Transcript Natural selection can only operate on the material

Chapter 9.
The evolution of communication.
Hyena social behavior
Hyenas live in social groups called
clans.
Clan members defend a territory and
hunt in groups.
Females are the dominant members
of the clan and have a clear
dominance hierarchy.
Dominant females have higher
reproductive success than other females.
Major feature of social interaction in hyenas
is penis sniffing. Both males and females
possess “penises”
Female penis really a pseudopenis, which
is an enlarged clitoris.
Pseudopenis
Picture here
Enlarged pseudopenis is costly.
.
Birth canal
passes through
pseudopenis
Pseudopenis is costly
 10-20% of females die giving birth
first time and 60% of first-born pups
die.
 Pseudopenis must provide big
selective advantage to balance this.
What is benefit to female?
Advantages of pseudopenis
There has been considerable debate on
the issue.
Initial research focused on possible role of
male hormones in masculinizing clitoris.
Speculated exposure to hormones
increased aggression in females, which
increased their social status and access to
food, and enlarged pseudopenis was a
byproduct of that.
Advantages of pseudopenis
Hormonal side-effect hypothesis does
not explain why females are dominant
over males or why selection would not
have favored a reduction in
pseudopenis size.
Advantages of pseudopenis
More recently attention has focused
on usefulness of pseudopenis in
communication. Sniffing appears to
enhance cooperation among hyenas.
Hyenas presumably communicate
information during sniffing events and
these may affect dominance
interactions between females.
Advantages of pseudopenis
Sniffing may enable dominants to
monitor hormonal status of other
females.
Dominants’ benefit: know if challenge
likely.
Subordinates’ benefit: allowed to
remain in pack.
Evolution of complex traits
All complex traits evolve from less
complex ones as a result of a long
sequence of small developmental
changes.
 This sequence of changes is an
evolutionary pathway.
Things to remember about evolutionary
pathways.
(1) Natural selection can only operate on
the material available to it.
Panda’s strip bamboo with “thumb.”
Thumb is modified wrist bone (radial
sesamoid).
True thumb committed as part of foot.
Natural selection forced to use available
material.
(2) Evolutionary intermediates must be
improvements over what preceded them.
An organism cannot get worse to ultimately
get better.
(3) Intermediate structures can have
different functions to their current ones,
but should be useful in some way.
Human ear bones were once jaw bones.
Had to work effectively at both jobs.
Evolution of hearing in Noctuid
moths
 Whistling moths signal to females by
banging “castanets” on their wings
together. Sounds are ultrasonic
castanet
(approx 30KHz).
Evolution of hearing in Noctuid
moths
Most moths cannot hear.
How did ability to hear ultrasound
evolve in these Noctuid moths?
Ears on side of thorax.
Ear: thin tympanic
membrane covering
an air sac.
Vibration of air sac
stimulates sensory
receptors.
Ear
Evolution of hearing in Noctuid
moths
Non-hearing Sphingid moths have
sensory cells attached to the cuticle
as in Noctuids.
These supply positional information
when moth vibrates wings.
Sensory9.7
cells
Ear
Evolution of hearing in Noctuid
moths
 In ancestral Noctuid, sensory cells
could have provided ability to hear
those sounds loud enough to move
cuticle.
Ear evolution pathway
 (1) Thin cuticle to enhance vibrations.
 (2) Enlarge air space.
 (3) Tune sensory cells to the desired
frequency.
What was likely selection pressure on
noctuids to hear ultrasound?
Noctuids evolved hearing to avoid bats.
Later evolved signaling ability.
Evolution of insect flight
What were the precursors of wings?
Gill plates of extinct immature aquatic
insects are plausible pre-wing
structures (function to move water
over the gills).
Fig 9.9
Evolution of insect flight
Gill plates appear to have evolved into a
wide variety of structures in arthropods
including wings, gills and lungs.
Fig 9.8
Evolution of insect flight
Gill plates, if retained in the adult, could
act as sails allowing the insect to skim
over the surface of the water.
 Increasing “wing” size would increase
skimming speed.
Beating wings would increase speed still
further so adding musculature would be
favored by selection.
Stonefly
skimming.
Evolution of insect flight
Modern stoneflies include a variety of species
that use different ways of moving over the
water surface that allow the insects to move
progressively faster.
Stonelfies include species that sail, row skim
and fly. There are also species that differ in the
number of legs they keep in contact with the
water while skimming.
Evolution of insect flight
The fewer legs in contact with the water
the faster the stonefly can move.
From hind-leg skimming, it is only a
short step to true flight.
Fig 9.11
Exploitation of preexisting biases
in evolution of communication
In whistling moths evolution exploited
existing sensory system to develop
communication system.
Remember: Complex structures not
evolved from nothing (e.g. Panda’s
thumb)
Exploitation of preexisting biases
in evolution of communication
Sensory biases/preferences may
precede evolution of many signals.
