Observations of a Nerd

Evolution: A Game Of Chance?

One of the toughest concepts to grasp about evolution is its lack of direction. Take the classic image of the evolution of man, from knuckle-walking ape to strong, smart hunter:
i-8c66c4d51330345ea25f9764619ec10e-human-evolution.gif
We view this as the natural progression of life. Truth is, there was no guarantee that some big brained primates in Africa would end up like we are now. It wasn’t inevitable that we grew taller, less hairy, and smarter than our relatives. And it certainly wasn’t guaranteed that single celled bacteria-like critters ended up joining forces into multicellular organisms, eventually leading to big brained primates!

Evolution isn’t predictable, and randomness is key in determining how things change. But that’s not the same as saying life evolves by chance. That’s because while the cause of evolution is random (mutations in our genes) the processes of evolution (selection) is not. It’s kind of like playing poker – the hand you receive is random, but the odds of you winning with it aren’t. And like poker, it’s about much more than just what you’re dealt. Outside factors – your friend’s ability to bluff you in your poker game, or changing environmental conditions in the game of life – also come into play. So while evolution isn’t random, it is a game of chance, and given how many species go extinct, it’s one where the house almost always wins.

Of course chance is important in evolution. Evolution occurs because nothing is perfect, not even the enzymes which replicate our DNA. All cells proliferate and divide, and to do so, they have to duplicate their genetic information each time. The enzymes which do this do their best to proof-read and ensure that they’re faithful to the original code, but they make mistakes. They put in a guanine instead of an adenine or a thymine, and suddenly, the gene is changed. Most of these changes are silent, and don’t affect the final protein that each gene encodes. But every once in awhile these changes have a bigger impact, subbing in different amino acids whose chemical properties alter the protein (usually for the worse, but not always).Or our cells make bigger mistakes – extra copies of entire genes or chromosomes, etc.

These genetic changes don’t anticipate an individual’s needs in any way. Giraffes didn’t “evolve” longer necks because they wanted to reach higher leaves. We didn’t “evolve” bigger brains to be better problem solvers, social creatures, or hunters. The changes themselves are random*. The mechanisms which influence their frequency in a population, however, aren’t. When a change allows you (a mutated animal) to survive and reproduce more than your peers, it’s likely to stay and spread through the population. This is selection, the mechanism that drives evolution. This can mean either natural selection (because it makes you run faster or do something to survive in your environment) or sexual selection (because even if it makes you less likely to survive, the chicks dig it). Either way the selection isn’t random: there’s a reason you got busier than your best friend and produced more offspring. But the mutation occurring in the first place – now that was luck of the draw.

Mistakes made by genetic machinery can lead to huge differences in organisms. Take flowering plants, for example. Flowering plants have a single gene that makes male and female parts of the flower. But in many species, this gene was accidentally duplicated about 120 million years ago. This gene has mutated and undergone selection, and has ended up modified in different species in very different ways. In rockcress (Arabidopsis), the extra copy now causes seed pods to shatter open. But it’s in snap dragons that we see how the smallest changes can have huge consequences. They, too, have two copies of the gene to make reproductive organs. But in these flowers, each copy fairly exclusively makes either male or female parts. This kind of male/female separation is the first step towards the sexes split into individual organisms, like we do. Why? It turns out that mutations causing the addition of a single amino acid in the final protein makes it so that one copy of the gene can only make male bits. That’s it. A single amino acid makes a gene male-only instead of both male and female.

Or, take something as specialized as flight. We like to think that flight evolved because some animals realized (in some sense of the word) the incredible advantage it would be to take to the air. But when you look at the evolution of flight, instead, it seems it evolved, in a sense, by accident. Take the masters of flight – birds – for example.

There are a few key alterations to bird bodies that make it so they can fly. The most obvious, of course, are their feathers. While feathers appear to be so ideally designed for flight, we are able to look back and realize that feathers didn’t start out that way. Through amazing fossil finds, we’re able to glimpse at how feathers arose, and it’s clear that at first, they were used for anything but airborne travel. These protofeathers were little more than hollow filaments, perhaps more akin to hairs, that may have been used in a similar fashion. More mutations occurred, and these filaments began to branch, join together. Indeed, as we might expect for a structure that is undergoing selection and change, there are dinosaurs with feather-like coverings of all kinds, showing that there was a lot of genetic experimentation and variety when it came to early feathers. Not all of these protofeathers were selected for, though, and in the end only one of these many forms ended up looking like the modern feather, thus giving a unique group of animals the chance to fly.

