Sex and Mating Systems
As best we can tell, life originated without sex. Although asexual reproduction would arguably be considerably less wasteful than sexual reproduction in many respects, the great majority of species have prospered with reliance on latter. in sexually repoducing organisms half of the population is lost as a reproducing unit (i.e., the cost of males). In addition, both individuals contribute only half of their genetic material to the offspring, while the other half is lost. In addition, there is the danger associated with combining your genome with a largely unknown and untrusted copy from a partner. Several possible reasons have been advanced to explain the presence of sexual reproduction:
- Muller's Ratchet: Mutations accumulate in asexual lineages as there is little chance to get rid of them again. In asexual reproduction genomes are inherited as indivisible blocks. Over time the population acquires a mutations without an opportunity to get rid of them (except as a result of back mutation). This results in an eventual accumulation of mutations known as genetic load.
- Vicar of Bray hypothesis: Ability to combine individually advantageous genes allow for rapid adaptations to change
- Red Queen Hypothesis: By constantly reconfiguring the precise genetic set that a parasite or pathogen will face, animals with longer lifespans can provide a moving target.
Asexual lineages tend to disappear over time while sexual lineages tend to persist much longer.
Evolution of Diploidy
Most eucaryotic groups exist as diploids (i.e., those carrying 2 sets of genetic information), some occur primarily in haploid forms with a single set of genes, and others alternate between haploid and diploid phases. Different strains of yeast, which may be haploid or diploid, show advantages depending on the specific envrironmental conditions. In a favorable setting, when nutrients are present in excess, the two strains show little difference in reproductive rates. However, when growth was limited by the concentration of a single nutrient, haploid strains of yeast were able to out-compete diploids. This is consistent with the idea that haploid cells are more efficient as simple replicators. The main advantage of diploid cells emerges with a greatly improved resistance to damage. Sexual haploids may combine the advantages of both: spending much of the life cycle in the haploid state, then temporarily fusing to become diploid, followed by splitting to the haploid state.
Diploid organisms appreciate several key advantages:
- A second set of genetic information provides a backup in case of somatic mutations. With two copies of every gene, a diploid organism is able to mask recessive deleterious mutations. DNA damage can be repaired in a diploid state, robinsince there are two copies of the gene in the cell and one copy is presumed to be undamaged.
- A second set provides one copy as a substrate that is free to be altered by mutation, while a second, trusted copy remains available for basic cell functionality
- Increases genetic diversity that may or may not be expressed
- Provides conditions that allow for sexual reproduction.
Evolution of Anisogamy
Disruptive selection in gamete characteristics underlies the evolution of Anisogamy. Anisogamy is the condition in which one type of gamete becomes increasingly large (i.e., the egg) to provide the zygote with storage for a head start in development. As size increases, gametes becomes less mobile. In turn selection produces another type of gamete (i.e., sperm) that is optimized for locomotion to find and fuse with the egg. The larger gamete is termed the female sex, the smaller gamete is that of the male. Compared to isogamy this allows for both increased provisions for zygote and optimized mobility.
Bateman's Principle: increased reproductive investment is accompanied by increased selectiveness for mating partners. While female gametes are energetically expensive, large and limited in number, male gametes are cheap, mobile and available in large numbers. This asymmetry leads to reluctant females as the choosy sex, while ardent males are the competing sex. A.J. Bateman (1948) phrased this rule based on work where he had examined variation in reproductive succcess between male and female fruitflies. He had placed 3 male and 3 female fruitflies into a container and subsequently assessed the reproductive success of all individuals. Most females in contrast had reproduced with little variation, while only a few males had accounted for all reproduction and most males had no success at all. Limits to reproductive success thus differ between sexes where males are generally limited by the number of successful matings, while that of females is primarily determined by the rate of egg production. This favors behavioral traits for choosy females and ardent males. Differential reproductive investment will polarize operational sex ratio (i.e., most females and only a few males get mating opportunities).
The Coolidge effect describes phenomena whereby males show high sexual performance given the introduction of previously unknown, receptive females.
Trivers-Willard Hypothesis: Moreover, as only the most competitive males achieve mating success, a bias in sex ratio may emerge where dominant/healthy females produce a relative excess of males which stand a disproportionate chance to develop into the successful male. Female offspring are the safe bet, while males may range from exceptionally successful to a spectacular bust.
