Animals have evolved a remarkable range
of traits.
Animals live in a diverse
and incredibly complex set of different environments.
And within those environments they
employ features
that enable appropriate behaviours to be
performed.
Hello everyone, my name is Dr Paul Rose
and
in this short video I'd like to explain
some of the differences between
natural and sexual selection and I'm
going to do that by using a range of
natural history artefacts that I have
in my own kind of geeky natural history
collection.
So I'd like to talk a bit about traits
that we see in animals
that are naturally selected which convey
an adaptive or a fitness benefit to
individuals
within the population, so natural
selection works at a species level
and it drives to make that population
fitter
and more evolved to a particular habitat,
so the animals or the individuals within
the habitat
can best survive and breed in the future.
And I'd like to compare that to some
examples of sexually selected traits
which convey an advantage for an
individual's lifetime reproductive
success,
they don't necessarily enhance the
individual's longevity
but what they do do is enable those
individuals that possess them
to be more successful at attracting or
finding a partner
and therefore have a greater influence
over the genetic qualities
of the future generation.
So there are some classic examples that we commonly find in the textbooks around natural
selection and
sexual selection and some of those
species will make an appearance in this
video.
So we're all familiar with the peacock,
and we're all familiar with the peacock's
train
and the eye spots on the peacock's train
are one of these textbook examples
of a sexually selected response so we've
got a range of different colours and
patterns
as well as different structures to the
feathers that are used for the peacock's
courtship display,
so this is again a classic example of
sexual selection.
Behind those train feathers are these
dowdy but more structurally important
plumes
and these help support the tail when the
peacock raises his train
so here we've got sexual selection
and here we've got natural selection,
so we've got two examples
of an adaptive trait within the same
species;
we've got these stronger more supportive
feathers
that without them the peacock wouldn't
be able to raise his train
to perform his courtship display.
The train of the peacock is incredibly
long, these feathers
are almost as tall as me and this is a
classic example of sexual selection
although as I've said previously we're
still not completely aware
of their function, or their single
function,
but we know that they do have a role in
mate choice
and they are used by the peahen in some
capacity to help choose a partner
and you can see the differences in colour
that is exhibited by these peacock train
feathers
as they catch the light, so imagine much
more of these feathers
all grouped together in the peacock's
train, that's a
real big handicap for the bird to carry
around
and it's something that's obviously
going to influence that individual's
longevity, fitness, as well as his
breeding success
throughout the course of his life.
So classic example of something
where evolution has made this not for
the adaptive capabilities of the animal
to survive in his habitat
but for something that's definitely
going to show off his abilities
and actually being a really, really fit
and strong individual
that's going to make the best breeding
partner.
You wouldn't evolve this to keep you
safe in the wild.
Long tail feathers are not just a
feature of the peacock,
they're a feature of many different
birds and again
they can have different functions based
on the habitat or the environment that
the animal finds itself in
as well as the role that the feather
plays to the bird's behaviour.
Parrots for example, these are the tail
feathers of a blue and gold macaw,
these incredibly long plumes help
guide the animal whilst it's flying, they
have a really good support and balance
and they are a very long rudder for
when it's flying to the rainforest
and also because parrots like to climb,
they're very, very good at climbing with
their beak and their feet,
if you're climbing around at the tops of
trees and very thin or very narrow
perches
and you want to maintain your balance so
do these long plumes
enable you to maintain your balance.
And you can tell this is a tail feather
because
it's equal along the central shaft down
the middle of the feather
we've got equal filaments on this side
and we've got equal filaments on that
side
so this type of feather is clearly
designed for posture,
orientation or balance.
Let's compare that to another tail
feather,
this is a tail feather from a golden
pheasant,
it's relatively long and it has less
structure
than what we can see in the macaw tail
feathers. The macaw tail feather
is relatively rigid, the golden pheasant
feather
is quite floppy, this is another example
of courtship display,
this is very, very similar to the
peacock's train, so this is another
handicap principle
where we've got the individual growing
this
long plume for
the same reason why the peacock grows
his train, to use this in his courtship
display to impress the female.
The peacock is not the only species of
birds to take advantage of what we call
ocelli or eye spots
on these display plumes, and this rather
moth-eaten example
is from an argus pheasant, and you can
see the row of eye spots
that is used in the male's courtship
display. Argus pheasants have the most
incredible display
where the male basically turns his wings
around the wrong way
to show off this dazzling array of eye
spots to the female,
and Argus, the Greek god of eyes, is where
the bird gets its name from.