There’s a lot of variety in what scientists think these early feathers were used for, too. Modern birds use feathers for a variety of functions, including mate selection, thermoregulation and camouflage, all of which have been implicated in the evolution of feathers. There was no plan from the beginning, nor did feathers arise overnight to suddenly allow dinosaurs to fly. Instead, accumulations of mutations led to a structure that happened to give birds the chance to take to the air, even though that wasn’t its original use.

The same is true for flying insects. Back in the 19th century, when evolution was fledging as a science, St. George Jackson Mivart asked “What use is half a wing?” At the time he intended to humiliate the idea that wings could have developed without a creator. But studies on insects have shown that half a wing is actually quite useful, particularly for aquatic insects like stoneflies (close relatives of mayflies). Scientists experimentally chopped down the wings of stoneflies to see what happened, and it turned out that though they couldn’t fly, they could sail across the water much more quickly while using less energy to do so. Indeed, early insect wings may have functioned in gliding, only later allowing the creatures to take to the air. Birds can use half a wing, too – undeveloped wings help chicks run up steeper hills – so half a wing is quite a useful thing.

But what’s really key is that if you rewound time and took one of the ancestors of modern birds, a dino with proto-feathers, or a half-winged insect and placed it in the same environment with the same ecological pressures, its decedents wouldn’t necessarily fly.

That’s because if you do replay evolution, you never know what will happen. Recently, scientists have shown this experimentally in the lab with E. coli bacteria. They took a strain of E. coli and separated it into 12 identical petri dishes containing a novel food source that the bacteria could not digest, thus starting with 12 identical colonies in an environment with strong selective pressure. They grew them for some 50,000 generations. Every 500 generations, they froze some of the bacteria. Some 31,500 generations later, one of the twelve colonies developed the ability to feed off of the new nutrient, showing that despite the fact that all of them started the same, were maintained in the same conditions and exposed to the exact same pressures, developing the ability to metabolize the new nutrient was not a guarantee. But even more shocking was that when they replayed that colony’s history, they found that it didn’t always develop the ability, either. In fact, when replayed anywhere from the first to the 19,999th generation, no luck. Some change occurring in the 20,000 generation or so – a good 11,500 generations before they were able to metabolize the new nutrient – had to be in place for the colony to gain its advantageous ability later on.

There’s two reasons for this. The first is that the mutations themselves are random, and the odds of the same mutations occurring in the same order are slim. But there’s another reason we can’t predict evolution: genetic alterations don’t have to be ‘good’ (from a selection standpoint) to stick around, because selection isn’t the only evolutionary mechanism in play. Yes, selection is a big one, but there can be changes in the frequency of a given mutation in a population without selection, too. Genetic drift occurs when events change the gene frequencies in a population for no reason whatsoever. A massive hurricane just happens to wipe out the vast majority of a kind of lizard, for example, leaving the one weird colored male to mate with all the girls. Later, that color may end up being a good thing and allowing the lizards to blend in a new habitat, or it may make them more vulnerable to predators. Genetic drift doesn’t care one bit.

Every mutation is a gamble. Even the smallest mutations – a change of a single nucleotide, called a point mutation – matter. They can lead to terrible diseases in people like sickle cell anemia and cystic fibrosis. Of course, point mutations also lead to antibiotic resistance in bacteria.

What does the role of chance mean for our species? Well, it has to do with how well we can adapt to the changing world. Since we can’t force our bodies to mutate beneficial adaptations (no matter what Marvel tells you), we rely on chance to help our species continue to evolve. And believe me, we as a species need to continue to evolve. Our bodies store fat because in the past, food was sporadic, and storing fat was the best solution to surviving periods of starvation. But now that trait has led to an epidemic of obesity, and related diseases like diabetes. As diseases evolve, too, our treatments fail, leaving us vulnerable to mass casualties on the scale of the bubonic plague. We may very well be on the cusp of the end of the age of man, if random mutations can’t solve the problems presented by our rapidly changing environment. What is the likelihood that man will continue to dominate, proliferate, and stick around when other species go extinct? Well, like any game of chance, you have to look at the odds:

99.99% of all the species that have ever existed are now extinct.

But then again – maybe our species is feeling lucky.

* If you want to get into more detail, actually, mutations aren’t completely random. They, too, are governed by natural laws – our machinery is more likely to sub an adenine for a guanine than for a thymine, for example. Certain sections are more likely to be invaded by transposons… etc. But from the viewpoint of selection, these changes are random – as in, a mutation’s potential selective advantage or disadvantage has no effect on how likely it is to occur.