Evolution of Sex Ratio
Sex ratio describes the relative number of males to females in a population.
The primary sex ratio refers to the number of males vs females at conception. The distribution is most commonly around 50:50. Such a sex ratio might not seem very efficient considering that a male can fertilize several females. Fisher's theoretical work illustrated that such a balance forms a general equilibrium point. If there are fewer males than females, then males face better odds in mating. An advantage for males favors females who produces extra sons. The same argument follows for deviations in the opposite direction. Thus, if a population ever comes to deviate from a ratio of 50:50, natural selection will tend to drive it back to that balance. The secondary sex ratio, the ratio at time of birth, in humans usually shows a slightly bias towards boys (p=0.53%). With a somewhat higher mortality of males the tertiary sex ratio, the ratio of mature organisms, becomes increasingly skewed towards females.
The operational sex ratio refers to the relative number of receptive males per receptive female in the population. The operational sex ratio is usually biased heavily towards males indicating that females have many more males to choose from than the other way round.
Several types of Mating systems can be recognized
- Monogamy (one male, one female partner)
- Polygyny (one male, multiple female partners)
- Polyandry (one female, multiple male partners)
- Polygynandry (multiple female, multiple male partners; Promiscuity)
Mating systems are highly fluid evolutionary entities. They are dynamic and highly optimized systems where many different factors figure into the equation. Selection will always foster the most successful tradeoffs for these parameters, but many strategies may lead to success. Mating partners are rarely selected at random. In most species it is strongly controlled by diverse factors. Although many individual strategies may prove successful, a variety of considerations provide predictive power for the type of mating system that is present:
Reproduction is the sole method through which a species may continue, and reproduction manifests itself in numerous deviations throughout nature. The ultimate goal of each species is to produce the maximum amount of offspring while exerting the least amount of energy. Predation pressures, resource accessibility, and competition with the species for attracting mates greatly influence the likelihood of an individual within a species producing viable offspring and donating genes to subsequent generations. Different fish species may be observed in order to learn the unique reproductive strategies employed throughout nature from fertilization tactics to parental care methods that have adapted these fish in continuing their species.
Females typically exert much more energy and time into reproduction than males, and therefore, females are usually more selective in choosing mates. Females typically favor males that are larger in size, possess more elaborate physical traits than the other males, and display more energy in courtship activity. However, smaller and less desirable males must still be able to fertilize the eggs, or, according to natural selection, they would have been eliminated from the population. Although slightly more unsettling, sexual coercion results in the fertilizing of eggs just as sexual courtship does. The term “sneakers” refers to smaller males who opt to race into the nest more desired males and fertilize the eggs while the nesting male pursues the female. The term “satellites” describes cryptic males mimicking females in appearance and behavior so that they are able to fertilize the eggs while the unsuspecting couple spawn.
Although fish species exhibit both internal and external fertilization, fish also exemplify many unique strategies for parental care. Most fish species utilize broadcast spawning, which is a method a releasing gametes into the water and providing no parental care. The goal of this reproductive method is to produce the maximum amount of progeny in hopes that as many offspring as possible will survive. Many fish species, however, opt to employ parental care in order to supply their smaller number of offspring a greater chance of survival. Oral brooding and sex role reversal are two unique methods of parental care that have proved successful in certain fish species.
Oral brooding, although quite rare in nature, is a parenting method adopted by many fish in the Family Cichlidae. After the successful fertilization of the eggs, cichlids, such as Tilapia, place the egg clutch into the mouth of the female. The fecundity is significantly lower in oral brooders; however, the eggs tend to be larger and receive more nutrients. By placing the eggs into the oral cavity, the female is able to provide protection for the eggs, and the churning ability of the female rotates the eggs so that each egg is exposed to oxygenated water.
Similar to oral brooding, sex role reversal in fish is rare but is utilized in the Family Sygnathildae. In sea horses, the female inseminates the male by inserting the oviduct into the male brooding pouch several times to ensure fertilization. After fertilization is complete, the female departs, and the male attaches itself to a nearby object with its tail waiting for the eggs to mature.
Nature exhibits many alternatives that species have adapted in order to reproduce. By utilizing these methods, these organisms are able to reproduce viable offspring and continue their kind.
