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10/12/11
SF Evolution Module Lecture 16
Case Study of Co-evolution:
Aposematism and Mimicry
Co-evolution is when changes in one species cause
adaptations in another, which further change the selective
pressures on the first, altering its evolution, and so on.
This may be:
A mutualism = everybody benefits from
a change in either species
e.g. signalling by poisonous prey to their
predators
An Arms Race = adaptation by one
species is detrimental to the fitness of
the other
e.g. increasing crypsis in camouflaged prey and
increasing ability to find them by the predator
We will look at co-evolution by considering just one system:
predator-prey interactions.
Most edible animals are cryptic
(= camouflaged).
Perfection of crypsis argues that
conspicuousness carries a heavy cost
Some which can’t hide by crypsis, use
masquerade, by appearing to be something
inedible e.g. a bird dropping or a leaf
But some are bright colours,
conspicuous shapes, very visible
to predators.
Why?
They are usually toxic and
defended by their “warning
coloration”.
They also have warning smells,
movements etc. so it’s better to
say “aposematic”.
Aposematism = signalling
unprofitability to potential
predators.
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For aposematism to work, the predator needs
to know the meaning of the signal already.
If aposematism is a mutualism, then we might expect the
signal to be designed to help the predator avoid the prey.
It must also remember which signal meant
poison.
We might also expect the predator to be adapted to learn
this signal easily.
So the aposematic signaller wants the
predator to see and recognise it.
It’s to the
predator’s
advantage to learn
too – it doesn’t
want to make a
mistake and lose
its dinner!
Aposematism relies on the predator recognising the
signal from its early encounters with it, and learning to
avoid touching prey that look the same.
So factors which enhance the speed of learning are of
benefit to aposematic animals.
So aposematic signalling is a co-evolved trait, to both birds’
and insects’ mutual benefit = a mutualism
Factors which aid learning -  Conspicuousness (also gives more time
to remember signal’s meaning)
-  Novelty of signal
-  Repeated signal
-  Truthful about outcome
-  Consistent effect following signal
-  Signal quickly followed by the
punishment or reward
-  Confirmed by other signals
So aposematic animals should be bright, conspicuous, preferably
aggregated and nasty tasting, with other cues e.g. odour, taste,
smell to back up their message… and they are.
The cost of aposematism to the prey
species is training naïve predators into the
meaning of the signal.
New baby birds eat the prey because they
don’t know it’s dangerous.
They learn by this, but they kill the prey
insect. So the insect pays the cost that it may
be the one who dies training the predator.
Aposematism therefore carries an intrinsic
risk.
This risk is lower, if you share a pattern with
lots of others so the chance of it being you
who does the teaching is lower.
So there is a second reason why aposematic
insects should aggregate – dilution of the
training cost.
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In addition, the risk to each individual is
reduced if several species share the
same signals.
Mullerian mimicry was named after
Fritz Muller 1878
Mullerian mimicry ring: Ladybirds
This reduces the number of signals the
predator must learn.
Red ring:
7 spot
Thus the different prey species co-evolve
their colour patterns
So there’s a second mutualistic coevolutionary relationship in the system
with different prey species sharing the
same warning signal.
11 spot
Black ring:
Pine
Kidney spot
5 spot
harlequin
eyed
24 spot
Yellow ring:
harlequin
16 spot
14 spot
22 spot
Sharing of signals between defended
prey species = Mullerian
Mimicry
Mullerian mimics experience normalising selection
pressures
Range of pattern found in
Different Mullerian mimics experience
convergent evolutionary selection pressures
Patterns more like the most protected
central pattern are at an advantage so
normalising selection occurs, reducing
variation in the signal.
Abundance of
pattern
Level of protection
given by that pattern
the species
Halo of protection around that
pattern, which birds also mistake
for the defended pattern
Species A and B start off
with different patterns
Their areas of protection
overlap as birds can’t
always tell them apart.
Prey with patterns in this overlap zone are protected by both
patterns, so pay a reduced cost of educating predators, and are
therefore at a selective advantage.
Thus the two patterns converge until all individuals of both
species have the intermediate pattern.
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Best studied mimics =
Heliconius butterflies
Mullerian mimicry ring: Viceroy
butterfly and models
Monarch is very toxic. Viceroy is very
nasty tasting, and a very good mimic of the
monarch in most of USA, despite being a
different genus.
In Florida Viceroy resembles the Queen
butterfly, a sister species of the Monarch.
Similar looking
butterflies aren’t closely
related.
Viceroy
Closely related ones
have very divergent
patterns even if they live
in the same place.
Monarch
In Mexico, it mimics another monarch
sister species, the Soldier.
The viceroy is therefore in 3 different
Mullerian mimicry pairings, copying the
local species of Danaus in each place.
Suggests convergent evolution occurred
several times in the Viceroy
Henry Walter Bates 1861 proposed Batesian
Mimicry = edible prey copying the signal of
aposematic animals.
Hoverflies, beetles and stingless wasps all
copy the signal of the true wasp, called the
model. Each mimic is completely harmless.
Batesian Mimics confuse the message, as
they are edible, and teach the wrong
message to the naïve predators.
The models lose out by this because more
of them are killed re-educating the predator.
The predators lose out, because they end
up eating more toxic prey.
Batesian mimics are therefore parasites of
the aposematic signalling system.
Why don’t they all
converge to the same
pattern?
