Males and Females of a species look different \

Does not make sense in light of natural selection

Natural selection pressures apply equally to male and female so ………

Why aren’t both sexes selected for in the same way

Darwin recognized that individuals differ in their success in obtaining a successful mating. He called this sexual selection

Sexual selection Sexual selection is differential reproductive success due to variation among individuals in success at mating.

Evolutionarily speaking failure to mate and leave offspring is equivalent to dying young

Females typically make a larger parental investment. It takes more effort to

produce eggs or pregnancies than to produce sperm

in 90% of mammals the female provides significant parental care while the male provides none

In most species neither parent cares for the young but…

Females make a much larger investment in constructing the young

determined by number of eggs she can make or pregnancies she can carry

Not determined so much by the number of males she can convince to mate with her

more likely to be limited by the numbers of females he can convince to mate with him

Not as much by the number of sperm he can provide

Access to females will be limiting factor to the success of the male but access to males will not be limiting to females’ success (example of the rough skinned newts in text page 405-406)

Leads to an asymmetry in the fitness limits of the two sexes ….

which leads to different mating behaviors by the two sexes.

Males should be competitive with other males

Females should be choosy in the males they select

These differences are not inherent to maleness and femaleness so much as they are dictated by survival and fitness.

IntrasexualIntrasexual – where males can directly monopolize access to females – males will fight with each other. Females mate with the winners

IntersexualIntersexual –Where males cannot control access to females – the males advertise for mates the female chooses

Combat

Sperm Competition

Infanticide

large body size weaponry

armor

Females make a large investment in producing and raising young (20% of body mass into the eggs themselves) digs nest, buries eggs and guards a few days

Males do nothing In Iguanas, males can monopolize females

by staking out territories in which the females graze and sun themselves

Females are not choosy

Males fight over territories, winner takes all females in the territory

Territories are only a few square meters. Numbers identify the males which own the territory. 59 and 65 were the most successful malesPrime territories are near the water

Although large body size is disadvantageous in and of itself on the islands because it takes so much energy to maintain a larger body size

In fact selective forces are always working against larger body size

Male iguanas get big because bigger males get more mates and pass on their big-male genes

Sexual selection actually works in opposition to natural selection in order to choose for characteristics which give males a chance to mate successfully

Occurs with internal fertilization where one female mates with more than one male within a short period of time

Sperm are in a race to the egg Animals in this situation typically have

larger ejaculates with many more sperm than males that are not in this type of competition ( Example of the medfly in the book)

prolonged copulation copulatory plugs application of pheromones to the

female to reduce her attractiveness to other males

Scoop out the sperm left by a previous suitor

Animals that live in groups. Such as a pride of lions

Pride composed of a group of closely related females with 2 or 3 adult males related to each other but unrelated to the females

average time a male holds a pride is a little over 2 years

When a new male moves in to a new pride, he will usually kill any young who are still nursing. 25% of cub deaths in the first year are due to infanticide. (10% of all deaths)

Spontaneous abortions also happen often when new males move in. Female cuts her losses because cub would be killed shortly after birth anyway

Based on Female Choice Female choice leads to elaborate displays

by males Occurs when males cannot monopolize

the access to females Females are highly selective in these

situations

male and female build nest together and both feed the chicks

since they both care for the young seems to equalize parental investment but other factors suggest they do exhibit sexual selection not a monogamous species sexual dimorphism suggests that there is female choice involved The quality of the potential parents may vary

He displays his tail while perching and flying to the females after he sets up a territory and builds a nest

Males with elongated tails attracted more mates and mated more quickly. The longer the tail the bigger the difference.

Also showed that if the female mated with a shorter tailed male they were more likely to seek other males to mate with as well

Also the longer tailed males were more likely to convince more than one female to mate with them

Males advertise with calls both the length and speed of calls seem to be relevant to the female choice

When males hear other males calling nearby they increase both the speed and length of their calls

Researchers have observed females actually going to the more distant caller suggesting selection was occurring

Used loud speakers to present calls to frogs

75% of females preferred long calls to short calls, even when short calls were louder

Also 72% of females went past the closer speaker to the more distant speaker which was giving longer calls

Frogs from the two groups were maternal half siblings

Five aspects of fitness related performance were monitored

larval growth rate time to metamorphosis mass at metamorphosis larval survival post-metamorphic growth

Table 11.2

Hangingflies example males present food to females if she accepts the food they mate while

she is eating the food

If not enough food she breaks off and goes looking for another male bearing gifts

What benefits are there for the female when she chooses males bearing large gifts?

1. Provides her with more nutrients, so she can lay more eggs

2. Saves her from having to hunt herself, hunting is dangerous. Males die in spider webs at more than twice the rate of females.

