Download Mating Systems

Sexual Selection, Mating
Systems and Parental Care
Reading; Freeman, Chapters 48 and 51
Why Have Sex?
Some Species Get By Without It
• Some very successful groups do not reproduce sexually every
generation
– these include aphids, parasitic wasps, mites, certain crustaceans,
the majority of vascular plants, and many others.
• A diverse collection of species never reproduce sexually;
– These include several whiptail lizards Cnemidophorus sp.,
many geckos, the stream fish Pocelliopsis sp., many snails, a
great many fungi and protozoa.
• At least one entire animal phylum never reproduces sexually
– the bellidoid rotifers (small freshwater invertebrates) never reproduce
sexually.
A real evolutionary mystery; How did
sex evolve?
• Sex is a complicated and pervasive aspect of the life histories of
nearly every eukaryote…
– since natural selection is the only known mechanism capable of
producing an adaptation like this, we must conclude that some
form of selection led to its origin, and to its continued existence.
• Natural selection favors individuals that pass on their genes to the next
generation.
– In most species, however, males contribute nothing but sperm to
reproduction.
• A parthenogenic female should have a distinct advantage, she passes
on twice as many of her genes with each offspring.
• The asexual route,
pictured to the left,
propagates twice as many
of the female’s alleles.
• If there were a genetic
polymorphism in a
population, between
asexual females and
sexual females, asexual
females would be
expected to have an
enormous advantage in
terms of fitness, especially
since, in most species,
males contribute nothing
but sperm.
Sex is Expensive
• Sexual displays and rituals can be enormously
expensive in terms of energy, time, and resources.
– Example: male bower birds in Australia spend weeks
constructing enormous “bowers”, which they decorate
with found objects, solely for the purpose of attracting
mates.
– Female water striders are “ridden” by the males after
copulation, sometimes for many hours. This process
interferes with the foraging of females and places them at
increased risk of some forms of predation.
Sex is Dangerous
• Seeking mates is dangerous for many species:
– Example; tree frogs in Central America call to attract mates.
Females are strongly attracted to the calls, it is much easier to get a
mate if a frog invests a lot of time calling.
– Carnivorous bats are also attracted to the calls. They come in on
the calling male and pluck them right off of the tree.
– Calling crickets suffer a similar situation. Calling males are much
more attractive, but parasitic flies home in on the sound and
deposit their eggs into singing males.
Reasons a SPECIES benefits
from Sex
• It is easy to come up with reasons a species, or an
evolutionary lineage benefits from sex.
• The problem here is that we lack a real understanding of
how selection at the “species level” works. Natural
selection, as we know it, acts on alleles, not species.
– Genes that act for the good of the species could be undermined
by “selfish” mutations that crop up within individuals, and
spread.
• Two VERY compelling reasons species benefit from sex:
• Recombination provides the genetic diversity necessary
for the survival of the species “The “Red Queen
Hypothesis”
• Without recombination, mutation can drive an asexual
species extinct-”Muller’s Rachet”.
The “Red Queen” hypothesis
• Even species that do not seem to be changing are
undergoing rapid evolution.
• This is because the natural world is so challenging
that, without constant evolution, species go
extinct.
• Sex and recombination provide the necessary
variation that keeps evolution going…without
constant evolution, pathogens, competitors, and
environmental change would lead to the extinction
of all species.
The parasite-host
arms race is a real
evolutionary
interaction, for
instance, smut are
parasites of vascular
plants.
The fungal parasite
evolves quickly, and
susceptible parasites
are “sitting ducks”,
only by constant
evolution do the host
survive, because the
parasite cannot
exploit a host with
novel defenses.
• The red queen hypothesis has been
supported by several interesting studies, in
which the fitness of asexual strains was
compared to the fitness of members of the
sexual species that gave rise to these
strains (they often live in exactly the same
environment).
• Best known are studies of the stream fish,
Pocelliopsis, in Mexico.
– Asexual strains suffer much more parasitism
than sexual individuals that live in the same
streams.
– This advantage to sexuals disappears when
genetic variation among them is lost, implying
that it is the genetic variability inherent in
sexual reproduction that keeps parasites at
bay.
Muller’s Ratchet
• Without occasional recombination, mutation
alone can drive an asexual species to
extinction.
• Many mutations are harmful. Eventually, in
an asexual species, every lineage
accumulates a few harmful ones. Without
recombination, there is no way to create a
lineage that is free from harmful mutations.
• Likewise, without recombination, there is
no way to collect favorable mutations into a
single lineage.
Reasons an Individual Might Benefit from
Reproducing Sexually
• Bet-Hedging. Sexual offspring will be more varied than
asexual offspring. In an unchanging environment, this is
a disadvantage, because the parental types were obviously
successful enough to reproduce. In a changing
environment, variable offspring ensure that at least a few
will survive.
