Table 1.

Common costs of sex and their sources of variation

Cost	Assumption	Violation	Example of violation	Refs.	Effect
Cost of males	Males contribute only genetic information	Males provide extra benefits	Paternal care, males provide direct benefits to females	18	↓
		Males interfere with female fitness	Sexual conflict	19, 20	↑
	Genetic information provided by males is of average quality	Males carry sexually antagonostic genes	Intersexual ontogenetic conflict	22	↑
		Males provide genes with above average fitness	“Good genes” sexual selection	21	↓
	1:1 sex ratio	Female-biased sex ratio	Local mate competition	23	↓
	“All else is equal”	Asexuals have lower lifetime reproduction	Automixis in previously outbreeding population	11	↓
		Asexuals have higher lifetime reproduction	Heterosis in asexuals of hybrid origin	31	↑
Cost of genome dilution	Gene pool of asexuals still “connected” to sexuals	Asexuals have 'female-only' populations	Most species with separate sexes	32, 33	↓
	Viable males or male function	Fertility of asexual males is compromised	Distorted meiosis produces inviable pollen in asexual plants	36	↓
	Allocation to male function 50%	Allocation to male function <50%	Hermaphrodites with reduction in male function	23	↓
Costs of recombination	Population highly adapted to environment	Adaptation prevented by rapidly changing environment	Environmental changes, migration, coevolutionary arms races		↓
	Adaptation is mainly due to nonadditive gene interactions	Adaptation is mainly due to additive gene interactions	Sexual ancestor had sex every generation (vs. occasional sex)	40	↓
	Asexuals lack recombination	Asexuality involves some sort of recombination	Automictic pathenogenesis in a previously outbreeding sexual population	11	↓
Costs of mating	Sexuals invest 100% into mating activities, asexuals 0%	Asexuals still engage in mating activities or have costly physical structures for mating	Showy flowers in asexual plants	47	↓
			Dependence on fertilization (gynogenesis, hybridogenesis)	48	↓
			Sexual males avoid fertilizing asexuals (which depend on it)	48	↑
Cellular mechanical cost of meiosis	Minimum duration of meiotic processes limits generation time	Long generation times (relative to meiosis)	Most multicellular organisms have long development times, or egg-to-egg times, relative to the time needed for meiosis	50	↓