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Hello and welcome. You are listening
to a podcast by the Milner Centre for
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Evolution at the University of Bath.
I'm Professor Turi King, your host,
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and today I'm talking to Matthew Wills, who is
professor of evolutionary paleobiology. And today
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I'm going to be chatting to him about crabs and
an evolutionary phenomenon known as carcinization,
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which basically means to become more crab like.
Matthew, I know this phenomenon of carcinization
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is a form of convergent evolution.
But before we get into what that is,
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let's start with evolutionary trees. What
are they and how did people develop them?
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Well, evolutionary trees are effectively
a family tree of species. So, they show us
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a picture of how species are related to one
another, and we infer them from two types of
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data principally. Historically, people would
rely much more on comparative anatomy. So,
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they would look at organisms, they would try
to encode aspects of the morphology, and then
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they would produce a tree from that. So, things
like, for example, how many pairs of legs does
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something have or does it have eyes, and so on.
More recently, in the last 30 years or so,
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people have relied much more on sequence
data, molecular data, sequences of amino
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acids and sequences of bases within genes.
And that's basically turned things a little
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bit on its head, hasn't it? The whole
genetic stuff has started to change what
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we know about the Tree of Life, hasn't it?
Yeah, hugely. There's a problem in looking
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at anatomy and morphology, and it's a problem
called convergent evolution. This is a phenomenon
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whereby organisms that are not particularly
closely related can evolve in such a way
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that they look remarkably similar. And they do
this because there's a similar set of physical
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or biological constraints acting on them.
So, for example, within mammals, there are two
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major lineages of mammals. One are the placentals.
So, we're placental mammals, we give birth to
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relatively large young. And the other lineage are
the marsupials; they give birth to a little tiny
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neonate about the size of a little jellybean.
And this crawls to the pouch in the mother and
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there it suckles and grows for a very long time;
the rate of development is much slower. And these
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two lineages, these two big branches of the mammal
evolutionary tree, diverge back in the Cretaceous,
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or possibly even back further into the Jurassic.
And so, they're evolving in different regions,
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in different continents to a great extent.
And they're starting from a very sort of
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modest ground plan, something a little bit
like a mouse or a rat, and they're diverging
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and they're diversifying morphologically in
different places and doing so independently.
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And the remarkable thing is how similar certain
species of marsupial look to certain species of
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placental mammal. A best-known example is probably
the thylacine or the Tasmanian tiger or Tasmanian
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wolf. But the remarkable thing about the
thylacine is it looked almost identical,
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particularly if you looked at a skull to a dog
or to a wolf. The convergence in the morphology
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is so striking, the shape of the teeth, the shape
of the skull just absolutely stunning. And this
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has happened through 100 million years or more
of independent evolution in these two lineages.
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So, it really is utterly remarkable, it doesn't
stop there. There are marsupial mice that look
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very similar to the placental mice. There are
marsupial moles that look almost identical to
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the placental moles. Flying squirrels, they've
arisen in both lineages, honey gliders and so on.
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These are just remarkable instances of
convergent evolution. And it's almost
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like the two lineages have invented the same
things, because there are similar ecological
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and evolutionary pressures on both groups.
And this is kind of converse to what people
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often think of with evolution. So, like
Darwin's finches, right. So, he goes to
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the Galapagos Islands, and he sees these various
types of finches, you know, some have got really
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long pointed beaks, a bit like tweezers that are
great for kind of picking insects off of leaves
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and branches. And then you've got these ones with
a broader beaks for cracking nuts and seeds. So,
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the idea is that they originally arrived on the
islands as one species, and that over time they've
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evolved these little differences that allow
them to fill specific sort of ecological niches.
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So, this is divergent evolution coming
from a common ancestor, evolving into sort
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of different types like twigs on the branch
of a tree. Convergent evolution is actually
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when the common ancestor is much further back, but
they've basically evolved to look awfully similar.
