products of evolution

• Sexual selection


  • Mode of natural selection with 2 forms:
  • 1. Intersexual selection: Members of one biological sex choose mates of the other sex to mate with
  • 2. Intrasexual selection: Members of the same sex compete for access to members of the opposite sex
• How does sexual selection benefit different sexes?

  • Successful males benefit from frequent mating and monopolizing access to one or more fertile females
  • Females can maximise the return on the energy they invest in reproduction by selecting and mating with the best males
• The Red Queen hypothesis

  • Hypothesis in evolutionary biology proposed in 1973
  • Posits that species must constantly adapt, evolve, and proliferate in order to survive while pitted against ever-evolving opposing species
  • Helps explain explains the advantage of sexual reproduction (cf. asexual reproduction) at individual level
• The Fisherian runaway


  • Sexual selection can lead males to extreme efforts to demonstrate their fitness to females, producing sexual dimorphism in secondary sexual characteristics
  • This is caused by a positive feedback mechanism known as a Fisherian runaway, where the passing-on of the desire for a trait in one sex is as important as having the trait in the other sex
• Sexy son hypothesis


  • States that a female's ideal mate choice among potential mates is one whose genes will produce males with the best chance of reproductive success
  • Means that other male qualities are less important (e.g., capacity as a parental caregiver, territory and any nuptial gifts)
  • Although the sexy son hypothesis indicates that females would prefer male offspring, work by Fisher explains why the sex ratio is most often 1:1. Why?
• Desire of a trait is as important as the trait itself

  • If females choose attractive males, they will tend to get attractive sons, and, thus more grandchildren, because other choosy females will prefer their attractive, sexy sons
  • Theory functions regardless of the trait a female chooses, as long as it is heritable (trait varies between individuals of the population)
  • Possessing the trait that makes males attractive is more important than the qualities of the trait
• Evolution of the trait and preference

  • Once a preference becomes established, females choosing males with elaborate secondary sexual traits will produce sons that carry alleles for the trait
  • Those males will produce daughters that carry alleles for the preference
  • This generate genetic coupling that will drive self-reinforcing coevolution of both trait and preference
  • Positive feedback due to the mating advantage of males with the trait creates a Fisherian runaway sexy sons process
• Female choice and runaway selection:

  • Long-tailed widowbird
  • Males have long tails selected for by female choice
  • Female preference may be passed on genetically, in conjunction with genes for the long tail itself
  • Offspring of both sexes inherit both sets of genes, with females expressing their preference for long tails, and males having the long tail
• Post-Fisherian thinking


  • Since Fisher's initial 'runaway' concept, other researchers have mathematically defined the circumstances under which runaway sexual selection can take place
  • Andersson's definition: "sexual selection is the differences in reproduction that arise from variation among individuals in traits that affect success in competition over mates and fertilizations"
• Modern sexual selection theory

  • Reproductive success
  • Honest signalling
  • Intrasexual selection
• Reproductive success


  • Sexual selection exists because females produce few large gametes and males many small gametes
  • This imbalance creates a conflict between the reproductive strategies of the sexes
  • Females suffer disproportionate fitness costs if they mate with the wrong male - see Bateman's principle
  • If males can mate frequently with little cost to their fitness, then females become a limiting resource
  • For females, males are not a limiting resource, so they can be more "picky"
  • Because males can mate multiple times, the variation in mating success is much greater than in females
  • This variation is a measure of the intensity of sexual selection
• Bateman's principle


  • Bateman's principle states that the sex which invests the most in producing offspring becomes a limiting resource for which the other sex competes
  • Illustrated by the greater nutritional investment of an egg in a zygote, and the limited capacity of females to reproduce
  • E.g., in humans, a woman can only give birth every ten months, whereas a male can become a father numerous times in the same period
  • However, recent research has cast doubt on Batemans principle
• Honest signalling


  • The handicap principle: a male's survival to a reproduction age with seemingly maladaptive traits (e.g., ornamentation) is taken by the female as a signal of his overall fitness
  • Indicator of his disease resistance, greater speed, and physical strength
  • It suggests that costly signals must be reliable signals
  • Controversial and not universally embraced, but influential
• Male intrasexual competition