• Offspring will be different from parents. If predators or
pathogens are part of the environment, offspring which
are different from their parents have an advantage,
because pathogens are adapted to attack the parental
generation.
Abundant evidence supports the notion
that sex is advantageous in the face
of environmental change. Many
species that can switch between sexual
and asexual reproduction turn to sex
when faced with an imminent change
in the environment.
Sex Determination Mechanisms
Haploid-Diploid Sex Determination
• In ants, bees, wasps, some mites, sex is determined by the
number of SETS of chromosomes. Males are haploid, and
descend from unfertilized eggs. Females are diploid, they
result from the genetic contribution of a mother and a father.
• Females of these species can either fertilize each egg, or let
it remain unfertilized, thus controlling the sex of their
offspring.
• This sometimes results in sex ratios that are skewed in favor
of females, because mothers ensure that their sons will not
compete with each other for mates by laying fewer male
eggs.
Environmental Sex Determination
• For most turtles and alligators, the sex of an individual is
determined by the incubation temperature of the eggs.
• In many turtles, for instance, eggs that incubate at low
temperatures become females, and the eggs that incubate at
high temperatures become males (in other species, the
pattern is reversed).
• In some cases, sex can be determined by the social or
physiological status of the individual. Small or poorly fed
individuals are male and larger individuals are female, or
vice-versa. This is especially common in sequential
hermaphrodites.
• In some cases, cytoplasmic parasites such as Wolbacchia sp.
Can feminize their hosts, converting males into females...
Chromosomal Sex Determination.
• In humans, XX is female vs. XY male
• In butterflies and birds, females are the heterogametic
sex (females are ZW, males are WW).
• In chromosomal sex determination, the assortment of
chromosomes during metaphase of Meosis I ensures
that exactly half the male gametes contain X
chromosomes, and exactly half contain Y.
• This system produces 50% males and 50% females.
• In humans, there is actually a slight male bias at
conception, because sperm containing Y chromosomes
have a smaller “payload” and swim faster, thus resulting
in more conceptions
Hermaphrodites
• Many fish, annelids, and plants can function as both
sexes. These are called hermaphrodites.
• Sequential hermaphrodites can switch from one sex
to another during the course of their lifetimes.
• Simultaneous hermaphrodites can be both sexes at
once. Many angiosperm plants, for instance, are
simultaneous hermaphrodites, as are many annelids.
• Generally, individuals switch sexes when they would
have a higher fitness as a member of the opposite sex:
for instance, if males fight for access to females, a
female might switch over to being a male when she
grew large enough to win fights with rival males.
Sexual Selection
• The two basic mechanisms of sexual selection
male competition and female choice.
• Because of inherent differences in the sexes, it is
usually the female sex that is sought after by the
male.
• Male competition is the process by which males
compete with each other for access to females.
• Female choice is selection imposed by females
when choosing males as mates.
Sexual selection is natural selection imposed on individuals
based on their ability to obtain mates.
It is a pervasive and important mechanism of evolution in all sexual
species.
The idea was first proposed by Charles Darwin, in “The Descent of
Man”
He saw it as antagonistic to natural selection, though nowadays, it is
usually viewed as one more aspect of selection.
Even species for which there is no overt competition among
individuals for mates experience sexual selection.
For instance…sperm competition is natural selection based upon the
differential ability of sperm to reach and fertilize mates
Flowering plants compete quite strongly for pollinators, driving the
evolution of colorful flowers and floral “rewards” such as nectar, oils,
and other compounds.
Example of Male Competition:
– Stag beetles establish
territories on
branches and logs
where females are
likely to be (females
mate before
ovipositing in woody
substrate).
– Males will fight to
control a territory,
trying to seize their
opponent and throw
them off the branch.
Example of Female Choice
– Birds of paradise are a
group of species of birds
distantly related to
blackbirds.
– Males “display” their
bright plumage to females
of the species (which are
drab colored)
– Females mate with the
males that have the most
attractive plumage and
most interesting displays
• Sexual Dimorphism, a
difference in appearance
or behavior between the
two sexes, is frequently
due to sexual selection.
• It can also be due to
differing selective
pressures between the two
sexes-I.e., males increase
their fitness by obtaining
more mates and females
increase their fitness by
laying more eggs.
• Behavior can also be
sexually dimorphic.
Mating Systems
• The effects of sexual selection depend upon the mating
system of the animal.
• In monogamous species, males and females pair for the
breeding season. In some cases, the individuals may pair
for life.
• In polygynous species, males have multiple female
partners in a single breeding season. In some cases a
male may control a large group of females. In other
cases the male may practice sequential polygyny, in
which a male visits with successive females.
• The opposite of polygynous is a polyandrous mating
system in which females have multiple male partners.
Example of a polygynous species:
• California elephant
seals defend territories
on the beach. They
mate with all the
females in their
territories.
