your speculation regarding the coevolution of mammal sexes is well-founded. several evolutionary factors have likely contributed to the maintenance of a balance between the sexes, preventing one from completely outcompeting the other in terms of reproductive success. here are some of the critical factors that played a role in this dynamic equilibrium:
- sexual recombination: sexual reproduction itself relies on genetic diversity generated by combining genes from two individuals. if one sex were to outclass the other, the benefits of recombination - critical for adapting to environmental changes and avoiding the accumulation of deleterious mutations - would be lost, ultimately reducing the overall fitness of the species.
- parental investment and sexual selection: the differing reproductive strategies of males and females balance each other out. for example, in many mammalian species, females invest heavily in offspring due to gestation and postnatal care, while males often compete for access to mates. this leads to a system where male reproductive success is constrained by the availability of females, and female reproductive success is constrained by the resources and care they can provide. thus, each sex has an evolutionary 'niche' in reproductive success.
- genomic imprinting: some genes are expressed differently depending on whether they are inherited from the mother or the father (genomic imprinting). this phenomenon suggests that genes from both sexes exert influence over reproduction and development, creating interdependence between the sexes in transmitting genetic material. evolution favours mechanisms where both paternal and maternal genomes contribute optimally to offspring success.
- mate limitation and mate choice: in many species, especially those with internal fertilization like mammals, finding and securing a mate is a non-trivial challenge. if one sex were to dominate reproduction, it would reduce the availability of suitable mates, introducing a natural limit. mate choice also reinforces the balance, as sexual selection ensures that certain traits in both males and females are selected for based on fitness criteria, preventing extreme skewing of reproductive success toward one sex.
- mutual benefits in parenting and social structures: in species with biparental care, the cooperation between males and females in raising offspring is critical. both sexes often provide different but complementary contributions to offspring survival. evolution has favoured the retention of both sexes, as their coordinated efforts improve the chances of offspring survival, thus propagating both sets of genes.
- runaway selection: if one sex were to exhibit traits that completely outclass the other in reproductive capability (for example, males monopolizing all reproductive success), it could lead to a collapse in genetic diversity and cause negative evolutionary feedback. this is somewhat mitigated by processes like runaway sexual selection, where the preferences for certain traits by one sex are balanced by the reproductive success of individuals with those traits.
- these factors, combined with the inherent genetic and reproductive advantages of biparental inheritance, ensure that neither sex outclasses the other completely, maintaining an equilibrium in reproductive success.