E.g. Many sex pheromones of
insects have floral odor (exploits sensory
bias towards food finding). Examples:
Example 1: Tungara Frog
Tungara frogs attract mates by calling.
Males give whining call sometimes followed by
one or more “chucks.” Females prefer males
who give chucks.
Fig 9.30b
If females prefer males who chuck,
why don’t all males chuck all the time?
Because bats prey on calling males.
Fig 9.30 a
Tungara frogs
Close relatives of tungara frog don’t
chuck.
However, females of these species prefer
the calls of males to which chucks have
been added
 Females have an innate preference for
chucks.
Example 2.
Female swordtail fish prefer males
with long tails.
Platyfishes close relatives of swordtails.
Males have short tails. Female
Platyfish prefer males with artificially
elongated tails.
Elongated tail in swordtails evolved
after preference in place.
What is an adaptation?
Characteristic of an organism that is
maintained or spread by natural selection.
Adaptationists try to figure out the value
of traits. E.g. what is the value of
signalling?
Risk avoidance by signalers
Signaling can be risky (recall Tungara
frog). Illegitimate receivers use signal to
detriment of signaler.
Many signalers try to reduce risk.
High pitched alarm calls of birds are
very similar in sound and hard to locate.
Risk avoidance by signalers
In contrast, mobbing calls have
evolved to attract other birds and are
easy to locate.
9.31a
Risk avoidance by signalers
Selection to avoid attracting predators
has led to the evolution of very similar
alarm calls in different species of bird.
9.33
Adaptationist approach to
signal receivers
Ritualized fighting is widespread.
Why do receivers believe the signals they
are receiving?
Why do animals not fight harder for
resources? What is the benefit to
giving up a fight?
Adaptationist approach to
signal receivers
By giving up, organism avoids
engaging in fights it probably
will lose.
Toad mating behavior
In mating toads male sits on females
back and grips her.He fertilizes eggs
as she releases them.
Other males may attempt to displace
male.
 Male croaks when touched.
Upon hearing croak usurping male
may cease attack.
Toad mating behavior
Hypothesis: croak is signal of
defending males size.
Deeper croaks indicate larger males.
Smaller males deterred by deep
croaks.
Toad mating behavior
Tested by playing taped call when
silenced defender touched.
Pitch of call significantly affected
amount of time attacker spent
attacking.
9.34
Toad mating behavior
Toad call appears to be an honest
signal.
Small toads cannot fake a deep
croak.
Honest signals
Expect signals need to be unfakeable
because otherwise cheaters would
invade the system.
 Honest signals provide useful
information and benefit both the
signaler and the receiver.
Honest signals
Expect threat display signals to be
expensive and difficult for
smaller/weaker individuals to imitate.
Should be good indicators of fighting
ability.
Male red deer control groups
of females.
Males compete to control groups,
but rarely resort to pushing matches.
Instead, first engage in roaring matches.
Roaring is energetically expensive. Only
males in top condition can roar for a long
time. Only if roaring match is indecisive
does escalation to fighting take place.
Antlered flies display antlers to rival males.
Males with smaller antlers usually retreat.
Antler size
closely
correlated
with body size
and so it is
an honest
signal.
Honest signals
 As one might expect not all signaling
is honest and some organisms exploit
receivers.
Dishonest signals
Male and female fireflies signal each
other by flashing in distinctive
patterns.
Female Photuris fireflies mimic signal
of female Photinus to attract males,
whom they catch and eat.
Responding to a deceptive signal
maladaptive.
Why respond?
Dishonest signals
If responding to a dishonest signal is
maladaptive, why respond?
Dishonest signals
 Possible explanations include:
 (1) Novel environment hypothesis.
 Mistake made because environment
has changed. Response once
adaptive, is now maladaptive.
Maladaptive mate choice in an Australian beetle
Dishonest signals
(2) Exploitation hypothesis.
Response adaptive on average, but
sometimes exploited.
Risk outweighed by benefits.
Dishonest signals
On average, male Photinus flies do
better by responding to light flashes.
Males who respond mate. Males
who don’t respond aren’t eaten, but
don’t mate either.
Dishonest signals
Successful illegitimate signalers
should exploit responses that are
usually adaptive.
E.g. predatory spiders lure out other
spiders by producing prey-like
vibration on web.
Anglerfish attract
small fish by waving
a fish-shaped lure.
Benefit of attacking
likely prey outweighs
low risk of predation
for small fish.
Dishonest signals
White-winged tanager-shrike forages in
mixed-species flocks with other birds.
Scans for prey from perch. Often first to
spot approaching predatory hawks.
Gives alarm call that other birds respond
to.
Dishonest signals
Sometimes tanager-shrike gives call
to distract another bird from prey item
it is chasing.
Usually, other bird aborts chase and
tanager-shrike gets item.
Dishonest signals
For other bird cost of ignoring alarm
call potentially is very high (i.e.
death).
Usually best to give up chase.