Queen
One answer relates to
Batesian mimicry…
Soldier
Batesian mimics e.g. hoverflies, are in
two co-evolutionary relationships:
One with their models (the
aposematic species they copy)
e.g. wasps
One with the predators e.g birds
Both of these are arms races because:
an evolutionary step by the Batesian mimic reduces the fitness of the model
or predator
…and an evolutionary step by the model or predator reduces the fitness of
the Batesian mimic.
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Using our curves of protection we can look at the coReal
evolutionary trajectory:
patterns
Halo of
protection
A is the model and B is
the Batesian mimic, with
a similar but not
identical pattern.
2) Batesian mimics are in an arms race with the predators.
As Batesian mimics confuse the signalling system, the
predators end up unsure whether the signal means danger.
They end up taking longer to learn avoidance of it, so end up
eating more toxin.
model
mimic
Birds mistake mimics in the overlapping zone for the model,
so these individuals get good protection from the birds.
So there is directional selection on mimic towards patterns most like the
model, and the curve will gradually shift left.
But patterns of the model which are in the overlapping zone will suffer most
from the confusion caused by the mimic. Patterns on the extreme other side
will do best, so species A will gradually move left too.
B moves faster, because as A gets more unlike the original pattern, it loses
some of its protection from predators who already know that pattern. B loses
nothing by changing pattern.
Frequency Dependence
Mullerian Mimics are positively
frequency dependent = they do better
the more of them there are
because it allows them to share the cost
of educating predators.
Batesian mimics are negatively
frequency dependent = they do better
the fewer of them there are…
..because they ruin the signal they are
parasitising.
Problem for their evolutionary success!
Any mutation of the Batesian mimic
to look more like the model is still
favoured
But any mutation of the bird which
makes it better able to distinguish
the Batesian mimic from the model is
favoured.
This makes the halo of protection
around the original pattern narrower,
as the bird’s error around the signal
reduces.
Overlap much
reduced
Halo of
protection
around the
signal
Less overlap produces fewer errors.
Batesian mimics are often polymorphic to get out of this
problem. Being polymorphic allows them to spread the load
between more than one signal, so increase in total number.
2 spot ladybirds have both red and black
morphs, so they parasitise both the red
and black Mullerian mimicry rings.
10 spot ladybirds have 3 morphs,
one in the red ring, two in the black.
NB 2 spot and 10 spot ladybirds are
perfectly edible.
Their models, 7spot (red model) and
pine ladybird (black model), are toxic
Red ring model
Black ring model
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Disruptive selection
So 2spot and 10spot ladybirds experience
disruptive selection, i.e. working to tear the
population apart into two distinct morphs.
Note: The black model is less common than
the red model, so gives less effective
protection. Consequently black 2spot
ladybirds are less common than red ones.
Who is the model?
The coral snake is deadly poisonous.
The milk snake is much less toxic, but copies the
signal of the coral snake.
“red touches yellow will kill a fellow” … but the
signal is accurate enough to fool a predator in a
hurry! Even less good mimics are ok.
But most models are not deadly. A trained
predator is a safe one – killing it means having to
teach another naïve one (at a cost to the models
and the Batesian mimics).
It has been argued that actually the coral snake is
mimicking the less poisonous milk snake, who is
training predators, not killing them.
(= Mertensian mimicry)
So why don’t
Mullerian mimics all
converge on the same
pattern?
Polymorphic
model H.
melpomene
Polymorphic
Batesian mimic
H. erato
The models may end up
with several patterns, if
some patterns carry a
huge Batesian load
while others do not.
The cost of teaching the predators a new pattern
may be less than the cost of working with a signal
which the predators are confused by.
Automimic = member of an
aposematic species which lacks the
defensive chemical so are effectively
Batesian mimics of their own species.
e.g. monarch butterfly usually lays its
eggs on milkweed, from which its
caterpillars sequester toxins.
If no milkweed available, they lay on
plants without the toxin.
These individuals are automimics, reducing the fitness of their
own better protected relatives by degrading their own signal!
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Summary:
Co-evolutionary associations may be very
complex, multi-species, and driving evolution in
different directions at the same time.
There are many more; it’s not only predator/
prey associations.
Mutualisms exist in which all the species benefit
by adaptation in any of them
Arms races are also common, where adaptation
by one species is detrimental to the others.
Mullerian and Batesian mimicry systems provide
examples of both mutualistic and arms race coevolutionary associations which combine to drive
evolutionary change in a predictable direction.
Key words and glossary:
Coevolution - when two or more species influence each other's evolution
Aposematism - toxic and brightly coloured organisms (= Warning
Colouration)
Convergent evolution - the development of similar traits in distantly
related organisms.
Mullerian mimicry - toxic organisms which share a colour pattern with
other toxic ones.
Batesian mimicry - edible organisms which copy the signal of toxic
models (usually Mullerian mimics).
Frequency dependent selection - when the selection pressure depends
on the relative numbers of two types of individual e.g. Batesian mimicry
It is negative when more of one species means less survival for that
species.
Background Reading:
Barnard “Animal Behaviour” Chapter 8.3 “Antipredator
behaviour”
Alcock “Animal Behavior” 8th Ed. Chapter 6 “Behavioural
adaptations for survival”
Krebs and Davies “Introduction to Behavioural Ecology”
Chapter 4 “Predators versus prey”
And look up any of the key words in the web or behaviour text
books.
If you get into it:
Speed MP 2000 Warning signals, receiver psychology and predator
memory. Animal Behaviour 60, 269-278
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