Sexual Selection and Mating Systems PDF

Sexual Selection and Mating Systems Reading – Freeman, Chapter 23 Systems and Parental Care -learning goal: students should understand the genetic consequences of asexual vs sexual reproduction, understand the mechanisms of sexual selection, and understand the consequences of paren...

Sexual Selection and Mating Systems Reading – Freeman, Chapter 23 Systems and Parental Care -learning goal: students should understand the genetic consequences of asexual vs sexual reproduction, understand the mechanisms of sexual selection, and understand the consequences of parental investment and mating systems on evolution. Why Have Sex?  From a Human Perspective, the Answer Seems Obvious, but it Isn’t. Sex is an evolutionary puzzle. In most sexual species, males make up half the population, yet they bear no offspring directly and generally contribute little to the survival of offspring. In many species, asexual reproduction is an option, or is the rule. Asexual lineages, called microspecies, arise spontaneously, and exist within many populations of sexually reproducing species. Many species can toggle between asexual ands sexual modes of reproduction. Asexual reproduction has advantages.  Some Species Get By Without Sex 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.  Right-the reed killifish. When males are not around, it reproduces by autogamy- a form of selfing. Below-the Amazon molly, a fish that reproduces asexually thought sperm- dependent parthenogenesis. Both pass ONLY their alleles to the next generation. Asexual lineages arise spontaneously within many sexual organisms. thelytoky, the asexual formation of diploid zygotes from a single individual, via apomictic parthenogenesis, is one mode. These asexual lineages have some advantages, but ultimately, seem to have a short evolutionary lifespan. For example, asexual whiptails are still tied to the need for courtship. 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 and Dangerous 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. The “Red Queen” Hypothesis Leigh Van Valen proposed the Red Queen hypothesis as a mechanism for the continued existence of sexual reproduction. It proposes that: 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 argument is essentially a mechanism of evolution that occurs at the level of species.  Every species on the planet is locked in what JBS Haldane referred to as an “Evolutionary Arms Race”. The natural enemies-parasites, predators, and competitors, of any species evolve every generation to better exploit their hosts, capture their pray, and push aside their competitors. This produces a selective pressure among the hosts, prey, and competitors, to negate these advantages. Evolution must be constant, or a species will go extinct. The Red Queen hypothesis can be seen as a natural outgrowth of the evolutionary arms race. Braconids parasitizing a hornworm larva A harvestman parasitized by mites.  Long-term evolutionary arms races are suggested in the fossil record. For instance, in Cenozoic mammals, there is a correspondence between the fastest running herbivores and the fastest-running carnivores The rise of jawed fishes seems to induce a cascade of shells, armor, and escape abilities among marine invertebrates in ancient, paleozoic seas. In shorter time scales, apparent cases of evolutionary arms races are easy to find. For instance, the rough-skinned newt, one of the most toxic amphibians on the planet, co-occurs with garter snakes that are just barely able to detoxify the poison.  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, Poceliopsis, 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. 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.  Polling Question; According to the “Red Queen Hypothesis”, what would you expect to the descendants of an asexual female, over time? A) Initially, they would pass on more copies of their genes than sexual females. B) Lacking genetic variation, they would accumulate parasites, or be vulnerable to predators, as natural enemies evolved to exploit them. C) Recombination would drive their genes extinct. D) A and B E) All of the above  Sexual Selection Sexual Selection is the subset of selective pressures concerned with attracting a mating partners and achieving copulations or pollination events, and also, choosing the best mating partners given the circumstances. 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.  Charles Darwin viewed sexual selection as a phenomenon quite different from natural selection. A more modern view is that sexual selection is really a subset of selection, in a larger sense. Some of its effects are counterintuitive from the perspective of “survival of the fittest” (which caused Darwin to classify it as a separate phenomenon). Selection, as we know it today, is a combination of the ability of an organism to survive and to ensure that its genes are passed into the next generation. Right-Alex Basolo and Deborah Charlesworth, two evolutionary biologists who studied the evolution of sex and sexual selection. A Fundamental Dichotomy A fundamental inequality exists between the sexes. If it is called “male” it has a small gamete, relative to the larger “female” gamete. This dichotomy probably evolved many times, from ancestral lineages which contributed more equally to the process of syngamy. “Female”, almost by definition, means contributing more. The metabolic cost of producing one egg is equivalent to the production of hundreds or thousands of sperm. Therefore: there are more sperm out there than eggs, and in one way or another, sperm compete to fertilize eggs.  