X-Birds: Genetic Mutations and the Evolution of a Species
by Bree (with Greg West)
Yesterday, Greg sent me the following satirical story to review and I think that for the most part he’s spot on.
“I’m not anything even close to being a biologist (and after reading this, I’m sure many of our skeptic visitors will agree), but even so, I have endeavored to understand the concept of Neo-Darwinian evolution. I’m going to use the example of woodpeckers to demonstrate my understanding thus far of how it works…
Once upon a time there was a species of birds (for the sake of our story we’ll call them, ‘protopeckers’) that contentedly roosted in the forest and flew around eating the plentiful varieties of bugs that also lived in the forest. The protopeckers merely had to expend the effort of flying around and catching these bugs in their beaks or catch them off-guard crawling around on the forest floor. Life was pretty simple for the protopeckers; eat, sleep, drink, fly, mate— things were going well, and like I said, the protopeckers were about as content as any birds could be-- and they were also thriving, despite the existence of some larger birds that would occasionally eat one of them.
Then one day a natural disaster occurred. It was terrible for the protopeckers because this disaster killed off most of the bugs that were the major part of their diet. Those bugs that were not killed off ran away to another unknown part of the forest, far away from where the protopeckers lived. The only bugs that remained where the protopeckers lived where the ones that lived underneath the bark of trees or could actually burrow into the trees themselves.
The birds had never attempted to eat these particular bugs before because they were so much more difficult to get at than the bugs they had been previously eating. The few that had tried had ended up with bad concussions, severely bent beaks, or both, which resulted in brain damage or the inability to mate because potential mates were repulsed by their bent beaks.
But now, because of the shortage of any other food, all of the protopeckers were forced to make the attempt to get at these tree bugs. Things were no longer easy for the protopeckers, and they began to die off. The ones that didn’t die off right away were forced to mate with brain damaged protopeckers or ones with badly damaged beaks, and their offspring had even a harder time surviving than their parents did.
Fortunately, there was one protopecker that was born with some genetic mutations which caused him to look rather peculiar to his fellow protopeckers. His parents named him ‘Woody’. Woody’s mutations caused him to have a long narrow pointy beak which was also quite sturdy. These mutations also gave him an unusually long tongue, with which he could snatch the tree bugs out of their deep holes in the trees. Luckily, Woody also discovered that he could hammer away at a tree with his super-beak without getting a concussion because there was a strange piece of cartilage between his skull and his brain which prevented that from happening.
Since Woody was now the only protopecker that could reach the previously unreachable food he was able to survive, but because there was no food for the other protopeckers, they all began to leave in search of food. Woody now has to leave as well, because all good little protopeckers must mate and produce more little protopeckers.
Because of his peculiar appearance, Woody had to travel far and wide in search of a suitable mate. He was about to die of sheer loneliness when suddenly, in the distance, he heard a strange knocking noise: “rattattatatatattatattat!” At first he wasn’t sure what it was, but then he heard it again, closer this time: “rattattatatatattatattat!” And then he heard it again… and again! It reminded him of the exact same sound that he made when he was hunting and eating the tree bugs.
Woody followed the sound until he saw that it was being made by a beautiful… Woodpecker. She noticed Woody coming her way and it was love at first sight. Woody introduced himself and she told him that her name was Winnie. Woody and Winnie made lots of babies, now known as ‘Woodpeckers’, and lived happily ever after.”
Now obviously, this is a story. This is not how Woodpeckers actually evolved (although it could be, but research in 2006 even suggests that two species of woodpeckers found in North America and in Cuba are very distantly related, having diverged over 1 million years ago1).
Let’s talk for a moment about adaptation. Woody has cartilage protecting his brain, and stronger beak, through a series of genetic mutations. First of all, as nice as that sounds, most mutations don’t just magically confer a really handy tool like that. Even the replacement of a single base pair can cause severe damage (just look at Cystic Fibrosis). If Woody actually had one useful mutation, it’s a miracle, but here, he has at least two, and didn’t die from the massive rearrangement of genes that must have taken place in his mother’s egg or father’s sperm, or both.
Don’t believe me? If you’ve heard of knockout mice, you might understand what I’m talking about. When researchers “knock out” a gene, they make use of some really neat machinery and physically remove a gene from an embryo (a gross oversimplification of the process). It’s not that simple though. They have to produce heterozygotes and then cross-breed them, hoping to produce a recessive offspring. Sometimes, the offspring simply aren’t there. What happened? They died in utero. Either they failed to develop, or a huge congenital deformity caused their untimely deaths.