And then these are the display plumes of
the grey peacock pheasant
and clue its name, it's got eye spots
on its plumes
just the same as we see in the peacock.
So the argus pheasant,
the peacock pheasant, and the peacock
itself
are all employing a very similar type
of behaviour based on the anatomy
and the structure of their feathers, the
colour and patterning of their feathers
are all conveying a similar message to
the female,
I'm a good quality mate, I can put on a
really good courtship display
and therefore you should choose me to be
the father of your offspring.
That long, rigid tail that we saw in the
macaw
is mirrored in even smaller parrots too,
these are the tail feathers of a
budgerigar. This is one of the world's
largest parrots,
this is one of the world's smallest
parakeets, yet the function of the tail
feather
is exactly the same, so within
family resemblances, habitat similarities,
behavioural ecology similarities, the same
evolutionary pressures
are exacting the same forces on the
traits those animals possess
because they need to be doing the same
type of behaviour;
long rigid tail in the rainforest, long(ish)
rigid tail in the Australian bush. The
birds
fulfill a similar ecological function,
therefore their feathers need to do the
same thing.
So this is natural selection.
Birds can use their tail feathers for
display without those tail feathers
being full of ornate colouration or
patterning or eye spots.
These are the bustle feathers, which I
think is a brilliant word,
of a red-crowned crane, so called because
when
all of these feathers are together
they reminded scientists of the
bustle,
the dress style in Victorian ladies.
Red-crowned cranes have a very elaborate
courtship display,
they jump up and down spreading their
wings out, throwing sticks around,
and it's a very life-long partnership
between the male and female
which is reinforced by this dancing
every year, so the red-crowned crane's
bustle feather has to be part of that
courtship dance, it has to add to that
courtship dance,
but because they have this lifelong bond,
they don't need to attract a new mate
every year,
so unlike the dancing of the peacock,
they don't need to grow
the elaborate colours and patterns, which
is moulted every year and then regrown
for the next breeding season,
because all their feathers need to do is
to show their
show of strength, and fitness and
faithfulness to their mate,
so the trait here is still sexually
selected,
this helps the cranes perform their
courtship dance
but in a slightly different way to the
sexual selection
that has evolved the peacock's tail feathers.
But when a feature works well, when it's
under the control of natural selection
that has enabled it to be successful
across species
you will see that feature be highly
conserved across
different types of animal, if we look at
the flight feathers of birds,
this is a swan, this is a buzzard,
this is a parrot, and this very long
feather
is from a vulture, you can see that they
all have a similar feature, they're all
asymmetrical
around the central shaft of the feather,
we've got more filaments,
more barbs on this side than we have on
that side.
This makes the feather aerodynamic, it
gives the leading edge of the wing
an ability to gain lift and therefore it
keeps the bird in the air.
So regardless of your type of bird, swan,
buzzard, parrot,
vulture, you have this same aerodynamic
shape
to your flight feathers because
evolution,
the way in which the feather has
evolved to fit on the bird wing,
generates the best possible amount of
lift.
So this shape makes the bird as
aerodynamic,
as streamlined and as efficient as
possible in flight,
and that's why all of these flight
feathers look exactly the same.
So let's put all of that together and
look at the anatomy of a bird's wing.
This is a duck's wing, don't worry it
didn't die for the purposes of this
video,
you can see the long primary feathers,
they're asymmetrical,
they all fit together to give this nice
leading edge,
then we've got these patches of colour
here particularly this reflective green
patch
that is used for communication between
individuals when they're flying.
The different feather structures and
colours have a different role to play
in that behaviour.
Primary feathers keep
the bird in the air,
these brighter coloured feathers
here,
including the reflective green patch,
which we call a speculum,
allow individuals to see the movement
patterns of others and can communicate
with each other
whilst they're in flight
So wing anatomy is really naturally
selected,
it has evolved for the bird's behaviours
based on the ecology of the habitat that
the animal fits
into and the things that it needs to do
with its wings.
If we look at my duck, which is a long
distance flyer, we've got this long wing
that's capable of carrying it very fast
over large distances.