Possible that selection on another trait may make a female more responsive to certain cues from males

In these cases the female preferences evolve first and then the male mating display follows and takes advantage of female biases already in place.

have poor vision rely mostly on smell and touch

Hunt by doing a “net-stance” where mite stands on a plant by its hind legs puts its front legs out to form a net and waits

Male water mites mimic the pray in order to find females.

The male gets the female’s attention by mimicking the vibration of the prey species. If the female turns and clutches at him he deposits the spermatophore.

Males induce females to accept their sperm sacks by fanning the water across the spermatophore towards the female

Moving water carries the pheromones towards the female

The female will then detect the pheromone and might pick up the sperm packet

It is possible female choice is simply arbitraryarbitrary no real selection going on for any apparent reason.

Once a particular male advertisement is favored by a majority of the females, selection by females will automatically reinforce selection for that trait in male offspring (sons)

AND...... Daughters (female offspring) will also

inherit the preference for that same trait passed on to them. Must show that preference is genetically based to prove this hypothesis.

This is sometimes called the “sexy sons” or “sexy sons” or “runaway selection” hypothesis. “runaway selection” hypothesis.

Females choosing more fashionable males will produce more fashionable sons and therefore more grandchildren than females choosing unfashionable mates

Example in book of stalk-eyed flies neatly shows that Selection by females for either long-eye stalks

or short eye stalks in males can produce an evolutionary response in the females which selects for the corresponding eyestalk length in future generations.

There is no one hypothesis that is better than another.

Individual species use different strategies all of these explanations are mutually compatible

and in many species more than one strategy may be involved

This is a very active area of evolutionary research today.

Sex role reversal

sea horses and pipefish In these organisms the female lays her

eggs directly into the male brood pouch and the male supplies the eggs with oxygen and nutrients until they hatch

In these cases: Males invest a greater amount of energy. access to male brood pouches limits the

reproductive success of females females should compete with each

other and males should be choosy

In one species of pipefish the females are larger and have dark blue stripes and skin folds on their bellies which appear to be advertisements for males

Skin folds are only present during mating season

In captivity females only develop skin folds when males are present

Females show no tendency to discriminate between males

Same sex-reversed effects are seen in other species as well

Number of males tested

Plants are often sexually dimorphic The seed parent (female) makes a much larger

reproductive investment than the pollen donor. Mating involves successful pollination Mating success = access to pollinators Principles of sexual selection we have been

studying would predict that the success of pollen donors is more limited by access to pollinators than is that of the seed parent(female)

self-incompatible white and yellow flowers; white shows

simple dominance over yellow study population of 8 WW (white) and 8

ww (yellow) plants Monitored pollinator visits to each color

of flower. ¾ of the pollinator visits were to yellow flowers.

Measured reproductive success through both male and female function

For females simply count the number of plants that produced fruits containing seed.

Essentially all plants produced seed.

The measure of the males success is a bit more tricky

Could not do it by individual plant Had to raise the seeds produced by the

yellow seed parent and determining how many white and how many yellow flowers

WHY?WHY? Because if the yellow parent (recessive)

had been pollinated by a yellow pollen it would produce yellow plants but if by a white pollen it would produce white plants

If male reproductive success is limited by pollinator visits then male pollen from yellow-flowered plants should have gotten ¾ of reproductive success, since they received ¾ of the visits.

¾ of the seeds diddid produce yellow flowered plants. So male success was directly related to the access to pollinators and where they delivered the pollen.

but seed parents (females) had equal success in producing seed and it did not matter which male was the pollen provider

Thus, reproductive success of males is more limited by access to pollinators than is the females

•Remember, Yellow flowered plants got ¾ of the pollinator visits

we would predict that dioecious plants would have dimorphic flowers and that ??? flowers would be showier?

Males do whatever they can to attract pollinators and assure that their pollen will be successful.

malemale

•Many studies bear this out

the size of flower parts is simply designed to protect the reproductive parts, the larger the reproductive parts, the larger the perianth (sepals + petals)

In animal pollinated, however, we see a variety of strategies …..

Males also seem to have stronger odors and more flowers per inflorescence

Not all flowers are dioecious. In this case larger flowers are usually more successful.

Larger flowers are visited preferentially by bees and butterflies.

Larger flowers will be visited first and their pollen, once delivered to the female, will have a head start on forming a pollen tube in the pistil of the flower it pollinates.

Meanwhile, females typically receive 4 times the amount of pollen needed to produce seed successfully

Male flowers actually train male bees not to visit other males

The orchid also assures that no other pollen coming to the same female flower at a later time will be able to successfully pollinate

How do they do all of this?

Males produce their pollen in a pollinarium which is held by a triggered mechanism

When a bee trips the trigger the pollinarium shoots at the bee and sticks to the bee’s back

After being shot the bees avoid going to other male flowers

When the bee visits the female flower the flower is smaller and rubs the pollinarium off of the bee

and it lodges on the receptive stigma In response the stigma swells and shuts out

any further pollination