Transcript Adaptive evolution

Evolution
Sexual Selection III
Monogamy, Polygyny, Extra-pair Paternity,
and Post-copulatory Sexual Selection
Why be monogamous?
Why be polygynous?
Monogamy
If females remain receptive after mating,
males that can prevent a partner from
accepting sperm from another male should
have higher fitness by mating
monogamously than by trying to be
polygynous
Males remain with a single female because
parental care and protection of offspring are
key
Monogamy
Monogamy
Timing maximizes RS
Polygamy and Promiscuity
Polygyny – One male, multiple females
Polyandry – One female, multiple males
Promiscuity – mixed mating system
generally involving extra-pair paternity
Why Seek EPCS?
• For males, easy opportunity for increasing
RS, but could face female abandonment
• For females, slightly more complicated, as
there are several downsides:
o Mate abandonment
o Decreased parental investment
o Harassment by other males/females
o Sexually-transmitted diseases
o No increase in offspring number
Evidence for Genetic
Compatibility
Evidence for Genetic
Compatibility
Extra-pair offspring sired by female
bluethroats are also more
heterozygous and have higher
immune responses than within-pair
offspring.
What about humans?
*cited 360 times….
Or….the infamous Sweaty T-shirt Experiment
Sperm Competition
Competition among males to fertilize
females doesn’t stop at ejaculation
1st male advantage – often seen in external fertilizing
species, as well as species with sperm storage
capabilities
Last male advantage – Common in species that can
remove competing sperm
Sperm competition
Selection can act on sperm morphology,
sperm number, as well as females’ ability to
control fertilization success
Drosophila bifurca have 58mm long sperm!
Wood mice sperm have an apical hook
used to attach to other sperm and create
mobile trains, which have higher
fertilization success than individual
sperm
A splendid fairy wren
male may have 8 billion
sperm at any given time
Across taxonomic groups, in species that have a high potential for sperm
competition, there are relationships between sperm competition and ejaculate
quality/sperm production
In birds, there is a direct relationship
between levels of extra-pair
paternity and testis mass
Sperm Competition
Selection on sperm can also occur within a
species
Older, territorial males nesting in the interior of a colony
produce ejaculates with more sperm that swim faster
than territorial males on the periphery, giving them a
fertilization advantage
Sneakers may release their sperm at exactly the same
time, but sneakers will fertilize more eggs
Sperm competition occurs across the animal kingdom
Removing competing sperm
Male black-winged damselflies use a spiky, modified penis to scrub out and
remove gametes from the female’s sperm storage organ before transferring their
own sperm.
Male dunnocks peck at the
cloaca of their partners if they
find another male near her.
This behaviour results in her
ejecting a droplet of ejaculate
from the other male.
Copulatory plugs
Observed in mammals, spiders, reptiles, and
insects, copulatory plugs are inserted just
after copulation in order to limit subsequent
copulations by another male.
The golden orb spider, Nephila fenestra males take this
to a whole new level
Females still hold the cards
Females may store the sperm of
their social partner, but instead use
recently received sperm from an
extra-pair partner to fertilize the
egg.
Tree swallows are socially monogamous migratory passerines
Both males and females provide parental care to offspring
Have one of the highest rates of extra-pair paternity in any bird species (83% of
nests, 47% of nestlings in an Ontario Population; Stapelton et al. 2007)
Thus, selection will act strongly on males to assure fertilization success.
Copulation attempts drop quickly after the
first egg is laid
Frequent copulations decrease the risk of
cuckoldry in presence of sperm competition
Life History Evolution
Life Histories
Life history traits: a trait directly associated with
reproduction and survival, including size at birth, growth
rate, age and size at maturity, number of offspring,
frequency of reproduction, and life span.
Life history traits
Darwinian demon - the ideal organism
matures at birth and leaves an infinite
number of immortal offspring
Conservation of mass states that this is
impossible
Life history evolution balances the life
history traits and trade-offs are selected
Life history: The big questions
How should it live its life?
When and how big should it be when reproducing?
How much energy should it allocate to reproduction?
Should it produce many or few, large or small
offspring?
How many offspring should be male or female?
Factors involved in life history evolution
1) The demography of the population
2) Risk of reproductive failure
3) Heritability and plasticity of the traits
4) Tradeoffs among traits
5) Phylogenetic history of the species
1) Demography
Connects age and size-specific variation in
survival and fecundity to variation in fitness
“Small organisms are usually small not because smallness
improves fecundity or lowers mortality. They are small because
it takes time to grow large, and with heavy mortality the
investment in growth would never be paid back as increased
fecundity” – Kozlowski 1992
Age
Early maturation reduces juvenile mortality
and generation time, but produces fewer
offspring with higher mortality
We can observe huge variation in life history
characteristics within lineages
Murellets
- Fledging date
- Clutch size
Marbled Murrelet
Ancient Murrelet
2) Risk of reproductive failure
If a bird puts all its eggs into one
nesting event and loses them to
a predator = zero reproduction
Best to reduce risk by spreading
risk across a set of independent
events
Seed types of Heterotheca latifolia
I’m a Darwinian
Dolt!
3) Heritability/Plasticity
Life history traits are controlled by many
genes (i.e., quantitative traits)
Heritabilities for life history traits: ~0.05-0.4
Trinidadian guppies
cichlid
Killifish
Guppy
4) Tradeoffs
Trade-off: a change in one trait that
increases fitness is linked to a change in
another trait that decreases fitness
- # offspring & survival rates of offspring
- attractiveness & predation
- reproductive investment & survival
- current & future reproduction
Detecting tradeoffs
If both traits improve fitness when increased
and both reduce fitness when decreased,
then a positive genetic correlation between
traits would not imply a tradeoff
If one trait improves fitness when increased
and the other improves fitness when
decreased, then a positive genetic
correlation between traits would imply a
tradeoff
5) Phylogeny
Phylogenetic effects are the contribution to
traits shared by all individuals because they
belong to a species or larger taxonomic
group.
Explaining the evolution of life-history traits
Comparative method
can help estimate
how much of the
pattern is attributable
to history and how
much is attributable
to microevolutionary
processes
Clutch size evolution &
reproductive investment
1. How does reproductive investment vary with age?
2. How large should each offspring be?
3. How many offspring should be produced in each
reproductive attempt?
4. How much parental care (if any) should be invested to
bring the offspring to independence?
Kiwi lays an egg that
is 5 times larger than
chicken egg
Can weigh ¼ the
weight of the female
Largest clutch sizes
Tiny eggs relative to body size
The eggs of the fly, Zenilla pullata, are only 0.027 by 0.02 mm
Orchid seeds weigh less than a
microgram and have a chance of 1
in 1 billion to survive
In contrast, dung beetles
raise 4-5 young that have
survival rates similar to
whales
Caeceilians can produce clutches of more than 100 eggs
weighing up to 65% of the mother’s post-birth weight. Females
also provide parental care to offspring.
Common pipestrel bats produce twins that weigh 50% of
the mother’s post birth weight.
Ageing
Lifespan and rate of aging
Lifespans evolve
The soma is mortal, and the germ line is
immortal
Death at old age does not impact fitness
Life spans evolve as trade-offs with life
histories:
o Extrinsic mortality
o Intrinsic mortality
Longer lifespans evolve if extrinsic mortality rates decrease in
Fixed effects
olderoorganisms,
increasing the value of older organisms
because of increased contribution to RS