How do we evolve?

When it comes to flies, scientists can measure changes in DNA and in theory, we could use the same method to see if humans are evolving...
05 January 2016

Interview with 

Simon Collier and Dr Sam Lewis, University of Cambridge


Initially, what drew Graihagh Jackson to Cambridge University's fly lab was to Fruit flylearn about how scientists measure evolution - flies are often used to model humans when it comes to this sort of stuff - but when Graihagh arrived, she was completely overwhelmed by fly lab manager Simon Collier's office...

Simon - Yes.  We definitely have a fly theme.  I've worked with flies now for 20 - 25 years and, through the places I've been, I've accumulated a number of sort of fly souvenirs and my students like to give me little gifts of models flies or pictures of flies that they've drawn, and they sort of surround us right now.

Graihagh - They are great.  I really like them.  I feel like I've seen more of a fly than I've ever considered before.

Simon - Yes, and people don't really look closely at the these animals.  I mean we just have them buzzing around and mostly we think they're a nuisance, but they are really quite spectacular when you get up close and see what they're really made of. 

Graihagh - Ok, back to the science and to see what flies are really made of, Simon and I 'took flight' to the flylab...

Simon - So we're now standing inside the main flylab and we are surrounded by benches which have microscopes on and this is where fly people work.

Graihagh - So I don't know quite what I expected but I think when I came here I did actually expect more of a pet shop but actually you're right, its desk after desk, microscope after microscope.

Simon - Yes.  So out here is does just look like a lot of microscopes, but if we walk over here into one of the constant temperature rooms and we'll see it looks quite different.

Graihagh - Oh.  It's like a sauna in here.  It's nice and humid.  Actually, I'd say more steam room than sauna.

Simon - Yes. We keep this at high humidity and the temperature is about 25 degrees centigrade, which is a warm summer day.

Graihagh - Tropical and there are rows after rows of little test tubes filled with flies.

Simon - Yes.  So you can have a look.

Graihagh -  Oh they're all jumping around.

Graihagh - At the bottom of each tube you can see something that looks green and that's the food.  And the food is very simple; it's just cornmeal, sugar, and yeast - very cheap too, and if you leave the flies in that tube you can essentially just walk away.

Graihagh - It reminds me of a little essay I wrote when I was very young - I was about 7 years old - where I write about what the perfect pet is, and I write it's a snail because they don't need much looking after, and you can go on holiday and not have to worry about it and they don't need much food.  And afterwards, when you get bored - you can eat it.  And what strikes me about these is they are also very low maintenance although you probably don't want to eat them.

Simon - Well I'm sure we occasionally do by mistake, there are so many around.  Yes and they are low maintenance and I think, when you do the sums, we can accommodate, I believe, 60,000 of these tubes in these four rooms.

Graihagh - So if you can accommodate 60,000 tubes - there must be what 30, 40 flies in one of these tubes?

Simon - Yes.  Probably several hundred actually flies in each of these tubes, so you can do the maths and work out just how many flies we can have here...

Graihagh - I don't trust my maths!  Well as long as they're contained, although I do feel a bit itchy.  So these are all, obviously, various experiments going on.  What sort of things would they be looking for and doing with them?

Simon - A whole variety of things. But I mean the fruit fly is used as a standard model for biomedical research, and what I mean by that is we use the fly as a model so we can understand some general processes which apply to us as much as to the fruit fly

Graihagh - One fly fan is Sam Lewis - a postdoctoral researcher at Cambridge who works on evolution and genetics...

Sam - Well these are fruit flies which are one of the main model organisms that we use to study evolution.

Graihagh - So if you look at them down a microscope - what do you see?

Sam - Well one of the easiest things to see is the difference between males and females.  Females have a very white abdomen and the males are a bit slimmer and have a kind of black bum, basically.

Graihagh - And I assume when you are looking at them, you don't look at them down a microscope and see their DNA, so how do you go then from fly to looking at it's DNA?

Sam - Unfortunately for the fly, the first step is to grind them...

Graihagh - Brutal...

Sam - Then you can extract their DNA and then you can work out the work out the sequence of As, Cs, Gs, and Ts that make up a certain gene.

Graihagh - Why are flies so good to use as models when you come to look at genetics and evolution?

Sam - Well, it's the combination really of the amount that we know about most of the genes that they have, but also the amount that you can manipulate them to find out more about those genes.  So you can do a lot of different experiments putting them on different diets, or under different temperatures, and then you can look and see how that changes their genetic makeup and how that might then be triggering evolution.

Graihagh - How would you look at that?  How would you determine whether a fly has evolved or not?

Sam - There are two possible approaches - well a number of possible approaches, but two main ones are either comparing the genetic makeup of flies from different habitats and seeing if there are differences in the genes that might help them respond to those habitats.  Or you could take flies that you know the genetic makeup of at the beginning of experiments, split them into different regimes, so maybe one in a hot environment, one in a cold environment, and then evolve them under that selective regime for a number of generations, and then look at time points through the experiment to see how the genes are changing and, given that this is the only difference between them, you get a good idea of how they are responding to that change.

Graihagh - So say a fly's been put in a progressively, or generations of flies have been put in progressively warmer and warmer environments, how quickly would a fly evolve to be more comfortable, let say, in a warmer environment.  Is it within a few generations?

Sam - Usually it's slower than that but, given the precision that you can measure these things at, you can pick up changes that occur in dozens of generations.  I mean you can get really large effects by just looking at a few hundreds of generations which, obviously, would be a lot harder to do in humans, but in flies that's not all that long.

Graihagh - Yes.  I was going to say a dozen generations to see some changes and then say a few hundred for quite large changes - that seems remarkably fast.  Is that comparable in humans?

Sam - I don't think we really know without doing those kind of experiments.  It would be very difficult to say how quickly you could have seen it if you were looking a bit earlier.

Graihagh - I suppose the difficulty is, is that we have not been looking at people's genomes for the last dozen generations, let alone hundreds of generations.  We're just not that easy to study like the fruit fly and I suppose the other thing is, at least in a fruit fly, you can just expose it to a warmer environment or a colder environment, whereas, I'm not sure any human being would be happy with that sort of condition.  But there are so many things interacting and going on with humans that it's really hard to pinpoint what's changing where.

Sam - Yes, exactly, and one of the hardest things, I think, about inferring human evolution is kind of with humans and with moving populations and intermixing of different genes, it makes it a lot harder to say for certain when a change occurred, or even if that change is causing the effect that you think it is.

Graihagh - But I suppose that doesn't mean we're not evolving.  We just might not be able to tell how we're evolving.

Sam - Yes. There are some examples where there are kind of relatively clear changes that correlate with alterations in diet, for example, in humans.  So a gene called amylase breaks down starch and if you look at populations which have a high starch diet, they have more copies of the amylase gene, so they would have more amylase protein than populations with a low starch diet, that's being acted on by natural selection to allow humans to break down starch.

Graihagh - I'm thinking of potato farmers in Peru...

Sam - Yes.

Graihagh - High starch diet.  There is starch in potatoes - isn't there?

Sam - Yes.  I'm not sure which populations they looked at exactly.

Graihagh - Well probably my mum as well, and myself are big potato eaters so we probably have that starch gene as well.  So does that mean we are still evolving?

Sam - It's probably very difficult to say we are still evolving currently but it would be very surprising if we weren't, and we certainly have been up until very recently when we last looked.

Graihagh -  Last looked!  When did we last look?

Sam - The last few generations.


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