BI2CV1 Lecture 5 - Sexual Selection PDF

BI2CV1: Comparative Vertebrate Biol Tuesday 29th October 17:00-17:55 Natural and Sexual Selection Dr Joanna Baker Evolutionary Biology Group [email protected] What is selection? Selection is the non-random differential survival or reproduction...

BI2CV1: Comparative Vertebrate Biol Tuesday 29th October 17:00-17:55 Natural and Sexual Selection Dr Joanna Baker Evolutionary Biology Group [email protected] What is selection? Selection is the non-random differential survival or reproduction of organisms. As defined by Darwin: “the principle by which each slight variation [of a trait], if useful, is preserved.”  Lecture objectives: By the end of this lecture, you should be able to:  Define sexual selection and differentiate it from natural selection  Explain key mechanisms of sexual selection  Provide examples of sexual selection across vertebrates Lecture objectives (1/1) Lecture outline 1. Types of selection 2. Mechanisms and examples of sexual selection. 3. Evolutionary consequences 4. Interactive discussion 5. Conclusions When you see this symbol, I will highlight further reading, questions to consider, and exercises to complete in your own time! These are not compulsory but will help you consolidate your understanding. I will upload the pdfs of any papers to Blackboard. Lecture outline (1/1) Lecture outline 1. Types of selection 2. Mechanisms and examples of sexual selection. 3. Evolutionary consequences 4. Interactive discussion 5. Conclusions Types of selection (1/10)  1. Types of selection Types of selection 1. Directional selection 2. Disruptive selection 3. Stabilizing selection 4. Sexual selection 5. Artificial selection Types of selection (2/10) Types of selection 1. Directional selection 2. Disruptive selection 3. Stabilizing selection 4. Sexual selection 5. Artificial selection Types of selection (3/10) Types of selection 1. Directional selection 2. Disruptive selection 3. Stabilizing selection 4. Sexual selection 5. Artificial selection Types of selection (4/10) Types of selection 1. Directional selection 2. Disruptive selection 3. Stabilizing selection 4. Sexual selection 5. Artificial selection Types of selection (5/10) Types of selectionSexual selection is the process through which both males and females attempt 1. Directional to maximize their chance of selection reproductive success. Acts differently on 2. Disruptive selection each sex. 3. Stabilizing selection 4. Sexual selection 5. Artificial selection Types of selection (6/10)  Types of selectionSelection by humans on a deliberately chosen trait or set of traits – the 1. Directional criterion for ‘fitness’ in this case selection becomes the most desirable trait by the 2. Disruptive selection selector. 3. Stabilizing selection 4. Sexual selection 5. Artificial selection “Domestic races of the same species, also, often have a somewhat monstrous character…” Types of selection (7/10) Natural selection? 1. Directional selection 2. Disruptive selection 3. Stabilizing selection Which of these are considered 4. Sexual selection to be natural selection? 5. Artificial selection Types of selection (8/10) Natural selection? 1. Directional selection 2. Disruptive selection 3. Stabilizing selection Which of these are considered 4. Sexual selection to be natural selection? 5. Artificial selection Types of selection (9/10) Is sexual selection a type of natural selection? Survivorship Access to mates Fecundity Mate choice Heredity Overall reproductive sucess Mating Fitness success What do you think? Is sexual selection a type of natural selection? Types of selection (10/10) Lecture outline 1. Types of selection 2. Mechanisms and examples of sexual selection. 3. Evolutionary consequences 4. Interactive discussion 5. Conclusions Sexual selection (1/20) A brief introduction to sexual selection Sexual selection exists because of sexual conflict: opposing reproductive interests between males and females. Are some mating systems more likely to face stronger sexual conflict? Why? Sexual selection (2/20) A brief introduction to sexual selection Mating success is a major component of fitness in sexually reproducing species. Competition for mates Mate choice Mating success Sexual selection (3/20) Mechanisms of sexual selection Darwin proposed two main forms of evolution via sexual selection:  Intrasexual selection: Access to mates within a species may be variable.  