Sexual selection 2 Flashcards
Narrow-sense sexual selection
Narrow-sense sexual selection:
*Selection favouring sex differences, especially fancy traits
Challenges to narrow-sense model
*Multiple mating in females – paternity confusion –Sarah Hrdy female langurs mate with all available males to protect them from infanticide due to the fact that males do not want to kill an offspring that may be their own
*Choice for complementary partners
*Male mate choice
*Female-female competition
Multiple mating in females: extra pair copulation
– 1968: David Lack: ‘Well over nine-tenths [93%] of all passerine subfamilies are normally monogamous.… Polyandry is unknown’
–The 1980s: molecular methods of determining parentage showed that this was far from true
– 2002: The number is now down to, at most, 14% (Griffiths et al. 2002)
(see socially monogamous species graph in notes)
Choice for complementary mates
*Self-referential mate choice: the “best mate” in terms of producing viable offspring, is different for different choosers
*At a genetic level, the alleles (e.g., at immune coding loci) that a choosing individual has informs which alleles in a potential mate would be “best”
Male mate choice
Male mate choice
*Two-spotted gobies (Gobiusculus flavescens)
–Populations that are initially male-biased (i.e., more males than females) become female-biased as the breeding season progresses (as males start to die off)
*Operational sex ratio: “the relative numbers of sexually active males to receptive females at any one time” - Clutton-Brock 2007 Science
*sex ratio equal at 0.5, beginning of season more males, end of season more females
*Resulting in shift in behaviour – high male male competition declines over time
*Female – female aggression increases as male quantitiy decreases end of season
*Courtship is more likely to be performed by females when males are scarce
Cryptic female choice: occuring after copulation has begun
*“[F]emale behavior, physiology, or morphology that selectively biases paternity in favor of some conspecific males and against others after copulation has already begun” Eberhard 1995
^Albo et al. 2013, Proc. Roy. Soc. B
Example: Nursery web spider (Pisaura mirabilis) females preferentially use sperm from males that present nuptial gifts. Males that bring gifts are allowed to copulate longer than males which do not.
Even if interrupted those males that provided gifts had more hatching success
Social selection
Social selection
Mary Jane West-Eberhard:
Selection that favours individuals bearing some trait relevant to social interactions (either competitive or cooperative) for access to resources such as food, territories, allies, and mates.
Parental Choice: American Coots (Lyon et al. 1991)
Bright colouration seen in young likely to have evolved by the process of parental choice – preferentially provisioning (feeding) young with bright colouring.
Feeding rate biased to investment in offspring with bright ornamental trait
In the experiment removing the bright feathers resulted in less feeding and lower survival rate
Territorial defence
e.g. Troupials (Icterus icterus)
Odom et al. 2016, Frontiers in Ecology & Evolution
In species with males and females that defend territories - male and females tend to be indistinguishable this Troupial could be male or female
Biological species concept
“Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups” -Mayr 1942
*Behavioral interactions can impact reproductive isolation.
Reproductive behaviour and speciation: 1. Reinforcement
*Process whereby selection against the formation of hybrids upon secondary contact leads to the evolution of reproductive isolating mechanisms.
*Often framed as the final phase of speciation—without reinforcement, species may collapse into a ‘hybrid swarm’
Allopatry - dividing event speciates
Parapatry - recombining event so that ranges overlap but unable to interbreed
Sympatry - overlapping of species results
Reinforcement in Drosophila
*Coyne and Orr (1989, 1997) compiled measurements of pre- and post-zygotic reproductive isolation between many different species of Drosophila, as a function of genetic distance
*They found that pre-zygotic isolation (i.e., behavioural isolation) evolves faster than post-zygotic isolation
*Sympatric taxa evolved pre-zygotic isolation at a much faster rate than allopatric taxa, supporting the hypothesis that reinforcement has occurred
*Prezygotic isolation faster than post
Evidence for reinforcement outside of this drosophila study not as strong/conclusive
Reproductive behaviour and speciation:
2. Mutation order speciation
Definition: speciation that arises from trait divergence resulting “from from the stochastic order in which traits arise and become fixed”. (Mendelson et al. 2014, Ecology Letters)
Randomness in mutation leads to divergence and speciation
*Phenotypic diversity results from trait divergence in allopatry, rather than sympatry.
*In the context of sexual selection, signal traits might evolve arbitrarily in separated populations, resulting in reproductive isolation upon secondary contact.
Example:Winger & Bates 2015: Compared species varying in divergence time across the Río Marañon (a large barrier to migration)
^ Mutation order speciation in birds in habitats of high altitude separated by a large river valley.
Plumage, morphometric and calls studied superspecies black and monospecies white
In voice and morphometric traits overlaps between super and mono
However in plumage time results in bigger differences in plumage between superspecie.
See speciation diagram in notes
Mutation-order speciation by sexual selection
Hypothesis:
*sexual selection should accelerate the process of speciation
*via divergence in preferences and stochastic changes in signal traits
*independent of differences between diverging populations in other factors (e.g., ecology)
*What evidence supports this?
Example: Sexual dichromatism as an index of the strength of sexual selection
Sexual dichromatism as a proxy for sexual selection
Dunnocks - similar looking sexual selection low
Teal ducks - diff looking sexual selection high
^Sexual selection accelerates the rate of male plumage colouration evolution
*Sexual selection accelerates the rate of secondary contact
*Together, these results support a model wherein sexual selection accelerates speciation process
Sexual selection leads to higher evolution rate in males but not female
3.Cascading effects of character displacement & reinforcement
Character displacement drives divergence
*Species interactions in sympatric populations can drive divergence that results in reproductive isolation between conspecific populations in a zone of sympatry and a zone of allopatry.
Mate recognition
“The process of discriminating among
different classes of individual in relation to levels of mate suitability”- Grether et al. 2017
Competitor recognition
“The process of discriminating among different classes of individual in relation to levels of competition for resources or mates”- Grether et al. 2017
e.g. between Rainbow darter and orange throat darter fighting and mating between species is rare
Resulting in reproductive isolation between sympatric and allopatric conspecifics across diff populations
THIS SUGGESTS THAT CHARACTER DISPLACEMENT HAS DRIVEN SHIFTS IN RECOGNITION IN ALLOPATRY
*Orangethroat darters from populations that are sympatric with rainbow darters have undergone character displacement in male mate recognition and male competitor recognition.
*Male orangethroat darters from these sympatric populations exhibit heightened preferences for mating and fighting with members of their own populations.
*Male orangethroat darters from allopatric populations readily mate and fight with other orangethroat darters.
*Character displacement between lineages has driven divergence within lineages, perhaps kick-starting speciation
Summary
1.Sexual selection can occur via several affiliative and agonistic mechanisms, including:
-a.within sex “clubs” such as female affiliative networks or leks
-b.intrasexual competition, such as fights or sperm competition
-c.mate choice, which may confer direct, indirect, or no benefits
-d.sexual conflict, where a trait that increases fitness in one sex decreases fitness in another sex
2.Increasingly, empirical studies unearth examples of sexual selection that call for a re-examination of a ‘narrow-sense’ approach.
5.Behavioural interactions can have impact the speciation process.
6.Reinforcement occurs when hybrid inviability drives selection for pre-zygotic isolation.
7.Mutation-order speciation theory posits that differences between populations arise stochastically.
8.Divergence driven by character displacement or reinforcement can have cascading effects across populations.
