Sexual Selection and Reproductive Behaviour

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Sexual Selection and Reproductive Behaviour
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Sex is the queen of problems in evolutionary
biology. Perhaps no other natural phenomenon has
aroused so much interest certainly none has
sowed as much confusion. The insights of Darwin
and Mendel, which have illuminated so many
mysteries, have so far failed to shed more than a
dim and wavering light on the central mystery of
sexuality, emphasizing its obscurity by its very
isolation.
Graham Bell The Masterpiece of Nature The
Evolution of Genetics and Sexuality
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Sex vs. Reproduction
Sex - process by which a genome is changed by
rearrangement and combining of genes
Reproduction - process by which individuals are
added to a population
Sex -changes in kinds of individuals in
populations
Reproduction -changes in the number of
individuals in populations
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Reproductive Behaviour
1. What is sex?
2. What are some of the costs and benefits of sex?
3. Why are the sexes generally in a 11 ratio?
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Reproductive Behaviour
What is the one feature that serves to separate
male from female in all animals?
Anisogamy
Anisogamy (heterogamy) - a form of sexual
reproduction involving the union or fusion of two
dissimilar gametes
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Reproductive Behaviour
Anisogamy how did it come about?
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Reproductive Behaviour
Anisogamy
Frequency of producers
Gamete size
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Reproductive Behaviour
Anisogamy
- lots are produced by not enough mass for cell
division
- fewer produced but are competent
- very few produced but are very competent
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Reproductive Behaviour
Anisogamy

- lots of contact but wont divide properly

- will divide but fewer contacts

- will divide but very few contacts

• greatest success
• best combination of numbers and competence

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Reproductive Behaviour
Costs of sex
Sex is not only unnecessary, but it ought to be
a recipe for evolutionary disaster. For one
thing, it is an inefficient way to reproduceAnd
sex carries other costs as wellBy all rights,
any group of animals that evolves sexual
reproduction should be promptly outcompeted by
nonsexual ones. And yet sex reigns Why is sex a
success, despite all its disadvantages?
Carl Zimmer Evolution The Triumph of an Idea
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Reproductive Behaviour
Costs of sex
1. Cost of meiosis

• must combine genes with another organism
• lose 50 every generation relative to asexuals

-often referred to as cost of males - females
can always do better asexually
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Reproductive Behaviour
Costs of sex
2. Cost of recombination

• take a functioning genome, split it and combine
  it with another one

-chance of deleterious combinations
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Reproductive Behaviour
Benefits of sex
1. Produce new combinations
Unique offspring can cope with different niches
x
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Reproductive Behaviour
Benefits of sex
2. Mullers Ratchet
- in asexual species, deleterious mutations
accumulate
A
AB
Frequency of mutant gene
Mutation A
ABC
Mutation B
BC
Mutation C
C
Time
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Reproductive Behaviour
Benefits of sex
2. Mullers ratchet
- in sexual species, such mutations tend to be
eliminated
Frequency of mutant gene
Mutation A
Mutation B
Mutation C
Time
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Reproductive Behaviour
Sex ratio -why 11?
Excess males
-some males wont mate -females that produce
more females are favoured
11 sex ratio
-some females wont mate -females that produce
more males are favoured
Excess females
Time
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Darwin - Theory of sexual selection
Natural selection - Observations and deductions
Observation - all species can increase
exponentially but dont - more are produced
than can survive
Deduction - there is a struggle for existence
Observation - there is variation in all
organisms that is heritable
Deduction - some are better equipped to survive
than others - advantage is passed to offspring
Differential survival Natural selection
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And this leads me to say a few words on what I
call Sexual Selection. This depends, not on a
struggle for existence, but on a struggle between
the males for possession of the females the
result is not death to the unsuccessful
competitor, but few or no offspring. Sexual
selection is, therefore, less rigorous than
natural selection. Generally, the most vigorous
males, those which are best fitted for their
places in nature, will leave most progeny.
Charles Darwin, 1859. Origin of species
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Darwin - Theory of sexual selection
Two parts
1. Intrasexual selection (Male-male competition)
But in many cases, victory will depend not on
general vigour, but on having special weapons,
confined to the male sex.
The war is, perhaps, severest between the males
of polygamous animals, and these seem oftenest
provided with special weapons.
2. Intersexual selection (Female choice)
but if man can in a short time give elegant
carriage and beauty to his bantams, according to
his standard of beauty, I can see no good reason
to doubt that female birds, by selecting, during
thousands of generations, the most melodious or
beautiful males, according to their standard of
beauty, might produce a marked effect.
Charles Darwin, 1859. Origin of species
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Darwin - Theory of sexual selection
Two parts
1. Intrasexual selection (Male-male competition)
All those structures and behaviour patterns
employed by males to fight other males for the
chance to mate with females
- claws, antlers etc.
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Darwin - Theory of sexual selection
Two parts
2. Intersexual selection (Female choice)
All those structures and behaviour patterns
employed by males to attract females
- plumage, song.
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Reproductive Behaviour
Males and females have different reproductive
interests
What does each sex have to provide to produce
offspring?
Females
Males