Intersexual selection: Mate choice – one sex (usually female) prefers to mate with individuals possessing certain characteristics or qualities. Mechanisms of sexual selection (4/20)  Mechanisms of sexual selection Many bases for sexual selection have since been recognized: Mechanism Characters favoured Same-sex contests Traits improving success in confrontation (or avoidance) Scramble competition Sensory and locomotory traits which aid finding and accessing mates Endurance rivalry Reproductive seasonality –outlasting the competition Sperm competition Sperm abundance, sperm displacement, mate guarding behaviour. Coercion Coercive behaviours and features improving forced copulation success Infanticide Traits improving success in confrontation (or avoidance) Mate preferences Attractive features (not linked to individual fitness) Further reading: Evolution (5th) edition by Douglas F. Futuyma, chapter 12. Mechanisms of sexual selection (5/20) Same-sex contests Where male-male competition exists, males tend to evolve weapons … or muscle. Mechanisms of sexual selection (6/20) Same-sex contests: example Large male elephant seals monopolize harems of females. This leads to intense competition where size and strength are selected for. Mechanisms of sexual selection (7/20) Same-sex contests: example Male elk compete with rival males for available females. However, female elk are more likely to choose males with larger and more elaborate antlers. Larger antlers are therefore driven by two mechanisms of sexual selection: intrasexual competition and intersexual selection. Mechanisms of sexual selection (8/20)  Where female mate choice exists, males tend to evolve ornaments and weapons. Mate choice Mechanisms of sexual selection (9/20)  Mate choice Where female mate choice exists, males tend to evolve ornaments and weapons. Further reading: Hosken & House (2011), Current Biology. Mechanisms of sexual selection (9/20)  Mate choice Bird song is an example of ornamentation: males with more elaborate song repertoires are more attractive to females Further reading: Catchpole (1987), Trends in Ecology Mechanisms of sexual selection (10/20) & Evolution.  Mate choice: examples from the Tail vanes in pterosaurs and frills in ceratopsian dinosaurs are putative sexual signals. past Mechanisms of sexual selection (11/20)  Mate choice: examples from the past Positive allometry hypothesis: sexually selected traits will scale steeply with body size Further reading: O’Brien et al, Animal Behaviour. Mechanisms of sexual selection (12/20)  Male mate choice: less well understood Conceptually different: unlikely that female traits will be detrimental (unlike costly ornaments such as antlers/tails etc.). Overall, males seem to prefer, larger and more fecund females. When females choose large males: indirect benefits to overall success. When males choose large females: direct benefits to males via improved reproductive success. Further reading: Schlupp (2018), Current Mechanisms of sexual selection (13/20) Zoology.  Male mate choice: less well understood Oestrus success. advertisements in baboons are an honest signal of female reproductive Males increase effort to mate with females with larger swellings. Females Male-male breed with competition multiple is intense males Females Increases advertise competition their fertility among males Further reading: Domb & Pagel 2001, Nature. Mechanisms of sexual selection (14/20)  Mutual mate choice Models of sexual selection often assume dichotomous sex roles. However, reality can be more complex. Female pot-bellied seahorses prefer males with different chemical scents. Male pot-bellied seahorses prefer larger females. Further reading: Bahr et al (2012), Behavioural Ecology. Mechanisms of sexual selection (15/20)  Mate choice Further reading: Bahr et al (2012), Behavioural Ecology. Mechanisms of sexual selection (15/20)  Coercion and sexual aggression  Coercive mating is a commonly employed strategy to avoid sexual competition.  It is observed in a wide range of animal species.  It can lead to arms races (but more on this later).  Benefits of sexual coercion are high in males – but may result in substantial costs to females.  Sexual disturbance and copulatory injuries are most frequently associated with female retention (e.g. mate guarding) Further reading: Snow & Prum (2023) Mechanisms of sexual selection (16/20)  Sperm Competition  A process where sperm compete to fertilize the same egg during sexual reproduction.  Higher levels observed in multi- male breeding systems.  