Additional Inequalities Gestation: Except in some rare circumstances (i.e., some members of the pipefish family, the best known of which are seahorses) gestation is the retention of eggs, or immature individuals, within the body of the female. This causes an additional drain on resources, because she must expend extra energy carrying them around.. Lactation: A mammalian parental care strategy. Only females lactate. Parental Care: Parental care can be split between both parents, or it can fall entirely upon one parent. In cases of uniparental care, it is much more common for the female to provide all of it (depends upon the lineage, uniparental male care is rather common in fish). Distribution of Parental Care in Various Lineages Bony Fish. Some groups. Male parental care much more common. Mammals. Universal. Female uniparental care much more common than biparental care. No uniparental male care. Birds. Virtually universal. Biparental care much more common than uniparental care, which can be only by female (most often), or only by male (rarely). Crocodilians. Common. Female care mostly. Snakes. A few tropical boas and pythons, exclusively female. Frogs+Salamanders. Uncommon. Male parental care occurs more often than female. Scarabs. Biparental care very common. Earwigs. Extensive parental care, always by female. Bees, Wasps. Extensive parental care by females. No male help. Assuming that there is this asymmetry in Reproductive Investment, male fitness depends very heavily on the number of matings, and female fitness depends more upon the resources she can amass for each reproductive effort. Therefore: Male fitness depends of finding as many mates as possible. Males compete for mates. Female fitness depends of finding the best possible mate. Females are choosy about who they mate with. Two Mechanisms The two basic mechanisms of sexual selection male-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-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.  Example of Male-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  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.  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. 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  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. Typically, a few males sire most of the offspring in a population. Few male winners, many losers. Below-Galapagos sea lion mating success histograms. https://www.ncbi.nlm.nih.gov/  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  Barn swallows are a North American, insectivorous bird that builds mud nests under overhanging cliffs and Example of a bridges. Monogamous Both sexes overwinter in warm place Species (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 Polyandrous Species Spotted sandpipers are a shore bird that breed along the coasts of Polyandry is rare in North America (they overwinter nature. South America). Here a female defends her Females arrive in temperate areas first, and establish mating territory territories. against a rival female. 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.  Female Choice Experiments Do females actually choose among males? Female choice has This idea was controversial been experimentally at first, partially because it confirmed in many is difficult to know why an species, and is thought animal is picking a to occur in many more. particular mate (are they Experimental tests really choosing, or going include: with what is available?), widowbirds and partially because of Guppies, swordtails Mice, voles subconscious biases Drosophila flies (sexism). http://jeb.biologists.org/  Guppi es documented in guppies (also Female choice is extensively 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.  Female guppies prefer brightly colored males, and males with more spots. Brightly colored males are more conspicuous to predators-but also more likely to assertively inspect their environment for threats. Females prefer bold males to timid males-a trait that might have survival value. Males go out of their way to prefer larger, thick-bodied females-body size is likely an indicator of fecundity. http://www.pnas.org/content/pnas/93/19/10262.full.pdf  Polling Question: In which scenario would you expect female choice to be the strongest mechanism of selection? A) Males gather in a lek and display for the benefit of females. Females mate with the most attractive male and leave. B) Orchids compete for pollinating bees by exhibiting bright colors and interesting smells C) Females come into oestrus and mate with every male in a very large social group. Males provide no parental care, nor do they fight for females. D) Male bighorn sheep fight for access to females E) None of the above. Alternative Mating Strategies Pacific Salmon Side- blotched lizard Redhead duck  Polling Question In which mating system would you expect male-male competition to be the most intense? A. Males invest more in offspring than females B. Males arrive at mating grounds and display to attract females C. Males arrive at mating ground and establish territories in areas where females are likely to breed. D. A and C E. Can’t Tell