Sexual selection

1. Sexual dimorphism

2. Sexual selection
Is sexual selection different from natural selection?
Darwin saw them as distinct - only sexual selection
could produce traits that compromise survival
The basic principle – selection favors
whatever gets more genes into the next generation
In sexual selection, fitness is measured relative to
members of the same sex
Two kinds of sexual selection
Intrasexual selection – mating success determined by within-sex interactions
e.g., male-male combat
Intersexual selection – mating success determined by between-sex interactions
e.g., female choice of males
(also referred to as epigamic selection)

3. Mating systems influence sexual selection
Monogamy Moderate Polygyny Strong Polygyny
Strength of sexual selection
Variance in mating success
All males have
same mating
success = 1 mate
Some males = 0 mates
Some males = 1 mate
Some males = 2 mates
Most males = 0 mates
One male = 8 mates
Mating systems

4. The peacock‟s tail
Extravagant male ornaments
The peacock’s tail greatly impairs his mobility…how could such a trait evolve?

5. His rationale, that conspicuous male displays persist and grow ever more
conspicuous because females prefer them, never fully explained the
benefit of choosing the brightest or loudest.
Darwin
"Thus a struggle produced by sexual selection in males exists not
because of the struggle for existence, but because it give the males that
possess it an advantage over other males in the competition for mates.―
Decrease in fitness (due to trait) compensated by increase in mating
success
Why a theory of sexual selection?
Darwin needed a theory to explain the many extravagant traits that
seem to reduce survival e.g. the peacock’s tail

6. What are Females choosing?
1) Males who possess good genes
2) Males who provide resources
3) Males whose signal matches a preexisting sensory bias
4) Males who are sexy and will thus produce sexy sons who will also
have success in mating

7. Parasitic Theory of Sexual Selection & Bright Coloration of Males
"General good health and freedom from parasites are often strikingly
indicated in plumage and fur, particularly when these are bright rather
than dull and cryptic."
Hamilton & Zuk
• females should choose parasite-free males
• why? pass on resistant genes to offspring
• bright colors = sign of health
•If disease resistance is heritable, females are making sure they have
healthy offspring if they choose such healthy looking males
• prediction: relatively more brightly colored males in more parasitized
species

8. Like a personal ad proclaiming: "Tall, good looking, disease-free," brightly
colored male animals may be advertising something of importance to their
prospective mates.
Should the female assume the gaudiest male has parasite-resistance genes that
will benefit her offspring? Or that she simply won't pick up bugs from the guy?
• decreases maneuverability & increases visibility to predators
• why does it exist? why is it not eliminated?
• natural selection cannot explain why traits that decrease survival exist
Many sexually dimorphic male display traits seem costly. Costly displays
will evolve if females actively choose males with brighter or more exaggerated
traits.

9. ELABORATED MALE TRAITS
MAY BE INDICATORS OF
HERITABLE GENETIC QUALITY
(I.E. FITNESS).
GOOD GENES MODEL
 Some males may have a heritable trait that reduces
viability.
 Only males with “Good Genes” can survive despite the
handicap.
 Females that mate with these males will have offspring
with higher fitness

10. Hypotheses for male ornaments
Fisher‟s “runaway” hypothesis
Mate choice originally evolved to facilitate adaptive choice for traits
conferring a survival advantage
Once female preference evolved, any genes that conferred survival
advantage but compromised attractiveness would not be passed on
because surviving males would fail to mate
The female preference genes will “hitchhike” onto the successful male
genes.
Zahavi‟s “handicap” hypotheses
Extravagant male traits are costly to develop and maintain
Choosing a mate with ―good genes‖ requires an honest signal of
genetic quality
Only males in good condition (those with good genes) will be able to
fully develop and maintain an ornament

11. Zahavi‟s handicap hypothesis
Some candidate “handicaps”
Note that the handicap itself need not be heritable…it need only provide a
reliable index of fitness, and fitness must be heritable
Traits that encumber the
owner are physiologically
costly (exertion in flight)
as well as being more
expensive to develop
Bright color honestly signals
immunocompetence and
parasite/disease resistance
Asymmetry is indicative
of developmental
instability and possibly
―bad genes‖. Symmetry
is chosen in some species

12. HANDICAP PRINCIPLE
 The bigger the handicap, the higher the
genetic quality of the male carrying the trait.
 Female choice evolves and the handicap
spreads and becomes elaborated.
 Assumes the superior quality is heritable
 This is an example of an honest signal since
there is a true cost to the elaborated trait that
prevents “cheaters”.

13. What does having a tail three times your
body length have to do with good genes?

14. Differential Costs of a Secondary Sexual Character:
An Experimental Test of the Handicap Principle
Moller and de Lope 1994
• Manipulated tail
lengths in males:
– Shortened by 20mm
– Extended by 20mm
– Cut & Glued
(treatment control)
– No manipulation
(control)
• Survival rate: return
rates to breeding
sites

15. MATING SUCCESS RELATED TO TAIL LENGTH
―Mock‖ altered
Unaltered
Barn swallow
Experimental manipulation of tail length
Males with elongated tails:
1. Obtain mates more quickly
2. Have greater reproductive success
3. Experience a cost in tail size the following year

16. Are these traits costly?
In the year after Moller’s (1988) experiment on barn
swallows, males with experimentally elongated tails
grew significantly shorter tails than the previous year
What does a long tail indicate?