• Manufacture eggs
• (expensive)

• Manufacture sperm
• (cheap)

2. Incubate eggs or fetus (expensive)
3. Post partum care (expensive)
Males invest less in any offspring
4. Loss of mating opportunity when
pregnant (expensive)
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Reproductive Behaviour
In more general terms
Females
Males
Difference in reproductive investment
higher investment
lower investment
higher potential reproductive rate
lower reproductive rate
lower levels of mating activity
higher levels of mating activity
Biased operational sex ratio
Selection among potential mates
Competition for mates
Best mate best fitness benefit
Achieve greatest number of matings
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Reproductive Behaviour
Operational vs Numerical Sex ratio
Numerical Sex Ratio The ratio of the number of
males to the number of females in the population
Operational Sex Ratio The ratio of the number of
males to the number of females in the population
who are available for reproduction
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Reproductive Behaviour
Operational vs Numerical Sex ratio
Numerical Sex Ratio (close to 11)
Males
Females
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Reproductive Behaviour
Operational Sex Ratio
Males
Females
Remove females for
Remove males for
1. Pre-puberty
2. Sterile
( Post-menopause)
3. Age
4. Pregnant
5. Lactating
Operational sex ratio
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Reproductive Behaviour
Operational vs Numerical Sex ratio
Numerical Sex Ratio (close to 11)
Males
Females
Operational Sex Ratio (very different from 11)
Competition in more abundant sex
Selectivity in less abundant sex
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Reproductive Behaviour
Evidence??
Bateman (1948)
-Drosophila melanogaster - nearly all females
mated but not all males
-variance in male reproductive success - higher
Males
Bateman gradient
of offspring
Females
of mates
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Reproductive Behaviour
Evidence?? - Lions
Females
of breeders
10
0
of surviving offspring
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Reproductive Behaviour
Evidence?? - Lions
Females
Males
of breeders
10
0
of surviving offspring
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Intersexual Selection
Genetic Models for Mate Choice
1. Direct Benefits
-females choose males that give them a
concrete resource
e.g. Nuptial gifts
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Intersexual Selection
Size of prey is critical
20 min
Duration of copulation
sperm transferred
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8
16 mm
Size of prey
Time in copulation
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Intersexual Selection

• 2. Good genes models

Female should look for best genetic complement in
male
Traits should reflect genetic quality
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Intersexual Selection

• 2. Good genes models

Fluctuating Asymmetry
Horns in oribi - Ourebia ourebia
Arese. 1994. Anim.Beh. 481485
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Intersexual Selection

• 2. Good genes models

3 2 1 0
Fluctuating Asymmetry
Harem size
Symmetric
Asymmetric
Damaged
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Intersexual Selection

• 2. Good genes models

Fluctuating Asymmetry
Symmetry
Ability to fight parasites (nematodes)
Lagesen Folstad, 1998. Beh.Ecol.Sociobiol.
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Intersexual Selection

• 2. Good genes models

Good genes and parasites
Number of matings
Clean
Infected
No choice
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Intersexual Selection

• 3. Runaway Selection

Assume 2 genes
Gene A - Trait X in males - more colourful
feathers
Gene B - In females - preference for trait X
A B
Expressed in males as the trait
A B
Expressed in females as a preference for the trait
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Intersexual Selection

• 3. Runaway Selection

Assume 2 genes
Gene A - Trait X in males - more colourful
feathers
Gene B - In females - preference for trait X
Frequency in males
Frequency in females
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Intersexual Selection

