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

streams. , in Mexico. – Asexual strains suffer much more parasitism than sexual individuals that live in the same – 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 male male 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

non resource 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 bird that breed along the coasts of North America (they 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.

Polyandry is rare in nature. Here a female defends her territory against a rival female.

Example of a Non-Resource Based Polygamy System • Sage grouse are hen like birds that live in steppe and sagebrush shrublandland.

• 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.