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Well, that's a really good contrast, because
within the placentals and within the marsupials,
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yes, they are diverging and they're diverging
and they're exploring the possibilities of what
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you can do as a mammal. But because they're both
doing that independently, they're both coming up
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with the same kind of solutions to similar sets
of problems, and that's the convergent bit. So,
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it's one from one side of the tree and
one from another side of the tree. So,
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there's a marsupial and there's a placental
each coming up with the same sort of solution.
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And it's not just body type as well isn’t it,
it can be like little characteristics. So,
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you've got whales and bats have both
separately come up with using echolocation.
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Oh, this is an amazing example because it's
mediated by convergence in the molecules in a
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gene called a prestin gene. And this has similar
amino acid substitutions in both bats and whales.
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Now you can't get much more different ecologically
really, than a bat in a whale. I mean,
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morphologically as well. You can't really imagine
two mammals that look more different. But because
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they are both using high frequency sound in order
to echolocate and in order to navigate the world,
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they've needed to increase that high frequency
responsiveness in the cochlea. And it's the
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same gene that's mutated in a very similar
way in those two lineages to enable them to
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do that. And that's really quite mind blowing.
So, this is the thing I find really fascinating
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because evolution is not directive. It's not
like it's working in a particular direction;
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it knows where it wants to go. It's little, tiny
mutations that will happen in genes spontaneously.
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And for whatever reason, a little mutation
might happen that might go, oh, well, okay,
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so that helps this animal survive that little bit
better to go on to have children that will also
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carry that little mutation, and then another
mutation happens. And what's so fascinating
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with the kind of the whales and the bats is
that's happened completely separately, just
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because it confers that little advantage. So, it's
evolved on two separate tree branches essentially.
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So yes. So, evolution typically take small
steps. And for something to be advantageous
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it has to be advantageous now. So, evolution
can't look forward and plan or it can't make
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a change now because it will make something
else that's good possible down the line.
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There's no foresight, there's no teleology.
But what does happen is certain changes
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probably make certain other types of changes more
likely, and they probably close down other options
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as lineages evolve. There's one particular
reason for this, and that's something called
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pleiotropy. And that's the fact that organisms
aren't constructed in the way that an engineer
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might construct them. Many structures and genes
have several functions, and those functions often
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aren't very obviously related. So, you can make a
change that has a beneficial effect, but it also,
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unfortunately has a deleterious effect.
So, the best example I can think of
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is the confirmation of neck vertebrae in
mammals. Now you and I have seven vertebrae,
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seven bones in the neck, but so does a giraffe.
And giraffes have enormously long necks. Now,
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mammals have got painted into an evolutionary
corner here. They are stuck with seven neck
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vertebrae, not because that's biomechanically the
best number to have. If you've got a very long
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neck you can imagine in other groups, like swans,
for example, they’re birds, they're not mammals,
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they have 20 odd vertebrae in the neck.
Giraffes are stuck with seven because if
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you fiddle around with the genes that change the
number of vertebrae in the neck, there are other
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bad things that happen to you, you produce lots
of tumours. You develop very wonky looking ribs;
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there is fibrous tissue between the ribs and
so on. And so, there are these deleterious
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consequences because of this phenomenon of
pleiotropy. Things don't just do one job;
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they often do a whole bunch of jobs. So,
it becomes more difficult to make radical
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changes. And that's why body plans can kind of
become entrenched or canalised through time.
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So, we're going to talk about carcinization, which
is basically meaning to become more crab like. So,
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this is convergent evolution.
So, what makes a crab a crab.
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So, crabs are within a larger group of
crustaceans called the decapoda. And
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decapoda means that they have ten legs.
Now, the decapods include animals like
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prawns and shrimps and also things like lobsters.
And those animals have a particular morphology.
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They've got a cephalothorax, which is a fusion
of the head and the thorax. Sticking out behind
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that they have an enormously large and muscular
abdomen. Now this allows them to move around in
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a number of different ways. So, they have these
five pairs of walking legs on the cephalothorax.
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Very often the first pair of those will be
modified as very large claws or pincers,
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and that leaves them four pairs of usually rather
smaller legs behind that they're able to walk on.
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But they tend to walk front to back, they tend
to walk in the direction that they're looking.