• Males will fight over
territories, the losers do
not get to mate that
year.
– Note that these
contests are an
example of malemale competition
• There are also subcategories of polygynous and
polyandrous mating systems that are defined in terms of
whether the mating systems are resource based or nonresource based.
– The elephant seal situation is a resource-based polygyny,
space on the beaches was the key resource. Resource based
systems intensify male-male competition as a mechanism of
sexual selection, though there is a little female choice.
– Females often choose to mate with “sneaky males” that
invade male territories and clandestinely copulate.
– In a resource based polygynous mating system a male defends
some resource that might sustain several females.
– In a non-resource based mating system like a lek, females
aggregate in regions with sole purpose of choosing a mate.
Males aggregate in these locations and display in order to get
access to females. Female choice is intense in this situation.
Caullauthidium is
a small solitary bee.
Males defend
the flowers that
females specialize
on.
They mate with
females on the
flowers.
Thus, the flower is
a resource and males
ensure mates by
defending the
flowers
• Note that in polygynous species, the male does not
generally provide anything toward parental care.
– Even in resource-based systems, he is basically
monopolizing a resource she needs, and protecting it
from males, he does not create or provide it.
• Thus, males can potentially mate with many
females. Male fitness is proportional to the number
of mates.
– The basic inequality between the sexes is increased, since
the female provides all the parental care. Females do not
benefit from more mates because their fitness is limited
by time and resources.
• In these systems, there is strong sexual selection on
males to obtain more mates.
• Barn swallows are a North
American, insectivorous bird that
builds mud nests under overhanging
cliffs and bridges.
– Both sexes overwinter in warm place
(Africa, S. America)
– Males return to temperate areas first,
and display to attract females into their
breeding territories.
– Males and females pair up early in the
mating season, mate, build a nest, and
cooperate to raise young
– Monogamous systems such as this
favor “choosiness” in both sexes.
• The result-good mates tend to end up with
good partners.
Example of a
Monogamous
Species
Example of a Polyandrous Species
– Spotted sandpipers are a shore Polyandry is rare in nature.
bird that breed along the coasts Here a female defends her territory
of North America (they
against a rival female.
overwinter South America).
– Females arrive in temperate
areas first, and establish mating
territories.
– Females compete to attract
males to their territories
– Males provide the parental care
in this species. A female will
mate with several males, laying
clutches in their nests (the two
cooperate to build the nest).
– A female who has more male
mates has a higher fitness.
Example of a Non-Resource Based Polygamy System
• Sage grouse are henlike birds that live in
steppe and sagebrushshrublandland.
• Males gather in places
that become leks.
There is no important
resource where the lek
forms, it is simply a
place males display.
• The males with the
best display get the
most mates.
Many males get no mates at all.
A few males get ten, twenty or more.
It seems to depend upon how attractive
the booming display and dance is.
Female Choice Experiments
• Do females actually
choose among males?
This idea was
controversial at first,
partially because it is
difficult to know why an
animal is picking a
particular mate (are they
really choosing, or going
with what is available?),
and partially because of
subconscious biases
(sexism).
• Female choice has
been experimentally
confirmed in several
species, and is thought
to occur in many
more.
• Experimental tests
include:
• widowbirds
– guppies
• Female choice is well documented in guppies (also the
related fish, swordtails)
–
–
–
–
place the female in a box with a “choice” between a short-tailed
give her a chance to check them out for a while
remove the barriers and see who she mates with
Result-females prefer males with longer tails.
Females who mate with
long-tailed males have male
offspring with long tails.
Tail length is heritable.
Also, the preference for long
tail length, and long tail length
itself, are likely to be
correlated.
This is because the preference
for long tails is most likely
heritable as well.
Sexual Selection in Widowbirds
• Widowbids are an
African species of
polygynous bird that
nests in open grassland.
• Males have brightly
colored epaulets and long
tails. They display to
attract female mates,
flying in looping flights
above the grass.
• Females are drab colored
and lack the long tail.
Experiment
• To test whether the long tails are really subject to
sexual selection, researchers conducted an
experiment on tail length.
– Birds with known display territories were observed, and
their mating success was noted.
– Some birds were left alone (control)
– Some birds were caught, had their tails cut off, and
reglued (another control)
– Some birds had their tails cut short
– Some birds had tail length added to make a longer tail
• Result-longer tails mean more mates, shorter tails
mean fewer mates.
• Notice that the control birds had fewer mates as well,
possibly because the long-tailed birds were getting all the
mates.
• There were a few flaws with the study.
• Males use the epaulets to signal other males
to stay out of their territories. Thus, their
fitness is a function of both male-male
competition and female choice.
• Also, researchers measured fitness
indirectly, by counting the number of nests
in the territories of males. Usually, a female
will nest in the territory of a male after
mating with him, but it is possible that some
females moved and established nests
elsewhere.