In the case of this tawny owl, I hope you
can see that the wing looks a bit more
fluffy,
it's got a leading edge to the wing
that is not as smooth as in the duck
and if we look across the feathers of
the owl we can see
that this edge to them, this kind of
serrated edge almost, allows the animal
to fly silently by disrupting the air
patterns
as the bird flies along which gives it
silent flight.
So the behavioural ecology of the owl
has caused the wing shape to look like
this
with a particular feather structure, and
the behavioural ecology of the duck
has caused the wing shape and feather
structure to look like this.
Same basic design but different
principles
to allow for different life history strategies.
Birds use pigments for very many
different things in their feathers,
these red feathers are from a turaco which
is an
African fruit-eating bird from the
tropical rainforests,
turaco feathers are unique in that their
pigments
within their feathers, these red pigments
are copper based,
and that's a very unusual thing to find,
and these copper-based pigments are used
by the birds for communication between
individuals in the rainforest
and we can see these different types of
pigments being really important
for individual species
based on their natural ecology and what
messages they need to convey to other
individuals
and birds that might appear really
colourful actually in their natural
habitat
can blend in with their surroundings.
These are the feathers from amazon
parrots.
Now a parrot might look incredibly
colourful if you see it in a cage or an
aviary
but actually in its natural environment
where it blends in with the dappled
light of the rainforest
this mixture of colours, these greens,
blues, oranges and yellows
break up the outline of the bird and
make it very difficult for predators to
spot
in dense green leaves within rainforest trees.
So these and these
are naturally selected traits that
enable the animal
to best fit within its natural habitat,
to live for as long as possible
and to be as successful as possible,
keeping itself alive,
finding enough food, communicating with
others, as it possibly can
within its natural environment.
In some species we're really 
confused about
why they have particular colours or
structures of colour in their feathers.
These are the feathers from a roller,
another African species of bird,
rollers are real aerial acrobats and
they can perform the most amazing
different types of manoeuvres whilst
they're in the air.
I like to think that these look like
they've been dipped in two different
shades of blue,
and again like the turaco, it may be
that these particular types of colour are
really useful
for communication and visual signaling
between individuals
whilst they're moving around their
natural habitat, so this would be an
example
of a naturally selected trait.
Some types of pigments directly come
from diet.
I've got three different types of pink
feather here; this is from a roseate spoonbill,
this is from a Caribbean flamingo and
this is from a scarlet ibis,
but what they all have in common is the
pink pigment within the feather
comes directly from the bird's diet.
It's a carotenoid pigment
and carotenoids are the same types of
chemicals that we find in carrots,
they give carrots their orange colours.
These carotenoids that come
directly from diet are a direct
reflection of the bird's qualities as a
forager, that means this is an honest signal.
The pinker my feathers, the better
I am at foraging and therefore the
better quality individual I am.
So not only have we got a sexually
selected trait
(look at how good a quality I am, you
should come and breed with me)
but we've also got a naturally selected
trait,
because lots of these carotenoid-rich
foods can be toxic
so by shunting that chemical into your
feathers
you get it out of your body and
consequently the toxic side effects of
eating that diet
don't have any effect on you. So honest
signals
can be displayed to individuals within a
social group
based on the colour of their feathers.
It's really easy to work out the natural
selection properties
of some types of features that animals
have.
In this little petri dish here I've got
some penguin feathers.
These penguin feathers are tiny and when
they all fit together on the bird itself
it's almost like the penguin is wearing
its own duvet,
so these tiny feathers trap air next to
the body,
keep the penguin warm and also
waterproof when it's underwater.
So penguins have many, many
thousands of these little feathers and
consequently
they have evolved because of the very
chilly
environment that penguins live in.
But we also see this insulation properties of
feathers in other species as well.
Another flightless bird- the rhea, which
is the South American equivalent of the
ostrich.
Rheas have these beautiful long plumes,
they lack the structure that we see in
flighted birds,
so if you look at my spoonbill feather
you can see
that all of the filaments are zipped
together by barbs
between the filaments themselves, this
creates a beautiful smooth aerodynamic
edge to the feather.
The rhea being flightless doesn't have
that;
the filaments are all loose and the
feather is really floppy,
this keeps the bird warm by acting like
thatch.
So all of these feathers sit on top of
each other and basically
form a protective layer against the
elements just like the thatching material
that we use in old-fashioned cottages.
And we can see that across different
species of flightless birds.
