Table 1.

The big picture of how gamete size evolution influences sexual selection.

	Ancestral isogamy	Very low anisogamy ratios (low female/male gamete masses)	Typical very high anisogamy ratios (high female/male gamete masses)	Organisms with parental care	Overall broad-scale predictions
Predicted effect of sexual selection	No consistent difference in intensity of sexual selection between mating types.	Divergence of gamete sizes and numbers provides initial impetus for mainstream flow of sexual differentiation. Theory predicts a clear correlation between anisogamy ratio and strength of precopulatory sexual selection only when anisogamy ratio is very low.	No predicted overall correlation between anisogamy ratio and intensity of precopulatory sexual selection, because the direct effect of gamete number asymmetry saturates quickly and also gamete sizes typically optimized independently of sexual selection under size/number trade-off.	Higher intensity of sexual selection likely on the sex with less extensive parental care.	1) The isogamy-anisogamy transition causes sexual selection such that on average, sexual selection is stronger on the microgamete producer. 2) A correlation between anisogamy ratio and intensity of precopulatory sexual selection is not expected under typical anisogamy ratios. 3) A correlation between the extent of parental care and the intensity of sexual selection is expected, such that precopulatory sexual selection is generally stronger in the sex with less extensive parental care.
Biological occurrence	Found in many unicellular organisms and some multicellular algae (e.g., Lehtonen et al., 2016a).	Such low anisogamy ratios very rare in Metazoa (e.g., a few Drosophila species: Bjork & Pitnick, 2006); found in some algae (e.g., Monostroma angicava: Togashi et al., 2015).	Found in almost all Metazoa, vascular plants and most multicellular algae (e.g., Lehtonen et al., 2016a)	Found in Metazoa, several taxa show maternal care, some show paternal care.	General conformance with empirical findings: mobile anisogamous metazoans typically show more intense precopulatory sexual selection in the microgamete producer, i.e., males [Janicke et al., (2016)], but beyond this binary relationship there is no sustained relation between anisogamy ratio and intensity of precopulatory sexual selection across typical anisogamy ratios [Mokos et al., (2021)].
					Sexual selection has been found to be more intense on the sex providing less parental care (Mokos et al., 2021).