ResearchBlogging.orgReferences:
Airoldi, C., Bergonzi, S., & Davies, B. (2010). Single amino acid change alters the ability to specify male or female organ identity Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1009050107

XU Xing, & GUO Yu (2009). THE ORIGIN AND EARLY EVOLUTION OF FEATHERS: INSIGHTS
FROM RECENT PALEONTOLOGICAL AND NEONTOLOGICAL DATA Verbrata PalAsiatica, 47 (4), 311-329

Perrichot, V., Marion, L., Neraudeau, D., Vullo, R., & Tafforeau, P. (2008). The early evolution of feathers: fossil evidence from Cretaceous amber of France Proceedings of the Royal Society B: Biological Sciences, 275 (1639), 1197-1202 DOI: 10.1098/rspb.2008.0003

Marden, J., & Kramer, M. (1994). Surface-Skimming Stoneflies: A Possible Intermediate Stage in Insect Flight Evolution Science, 266 (5184), 427-430 DOI: 10.1126/science.266.5184.427

DIAL, K., RANDALL, R., & DIAL, T. (2006). What Use Is Half a Wing in the Ecology and Evolution of Birds? BioScience, 56 (5) DOI: 10.1641/0006-3568(2006)056[0437:WUIHAW]2.0.CO;2

Blount, Z., Borland, C., & Lenski, R. (2008). Inaugural Article: Historical contingency and the evolution of a key innovation in an experimental population of Escherichia coli Proceedings of the National Academy of Sciences, 105 (23), 7899-7906 DOI: 10.1073/pnas.0803151105

Comments

  1. #1 kleer001
    November 1, 2010

    Good article.
    In addition I would say there are lots of types of selection. And that natural disasters is one of them, just like you illustrated with a massive hurricane. Selected by luck.

  2. #2 Raven
    November 1, 2010

    “What use is half a wing?” As you correctly note, baby chicks flap their little wings when they want to run faster.

    Adult chickens do this too, although not as adorably. All that flapping and running has the bonus effect of making a pretty big commotion. All the better to confuse you with, my dear!

  3. #3 CW
    November 1, 2010

    Excellent article. I really enjoy nice overviews of evolution that give examples, analogies, and emphasis of important points. Thank you!

  4. #4 megan
    November 1, 2010

    Well as they say life’s crap shoot, but what about how environment puts selection pressure and results in similar physiology. Is it because most life works off of a central chordate model that all subsequent evolution of carnivores have similar teeth claws, or when they take to the air develop the same physics based wing shape structures?

    What if the Pre-Cambrian extinction never happened and all the other myriad of cellular structures and chemistry competed and co-evolved. That’s why I like the Star Trek guesses on alien life, except for the ones about ancient beings seeding the universe with their DNA physio type to explain humans, vulcan, romulans, klingons etc looking alike.

  5. #5 Birger Johansson
    November 2, 2010

    -Unfortunately, a huge number of people think in the manner of an infamous pundit: “why aren’t the apes evolving into humans today” ? Evolution has NO DIRECTION!!! (alas, yelling does not cure wilful ignorance).
    — — —
    An interesting result of selection: “Why women live longer than men” http://www.physorg.com/news/2010-11-women-longer-men.html

  6. #6 darwinsdog
    November 2, 2010

    All cells proliferate and divide, and to do so, they have to duplicate their genetic information each time. The enzymes which do this do their best to proof-read and ensure that they’re faithful to the original code..

    Are you sure about this? Aren’t there selection pressures on the genes that code for replicase & proof-reading enzymes that optimize their replication fidelity, ensuring that it is accurate enough but not too accurate? After all, if there was 100% fidelity there would be no point mutation.

  7. #7 steve miller
    November 2, 2010

    You should consider adding a “print” button as these are nice essays and the letters are small.

  8. #8 Omar
    November 2, 2010

    Good explanation of evolution, thanks!!

  9. #9 Excited Neuron
    November 2, 2010

    REALLY nice article. I’ll try to use it this year in my high school biology classes. I especially like the poker analogy. Thanks for sharing!

  10. #10 Paulo
    November 3, 2010

    I especially enjoyed reading this article. Evolution has always been one of my passions and without a doubt the hardest thing to explain to somebody is exactly this tenet that the mechanisms of evolution have no direction, that whilst randomness and chance play a big part it’s not wholesome random.
    Whilst this is all true I also believe that there are constrictions, that channel species into certain niches and appearances, such as embryological conditions and mathematics (look at animals body plans), though there is certainly enough space there for the marvelous variety of life.

  11. #11 4liberty4all
    November 3, 2010

    the image that came to mind as I read was fractals..