In emus for example, we have this very
strange
two pronged approach, each feather has
two shafts that come off the central
base,
and again when we put all of these
feathers together
we can create a thatch-like structure,
which helps keep the bird insulated
and therefore that's the best way that
the bird
can ensure it maintains homeostasis
without having
to oil or preen its feathers in the same
way as flighted birds do,
they still look after their feathers,
they still preen them
and keep them clean and tidy, but they
don't have to worry
about keeping them oiled and waterproof
for flight so the feather has evolved
a different structure.
I've talked a lot about birds so far in
my examples of sexual selection
but obviously it's not just birds that
show the processes of sexual selection
in the traits they carry,
this is seen across a whole range of
species in the natural world
and one of the species that is most
incredible I suppose in the sexually
selected traits that it carries
is one that we can find close to home in
this country, it's the stag beetle.
These are two dead specimens, they don't
live very long as adults
so if you live around London or in the
south east and in the summertime,
June and July, you might be able to pick
up the
the spent cases as it were of the
beetles themselves
at the end of their breeding season.
Stag beetles have these huge mandibles,
these huge jaw-like appendages, which
they use for the purposes of courtship
display.
The males grapple with each other to
show
strengths with each other and therefore
control access to the females,
so those appendages, those enormous jaws
are not for feeding
and nothing to do with foraging
behaviours, they're purely sexually
selected.
The female stag beetle is much smaller,
here she is,
and she lacks any of those mandibles, she
lacks
that jaw, so consequently the traits
that are developed by the males are
purely for sexual selection,
purely for impressing her and
consequently
it's almost the insect example of the
peacocks train.
The stag beetle spends a very long time
as a grub,
number of years eating rotten wood so if
you have
old forests with lots of fallen trees,
with lots of dead wood around,
you'll find stag beetles.
This male is much larger than this male and we've
got these size differences even within
the same population of the same species
and that's partly dependent
on the nutrition, the quality of bark
that the animal was eating when it was a
youngster,
so that's going to have an effect on its
courtship display,
on the size of its appendages and
therefore on its likelihood
of being more successful by attracting females.
And then we've got this, which is the
antler of a fallow deer,
it's calcified bone grows deciduously,
so that means it's regrown and shed each year.
The deer has two of these on top of his head
and this is only a small specimen
but it's still relatively weighty,
so like the peacock's train
we've got something that is a real
investment in the animal,
it's something that is going to be
costly to produce and costly to maintain
and will have other costs based on how
he can move around the habitat,
he's foraging efficiency, the likelihood
he can run away from predators,
all so he can compete with other males
for the best access to females.
So this type of ornamentation,
which we see in these animals that are
directly competing with each other for
females
is another example of a sexually
selected trait.
You can see at the bottom of this antler
the attachment where the antler would
grow out of the skull
and you might be able to see the little
indentations in the base of the antler
which is where the blood supply would
flow into the growing bone.
So the deer has to eat a lot
over the summer, fresh grazing, which
gives it the most
energy, sweetest sugar-rich grass
and young leaves are going to be put in
in
a very short space of time to growing
this,
so the foraging efficiency of the deer
also
has an influence on the strength and
quality of his antlers.
You can see this antler has been used,
it's damaged and scarred
by combat with other males, so let's hope
that this was a successful stag after
putting all the investment in
to growing two of these on the top of
his head.
And then we've got some extreme examples of shapes and sizes.
These are the sail feathers from a
mandarin duck, he uses these
purely for courtship display, they are
orange with a beautiful blue bottom
which catches the light, this
like the stag's antlers and like the
mandibles on the stag beetle,
and like the peacocks train, is a lot of
investment,
these are grown new every breeding
season.
And the mandarin uses these in his
courtship display
so that he can impress to the females
with a range of movements that he makes
around the female that he wants to breed with.
Because the mandarin invests in these
feathers,
just like the peacock, so he's
compromising his
own effects at longevity and attempts at
evading predators
purely to show off that he's a good
quality mate, so again this is a sexually
selected trait
that is purely for the purposes of
courtship display and breeding.
You can now see that the features
possessed by different species of animal
are under the pressures from natural
selection,
which ensures that population, that
species as a whole,
can survive as best possible under
prevailing conditions.
Sexual selection is a form of natural
selection,
which promotes the likelihood of an
individual
having the best possible lifetime
reproductive success
and consequently individuals will have
traits
that don't necessarily enhance longevity
but will enhance their chances of being
successful
during mating and breeding.
