Sexual selection 2 Flashcards
(19 cards)
Why should females care about advertisements?
If females choose males with costly signals, they will be passed on to their sons, which appear disadvantageous – she produces inferior sons!
Fisher’s sexy sons hypothesis
If a mutation arises in a females to remove the preference, she will pick mates without the signal…
Her sons will lack the costly signal, but have increased survival…
In a population with a majority female preference her sons attract no mates – condemned to celibacy!
The mutation for no preference is lost…
Describe Fisher’s Runaway hypothesis
Fisher (1915 & subsequently) one of our key movers in the Modern Synthesis
Female mate choice for a male signal may begin for any reason (according to another model? Genetic Drift?)
Fisher suggested that at early evolutionary stage a female preference could initially begin for a character that actually improves survival
Stage 1 of the runaway process
Imagine that at an early stage of evolution peacocks had shorter tails
Mutation arises for longer tails
This increases survival (e.g. better fliers) – acted on by natural selection
Stage 2
Another mutation for females to pick males with longer tails also arises
These females have sons with longer tails & increased survival
Mutation for preference spreads – more females with preference
Males with long tails survive better & gain a mating advantage
Evolution of long tails in males and female preference reinforce each other
Offspring from females with the preference gene and males with the long tailed gene will have both of these genes
Within the population these genes become non-randomly associated (linkage disequilibrium) – the two are correlated
Every time a new gene for female preference arises it spreads, increasing the strength of the female preference, driving a correlated, increased response in the male signal
Stage 3
Tail length grows past the optimum for survival (the signal has become costly!)
The mating preference alone is driving the elongation of the tail
The preference compensates for the lower male survival (through higher reproductive success)
Describe Zahavi’s handicap theory
Zahavi (1975), females should prefer males displaying honest, costly signals that indicate superior survival and better genetic quality
Selection operates indirectly via the production of higher-fitness offspring
Choosey females secure ‘good genes’ for their offspring
In the simplest form –
if only males with good genes can survive processing a handicap, then a female who preferentially mates with such handicapped males will only mate with males with good genes
Why is being costly important?
If a signal is not costly isn’t then males can simply cheat the signal!
If a signal is cheap/easy to produce then even males of poor genetic quality would rapidly be under selection to produce the signal…it becomes dishonest
i.e. if the criterion favoured by female is a blue spot (or any trait without a cost), selection will favour all males to develop it, regardless of their quality
But if the cost of growing a handicap is less for a truly high quality male than for a low quality male, handicaps will only be grown by high quality males and will be reliable signals for females to use…it is honest
i.e. the criterion favoured by females is grow a long tail (perhaps the longest tail you can!), then selection cannot so easily favour cheats, as the highest quality males can grown the long tails, despite the cost
What are the problems with the concepts associated with ‘good genes’
If it is possible for males to cheat then a handicap becomes useless
Selection is very efficient at fixing good genes – it will act to remove variation in genetic quality (i.e. fitness) – Zahavi’s model would not work if all males have good genes
Models suggest the correlation between a signal and genetic quality also tends to break down over time…
Three factors could maintain sufficient genetic variation in fitness for sexual selection to work;
- Spatial variation - Temporal variation - Mutations (especially deleterious mutations)
Describe Hamilton and Zuc’s mechanism
Temporal variation in fitness.
the perpetual arms race between hosts and pathogens/parasites – element of the Red Queen Hypothesis (Van Valen 1973)…as one allele becomes common it becomes less fit as parasites/disease adapt or overcome it…
What if male signals advertise their resistance to disease?
Male fitness decreases with increased parasite infection
Signal condition decreases with increased parasite burden; maintaining this signal must be costly
There must be heritable variation in resistance (fitness)
Females should choose the strongest signal and the least parasitized males
Describe genetic compatibility
Idea of how well the genes of two parents function together in their offspring…
A newer and perhaps more controversial idea/theory driving mate choice - The female must know their own genotype and their partners to choose an optimum
Perhaps only applies in limited situations or traits (but potentially important ones), like immune genes – the major histocompatibility complex (MHC)
Genes code for cell surface proteins essential for the adaptive immune system, immune response and effective resistance against pathogens. They have extremely high level of allelic diversity in many animals
MHC and inbred lab mice
Inbred strains of lab mice with known MHC variation/fixed alleles show preferences based on MHC…initially supporting male mate choice (not female choice!)…(Yamazaki, K., et al. 1976)
Other inbred lines with known MHC variation - females chose males based on MHC dissimilarity (Egid & Brown 1989)…increasing the diversity of MHC alleles in their offspring
Further experiments with cross-fostered mice – soon after birth individual mice were fostered with families with a different MHC variation - also suggested preferences could be reversed, perhaps learned? (Eklund et al 1997)
Human T shirt experiment
Female and male students were typed for their HLA-A, -B and –DR (MHC genes/antigens).
Each male student wore a T-shirt for two consecutive nights.
The next day, each female student was asked to rate the odours of six T-shirts.
They scored male body odours as more pleasant when they differed from the men in their MHC than when they were more similar.
The theory remains contentious, especially in human populations….they are criticisms over the original paper, especially over sample size
Subsequent investigations do not all come to the same conclusion…in different populations…sexes…
In other animals tests find a variety of outcomes – in some publications females seem to show no preference, a preference for similar MHC variation or intermediate variation!
Important points about genetic compatibility
Mate choice for compatibility differs from other forms of choice for genetic benefits (such as ‘good genes’) because individuals are expected to differ in their mate preferences, changing the evolutionary dynamics of sexual selection
Huge levels of variation at MHC genes are thought to be driven by parasites/disease, oscillations in genotype frequencies are observed between parasites and hosts in an antagonistic co-evolutionary way – as in the Red Queen
Describe sensory bias
This theory emerged in the 1980s (e.g. West-Eberhard 1983, Ryan 1990, Endler also), difficult to attribute, different formulations, similar ideas…
Which male trait exaggerated by sexual selection may depend on pre-existing features of the female sensory system
Why would females have pre-existing sensory bias?
Females use sensory systems for many other reasons
Natural selection acts on traits relating to finding food, avoiding predators etc. and may make females especially responsive to specific cues
This theory focuses on the initial origin of female choice
Perhaps powerful when combined with runaway selection (helps to provide an initial reason for female preference to occur…)
Direct benefits
The least controversial way in which female mate choice can evolve is via direct benefits to the female (that increase her fitness and/or fecundity)
Examples; food, higher quality territories, parental care, protection from predators…
If the competitive sex displays a signal that reliably indicates some direct benefit then strong selection will favour mating bias
Having a mating preference is advantageous in this situation because it directly affects reproductive fitness
Examples of direct benefits