  12. #12 supertec
    November 4, 2010

    Very good, succinct article. I think words like “evolved”, as you rightly imply, are bandied about too often without the deeper understanding of the term. And like Excited Neuron, I especially like the poker analogy.

  13. #13 dave
    November 4, 2010

    hate to be a dick but there is evidence that us becoming smart was basically inevitable because there was parallel evolution going on there. there were several ancestors on different continents going in the exact same direction; all with the same “direction”

  14. #14 Kevin
    November 4, 2010

    Excellent article!

  15. #15 Charles J. Slavis, Jr.
    November 5, 2010

    In it’s basic form evolution is merely change. If change happens for the better it is more likely to continue but there is no guarantee. We are proof that we can exist and given forever in an eternally evolving situation we can and will probably exist again….and again. Start with nothing. Fill it with an expanding universe. That universe will always be a singularity when compared to the vastness of nothing.

  16. #16 natalia
    November 5, 2010

    love your blog!

  17. #17 Cordelia
    November 6, 2010

    Evolution is a fascinating topic. There are many ways to explain it. Your observations about it being up to chance made me wonder how different some species could have turned out. What if instead of birds developing feathers that would allow them to fly, large dinosaurs did? It would have been unlikely, but in the theory of chance it could have happened.

    Another thought that this post sparked was that negative traits can outweigh positive. I have bottom wisdom teeth that are growing in horizontally and non-existent upper wisdom teeth. My great-grandma died of a wisdom tooth infection because it was growing against another tooth. Even though I inherited no top wisdom teeth, the bottom ones could still kill me if I don’t get them removed.

    Also, the example about how our bodies evolved to store fat to combat starvation is now a negative trait in populations that eat the amount and/or type of food that has been leading to an “epidemic of obesity” is interesting. If those populations eventually lost that trait, would that be considered de-evolution?

  18. #18 Bjørn Østman
    November 18, 2010

    Christie, note that Lenski’s E. coli have been growing for only 50,000 generations.

    Also, how come you write that the bacteria “developed the ability”? It is specifically “evolved the ability”.

    Kleer wrote,

    In addition I would say there are lots of types of selection. And that natural disasters is one of them, just like you illustrated with a massive hurricane. Selected by luck.

    Natural disasters do not count as selection. You may say so, of course, but you will be alone.

    Cordelia wrote,

    If those populations eventually lost that trait, would that be considered de-evolution?

    No, nothing is considered de-evolution. It is common for species to lose traits (e.g. eyes, hair, tails, limbs, wings, appendices), and evolution does not imply some kind of progress by only adding traits.

  19. #19 Mark
    November 19, 2010

    Hey Christie, just wondering if you fancied entering our #evolution blog contest http://www.science3point0.com/bloggingcontest

  20. #20 billnut
    December 13, 2010

    Megan asked,”What if the Pre-Cambrian extinction never happened and all the other myriad of cellular structures and chemistry competed and co-evolved.”

    Actually, the chemistry and cellular structures were well established a billion years before the pre-cambrian extinction. What was lost was a great diversity of body plans. See Stephen J Gould’s book “Wonderful Life” for a great exploration of the topic.

  21. #21 billnut
    December 13, 2010

    Bjørn Østman wrote, “Natural disasters do not count as selection. You may say so, of course, but you will be alone.

    Of course natural disasters count as natural selection. That is why “fitness” can only be defined in retrospect.

    I guess we’ll have to be alone together Kleer ;-)

  22. #22 me
    May 15, 2011

    billnut wrote, “Of course natural disasters count as natural selection. That is why “fitness” can only be defined in retrospect.”

    Tautologically, it is only selection if the mortality rates differ by phenotype. Otherwise it is simply a random subset of the pre disaster population with no selection going on.

  23. #23 Stephen
    October 12, 2011

    Well written article, but something has always bugged me and I’ve never received a good answer. If 99.99% of all species that existed are now extinct, why do we not find more evidence of them in the fossil record? It seems that most fossils we find match up with a known species. Some, indeed, are extinct, but not 99.99% of the fossils! How is that possible?

  24. #24 Peter Bedson
    November 29, 2011

    Stephen 99.99% of species in the fossil record are extinct – obviously you can often see either their decendents or much less often very long-lived species today but at the species level extinction is the norm. Also, dont forget fossilisation is a very rare event and it only usually preserves part of the animal or plant; so unless the structure of what is preserved is characteristic of that species you may well be identifiying lots of different animals eroneously as the same species (though sexual dimorphism plays in the opposite direction). I guess fossils of many modern bird species would be very easily confused.