Download Sexual Selection

http://www.youtube.com/watch?v=
YTR21os8gTA
Sex al Selection
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The fundamental difference between the sexes and its implications:
- parental investment and sexual selection
- sexual conflict: between and among the sexes
- the evolution of bizarre morphology and behavior for the purpose of attracting mates
- mate choice and why females are the choosier sex
- an understanding of what each sex looks for in a mate
- signaling theory and handicaps
- mating strategies
- why some species change their sex in their lifetime
- why are males often the first to abandon their mates/young
Sexual Selection – differences in reproductive success caused by competition
over mates that is related to the expression of traits for such competition
Sexual Selection is not defined in relation to reproductive success – e.g., many
traits such as a brood pouch or parental alarm calls increase survival of the young but rather it is defined by improving reproductive success via competition over mates,
whether by aggression, mate choice, or some alternative
Rewriting Darwin’s postulates for sexual selection:
- struggle for reproductive “existence” (i.e., access to/competition for mates)
- variation in traits influences the struggle
- that said traits are heritable
Sexual Selection is a subset of Natural Selection – but the evolutionary
consequences differ in remarkable ways. Traits that evolve via Sexual Selection
may be disfavored by natural selection, that is, selection that ignores competition
for mates.
By competition we mean the use of a resource that reduces its availability to others
Sexual Selection is based on variance in mating success. To
demonstrate sexual selection requires that Variance in the trait leads to
Variance in mating success due to competition among rivals, mate choice,
or some other mechanism
https://www.youtube.com/watch?v=DU4xW
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3 examples:
Elephant seals
8-348
Red Deer
Percent contributions to LFR
Reproductive lifespan
Fecundity/mating success
Offspring survival
F
27
8
57
M
7
32
20
Sage grouse
50
Percent of
copulations
(105 observations)
0
1
2
3
4
5
6
7
8.............14
These 3 examples illustrate:
(1) strong variation in reproductive success – many males in fact may never mate
at all
(2) Variation is influenced age, which often serves as a correlate for experience
or body size/size of fighting appendages
(3) That the strength of sexual selection – inferred from the degree of variance in
reproductive success – is most intense among males
How did it all begin?
Sexual reproduction – gamete formation by meiosis and fusion of genetic material
from two individuals almost always involving 2 sexes: male and female
sexes often identified via 1° and 2° sexual characters –
the fundamental difference is:
Females = produce large, immobile, food-rich gametes called eggs
Males = produce tiny, mobile gametes (sperm) that are little more than
self-propelled DNA
Anisogamous Reproduction
Anisogamous Reproduction was probably inevitable...
There are 2 selection pressures on gametes:
(1) larger zygotes improves zygote survival
(2) more gametes increases chances of fertilization
As resources for reproduction become limiting, these 2 pressures oppose
one another – the compromise solution is the evolution of 2 different sexes,
one producing few, large gametes the other many, small gametes.
This fundamental asymmetry in gamete size and associated investment in
offspring has led to the far-reaching consequences of sexual behavior …
• Females invest more resources than males into each offspring, both at the gamete
level and often in other forms of care
• Males can fertilize eggs at a faster rate than they are produced – 5 ml of human
semen has enough sperm to fertilize 2x the USA population!!
Thus, females are a scare resource for which males compete
Offspring versus number of mates
Drosophila melanogaster
120
Males increase their rep. success
by finding and fertilizing as many
females as possible
males
Whereas females can only increase
rep. success by turning food into eggs
or offspring at a faster rate
60
females
Or better yet, increase offspring survival
0
0
1
2
(after Bateman 1948)
3….
The relative payoff for a successful
male is much greater than for a
successful female – therefore males
are often under tremendous sexual
selection for competing for mates
Females invest more into parental effort
(investment over a lifetime)
Males invest more into mating effort
Males have a greater
maximum potential
rate of reproduction
Male Female
50
Elephant seal
Red deer
Man
Kittiwake gull*
Percent of
copulations
0
1
2
3
4
5
6
7
8.............14
100
24
888
26
8
14
69
28
Sperm competition – fights are risky activities and the benefits may not always
justify the costs involved, therefore many males compete with each other using
less conspicuous, but effective and bizarre means
Dragonflies (Odonates) are one of a numerous
number of inverts that store sperm (in the
spermatheca) for use at a later date
uninflated
inflated penis
Coccethemis erythraea
Horn-like structure repositions
sperm of the previous male into
the corners of the spermatheca
Orthetrum cancellatum
barbed, whip-like flagellum
used to remove sperm in the
narrow ducts of the spermatheca
Sperm displacement
in Odonates
Mate-guarding combats sperm displacement strategies
Other options are to dilute the effect of your neighbor...
Testes Mass (g)
against
Body Mass (kg)
250
100
multi-male groups
single male harems
monogamous
H
human – 20g
gorilla – 35g
chimpanzee – 120g
10
1
0.2
1
10
100
200
Other bizarre forms
of sperm competition
“Chastity Belts” (copulatory plugs) – Monoliformes dubius (Acanthocephalan worm)
– cements shut the female’s genital opening
Xylocoris maculipennis – fertilizes the female by simply piercing the body cavity;
sperm swim around until they encounter eggs to fertilize.
Occasionally males pierce other males – the sperm then
swim into the victims testes where they wait to be passed
on to a female at the next mating
male Heliconius erato butterflies deposit an
anti-aphrodisiac on females after mating to
discourage other males – an odor males use
to repel each other in other contexts
Mechanisms of Sex. Sel.
- Scrambles
early search and quick location of mate; well-developed
sensory and locomotory organs
- Endurance
ability to remain reproductively active for a longer time
- Contest**
traits that improve success in fights; alternative mating
tactics of inferior competitors to avoid contests
- Mate Choice**
traits that attract and stimulate mates; offering direct benefits
(territories, nutrition) to mate; alternatives such as forced
copulation
- Sperm Competition**
mate guarding, sequestering sperm, mating plugs; production
of abundant sperm to dilute out your neighbor