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Behind then the cephalothorax is this great big,
muscular abdomen, and underneath that they have
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little appendages that are a little bit like oars,
and they're called pleopods. And particularly in
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the smaller shrimp and so forth, they can beat
these through the water, and they can swim.
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So, they can walk with their thoracic
limbs, they can swim with their pleopods,
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and they've also got another amazing trick up
their sleeve, they can contract all of the muscle
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in the abdomen. And this is a big chunk of muscle,
it's the bit that you eat when you eat a prawn,
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and most of what you eat when you eat a lobster.
But when they contract this, they've got a fan of
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appendages, right at the back of the abdomen,
and that splays out like a huge great big fan,
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and they snap the abdomen down underneath the body
and they shoot off backwards at very high speed.
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So, it's an amazing way of escaping predators and
of moving really fast. So, they're really clever,
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they've got three ways they can move around.
Now we like eating shrimp and lots of other things
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do as well. So that's a really big meal for a
predator and makes them sort of quite vulnerable,
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quite attractive in that respect. So, what crabs
have done, crabs have done away with most of that
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great big chunk of muscle in the abdomen. And
they’ve folded the abdomen down underneath the
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body, back up under the cephalothorax. They've
also tended to make the carapace much broader
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and much flatter, so that they can slip between
crevices in rocks, and so on. This also makes
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their centre of gravity much lower. So, they're
more stable, they're more able to hunker down and
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to sort of grab hold of the substrate or grab hold
of rocks, and that makes them much more defensive.
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And the limbs, they're sort of jointed in such
a way that now it's much easier for the crab to
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move sideways than it is to move front to back.
And there's more to this than you might think,
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because if you're trying to catch a crab, you've
probably realized that it will do two things.
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Firstly, it can run in either direction.
It can escape to the left or to the right,
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and you don't know which way it's going
to go, and it's the same for a predator.
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The other thing is because it doesn't have this
great big, long abdomen sticking out the back,
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it can rear up, it can sort of push
itself forward, it can raise its pincers,
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and it can get at you very easily. So, it's
an extremely manoeuvrable and very agile tank,
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and it can literally run circles around
things that it doesn't like the look of.
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And so that's the process of carcinization. Comes
with one or two other things, it comes with fusion
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of the plates on the underside of the animal as
well. And there's another advantage actually. So,
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lobsters and shrimps, when they are carrying
an egg mass, it's carried by the pleopods,
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but it's still quite vulnerable. You can
see this mass of eggs sort of underneath
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the abdomen. Whereas crabs, because they've
tucked the abdomen underneath the cephalothorax,
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they're protecting it. So, it's kind of
armoured, they armoured the egg mass as well.
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So, these crab like things, they've
evolved five separate times?
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So, there is a group of carcinized decapods
called the brachyura, and these are the true
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crabs. And these originate once, and the process
of carcinization happens in those true crabs. So,
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the edible crab for example, the swimming
crab, the green shore crab, ones that we're
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probably most familiar with and most often tend
to eat, at least in the UK, those originate once.
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Now there is a group of brachyurans that's gone
back the other way, so these things can sometimes
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go in reverse. Those are the raninidea or the
frog crabs. They've started to untuck the abdomen
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and they've gone from being wider than they are
long back to being longer than they are wide. So,
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they're actually sort of working in reverse.
But you asked me the question about separate
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origins and the group that sits next to
the brachyura on the phylogenetic tree are
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the anomura, sometimes called the false crabs.
So, these include things like squat lobsters
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and hermit crabs. And hermit crabs, you probably
know, have a soft abdomen and it's coiled up a
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little bit like a snail. And that's because hermit
crabs use the shells of molluscs once they've been
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discarded, to armour their abdomen, and when
they need to grow, they find a bigger shell.
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But some of the anomura have gone the whole way,
and they've become carcinized. So, they've come
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to look very, very similar to the true crabs,
to the brachyura. So, there's a group called
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the porcelain crabs, and they've done it very
efficiently. The king crabs, which are a hugely
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important species for fisheries, and huge numbers
of king crabs are eaten, those are a separate
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carcinization event from the brachyura. And
indeed, there's a group called the, or… actually,
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it's only a single genus called the lomisidea,
the hairy stone crab from Australia. Again, that's
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become carcinized separately all on its own.
