\[ **up: [[Biology]] | [[Evolution]] | [[Sex and sexuality]]** ] --- # Evolution of sex and sexuality > [!attention] Terminology > Terms like *homosexuality* and *heterosexuality*, etc., are based on human self-identification and personal identity. Best practice, then, is to avoid applying these terms in other situations; e.g. when referring to animal sexual behaviour. Monk et al propose the more neutral terms: *same-sex sexual behaviour (SSB)* and *different-sex sexual behaviour (DSB)*.[^1] > [!quote] > ‘Our goal is that the ideas we present here lead to a productive, careful discussion of the diversity, functions and evolution of animal sexual behaviour and that this discussion is inclusive of people of diverse sexes, genders and sexualities.’[^2] --- ## Impact of cultural bias upon evolutionary research > [[Traditional considerations re. ‘sex and fitness in evolutionary biology have largely focused on sexual behaviours between individuals inferred to have fertilization-compatible gametes.’]][^3] --- ## How to explain the evolution of same-sex sexual behaviour? - *See also:* [[Same-sex sexual behaviour in animals]] That same-sex sexual behaviour is naturally occurring oughtn’t to really be up for debate.[[Same-sex sexual behaviour has been ‘recorded in over 1,500 animal species with a widespread distribution across most major clades.’]][^4] However, if [[Natural selection|natural selection]] is all about [[Fitness (Biology)|fitness]]—and fitness is all about propagating one’s genes/[[Reproductive success|reproductive success]]—then how does the evolution of same-sex sexual behaviour make any sense…? ### Common hypotheses #### Non-adaptive/maladaptive hypotheses Some non-adaptive or maladaptive hypotheses include: - mistaken identity - [[Prison effect|prison effect]] - infection.[^5] They all work on the assumption that same-sex sexual behaviour is a [[When explaining same-sex sexual behaviour via ‘non-adaptive or maladaptive hypotheses’, scientists are working on the assumption that SSB is a ‘fundamentally erroneous tactic’.|fundamentally erroneous tactic]].[^6] #### Adaptive hypotheses Some adaptive hypotheses include: - kin selection - overdominance - intrasexual conflict - sexual antagonism.[^7] [[Adaptive hypotheses re. evolution of same-sex sexual behaviour assume it must be ‘due to indirect fitness benefits.’]][^8] #### Implicit assumptions Both adaptive and maladaptive hypotheses contain the implicit assumption that the evolution of same-sex sexual behaviour is a ‘[[Assumption that same-sex sexual behaviour has arisen from ancestral populations where different-sex sexual behaviour was the default.|trait that arose in each study system from an ancestral population with exclusive DSB]]’.[^9] - Also seen in evolutionary models such as [[Female targeting|female targeting]] (§[[‘These models assume that because DSB is essential for sexual reproduction, selection will strongly favour high levels of DSB.’|13.19]].) *However*, > [[‘Factors such as mate competition, mating order, mate age, gamete quality, sperm concentration and post-copulatory choice by females play important roles in determining whether or not DSB results in the production of offspring.’]][^10] Furthermore, > [[Sexual behaviour is ‘not limited to unique occasions of fertilization and may also have many non-reproductive functions indicating that “excess” sexual behaviours need not significantly reduce fitness.’]][^11] (In fact, organisms determined to focus *only* on different-sex sexual behaviour have been shown to *miss out* on mating opportunities.[^12]) There is also the assumption that same-sex sexual behaviour is by default costly—‘[[Assumption that the costs of same-sex sexual behaviour are high (from a fitness viewpoint).|and thus adaptive hypotheses assume that benefits of SSB must be even greater to account for its persistence, whereas non-adaptive hypotheses assume that SSB will be strongly selected against when possible.]]’[^13] ##### Human cultural constructions - *See also:* [[Sex and sexuality--Evolution#Impact of cultural bias upon evolutionary research]] [[Monk et al. argue that the implicit assumption of different-sex sexual behaviour as the ancestral default is a poorly-examined artefact of cultural bias.]][^14] > [[‘Human cultural constructions of sexual behaviours have impacted hypotheses surrounding SSB evolution, particularly in social vertebrates and other primates.’]][^15] Even the *terminology* used by biologists (etc.) shows this; e.g., the so-called ‘[[Prison effect|prison effect]]’. There has also been a case of Choose Not To See re. the evolutionary pattern of same-sex sexual behaviour’s prevalence within clades: > [[When a trait is very prevalent, ‘a reasonable hypothesis to explain such an evolutionary pattern is that the trait likely arose near the clade’s origins.’|‘Indeed, when we observe a particular trait so prevalent within a clade, a reasonable hypothesis to explain such an evolutionary pattern is that the trait likely arose near the clade’s origins.’]][^16] [[Rather than question ‘why engage in SSB’, the real question should be ‘why not’.|Monk et al. propose ‘a shift from asking “Why engage in SSB?” to “Why not?”’]][^17] --- ## Monk et al.’s ‘indiscriminate ancestral condition’ proposal [[Monk et al. ‘propose that indiscriminate sexual behaviour, or sexual behaviour without sex-based mate identification resulting in the expression of both SSB and DSB, is the most likely ancestral condition of sexually reproducing animals.’]][^18] - Exclusive different-sex sexual behaviour requires a level of [[Mate recognition|mate recognition]] that ‘[[Since exclusive different-sex sexual behaviour ‘requires mechanisms of mate recognition … it logically can only occur subsequent to the evolution of perceivable sexual polymorphism.’|can only occur subsequent to the evolution of perceivable sexual polymorphism]]’.[^19] + If *indiscriminate* sexual behaviour is the ancestral condition, and selection would only act against same-sex sexual behaviour if there were ‘[[Selection would only act against same-sex sexual behaviour if it's costs became prohibitive.|prohibitive]]’[^20] costs, then there’s no reason for SSB *not* to be retained across the board.[^21] + This makes *both* same-sex and different-sex sexual behaviour legacy behaviours.[^22] + And SSB no longer needs to provide ‘advantages’; it simply needs to not *disadvantage*.[^23] [[If SSB does not have high costs, and may actually increase mating opportunities, then it ‘may be the norm for most animal species’ and the ‘legacy of an the legacy of an ancestral condition of indiscriminate sexual behaviour.’]][^24] [^1]: Julia D. Monk, et al., ‘[[Monk et al, ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, 2019|An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals]]’, *Nature Ecology & Evolution*, vol. 3, no. 2 (December 2019), p. 1623. https://doi.org/10.1038/s41559-019-1019-7. (§[[It is best practice to draw ‘a distinction between human attributes of gender and sexuality (both of which are categories based on self-identification …) and the scientific terminology of sex and sexual behaviours.’|13.10]].) [^2]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1623. (§[[‘Our goal is that the ideas we present here lead to a productive, careful discussion of the diversity, functions and evolution of animal sexual behaviour and that this discussion is inclusive of people of diverse sexes, genders and sexualities.’|13.16]].) [^3]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1622. [^4]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1622. [^5]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1622. [^6]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1623. [^7]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1622. [^8]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1622. [^9]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1622. [^10]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1626. [^11]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1626. [^12]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1626. [^13]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1622. [^14]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1622. [^15]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1623. [^16]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1622. [^17]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1623. [^18]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1623. [^19]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1623. [^20]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1623. [^21]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1623. [^22]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1623. [^23]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1623. [^24]: Monk et al., ‘An Alternative Hypothesis for the Evolution of Same-Sex Sexual Behaviour in Animals’, p. 1622.