The next bit of criticism we’ll see from our skeptics will tell us that we don’t understand how evolution works! It’s gradual change over time! Sure, I understand completely what it means. One bird had a teeny bit of extra cartilage. His children had a bit more cartilage, the next generation a bit more, etc. etc., until we have a bird with a massively large protrusion on his forehead. Okay, not really. But there’s a problem with that hypothesis. That one gene wouldn’t increase in frequency unless it had some function. Until it can confer function, it doesn’t increase fitness, and no increased fitness means no increase in mating behavior, which means no increase in gene frequency. And remember, we’re still assuming this mutation didn’t kill our bird to begin with. The point at which we pick Woody up in our story is past all of these gradual mutations, of course. He’s already an ancestor of the first mutated bird. Somewhere down the line, he and maybe a buddy or two have this weird cartilage, long beak, and long tongue. The rest of his family just thinks that he and these others are a little… weird looking. Which means no one would want to mate with him anyway.
Assuming the entire population of protopeckers is dying out, maybe all the males are gone except for Woody, it looks like now Woody’s fitness has drastically improved. He’s now hot stuff, and he has lots of babies. About half of them are blessed with Woody’s strange adaptations, and some of them survive. Two girls and a boy. And now these guys dominate the scene because they have improved “fitness” and the others don’t reproduce as much, if they even survive to childbearing age.
Another assumption that we should elaborate on is foraging behavior (finding food). If such a natural disaster occurred, the birds would start doing what's called sampling behavior. They had probably previously done this in order to find the cache of food supply they’ve already been making use of. They would search for food in new locations until they found them or died trying. Birds have the ability to remember where they sampled for food. When they do this, they systematically scan an area and remember where they found the food, and which area had the most food and was the closest or easiest to get to, etc.
Evolutionary ecologists have come up with formulas that mathematically calculate (model) how birds do this. But birds are not as smart as people - they just eat bugs, and keep eating bugs. In order to come up with a model, however, we have to do math (go figure). All of that gets very difficult to discuss… suffice it to say that the foraging behavior of birds is very complex. No one has really decided yet if behavior is a genetics thing or a learned thing. (Surprised?) So we can't really say for sure if birds learn their behaviors or know them by instinct. And we don't really know what drives instinct either… yet. (And it's my belief that we won't ever know. It's too complex and there are too many possible gene combinations to ever really study behavior without any other variables.)
When changes in behavior occur, it’s hard to know why. In general, “it is difficult to link genes with specific behaviors, especially in vertebrates” (2) which is why we use invertebrate models. Wait. So now, I’m studying the behavior of birds using fruit flies. Awesome. Because fruit flies and birds are just alike. Another problem is that sometimes behaviors aren’t all that variable (i.e. in our protopecker scenario above, all the protopeckers have similar behavior). Lack of variability also makes it difficult to study behaviors and their underlying genetic bases. Fortunately, we can engineer knockout mice and transgenic organisms to study behavior. Because transgenic organisms are just like “normal” organisms. Except that they have genes from other species.
Assuming all of these techniques are really telling us something relevant, we can learn how genetics drives behavior. Eventually. In the lab. But in natural environments, that’s where we come to the real difficulty. Studying behavior in natural environments and making inferences about evolutionary relationships from these studies is nearly impossible. It’s one thing to go bird-watching or bird-counting, and make an inference about the population’s movement, but another thing entirely to ascertain WHY they are feeding or moving or mating.
Now, back to Woody. Based on many studies of typical forest birds, we could say that Woody would probably not just up and leave his patch unless he A) couldn't get food, or B) couldn't mate. We’ve already assumed Woody had to leave due to the fact that he needed a mate. Once he found a mate and successfully reproduced, these birds would start an entirely new population of mutated protopeckers (woodpeckers). Mutation after mutation, eventually these birds would exhibit "speciation" - they are now a new species.
What does this all mean? Friends, there is no way to prove that woodpeckers received some mutations, either one bit at a time, or all at once, that conferred their ability to “peck wood.” Environmental changes as rapid as described here would most certainly result in the loss of that population. Slow changes in the environment would suggest that the population would move on to a new location in the process of sampling. So what really drove the evolution of the woodpecker to its current state? What would ever cause a bird to suddenly start to peck at the wood? If just one mutation were required to confer the extra cartilage, or lengthen the tongue, or strengthen the beak, could it happen? Over a long period of time?
My students all know about my scandalous love affair with proteins. Proteins are what makes us who we are – all of us, man and beast alike. Extra cartilage means that somewhere, a gene was changed, a protein was made differently, or a signaling pathway was modified – and all of these require proteins. Proteins are what makes the world go ‘round. Changing just one protein would require no less than seven gene mutations (3). Seven. That, my friends, is highly unlikely.
1 Fleischer et al. Mid-Pleistocene divergence of Cuban and North American ivory-billed woodpeckers. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1686174/?tool=pmcentrez
2 Behavioral Ecology, Danchin, Giraldeau, and Cezilly. 2007. Oxford Press.
3 Ann K. Gauger & Douglas D. Axe, "The Evolutionary Accessibility of New Enzyme Functions: A Case Study from the Biotin Pathway," BIO-Complexity, Vol. 2011(1) (2011).