Download Sexual Selection

Sexual Selection
 Sexual Dimorphism
 Reproductive Systems
Sexual selection
Selection that "depends on
the advantage which certain
individuals have over others
of the same sex and species
solely in respect of
reproduction" – Charles Darwin
 sexual selection arises through variation in
mating success
Fitness – determined by the number of
genes in future generations
Factors involved in fitness:
• number of matings
• survival of offspring
• fertility of offspring
• the environment
 Number of matings is only one part of
fitness
 The survival and fertility of offspring is a
major factor driving sexual selection
• Fundamental Question:
– What is male and female ?
• Females produce fewer, larger, more energetically
costly gametes.
• Males produce more, smaller, less energetically costly
gametes.
Sexual Dimorphism
• The occurrence of two distinct
forms among sexes within a
species
Gamete Dimorphism is Huge
• Male reproduction
success is limited by
mate access.
Mean # offspring produced
• Female reproduction
success is thought to
be limited by resource
access.
140
120
100
80
Males
Females
60
40
20
0
0
1
2
Number of mates
3
Sexual dimorphism - Birds
Sexual dimorphism - Reptiles
Sexual dimorphism - Fish
Sexual dimorphism – Weapons?
Sexual dimorphism
Weapons
female
male
male
Sexual dimorphism
Ornaments
Sexual dimorphism
Sexual size dimorphism
Extreme Dimorphism
The peacock’s tail
Osedax (bone-eating worms)
– a polychaet worm that lives on whale falls
(whale carcass at the bottom of the ocean)
O. frankpressi on
whale ribs
Scavengers feeding on whale fall
a typical female
Osedax roseus
Female
Males
 Males remain in a larval-like stage
 Live inside the females
 Essentially a sac of sperm, used to fertilize
the female’s eggs
Green spoonworm, Bonellia viridus
 Lives on the sea-floor (10-100 m)
 Lives in burrows/crevasses in rock and gravel
 Females are 15-cm long body with a 1.5-m
long extendable feeding tube
Male spoonworms
• 1-3 mm (mostly reproductive organs)
• live inside the genital sac of females
Sexual or Asexual Reproduction
• Asexual reproduction
–
–
–
–
Rapid
If fitness is high, matches organism to environment
If fitness is low, possible extinction
Stress can result in use of sexual cycle to give new gene
combinations
• Sexual Reproduction
– Greater energy commitment
• Specific organelles
• Mating rituals
• Feeding offspring
– Cost not equal between sexes
– Can produce new gene combinations able to cope with a
changing environment
– Greater loss due to selection against divergent genotypes in a
static environment
Mate Choice and Sexual Selection
in Guppies
• Given a choice, female guppies will mate
with brightly colored males.
– Brightly colored males attract predators.
– Color characteristics that have been shown to
confer mating advantage:
•
•
•
•
Brightness
Number of spots
Total pigmented area
Frogs
Mate Choice Among
Scorpionflies
• Adult scorpionflies (genus Panorpa) feed on
dead arthropods in the forest understory.
– Dead arthropod supplies are often limited.
• Thornhill found that male Panorpa will take
possession of, and guard, dead arthropods as nuptial
offering to female.
– Larger males are more successful.
– Small males offer spit.
Mating systems
Monogamy - One male mates exclusively with one female
Polygamy - Individuals mate with more than one partner
Polygyny - Some males mate with more than one females
Polyandry - Some females mate with more than one males
Promiscuity - Males mate with more than 1 female and vice versa
Monogamy
Polygyny
Promiscuity
Mating Strategies
• Different mating stategies have different advantages
and disadvantages
– Monogamy
• Seasonal or permanent
–
–
–
–
Allows sharing of cost of raising offspring
Increases survival chances of offspring
Many bird species and some mammalian species
Cheating does occur and has specific advantages to fitness
– Polygamy
• More than one mate of one sex for a single individual of the other sex
– Free individual to compete for resources and protect territory
– Better food etc for mates
– Some protection of offspring from competition
– Promiscuity
• Greatest number of offspring
• Large amount of competition
• Female only responsible for offspring in terms of resources
– Poorer survival change for offspring
Mating Success
Monogamy
Promiscuity
All males: 1 mating
All females: 1 mating
Many males: 0 matings
Some males: +++ matings
All females: 1 mating
Mating Success
Polygyny
Some males: 0 matings
Some males: 1+ matings
All females: 1 mating
Polyandry
Most males: 1 mating
All females: 1+ matings
Monogamy
• paring of a single male and single female
• can be for a short time or for life
• very rare among mammals (<10% of species)
• usually associated with male parental care
Monogamy
• 92% of the 9,700 species of birds are
monogamous
• often, the male is needed to help incubate the
eggs, provide food for the young, and
protect the nest from predators
Emperor penguins, Aptenodytes forsteri
Why might it be adaptive for a male to
mate with only one female?
1. Mate assistance hypothesis
2. Mate guarding hypothesis
3. Female-enforced monogamy
(monogamy is not adaptive for males)
Mate assistance hypothesis: Monogamy is adaptive
because male parental care greatly increases the
offspring’s survival
Effect of male parental care
on survival of the California
mouse
Male short-head
seahorse, Hippocampus
breviceps, giving birth.