Huge number of traits influenced by sperm competition: Further reading: Parker & Pizzari (2010) Biological Reviews and Wigby & Chapman Mechanisms of sexual selection (17/20) (2004) Current Biology  Sperm Competition Testes size is strongly linked with level of sperm competition Figure by Mark Maslin, Cradle of Humanity Further reading: Parker & Pizzari (2010) Biological Reviews and Wigby & Chapman Mechanisms of sexual selection (18/20) (2004) Current Biology  Sperm Competition  Sperm is highly variable across the animal kingdom.  Sperm morphology and ejaculate characteristics are highly influenced by sperm competition Further reading: Carballo et al (2019) J Evolutionary Biology Mechanisms of sexual selection (19/20)  Sperm Competition  Sperm is highly variable across the animal kingdom.  Sperm morphology and ejaculate characteristics are highly influenced by sperm competition  For example, parrots which breed in high densities have much longer sperm! Further reading: Carballo et al (2019) J Evolutionary Biology Mechanisms of sexual selection (20/20) Lecture outline 1. Types of selection 2. Mechanisms and examples of sexual selection. 3. Evolutionary consequences 4. Interactive discussion 5. Conclusions Evolutionary Consequences (1/7) Evolutionary consequences A combination of sexual selection and the mechanisms we have discussed here can be invoked to explain several aspects of evolution:  Sexual dimorphism  Alternative mating strategies  Sexual conflict  Human evolution Evolutionary Consequences (2/7) Sexual Dimorphism Sexual dimorphism is an apparent difference in appearance between the males and females of a species arising (largely) through sexual selection. Evolutionary Consequences (3/7) Sexual Dimorphism A combination of same-sex competition and mating preferences can give rise to drastic differences in the sexes. This can lead to “Fisherian runaway” selection: a process in which both preference and the sexual signal are linked. This causes a positive feedback: both trait and preference increase, resulting in pronounced sexual dimorphism. Ultimately, practical constraints will halt the process. Evolutionary Consequences (4/7) Alternative Mating Strategies Alternative mating strategies are behavioural, morphological, or developmental differences in a sub-set of species that give rise to mating behaviours that are consistently different to those from other individuals of the same sex. A common alternative mating strategy is “sneak” mating. In swordtails, the adult ‘sneaker’ males are indistinguishable from juveniles. Evolutionary Consequences (5/7)  Sexual Conflict and Antagonistic Co-evolution Earlier, I mentioned that sexual selection arises through sexual conflict. Males and females commonly have conflicting reproductive strategies. This can lead to evolutionary “arms races” between the sexes. For example, coercive breeding in waterfowl has led to antagonistic co-evolution of genital morphology. Further reading: Brennan et al (2009), Proceedings of the Evolutionary Consequences (6/7) Human Evolution  The study of our own species has received a lot of attention in this regard!  We exhibit mate choice preferences and have strong cultural influences on sexual selection (beauty standards).  Darwin himself conjectured that the male beard was the result of sexual selection.  Most evidence suggests that the effects of sexual selection within humans are small (but not zero). Increasing attractiveness? Evolutionary Consequences (7/7) Lecture outline 1. Types of selection 2. An introduction to sexual selection. 3. Mechanisms of sexual selection. 4. Evolutionary consequences 5. Interactive discussion 6. Conclusions Group Discussions (1/2) Discussion Activity (2 minutes per question) In small groups, or with the person next to you, discuss the following questions: 1) Can you think of an example from nature that I have not talked about today of a species that exhibits sexual dimorphism? How might that trait have evolved? 2) What are the trade-offs involved in males evolving elaborate displays or larger bodies? 3) How much of human behaviour is shaped by sexual selection? Can you identify a possible example of a trait or behaviour that might be an example in humans? Group Discussions (2/2) Conclusions  Sexual selection largely operates through mate choice and competition.  It often results in improved reproductive success, but may affect fitness and survival.  A number of key evolutionary features arise through sexual selection:  Sexual dimorphism  Sexual conflict  Arms races between the sexes Conclusions (1/2)  Conclusions Sexual selection is a powerful evolutionary force, shaping many traits and behaviours. Conclusions (2/2)