Lecture 10: Human and animal Flashcards
What is the two-fold cost of sex?
It’s the idea that asexual reproduction should be more beneficial as it’s easier and faster to reproduce and sexual reproduction costs in terms of recombination and mating. However, it increases variability.
Costs: Recombination; the break-up of successful genotypes and the recombination of their elements.
Mating; Time and energy is spent in investing and securing mates. Behaviour and elaborate ornamentation can be dispensed in asexual species.
What are the advantages of sex?
The Red Queen theory; This follows the idea that it takes all of the running you can do to stay in the same place. So antagonistic species (predator vs prey) constantly need to adapt and evolve counter ploys in order to survive. This also applies to humans and pathogens; the recombination process allows humans to provide a defences against the rapidly evolving pathogens. This is important as disease is the biggest threat to most species.
Why do we have two sexes?
This is the most common pattern in nature (but isn’t universal). Anisogamous gamete sizes define our sexes, the evolution of this is why we have two sexes. It started with two same sized gametes that produced a megazygote. Then a smaller gamete fused with a larger one to form a zygote with required less energy. Over time the male’s gametes got smaller and more numerous (as they required less energy to form) to produce microgametes and the female’s larger to produce macrogametes. This is disruptive selection as two characteristics survived, this is how anisogamy is maintained. Other sexual differences from this are just logical consequences.
Why is there roughly and equal ratio between males and females?
Because, the more males there are, the less fitness they have but if there was less males than females then the females’ fitness will be low. So for fitness to be equal, the ratio needs to be equal. This is frequency dependent selection.
Describe sexual selection and its issues
Sexual selection is when a male has exaggerated traits and the female chooses which male to mate with depending on this trait. The issues with this is that it goes against natural selection as these traits aren’t beneficial for survival and they can sometimes decrease the chance of survival.
Compare natural selection and sexual selection
They work in the same way, however:
Sexual selection focuses on the selective consequences of sexual interactions within a species.
Natural selection focuses on environmental factors.
Describe Bateman’s findings from his 1984 study about fruit flies
The variance in male reproductive success was much greater than females. This is because a males’ reproductive success depends on the number of mates; additional matings have reproductive payoffs however a females reproductive success doesn’t depend on number of males. From this he formulated the Bateman’s principle: the sex which invests more in the young becomes a resource for which the other sex (less parental) compete.
What are the two mechanisms of sexual selection?
Male-male competition and female choice.
Describe how sexual selection can lead to sex differences
Because in most species, the males compete for access to the females and the females choose. This means that the most exaggerated males will reproduce, resulting in increased sex differences involving morphology and behaviour. In general, the bigger the difference in parental investment, the bigger the sex differences are.
Discuss size dimorphism in terms of sexual selection
Increased polygyny leads to increased male-male competition. The more competition, the more sexual dimorphism there is as the males will compete to try and be larger. This is found in most mammal and bird species. For example elephant seals, LeBoeuf found that 5 males accounted for half of the matings in the population and they tended to weight three times more than the females.
Describe another type of intrasexual competition (other than size dimorphism)
Mating system covariation; One of the greatest constraints of male promiscuity is post-copulatory competition, aka sperm competition which results in cryptic female choice instead of lekking. Sperm competition results in mate guarding, sperm displacement (dragonflies scraping out others’ sperm), blocking the females’ reproductive tract (drosophila bifurca), cryptic female choice (the female tract is complex to ensure the best sperm is selected, there is a positive correlation between the frequency of remating and the complexity) and testis size dimorphism (in primates, chimps live in multi-male/multi-female groups so the competition is high, this is why they have such large testis).
Discuss female choice in terms of mating
Females tend to prefer males with exaggerated secondary sexual traits, this wasn’t accepted in Darwin’s time though. However,it’s now widely accepted as a force of evolution. Female african widowbirds prefer males with longer tails. Andersson did a study and found that normally the means nests number among groups is about the same, they defend their territories. He manipulated this and artificially changed the length of the different tails, the birds with the largest tails had the most nests (the males still held their territories though).
What are the three theories about why females prefer exaggerated traits?
Runaway selection
Good genes
Sensory exploitation
Discuss runaway selection
A females’ preference can vary slightly, this means that sons inherit the trait and daughters inherit the preference. This is genetic covariance and also positive feedback aka sexy sons. This argument is circular. This may have started because the trait was originally advantageous.
Discuss good genes
Females can gain directly from mating. For example, some arachnids make spermatophores which provide nutrition. However, most times females don’t benefit but are still choosy; the paradox of the lek, are they choosing genes? Sometimes females don’t gain anything other than sperm, but the secondary sex characteristic can show quality/good genes. For example fish with red bellies have good genes as the colour is a carotenoid-dependent trait showing they are good at gathering food.