Differences in Reproductive Investment
- Parental investment is any behavior toward offspring that increases the chances of the offspring's survival at the cost of the parent's. The presence of a large asymmetry in parental care paired with an ability to monopoluize mates favors conditions for male-male competition and polygynous mating systems where a small number of males account for a disproportionate amount of the breeding. As size matters as a cheat-proof arbitator, selection for increased body size and asymmetries between the sexes become common.
- Resource Distribution: When resources (e.g., nesting sites, food resources) can be dominated by a small subset of males, a polygynous mating system will likely emerge:
Certainty of Paternity
Consider mammals, birds, and fish and phrase your prediction as to the sex that you would be expected to invest more into the offspring.
- external vs. internal fertilization
- control of fertilization site
Social monogamy is relatively rare in mammals but common in birds. Eggs, which develop internally in mammals, restrict the females ability to shift parental duties to the male. Birds, where egg development is external, is more conducive to such efforts. Differences in constraints and costs between sexes in parental care.
Types of Mating Systems
Polygyny should be more common in patchy environments with variation in territory quality. e.g., honey guides; female fitness depends on quality of resource which is controlled by the male
When females aggregate at favorable sites, males tend to compete vigorously with each other for the right to be the only male present there, mate with this individuals from the cluster. Males receive most of possible copulations in a cluster they successfully protect, and females gain protection for themselves and their offspring. e.g., seals;
Scramble Competition Polygyny
Males attempt to outrace competitors for access to receptive females. Little to no effort is made to defend individual exclusive mating territories or in attempts to monopolize mates. Wood frogs breed explosively in spring ponds where resident males jostle with each other. As a female enters the pond she usually gets grabbed quickly in a mad-dash effort by all nearby males.
Lek polygeny is a mating system common in polygamous species of insects and birds in which the males provides no parental care to its offspring. The lek mating system is uniquely driven by the females’ pursuit of their mate, rather than the males'. Males of lekking species do not hunt for receptive females. Males form aggregates in neutral locations devoid any resources valuable to females. The group of males performs intricate vocal, visual or chemical displays to lure receptive females to their lekking site. In most lekking species, these group displays typically increase the ratio of visiting females per males. At the lekking site, visiting females are able to compare the males’ physiques and courtship displays, picking the most attractive male as their mate. Thus, the few, most attractive males will do the majority of the mating (about 99%), while the subordinate males do not get an opportunity to mate at all.
- Hotspot Hypothesis. According to the hotspot hypothesis, males form leks because females frequently visit certain "hotspots".
- Hotshot Hypothesis. The hotshot hypothesis predicts that males form leks because subordinate males congregate around highly attractive males to increase their chance of being noticed by receptive females.
- Female Preference Hypothesis. The female preference hypothesis predicts that males form leks because females like to visit large clusters of males consisting of a variety of potential mates from which she can quickly and safely compare the quality of her mating choices.
- Kin Selection Hypothesis. The Lek Polygyny mating system promotes a heavily skewed mating success rate among lekking males. Although most of the individuals in a lek never receive a mating opportunity, lek polygamy continues to flourish among various species of birds and insects. This suggests that the fitness of subordinate males must somehow be indirectly benefited by communal displays. A number of hypotheses have been proposed to explain the reasons behind lekking behavior. Widely recognized hypotheses include the Hotspot Hypothesis, the Hotshot Hypothesis, and the Female Preference Hypothesis (Alcock, 2001). An alternative hypothesis predicts that lekking behavior is driven by kin selection. If all the males in a lek where genetically related, then the males (even the subordinate males) would receive fitness benefits. The kin selection hypothesis was tested by determining the genetic structure of different lekking groups of the Whipsnade Park peacock population. Peacocks, a typical lekking species, form aggregates at neutral display sites. Peacocks use their calls to attract receptive peahens. Upon the arrival of a peahen, the peacocks cease calling and perform intricate plumage displays. As common in lekking species, the peacocks with the most appealing courtship displays have a high mating success while the subordinate peacocks have no mates. The lekking sites of peacocks are carefully chosen and after a male’s fourth year of lekking, he will establish his permanent lekking site. Peacocks will return to this same lekking site every mating season. Multilocus fingerprinting compared the genetic similarity of within and between the lekking groups. The results supported the proposed kin selection hypothesis. As predicted, the degree of band-sharing within the leks was substantially higher than the band-sharing between the leks. The band sharing within the leks was indicative of that of half-siblings suggesting that related peacocks tended to display together because their dispersal was concentrated around their natal sites This hypothesis was rejected when peacocks of mixed relatedness were reared away from their natal sites but, upon reintroduction into the Whipsnade Park population, joined leks with closely related peacocks. It was then suggested that peacocks’ tendency to congregate with relatives was due to a shared genetic preference for habitat-selection. This was rejected due to the homogenous environmental features of the park. Petrie et al. concluded that unique structure of the peacock leks was driven by kin selection based on self-referent phenotypic matching (“Armpit Effect”). Peacocks match heritable similarities of their own phenotype with those of other males. Because the authors believed that peacocks do not participate in the rearing of their offspring, they posit that a peafowl’s recognition of their father must be genetically innate. The tendency to form leks with relatives, therefore, occurs in the absence of social cues to their identity. By cooperatively forming leks with genetically related individuals, subordinate peacocks forfeit their chances of mating, but increase the chance that their genes will be passed on via successful relatives. The indirect benefits of fitness seen in peacock lekking displays outweigh the costs of communal displays.
