CH. 6 (Why Sex?) Flashcards
Why Sex?
Sexual dimorphisms
traits that differ between males and females of a species (often sexually dimorphic traits in males violate evolutionary theory as they can waste time, energy and reduce survival)
What is the currency of evolution?
Successful reproduction (not survival)
Sexual Selection
selection that increases an individual’s chance of getting a mate (it’s a subset of natural selection that produces “nonadaptive” adaptations).
Types of Sexual Selection
Direct competition between males and female choosiness
- direct competition produces armour, strong bodies, sharp spikes, larger body size etc (to help in a fight)
- female choosiness produces bright colours, ornaments, mating dances, etc
Polygynous
Male that mates with many females during one breeding season
- elephant seal males fight each other to mate w females. Alpha mates with up to 100 females which encourages evolution of fiercer males
Floaters
unmated males who try to invade established territories to sneak copulations with females
- red wing blackbirds flash red epaulets on their shoulders to ward off floaters
Post-mating Competition
After a male inseminates a female he must prevent other males from fertilizing her again and stealing paternity
- can involve physically guarding her, chemically blocking up reproductive tract, injecting with an anti aphrodisiac.
Marion Petrie and Tim Halliday
experimented on sexual advantage of peacock tail feathers in males by cutting off and gluing on spots.
Disadvantages of sex (6)
- individual loses 50% of its genetic contribution to the next generation (eggs and sperm only contain half of its genes)
- requires time and energy to find a mate
- increased energetic costs
- risk of predation and infection
- cost of producing males
- break up of adaptive gene combos
What is meant by the two-fold cost of sex?
asexually reproducing females can produce double the number of child bearing (female) offspring compared to sexual females.
Parthenogenetically
Producing eggs that develop without fertilization (seen in aphids, lizards, and fish)
Benefits of Sex
- random shuffling of genes produces new combos that can be really good or bad (bad are selected against purging them, good selected for (either way the gene pool is changing)).
- promotes faster evolution to deal with aspects of environment that are constantly changing (genetic lottery good in temporally/spatially heterogenous environments)
Differential Investment
Discrepancy in investment of time/resources/energy by the male and female. Investment in expensive eggs vs. cheap sperm. Also, in many species the females raise the young
Offspring Potential
Potential to have offspring, females have limited, males have essentially unlimited because they produce lots of sperm.
Why are females picky?
They invest much more time in offspring. In 90% of mammals, a male’s only investment in offspring is his sperm.
Socially Monogamous Species
Species where male and females pair up and rear young together HOWEVER adulteration is super common! The male display sexually dimorphic traits because 3/4 of the species mate w individuals other than their social partner. 90% of bird species.
Why would a male and female of the same species look similar?
Potentially because extreme dimorphism in size, colour, and behaviour are found in species where the males compete to mate with females (and only a few males get most of the mating). When males and females look alike the species tends to be truly monogamous.
- parrots, geese, penguins, pigeons
Polyandrous
one female mates with many males (seahorses, pipe fish)
- usually in these scenarios the differential investment is flipped.
What three types of genes will increase in frequency together?
- genes for male indicator trait reflecting that he has good genes
- genes that make a female prefer that indicator trait
- the “good” genes whose presence is reflected by the indicator
Females choose mates and can benefit…
Directly - a male that will help her produce more/healthier youth during ACT of CHILDCARE
Indirectly - a male that has better genes than other males (will give offspring a leg up)
How are female preferences and extravagant male traits related?
They co-evolve (gene frequencies increase together). The co-evolution of female preferences and ridiculous male traits only stops when the trait becomes so exaggerated that any further increase would reduce his survival more than it would increase his likelihood of getting a mate.
Sensory Bias Model
model that claims that evolution of sexual dimorphism is driven simply by pre-existing biases in a female’s nervous system. Females do not get direct or indirect benefits.