So yes, there are multiple instances of becoming
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crab like. And again, that's convergent evolution.
But it's something that can be lost again,
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you've got groups that do away with it. And
it's down to what's contingently valuable to
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the organism or to the lineage at the time.
And the reason pleiotropy becomes important
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is if you get locked into a particular body plan
and you can't then put things into reverse. So,
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although we like to think of evolution as
being this open-ended process where anything
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is possible, there are many cases where we see
lineages shutting down options as they evolve.
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One of the things that's gone through the media
a lot is a question, is everything evolving
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into crabs? And it's only within the decapod
crustacea that crabs have evolved, in fact,
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only within the group the meiura that contains
the anomura and brachyura where that's happened.
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There are other arthropods that have evolved
things that look a little bit crab like. So,
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there are some spiders that superficially
resemble crabs, and indeed they're adapted to
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living in flowers, and they have a flattened body
plan, and actually they move sideways as well.
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But this is a very superficial resemblance.
So, yes, humans are not going to evolve into
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crabs. We don't have the developmental
or genomic toolkit to do that. We don't
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have an exoskeleton. And because we've kind of
committed ourselves to a particular body plan,
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it's very difficult to imagine that however
many hundreds of millions of years you give
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us were never going to evolve in that way.
And I know this isn't the only thing that's
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evolved more than once. So, there's lots of
different things that have evolved more than once.
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Yes. I mean, convergent evolution is a phenomenon
that's been studied across the tree of life. The
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evolution of wings, for example, flapping flight.
There are birds, of course there are bats,
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and bats are mammals. So, they've evolved
flapping flight independently. The pterosaurs,
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a completely extinct group, the sister group
to the dinosaurs. And those evolved flapping
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flight again, completely independently.
And I guess you could also include insects,
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because although they're very, very distant
from the vertebrates, they've also evolved
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flapping flight. So, it's a similar set of
biomechanical solutions to moving through the air.
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There are only so many ways that you can do that.
Eyes is another classic example. So complex eyes,
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eyes like ours, for example, with a lens, a
cornea, a retina. Those have evolved convergent,
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famously within the cephalopod molluscs. So,
within octopuses and squid there are differences
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in the way those eyes are innervated, how the
retina is wired up, for example. In vertebrates,
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it's done in a very odd way. The optic nerve
comes through the back of the retina and splays
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out over the photoreceptors, in fact. So, we
have a blind spot that's part of our visual
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field where you can't see anything. And molluscs
and the cephalopods, the octopus, for example,
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innervates the eye from the back. So, there's
not a blind spot, much more efficient way of
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doing things. So, there are differences in
detail, but the overall form of the eye is
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just incredibly similar because it's doing a
similar job. And we're separated from octopuses
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by a good 600 million years of evolution.
But not just twice. So complex eyes have
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evolved perhaps 20, 25 times across the tree
of life. So, there are all sorts of different
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types of eye. There are eyes that can form an
image, our eyes can do that. The eyes of insects,
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compound eyes can form images, they're very
good at detecting movement. The eyes of a fly,
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for example, much better at seeing things move
than we are. We tend to see a bit of a smudge.
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So, lots of groups have evolved these complex
eye types. There are simpler eye types,
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there are eyes that can simply tell the direction
that light is coming from. So many flatworms have
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eyes that can just sense directionality. And
right down to simple structures that can just
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detect if a shadow has been cast over them.
But that elaboration, that building on those
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early forms of eye to produce something which
is image forming, yes that's happened many,
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many times because of the physics of focusing
light, the physics of forming an image. There's
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only so many ways that you can do it. And
that's been reinvented multiple times.
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00:21:07,520
Matthew, thank you so much for talking with
me. This was a podcast by the Milner Centre
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for Evolution at the University of Bath. I'm Turi
King and thank you for listening. If you have any
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thoughts or comments on this or any other
episodes, please contact us via our social
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00:21:22,450
media. For more information about the Milner
Centre for Evolution, you can visit our website.
