Lecture 6 - sexual selection Flashcards
describe darwins theory of natural selection
- the fittest individuals will survive and reproduce
- succeeding generations will become adapted to their environment
what was darwins problem with the theory of natural selection?
The problem with the theory of natural selection is that it can’t explain costly traits that lower an individual’s survival
e.g. peacocks tail - costly to clean, carry and attracts attention of predators
what is darwins theory of sexual selection
“a struggle between the individuals of one sex, usually the males, for possession of the other sex”
what is male- male competition?
intrasexual selection-
Competition between members of the same sex - can be in obvious forms such as males fighting or less obvious e.g. impressive characters
what is female choice?
intersexual selection-
Choice, by one sex, of mates of the other sex
why did it take a while for female choice to be accepted?
many old male scientists doubted that the course of evolution could be determined by females
what are secondary sexual characters?
characters not obviously used for reproduction e.g. size, fighting ability, courtship behaviour
what are primary sexual characters?
- directly linked to reproduction e.g. genitalia size
what caused the evolution of sexual dimorphism?
- competition of mates - only the ones competing have over the top, garish traits
how can you understand the basis of sexual selection?
to understand you must look at the fundamental differences between the sexes i.e. anisogamy = non identical gametes - females produce few costly eggs and males have small mobile sperm - males can generally fertilise eggs at a faster rate than females can produce them
describe bateman (1948) experiment with drosophila on potential reproductive rates
the number of offspring produced by the male drosophila increased linearly with the number of copulations
- Males have a higher potential reproductive rate than females
- So males compete and females choose (leads to sexual dimorphism) - females arent going to waste their costly eggs
- female= quality male = quantity
what is the most basic form of male-male competition?
direct physical contests among the members of one sex, over access to mates
describe male-male competition in elephant seals
- Male elephant seals can weigh 2000kg and they fight vigorously
- The winner (beachmaster) has a harem of dozens of females
- Other males get few or no matings
- in this case size makes a male dominant/successful as increased dominant rank results in increased copulations
- DNA analysis shows mating success is directly linked to paternity
what does mating in male elephant seals select for?
extreme selection to grow large - however some males dont both growing large and sneak copulations in the sea
when does large sizes in elephant seals only occur?
only occur when they can monopolise a large area of females otherwise its not worth it - e.g. a beach - cant monopolise an area in the sea
what did the experiment on male-male competition in Bufo Bufo toads demonstrate?
there was greater sexual dimorphism in species with alot of competition - the pattern wasnt the same in monogamous species
what is the link between body size and harem size in pinnipeds?
- as body size dimorphism increases harem size increases i.e. there is greater dimorphism between the sexes in polygynous species as there is less direct male-male competition in monogamous species
describe the link in antler length and mating systems in deer
- polygynous male deer species have larger antlers than expected for their body size and monogamous species have smaller antler than expected by body size
what are two important points about sexual selection and dimorphism?
- Sexual Selection doesn’t have to lead to Sexual Dimorphism
- Sexual Dimorphism can arise from processes other than Sexual Selection e.g. if they occupy different niches ad therefore have different feeding patterns - could lead to different beak shape in a bird for example
what is often hard to distinguish between when studying sexual selection?
- Often difficult to distinguish between male-male competition and female choice
- Need to carry out experimental manipulations to show female choice with any degree of certainty
- acceptance of partner isnt choice only rejection can be deemed as choice
describe one of the first experiments to successfully demonstrate choice
- Female long-tailed widowbirds come to a lekking and choose a dancing male
- was notices females preferred males with long tails
- to determine is this was because the long tailed males are better at competing or if its female choice they manipulated the males tail length and observed that females still had a preference for the elongated tails because the males themselves hadnt changed
describe how investment in offspring effects competion/choice
Low investmentin offspring = competition
high investment in offspring = choice
equal investment in offspring = both sexes compete and both sexes choose
describe sexual selection in crested auklets
- Both sexes invest in the offspring
- Both sexes have crests
- both sexes compete and both choose
- the crest ornament only grows in the breeding season
- experiment displayed both sexes perform more courtship displays to long crested models = mutual mate choice
- in both sexes long crested individuals are dominant = mutual competition
what is sex role reversal
In some species males invest more in reproduction than females
when is it predicted that sex role reversal will occur
- There will be sex role reversal in species in which males invest a lot in offspring.
- Females will compete and males will be choosy
describe sex role reversal in pipefish
- Males get “pregnant” and provide oxygen and nutrients to a clutch of eggs in an egg pouch
- During male pregnancy, females can produce eggs to fill 2 male pouches. Given an even sex ratio, male pouch space is limited.
- Males choose large females that produce more eggs than small ones
what is choosiness dependent on in katydids?
- males provide nuptial gift for female
- choosiness of males is dependent on availability of resources for making the gift
- when food is scarce males are more choosy and the females compete more
describe sex role reversal in mormon crickets
- mormon crickets and large and flightless
- Males transfer a huge (25% body mass) edible spermatophore to females when they mate so they can only mate once.
- Females can produce several clutches
- therefore the males choose larger females that produce more eggs to be fertilised
- mating aggregations occur where females compete for males - the operational sex ratio is female biased as the females can mate more than the males
3 facts about sexual selection
1) Any pattern of choice and competition can occur
2) Each sex can play both roles
3) The environment can influence the pattern
of sexual selection
