Sexual conflict
Sexual conflict or sexual antagonism occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly over the mode and frequency of mating, potentially leading to an evolutionary arms race between males and females. In one example, males may benefit from multiple matings, while multiple matings may harm or endanger females due to the anatomical differences of that species. Sexual conflict underlies the evolutionary distinction between male and female.
The development of an evolutionary arms race can also be seen in the chase-away sexual selection model, which places inter-sexual conflicts in the context of secondary sexual characteristic evolution, sensory exploitation, and female resistance. According to chase-away selection, continuous sexual conflict creates an environment in which mating frequency and male secondary sexual trait development are somewhat in step with the female's degree of resistance. It has primarily been studied in animals, though it can in principle apply to any sexually reproducing organism, such as plants and fungi. There is some evidence for sexual conflict in plants.
Sexual conflict takes two major forms:
- Interlocus sexual conflict is the interaction of a set of antagonistic alleles at one or more loci in males and females. An example is conflict over mating rates. Males frequently have a higher optimal mating rate than females because in most animal species, they invest fewer resources in offspring than their female counterparts. Therefore, males have numerous adaptations to induce females to mate with them. Another well-documented example of inter-locus sexual conflict are the seminal fluid proteins of Drosophila melanogaster, which up-regulate females' egg-laying rate and reduce her desire to re-mate with another male, but also shorten the female's lifespan, reducing her fitness.
- Intralocus sexual conflict – This kind of conflict represents a tug of war between natural selection on both sexes and sexual selection on one sex. An example would be the bill color in zebra finches. Ornamentation could be costly to produce, but it is important in mate choice. However, it also makes an individual more vulnerable to predators. As a result, the alleles for such phenotypic traits exist under antagonistic selection. This conflict is resolved via elaborate sexual dimorphism thus maintaining sexually antagonistic alleles in the population. Evidence indicates that intralocus conflict may be an important constraint in the evolution of many traits.
Sensory exploitation by males is one mechanism that involves males attempting to overcome female reluctance. It can result in chase-away selection, which then leads to a co-evolutionary arms race. There are also other mechanisms involved in sexual conflict such as traumatic insemination, forced copulation, penis fencing, love darts and others.
Female resistance traditionally includes reducing negative effects to mechanisms implemented by males, but outside the norm may include sexual cannibalism, increased fitness in females on offspring and increased aggression to males.
Some regard sexual conflict as a subset of sexual selection, while others suggest it is a separate evolutionary phenomenon.
Conflicts of interest between sexes
The differences between male and female general evolutionary interests can be better understood through the analysis of the various factors that affect sexual conflict. In situations involving a male and female, only the relative positions of the optimal trait values are important as it is their comparative positions that provide insight into the resulting conflict. The trait value bar at the bottom of the accompanying figure indicates the relative intensity of each trait. The left side represents the poorly developed end of intensity range, while the right side represents the strongly developed end of the range.Males and females differ in the following general components of fitness, thus leading to sexual conflict. Refer to the accompanying figure in this section.
Mating rate
Males generally increase their fitness by mating with multiple mates, while females are on the middle section of the range because they do not favor a particular side of the spectrum. For instance, females tend to be the choosier sex, but the presence of female sexual promiscuity in Soay sheep show that females might not have an established mating preference. However, Soay sheep are a breed of domestic sheep, ergo might not be a subject to traditional evolutionary mechanisms due to human interference.Female stimulation threshold
Generally, females benefit from being more selective than males would like them to be. For example, the Neotropical spider, Paratrechalea ornata, displays nuptial gift-giving behaviors during courtship as a part of their male mating efforts. These nuptials gifts allow the male to control copulation duration and to increase the speed of female oviposition.Degree of female fidelity
Because female fidelity depends on the species' particular mating system, therefore they are in the middle section of the spectrum. However, males seeking mates have different preferences depending on whether they are unpaired or paired. Paired males benefit from high female fidelity, while unpaired males benefit from low female fidelity in order to increase their mating frequencies.Toxicity of seminal fluid
Females benefit from low seminal fluid toxicity, while males benefit from a high toxicity level as it increases their competitive edge.Female fecundity
Males benefit from a high female fecundity as it means that females can produce more offspring and have a higher potential for reproduction. It is important to note that females also benefit from high fecundity, and thus this trait is probably more affected by classical natural selection.Maternal investment
In many species, males benefit from high maternal investment as it allows them to preserve more energy and time for additional matings rather than investing their resources on one offspring. Females are expected to invest a certain amount of time and resources, but it can also be detrimental to the female if too much maternal investment is expected.Sex-biased gene expression
Natural and/or sexual selection on traits that influence the fitness of either male or female give rise to fundamental phenotypic and behavioral differences between them referred to as sexual dimorphism. Selective pressures on such traits give rise to differences in expression of these genes either at transcriptional or translational level. In certain cases these differences are as dramatic as genes not being expressed at all in either of the sexes. These differences in gene expression are the result of either natural selection on reproductive potential and survival traits of either sex or sexual selection on traits relevant to intra-sexual competition and inter-sexual mate choice.Sex-biased genes could either be male- or female-biased and sequence analysis of these protein coding genes have revealed their faster rate of evolution which has been attributed to their positive selection vs. reduced selective constraint. Apart from sex specific natural selection and sexual selection that includes both intersexual and intrasexual selection, a third phenomenon also explains the differences in gene expressions between two sexes – sexual antagonism. Sexual antagonism represents an evolutionary conflict at a single or multiple locus that contribute differentially to the male and female fitness. The conflict occurs as the spread of an allele at one locus in either male or female that lowers the fitness of the other sex. This gives rise to different selection pressure on males and females. Since the allele is beneficial for one sex and detrimental to the other, counter adaptations in the form of suppressor alleles at different genetic loci can develop that reduce the effects of deleterious allele, giving rise to differences in gene expression. Selection on such traits in males would select for suppressor alleles in females thus increasing the chances of retaining the deleterious allele in the population in interlocus sexual conflict.
The retention of such antagonistic alleles in a population could also be explained in terms of increase in the net fitness of the maternal line, for example, the locus for male sexual orientation in humans was identified on subtelomeric regions of X chromosomes after studies conducted on 114 families of homosexual men. Same sex orientation was found to be higher in maternal uncles and male cousins of the gay subjects. An evolutionary model explained this finding in terms of increased fertility of the females in maternal lines, hence adding to net fitness gain.