Why anisogamy drives ancestral sex roles

  title={Why anisogamy drives ancestral sex roles},
  author={Jussi Lehtonen and Geoffrey Alan Parker and Lukas Sch{\"a}rer},
There is a clear tendency in nature for males to compete more strongly for fertilizations than females, yet the ultimate reasons for this are still unclear. Many researchers—dating back to Darwin and Bateman—have argued that the difference is ultimately driven by the fact that males (by definition) produce smaller and more numerous gametes than females. However, this view has recently been challenged, and a formal validation of the link between anisogamy and sex roles has been lacking. Here, we… 
Coevolution of parental investment and sexually selected traits drives sex-role divergence
Sex-role evolution theory attempts to explain the origin and direction of male–female differences. A fundamental question is why anisogamy, the difference in gamete size that defines the sexes, has
Fisher vs. the Worms: Extraordinary Sex Ratios in Nematodes and the Mechanisms that Produce Them
Recent research in nematodes that has characterized the mechanisms underlying highly skewed sex ratios in fully diploid systems are discussed, including self-fertile hermaphroditism and the adaptive elimination of sperm competition factors, facultative parthenogenesis, non-Mendelian meiotic oddities involving the sex chromosomes, and environmental sex determination.
Sex roles and the evolution of parental care specialization
The model predicts that efficient care specialization broadens the conditions under which biparental investment can evolve in lineages that historically had uniparental care, and contributes towards explaining widespread phylogenetic inertia in parenting and mating systems.
What do isogamous organisms teach us about sex and the two sexes?
This article asks what happens if there is a less than twofold benefit to sex, and argues that this could lead to a situation where lineages that evolve anisogamy—and thus the highest costs of sex—end up being associated with constraints that make invasion by asexual reproduction unlikely (the ‘an isogamy gateway' hypothesis).
Male mate choice, female competition, and female ornaments as components of sexual selection
Conceptually, male mate choice is not only connected to female mate choice, but also relevant to the understanding of female– female competition, and female ornamentation, as well as two studies in the Special Column discuss such scenarios.
Multicellularity Drives the Evolution of Sexual Traits
It is shown that species with greater multicellular complexity have a significantly larger number of derived sexual traits, including anisogamy, internal fertilization, and secondary sexual dimorphism, which may set the stage for the overall diversity of sexual complexity throughout the Tree of Life.
The 150th anniversary of The Descent of Man: Darwin and the impact of sex-role reversal on sexual selection research
The history and impact of a single profound insight from Charles Darwin’s The Descent of Man is reviewed: that, in some few species, females rather than males compete for access to mates.
Anisogamy selects for male‐biased care in self‐consistent games with synchronous matings
It is argued that the mating‐caring trade‐off for males is neither a necessary consequence of anisogamy nor sufficient to select for female‐biased care, and the factors excluded from the models—costly competitive traits, sexual selection, and partial parentage—may be necessary for the parental investment hypothesis to work.
Bigger testes increase paternity in a simultaneous hermaphrodite, independently of the sperm competition level
It was shown that worms with high testis investment sired 22% more offspring relative to those with low investment, corroborating previous findings in M. lignano and discussing the possible implications for the evolutionary maintenance of hermaphroditism in this species.


Gamete competition, gamete limitation, and the evolution of the two sexes.
Two classes of models explaining the evolutionary origin of males and females: gamete competition and gamete limitation are reviewed, finding they are not mutually exclusive, but two aspects of a single evolutionary process.
Parental investment, sexual selection and sex ratios
An integrative model shows how factors interact to generate sex roles and underscores the need to distinguish between the ASR and the operational sex ratio (OSR) if mortality is higher when caring than competing this diminishes the likelihood of sex role divergence.
Anisogamy, chance and the evolution of sex roles.
The evolution of sex roles in mate searching
This work correct and expand upon earlier models, and presents two novel hypotheses that might explain the evolution of male‐biased mate searching, and shed new light on classic arguments about sex role evolution.
The evolution of sexes.
Mating type evolution has been analyzed theoretically in population genetic models and explorations show that mating types may evolve as a consequence of selection for more efficient gamete recognition, and also as a result of intragenomic conflict between nuclear and cytoplasmic DNA.
Local Gamete Competition Explains Sex Allocation and Fertilization Strategies in the Sea
The first evolutionary model of egg retention and release is developed, which also considers transitions between hermaphroditism and dioecy as well as egg size evolution, and shows that all these patterns can arise as an evolutionary response to local competition between eggs for fertilization.
Sexual Selection: The Logical Imperative
The principles of modern sexual selection theory are outlined and their heuristic value discussed, and it is proposed that a sequence of evolutionary events flows inevitably from the early evolution of sexual recombination and gametes, to anisogamy and in dioecious organisms, to the unity sex ratio via Fisher’s principle.
Intra-sexual selection in Drosophila
Epigamic selection includes the major part of what Darwin meant by sexual selection, and is introduced to apply to characters which increased the fertility of a given mating and therefore had a selective value for the species as a whole.
Sex allocation and investment into pre- and post-copulatory traits in simultaneous hermaphrodites: the role of polyandry and local sperm competition
  • L. Schärer, I. Pen
  • Biology
    Philosophical Transactions of the Royal Society B: Biological Sciences
  • 2013
It is concluded that understanding the evolution of sex allocation in simultaneous hermaphrodites requires detailed knowledge of the different sexual selection processes and their relative importance, and quantifying sexual selection and identifying the underlying traits along the pre- to post-copulatory axis.
The theory of sex allocation.
  • E. Charnov
  • Biology
    Monographs in population biology
  • 1982
This book is the first comprehensive treatment of sex allocation from the standpoint of modern evolutionary theory. It shows how the determination of sex ratio, resource allocation to sperm versus