  • Male–male competition occurs when two males of the same species compete for the opportunity to mate with a female
  • Strong driver of male body size, as larger males tend to win competitive interactions
  • Male–male competition may also affect a female's ability to select the best mates, and therefore decrease the likelihood of successful reproduction
• Sexual selection and body size

  • Sexual size dimorphism can vary in direction and magnitude across populations
  • Traits such as territorial defence (a predictor of sexual selection for large male size) and clutch size (a predictor of fecundity selection for large female size) are associated with evolutionary shifts in sexual size dimorphism
  • Tradeoffs between male and female body size
• The Guppy: a model organism for evolution and ecology


  • Endler's guppy (Poecilia reticulata)
  • Small freshwater fish native to NE South America
  • Occupy shallow, clear-water rainforest streams
  • Males possess extensively polymorphic colour patterns
  • Males bear complex colour patterns due to female mating preferences and visual predators
• Male guppy colouration


  • Trade-off between:
  • 1. sexual selection, favouring mate attractiveness, and
  • 2. natural selection, favouring crypsis (camouflage)
• Endler's 1st experiment


  • Guppies transplanted from high predation habitat downstream to low predation habitat upstream above a waterfall
  • A field experiment to see if relaxing natural selection led to stronger sexual selection
• Endler's 2nd experiment


  • Two predators:
  • Weak predator - Rivulus (killifish)
  • Strong predator - Crenicichla (cichlid)
  • Common garden experiment (greenhouse)
  • Fish in benign habitats (little to no predation) acquired stronger male colouration
  • Fish in harsh habitats (strong predation) showed weaker colouration
• Guppies responded to natural selection
• Prof Matt Gage, University of East Anglia: '"Sexual selection was Darwin's second great idea, explaining the evolution of a fascinating array of sights, sounds and smells that help in the struggle to reproduce - sometimes at the expense of survival"'
• Lecture summary
• Sexual selection


  Mode of natural selection with 2 forms:
• Forms of sexual selection

  • Intersexual selection: Members of one biological sex choose mates of the other sex to mate with
  • Intrasexual selection: Members of the same sex compete for access to members of the opposite sex
• LAB4: Fancy versus Wild Guppies

  What conditions might promote the success of each phenotype?
• Mechanisms of competition for mates and traits likely favoured

  • Same-sex contests: Traits improving success in confrontation (large body size, strength, weapons, threats); avoidance of contests with superior rivals
  • Mate preference by opposite sex: Attractive/stimulatory features; offerings of food, territory, and other resources that improve mate's reproductive success
  • Scrambles: Early search and rapid location of mates; well-developed sensory and locomotory organs
  • Endurance rivalry: Ability to remain reproductively active during much of season
  • Sperm competition: Ability to displace rival sperm; production of abundant sperm; mate guarding or other ways of preventing rivals from copulating with mate
• Same-sex contests


  • Rutting red deer, elephant seal battles for dominance, and brown anole lizards defending territory
• Mate preference by opposite sex

  • Bird of Paradise, Twelve-Wired Bird of Paradise using "wires" to brush the face of the female during courtship
• Scramble competition


  • Eastern fork-marked lemur, males and females ranging widely in overlapping home ranges to maximise mating opportunities in a short period
• Endurance rivalry


  • Rhesus macaque males losing up to 10% of their body weight by focusing on mating and mate guarding rather than activities such as feeding
• Sperm competition


  • Black-winged damselfly, male's penis adapted as a scrub brush
• Mating systems


  The "special circumstances" in which reproduction occurs within species
• Types of mating systems

  • Monogamy
  • Polygyny
  • Polyandry
  • Polygynandry
  • Other (promiscuity, sperm competition)
• Monogamy


  Social monogamy: behavioural pairing of a single male with a single female
• Polygyny


  Association of one male with multiple females
• Polygyny


  • Resource defense polygyny: females are attracted to a resource — males then compete to possess resource, and, by extension, mating priority with females
• Harems


  A common type of polygyny, defended group of females associated with one male
• Lekking


  An aggregation of males each seeking to attract a mate, not associated with resources
• Lek paradox


  How is genetic variation maintained in lek mating species despite consistent female intersexual selection?
• Polyandry


  A group with one female and many males