• 3. Runaway Selection

Example - stalk-eyed flies - Diopsidae
Variation in males
Female
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Intersexual Selection

• 3. Runaway Selection

Example - stalk-eyed flies - Diopsidae
Two experiments
1. Standard disruptive selection

• every generation - allow shortest and longest
  stalked
• males to mate with randomly selected females

Short stalked
Long stalked
13 generations
Stalk length
Stalk length
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Intersexual Selection

• 3. Runaway Selection

Example - stalk-eyed flies - Diopsidae
Two experiments
2. Female choice
Female offspring prefer short stalked males
Female offspring prefer long stalked males
Stalk length
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Intersexual Selection

• Other factors affecting mate choice - Learning

1. Sexual imprinting -already discussed this

2. Mate choice copying
- chances of a male being preferred as a mate at
time 2 increases as a result of being preferred
at time 1.
Male 1
Male 2
Model female
Test female
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Intersexual Selection

• Other factors affecting mate choice - Learning

1. Sexual imprinting -already discussed this

2. Mate choice copying
Frequency
Near chosen male
Near non-chosen male
Time spent by test fish
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Intrasexual Selection - Male/Male Competition
Competition before mating
Fiddler Crab (Uca)
1. Direct Fighting
Female
Female digs burrow
Male
Male enters burrow and mates
Male defends burrow and female
No evidence of female choice
Male leaves when female ovulates
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Intrasexual Selection - Male/Male Competition
Competition before mating
2. Interference
-male tries to interfere with copulating pair
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Intrasexual Selection - Male/Male Competition
Competition before mating
2. Interference
Elephant seals (Mirounga) -females incite males
to fight via interference
Males fight for dominance -8.3 of males mate
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Intrasexual Selection - Male/Male Competition
Competition before mating
2. Interference
Logic - if subordinate male tries to mate -
female protests - dominant interferes -fema
le gets best mate
of matings protested by female
Alpha
Adult male
Subadult male
Rank of male
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Intrasexual Selection - Male/Male Competition
Competition before mating
3. Cuckoldry
Bluegill sunfish
Males set up territories where females lay eggs
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Sunfish Territory
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Intrasexual Selection - Male/Male Competition
Competition before mating
3. Cuckoldry
Three kinds of males 1. Parental - larger,
aggressive territory holders 2. Sneaker -
smaller 3. Satellite - look like females
spawning
Male digs nest
Female lays eggs
Male fertilizes eggs
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Intrasexual Selection - Male/Male Competition
Male digs nest
Female lays eggs
Male fertilizes eggs
Parental male
Parental - larger, aggressive territory holders
Female
Sneaker - smaller - rush in before parental male
and fertilize eggs
Satellite
Satellite - look like females - spawn with pair
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Intrasexual Selection - Male/Male Competition
Competition after mating
1. Sperm Competition

• Females of many species can
• Store sperm
• Mate with several males before fertilization

Raises probability that sperm will compete for
fertilizations
Males can a) reduce the chances that a second
males sperm is used (first male adaptations)
b) reduce the chances that a first males sperm
is used (second male adaptations)
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Intrasexual Selection - Male/Male Competition
Competition after mating
1. Sperm Competition - First male adaptations
a. Postcopulatory mate guarding
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Intrasexual Selection - Male/Male Competition
Competition after mating
1. Sperm Competition - First male adaptations
a. Postcopulatory mate guarding
Stick insects
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Intrasexual Selection - Male/Male Competition
Competition after mating
1. Sperm Competition - First male adaptations

1. Postcopulatory mate guarding
2. Antiaphrodisiacs
3. Mating plugs

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Intrasexual Selection - Male/Male Competition
Competition after mating
1. Sperm Competition - Second male adaptations
a. Sperm removal
Dunnock
Damselfly (Argia) Penes
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Intrasexual Selection - Male/Male Competition
Competition after mating
2. Bruce effect
- Mice - strange male (or male odour)
-females abort fetuses and become receptive
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Intrasexual Selection - Male/Male Competition
Competition after mating
3. Infanticide
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Summary - Sexual Selection
Intersexual Selection 1. Direct Benefits
Intrasexual Selection

1. Premating

2. Good Genes
a. Male fighting
3. Runaway Selection
b. Cuckoldry
c. Interference
2. Postmating
a. Sperm Competition
b. Bruce Effect
c. Infanticide