Download Giraffe Necks

How are adaptationist hypotheses
built and tested?
Examples
Why do we observe sexual
dimorphism in humans?


H1: Because males and females consume
different food resources as a consequence of
division of labor.
H2: Because males compete for females.
There is sexual selection for bigger males.
Sexual Dimorphism – Comparative
Method
Why do we observe sexual
dimorphism in humans?



There are many dots on the graph which
significantly deviate from the regression line.
Results neither confirm nor falsify
adaptationism. They only show that natural
selection is relevant in explaining dimorphism
in some species.
Adaptationism is tested in the long run.
Copulation time in dung flies:
Mathematical model

Given that:
–
–
–
–

Search and guard time is 156.5 minutes,
First male fertilizes 20% and second male
fertilizes 80% of eggs,
Fertilization rate increases with with time spent in
copulation with diminishing returns,
Optimal copulation time is the one which
maximizes # Fertilized eggs/unit time,
What is the optimum time of copulation?
Copulation Time – Mathematical Model
Giraffe’s Neck: Why is it so long?


H1: Giraffe necks have evolved to be long
because long necks allowed giraffes to reach
higher branches in feeding.
H2: Giraffe necks evolved as a fighting tool
for mating.
Giraffe Necks

Evidence against H1:
–
–
–
Giraffes usually don’t feed on tall trees. They feed
on short bushes.
They usually use their necks as a weapon in
competiton for females.
There should be a correlation between giraffe
neck length and mating success.
Giraffe Neck
Flies vs. Jumping Spiders: Mimicry?


Zonosemata vittigera performs a special dance
whence threatened. The dance resembles the
defensive dance of its hunter: jumping spider.
What is the function of this behavior?
–
–
–
H1: Flies do not mimic jumping spiders. In many fly species,
these dances are used as mating displays.
H2: Flies mimic jumping spiders not for threatening them
but for threatening other predators.
H3 Flies mimic jumping spiders in order to dissuade them
from hunting.
Flies vs. Jumping Spiders: Mimicry?





A: Normal Zonosemata
B: Wings cut and pasted.
C: Wings cut and replaced with transparent
wings of house fly.
D: Housefly with Zonosemata wings
E: Normal housefly
Predictions
A
H1
JS
Others
H2
JS
Others
H3
JS
Others
B
C
D
E
Attacks Attacks Attacks Attacks Attacks
Attacks Attacks Attacks Attacks Attacks
Attacks Attacks Attacks Attacks Attacks
Retreat Retreat Attacks Attacks Attacks
Retreat Retreat Attacks Attacks Attacks
Attacks Attacks Attacks Attacks Attacks
Zonosemata vs. Salticidae: Results
Why do some bats have bigger
testicles?




Female bats mate with multiple males.
Sperms of different males compete for fertilizing the
egg.
Greater ejaculation means a greater chance of
fertilization. Bigger testicles allow this outcome.
Predictions: We should find positive correlation
between group size and testicle size.
Bat Testicle Size – Comparison
Trade-off





Female begonias are pollinated by bees.
Male begonias offer bees lots of pollens whereas female ones
don't.
Female begonias look like males (same size, same color, etc).
H1: Female flowers are visited by bees more if they resemble
male flowers better. So, optimum phenotype for females is
resembling the average male flower.
H2: Pollination success (attracting the bees) depends on size,
there is a linear relation. The greater the better.
Trade-off
Host change in Ophraella: Constraint





Hosts produce toxins.
Ophraella should detoxify it.
Why has some host changes occurred and
others not?
H1: Chance and ecological factors. There
was adequate genetic variation.
H2: There wasn’t adequate genetic variation.
Constraint
Additional resources

Evolutionary Analysis (Freeman and Heron)
2nd edition

Pratt D., Anderson, V., “Giraffe social behaviour”,
Journal of Natural History, 1985, 19:771-781