Resource considerations may foster the emergence of monogamous mating systems. When resources are difficult to dominate (e.g., a scattered renewable food source) things get more complicated. Monogamy is a possible outcome which exists either over a lifetime (e.g., greylag geese) or serially (e.g., ducks). Also if females are widely distributed, males may not be able to monopolize them. As females may mate with another male, monogamous males may play a guarding roling towards the female. The male may derive some fitness benefits in situations where his parental contributions increased surviva; of his offspring. Under such conditions, female choice will encourage shifting part of the burden of parental care to the male.
Polyandry is defined as “the mating of one female with more than one male while each male mates with only one female.” Exclusive polyandry (as opposed to polyandry in concert with polygyny) is very rare, occurring in only about 1% of animal populations, most being shorebirds like the sandpiper. The basis of polyandry is a sex role reversal. The females compete for the males and are larger and more colorful, while the males take on the parental role. With the sex role reversal, a natural selection against older males evolves. This is accomplished by the females tending to select the males with the best sperm in order to give the female the most offspring possible. Younger males will more likely have fertile sperm; therefore impregnating the female on more instances than an older male with less fertile sperm. When multiple helpers are needed for successful reproduction, females specialize in egg production while pairing with multiple males in polyandry (e.g., Jacanas).
There are two major types of polyandry: simultaneous and sequential. Simultaneous polyandry is when the female controls a very large territory. Inside this territory, the female has multiple smaller nesting territories with different males. The female mates with all males simultaneously, keeping control of the smaller territories. Another form of simultaneous polyandry, cooperative simultaneous polyandry, is when the female only has one nesting area where she mates with multiple males producing a clutch of eggs of mixed parentage with all males contributing to the eggs. Sequential polyandry, the most common form, is where the female mates and produces a clutch of eggs with one male, then leaves the male to incubate and rear the eggs while moving on to another male in a different nesting territory. Here, the female moves from one male to another, leaving the male in full responsibility of the eggs instead of sharing the responsibility.
The genetic benefits of polyandry include fertility insurance. This hypothesis suggests that by mating with multiple males, the female is guaranteed to fertilize all of her eggs. The multiple partners potentially make up for one male that may not be able to fertilize the eggs. The good genes hypothesis states that the females have multiple mates because she is in search of the male that will pass along the best genes to her offspring. By finding this male, the female is increasing the survival rate of her offspring. The genetic compatibility hypothesis is one that suggests that the female finds multiple mates in order to find the most compatible genetic match for her eggs. While looking for a good match, she is also eliminating the males that are least compatible with her eggs.
The material benefits of polyandry can be seen through three hypotheses. The more resource hypothesis suggests that the more mates the female has, the more males she has to care for her clutch. The better protection hypothesis states that by having multiple partners, the female is better protected from predators. The infanticide reduction hypothesis is one that claims that since the female has multiple males, she has a lower infanticide rate because the males do not know which progeny belong to them. This prevents the males from killing other male’s young. Although some infanticide occurs between females and other female’s eggs, it is minimal among males.
Alternatively, in situations where social groups are able to form, females may mate with any male member of the group leading to Promiscuity (Polygynandry). Sperm competition will in this case most likely determine paternal identity (e.g., elephants, chimpanzees). In situations where food is scattered and limited, both parents need to provision the young.
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- Petrie M, Krupa A & T Burke. 1999. Peacocks lek with relatives even in the absence of social and environmental cues. Nature 401: 155-157