Table 1. Glossary.

Anisogamy	The within-species occurrence of gametes of two different sizes, which results in two sexes, males and females. Females produce the larger and males the smaller gametes.
Bateman Gradient	The slope of the linear regression of the number of offspring produced by an individual (reproductive success, or ‘fertility’) on the number of its reproductive partners (mating success). This represents the multiplicative component of the gradient of precopulatory sexual selection acting on a trait. It is named after the seminal study of Bateman (1948), which used fruit flies, D. melanogaster, to suggest that the relationship between fertility and mating success is stronger in males, and argued that in an anisogamous population males can have higher potential reproductive rates than females, resulting in more intense intrasexual competition over mating opportunities in males.
Benefits of mate choice	‘Direct’ benefits of mate choice are ‘non-genetic’ and include resources that will benefit the choosing parent or its offspring, for example access to food, a safe territory, or parental care. ‘Indirect’ benefits are ‘genetic’ in the sense that by choosing a mate, a parent will secure ‘good’ (viability-related) genes or ‘sexy’ genes (genes for traits that are attractive to the opposite sex) for its offspring, or genes that are compatible to the parent’s own genotype.
Generalization	Responsiveness (preference or aversion) to novel stimuli, generated by discrimination learning, and along the dimension(s) of the training stimuli. The resulting generalization gradients (e.g., a preference function) can be either a Peak shift (peak response to stimuli stronger than the positive training stimulus), or an Area shift (peak not shifted, but function asymmetric and biased towards the reinforced direction). Finally, if the gradient does not show a decrease within the interval considered, the preference or aversion can be called Open-ended (see e.g., Ghirlanda & Enquist, 2003; Ten Cate & Rowe, 2007 and Fig. 3).
Genic capture	Female preferences for costly male traits results in the evolution of a genetic covariance between male condition, dictated by many genes, and a target male trait expression
Lek paradox	The problem, commonly relating to female choice of males on leks, of how genetic variation for mate choice can persist despite directional selection for the problem, commonly relating to female choice of males on leks, of how genetic variation for mate choice can persist despite directional selection on traits relevant to choice. Under directional selection, the favoured genes should fixate, so that all individuals of the selected sex should have the gene(s) making them attractive, thus removing the basis for the choice.
Linkage disequilibrium (LD)	LD is the non-random association of alleles at different loci. The term often causes confusion and LD may exist without physical linkage or allele frequencies in equilibrium. The speciation-with-gene-flow process is characterized by the build up of LD and genome-wide LD is the footprint of speciation. LD in specific genomic regions reflects the history of selection, gene conversion and other forces that cause gene-frequency evolution.
Mating preference	A bias during mate choice which results in a skew towards mating with individuals that express specific phenotypic traits.
Mating system	Monandry –females mating with one male. Monogamy –both sexes mating with one mate. Monogyny –males mating with one female. Polyandry –females mating with multiple males. Polygamy (or polygynandry) –both sexes mating with multiple mates. Polygyny –males mating with multiple females.
Pleiotropy	One gene affects two or more traits (genetic pleiotropy), or one hormone affects two or more traits (hormonal pleiotropy).
Receiver bias	Used here and by some other authors (Ten Cate & Rowe, 2007) to include all biased responses (preferences or aversions), whether generated by peripheral sensory systems (sensory bias), neural processing (perceptual bias) or learning or imprinting (cognitive bias). Ryan & Cummings (2013) suggest that Sensory and Cognitive bias should be included in Perceptual bias. See Fig. 3
Receiver psychology	A phrase coined by Guilford & Dawkins (1991) “to encompass the cognitive mechanisms in signal receivers that process incoming information and could potentially influence signal evolution” (Rowe, 2013).
Recombination	The production of offspring with different combination of alleles at different loci than their parents. Recombination often refers to the exchange of genetic material between homologous chromosomes during meiosis (chromosomal crossover).
Red Queen	A theory proposing that organisms must constantly evolve in response to their ever-changing environment. The “Red Queen” analogy is derived from Lewis Carroll’s fantasy novel “Through the Looking-Glass” (1871) where the Red Queen tells Alice that “it takes all the running you can do, to keep in the same place”. The Red Queen theory has been applied to many forms of coevolution among species, for example the antagonistic interactions between parasites and their hosts, and the benefit of sex. In sexual selection theory, Hamilton & Zuk (1982) proposed that sexual ornaments signal the bearer’s resistance to parasites, which is a “Red Queen” model assuming a female preference for good genes. The “Red Queen” logic can also be applied to explain female preferences for rare or dissimilar alleles at immune genes that give a broader allelic repertoire and better pathogen resistance in the offspring, as argued here (the “Promiscuous Red Queen” hypothesis, see Fig. 2.)
Segregation	Pairs of alleles segregate (separate) into different gametes during meiosis. This is referred to as Mendel’s law of segregation.
Sensory drive	A model proposed by Endler (1992) which encompasses evolutionary interactions between the (abiotic and biotic) environment, sensory system and courtship signals, taking into account pre-existing bias and sensory exploitation. Sensory and signalling systems coevolve under the constraints of the environment which hence influence the evolutionary trajectory in a predictable direction (Cummings & Endler, 2018; Endler, 1992).
Sexual cascade	The set of sequential evolutionary transitions in sexual strategy of eukaryote organisms, each transition under appropriate conditions giving rise to the selective forces that generate the next. Some taxa remain ‘frozen’ at a given stage without further change. The cascade begins with isogamous syngamy in unicells. Development towards multicellularity favours anisogamy and generates a unity sex ratio. In early, sedentary marine organisms with broadcast spawning, sexual selection is restricted to sperm competition and sperm selection. Development of mobility permits diversion of expenditure on sperm into ‘female-targeting’ (moving to and release of sperm adjacent to spawning females), which may ultimately facilitate internal fertilization and the many forms of pre-copulatory sexual selection documented by Darwin (1871).
Sexual conflict	A situation in which the fitness of a male and a female cannot be both maximized separately and simultaneously, by the same trait or reproductive decision. This can arise as social conflict between prospective sexual partners, when a reproductive decision (e.g., whether to mate with each other or not) is adaptive for one individual but detrimental to the other. This conflict is often mediated by sex-limited traits and can give rise to sexually antagonistic patterns of intersexual coevolution in which the antagonistic effect of alleles at some loci is counteracted by the effect of alleles at other loci (inter-locus). Another form of ‘conflict’ can arise when there is a divergence in the male and female phenotypic optima, and gene expression is not sex limited. In this case a locus can segregate for different alleles which may have sexually antagonistic effects when expressed in males and females, i.e., an allele that is beneficial when expressed in females may be detrimental when expressed in males and vice versa (intra-locus).
Sexual selection	Selection that depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to mating and fertilization (Andersson, 1994; Darwin, 1871).