards in time millions of years and not taking any of the forks forwards in time. Eventually, he would reach a position where the ancestor he identifies looks so different to him that it would hardly be recognizable as a human being.  In fact, genetically, it might hardly be human at all. 

Still, he traces back even further and keeps going for around 3.2 million years. Then he arrives at someone like this:


This is Lucy, a very famous lady who lived and died in Africa about 3.2 million years ago. What is important here is that Lucy was not just a single individual. She was one of an entire population of individuals with siblings and cousins and aunts and grandmothers. So Lucy wasn’t THE common ancestor of modern humans, but members of that entire population of Lucies were undoubtedly the common ancestors of many humans alive today.

But my friend doesn't even stop here. He continues backwards in time following the forks of his ancestors for maybe 7 to 8 million years. Only then does he decide to move forwards in time again, heading back for 'home'. But imagine he gets a little lost along the way. Instead of going ‘home’ he takes unfamiliar forks in the path that take him through genetic changes, through ‘interracial marriages’, through shifts in geography, through changes in environment, through changes in the demands placed on populations by food availability and food shortage, and  through changes in the demands made on populations by different predators. 
Getting a little lost on the way back home from 7 or 8 million years ago.

Ultimately, the route my friend takes through all of his distant relatives leads him not to the USA and the comfort of a modern human existence with our Wide-Screen TVs, Krispy Kreme Donuts, Fake Plastic Trees and the Internet, but to Africa and to this:

The chimpanzee, my friend's 230,000th cousin, 10,000 times removed (approximately)!

And to stress again, the common ancestor my friend identified 7 or 8 million years ago was NOT a human and was NOT a chimpanzee. It did not look like a chimpanzee any more than my friend and his bog-snorkelling distant Irish relative look, sound or behave the same. But this ancient common ancestor possessed an ancient genetic blueprint that, over millions of years and via hundreds of thousands of small changes in that blueprint (with those changes occurring in DISTINCT populations of the descendants of this common ancestor), gradually led to the rise of different modern species - humans and apes. 

So this is common ancestry. No straight lines linking monkeys or apes to human beings, but an exquisitely complex family tree stretching back millions of generations and millions of years. We are not descended from monkeys or apes at all, but in the same way that it would be possible to trace back the family histories of all domesticated dogs to find the common ancestral wolf, it is theoretically possible to trace back any human's ancestry to a point where a common ancestor with apes can be identified.

So...... Moving beyond my friend and his relationship to all of humanity and the apes, and thinking about how we could possibly envisage an animal that gave rise to both cats and dogs, you can see - I hope - that we can trace family trees back millions and millions and millions of years, beyond the point where we find the common ancestor of apes and humans, back to the point where we find the common ancestor of bears and humans, dogs and cats and humans and, ultimately, the common ancestor of all mammals and humans.

And that will be the topic of a second post - Evolution for Dummies Part 2.

Watch this space!

No comments:

Post a Comment

Please feel free to leave comments. I try to invoke some thought by my posts, so please show that you have thought about what it is you'd like to say BEFORE posting. I will delete any comments I deem are inappropriate, offensive or funnier than mine.