Males take care of the
young, ensuring their
fitness
Mate guarding hypothesis: Monogamy is adaptive
because it ensures that the female will not mate with
other males; the male remains with the female while
she’s receptive
Male clown shrimp, Hymenocera
picta, remains with a female for a
long time because other females
are very scarce
Male bank swallow, Riparia
riparia, must stay with the
female to prevent her from
copulating with other males
Female enforced monogamy hypothesis:
Monogamy is not adaptive for males, but the female
forces the male to stick around and help raise the
offspring
• A male and female burying beetle
will bury a carcass on which the
female will later lay her eggs
• Once the carcass is buried, the
male will try to attract another
female partner by releasing
pheromones
• The female will push and bite the
male to get him to stop releasing
the pheromone
Polygyny
• paring of a single male and more than one
female
• very common among mammals
• two types:
• territorial defense
• dominant/fittest male
Territorial defense polygyny
Territory
An area that is defended for
exclusive use of the defender
against rivals
What is being defended?
• Resources (e.g., food, dens, hiding places)
• Females “choose” the male based on the
resources in his territory
Territorial defense polygyny
Male African cichlid fish, Lamprologus
callipterus, collect empty snail shells that are
used by females to deposit their eggs.
Territorial defense polygyny
 Advantage for males with good territories
 multiple matings
 many healthy offspring
 Advantage for males with poor territories – none
 Advantage for females – abundant resources,
protection from predators, fittest males
yellow-headed blackbird,
Xanthocepalus xanthocephalus
Dominant/fittest male polygyny
The fittest (i.e., strongest) male
mates with many females, while
keeping other males away
• harems
• leks
• scramble competition
plains zebra male with a harem
of females
Dominant/fittest male polygyny
 Advantage for dominant males
 multiple matings
 many healthy offspring
 Advantage for other males - none
 Advantage for females – the most fit male
will father offspring
Harems
Elephant seals - breeding females cluster together on
beaches; males defend the harem from
other males
Bull male elephant seals
engage in violent, bloody
fights over females –
large size confers an
advantage in combat
• male reproductive success is highly variable
• 8 individual males mated with 348 females in one study
Leks
Lekking - males aggregate in particular areas called leks;
display for females
Involves male competition
• males may fight for position in
center of the lek
Involves female choice
• females choose a mate…often
the dominant male
Lekking in Black Grouse and Fallow Deer
Scramble Competition
 Males can’t defend territories (e.g., females and
resources widely dispersed)
 Males still try to mate with as many females as
possible
 The fittest males are the fastest, most persistent,
and most aggressive
Horseshoe crabs mating
Polyandry
• paring of a single female and more than one
male
• very rare
Polyandry
Jacanas
• Males maintain small territories
• Males perform all parental care
• Females mate with multiple males
and then leave eggs with males
Sexual dimorphism
• Females larger than males by 60%
in mass (unusual for birds)
• Females aggressively fight other
females and also kill their chicks
(infanticide)
Polyandry
When is polyandry favored?
• When males become a limiting resource
for reproduction (females fight over the
few males)
• When offspring survival requires male
parental care
Saddleback Tamarins,
Spotted Sandpipers,
and Red-necked
Phalaropes are
examples of polyandrous
species
 Advantage for female
 multiple matings to ensure fertilization
of eggs many healthy offspring
 Advantage for males
 only way to ensure fitness; male
parental care needed for offspring to
survive
Yellow-toothed Cavy, Galea musteloides
• Females who mate with more than one male
will have more surviving offspring
3.0
# offspring
2.5
2.0
1.5
1.0
0.5
0.0
1 male
4 males
Number of mates
Inbreeding
• If a population is small then inbreeding occurs
– Self-fertilization
• Reassortment of genes but no new genes
– A population in terms of genetics always has some
degree of inbreeding
• Proportional to size of population
• Small population – Large inbreeding index
• Inbreeding depression
– Rare recessive alleles become homozygous and are deleterious
– Only when outbreeding between populations occurs
is this avoided
• More new gene combinations
• Outbreeding depression
– Maladaption to environment such as by colour
Hermaphrodites
– Exhibit both male and female function.
• Simultaneous
• Sequential
– Charnov et.al. identified (3) conditions
favoring hermaphroditic populations:
• Low mobility limiting male:male
competition.
• Low overlap in resource demands by male
and female structures.
• Sharing of costs for male and female
function.
Hermaphroditic Reef Fish
Simultaneous
Protogyny
=♀→♂
Protogyny
=♀→♂
Protogyny
=♀→♂ = ♂→♀
Protandry
END
Food supply is important to
reproductive success
• Birds in temperate
regions have a
larger clutch size
than tropical birds
• Animals in
temperate regions
have a larger litter
size than those in
tropics
– Food supply with
longer day length?
– Adaption to large
climate variation to
allow rapid
regrowth after a
poor season
– Greater mortality in
winter results in
more food next
spring
Nonrandom Mating Among Wild
Radish
• Wild radish flowers have both male
(stamens) and female (pistils) parts, but
cannot self-pollinate (self-incompatible)
• Marshall found non-random mating in wild
radish populations.
– Interference Competition - Aggressive or
inhibitory interaction between individuals.
– Some characteristic of the pollen from certain
individuals increases pollination success seed
plants.
Experiment: Comparing the effect of a “tethered” female
(i.e., one that can’t move) on the duration of pheromones
released by males to attract other females
Is sexual selection different from
natural selection?
• Darwin saw them as distinct - only sexual
selection could produce traits that
compromise survival
• The basic principles are identical – selection
favors whatever gets more genes into the next
generation
• In sexual selection, fitness is measured
relative to members of the same sex