17. • lower loads of blood-sucking mites on males with larger
tails
• placed 50 mites/nest of various males
• later date: offspring of males with longer tails had lower
mite loads
Therefore: genetic basis to resistance (good genes
passed on!)
Therefore:
1) trait is costly
2) tail maintained by female
choice

18. Parasite load of offspring
measured in biological
parent’s nest
Parasite load of offspring
measured in a foster
parent’s nest
Males with longer tails fathered offspring with lower
parasite loads

19. Runaway Model:
Trait initially advantageous
Preferences and trait become genetically correlated (♂ expressed, ♀ preference)
Requires heritability of trait and preference, trait cannot be too expensive.
Zahavi „s Handicap principle
Traits not chosen despite being costly, but BECAUSE they are costly they must
have negative effects on fitness, so only males of the highest genetic quality can
successfully develop the trait (i.e., the trait is costly to maintain)
Good genes model: ornaments are a sign of good genes, and have a cost to
keep the displayer honest.
Traits reveal male condition: females select males of the highest genetic quality
Conditional traits- expressed by high quality males
Viability: expressed by all males but tend to kill off low quality males
Problem: what maintains genetic variation for goodness? Would everyone not
soon have the good genes and therefore no variability for females to assess??

20. Hamilton-Zuk hypothesis: (the red queen returns)
Females prefer resistant/showy males
Correlation between quality (resistance) and the trait, or negative
correlation between parasite load and trait
Heritable variation in resistance (often difficult to prove)
Parasite load reduces host viability
Within a species: guppy infestations lowers male brightness, reduces
sexual displays, females prefer bright males. Heritable – unknown
If this is true: there should be a correlation between parasite load

21. Can development of these traits be condition-dependent?
Many secondary sexual traits do develop in proportion to male condition:
antler size (reduced by starvation in red deer)
orange spots in guppies affected by carotenoid content of diet—may
reflect food-finding ability
tail length in swallows and widowbirds
horn size in beetles—may depend on larval nutrition
hormone levels that regulate secondary sexual traits—sensitive to
nutrition

22. Carotenoid Modulation of Immune
Function and Sexual Attractiveness
in Zebra Finches
Blount et al.
One hypothesis for female choice predicts that availability of carotenoid
pigments is a potentially limiting factor for both ornament expression
and immune function. Males that can afford to produce more elaborate
carotenoid-dependent displays must be healthier individuals with
superior immunocompetence. Manipulation of dietary carotenoid supply
invokes parallel changes in cell-mediated immune function and sexual
attractiveness in male zebra finches (Taeniopygia guttata).
SCIENCE, April 4, 2003
Experimental Group
Control Group
Some indirect benefits

23. Plumage brightness in the house finch: condition-dependent
Plumage color may also have a genetic component: positive correlation between
plumage brightness of father and plumage brightness of sons
Brightly colored plumage—reliable indicator of male parental effort;
perhaps also an indicator of genetic quality?
Males returning to
the nesting grounds
more brightly colored
than the males that
did not return—
plumage indicates
viability?
Females who prefer most extreme traits will mate with males with most resistance
to parasites: offspring will therefore have high resistance to parasites as well

24. Distinguishing between runaway sexual selection and ―good genes‖
hypotheses
Necessary condition of runaway sexual selection hypothesis: genetic
correlation between male trait and female preference for that trait
--but this can also be true of good genes scenario
Runaway sexual selection: there is no association between the trait and
fitness viability of offspring—only enhanced mating success of offspring
Good genes hypotheses: require demonstration of heritable variation in
male genetic quality
Female choice of males with better developed traits results in enhanced
fitness of offspring (which has a genetic basis)

25. Good Genes
The "good genes" model describes the motivation for female preference. A
strong, dominant mate in good health, is strong and dominant because he
has a "good" genome capable of defending itself against predators, disease,
and competition with other males.
Male-male interactions: Males that win the most contests will be dominant,
and consequently will have greater exposure to mates. A secondary sex
character that allows a male to gain an advantage over competitors will mate
more and consequently his genes will out compete those of other males.
Over time, exaggerated traits such as antlers, spurs, or territorial song will
develop in the population.
Mate choice can be based on the search for "good genes" in a mate as well,
but the mechanism of choice is less clear. How is it that females choose one
male over another? What roles do the exaggerated secondary sex
characters displayed by many males play in this choice?

26. The Burden of Display
In the great majority of species displaying sexually selected characters, it
is the male who must compete with rivals for the opportunity to mate.
Why aren’t males as selective in the choice of mates as females tend to
be?
Male offspring number depends on # of inseminations.
Female offspring number remains constant regardless of # matings.
Promiscuous males will have a large number of offspring carrying his
promiscuity trait, whereas a promiscuous female gains nothing when
compared with a monogamous female. Extra time spent mating may be
costly in terms of time and energy- =reduced fitness.
Because of the fecundity constraints placed on a female, a female that
selects high "quality" mates will have more fit offspring and will
consequently out compete her less selective conspecifics. Males, on the
other hand, have virtually nothing to lose from mating with every
receptive female they encounter.

27. The Sexy Son
The selection of the "highest quality," or fittest mates: The fitness of a
female, will be enhanced if her male offspring are more likely to survive
and reproduce than other conspecifics.
Why do females consistently continue to choose the "preferred" male?
In the "sexy-son hypothesis" females choose the preferred male
because their male offspring will later be preferred, either because
they out compete other males in a dominance hierarchy or are simply
more attractive to females.
This benefit outweighs the cost of decreased fecundity caused by
single parenting.

28. Parasitic Theory of Sexual Selection & Bright Coloration of Males
"General good health and freedom from parasites are often strikingly
indicated in plumage and fur, particularly when these are bright rather than
dull and cryptic."
Hamilton & Zuk:
• females should choose parasite-free males
• why? pass on resistant genes to offspring
• bright colors = sign of health
•If disease resistance is heritable, females are making sure they have
healthy offspring if they choose such healthy looking males
• prediction: relatively more brightly colored males in more
parasitized species
Bright colours ===> GOOD?