Sexual dimorphism- sexual selection Flashcards
(33 cards)
when organisms don’t have sexes, what do they have instead and why are they needed?
there are normally mating types which differ at one or a few loci. two mating types can’t breed with each other- this is likely to be because of encouraging outcrossing
why is sexual reproduction good?
it increase heterozygosity- good for a constantly adapting population
what is isogamy?
the gametes that form a zygote are equal in size- they are usually both motile, normally one sex or mating types.
how many mating types do slime moulds have?
500
what is anisogamy?
two sizes of gametes, one larger and one smaller, two sexes, found in multicellular organisms and unicellular. can both be motile
what is oogamy?
one very large and one very small. the small is produced in a large number- used in almost all multicellular organisms that have sex
what is the definitive factor for the progression from isogamy to anisogamy?
the fact that in multicellular organism, the fitness of the zygote is proportional to size. this is not the case in unicellular organisms
how did fertilisation first occur?
in the sea, external fertilisation
what are the 4 theories for the size difference in anisogamy?
- the collision theory
- parasite theory
- sperm limitation
- disruptive selection
what is the collision theory os anisogamy?
? ensuring collision .. how?
what is the parasite theory of anisogamy?
that the less cytoplasm the sperm transfers to the egg, the less likely it is to transfer a parasite
what is the disruptive selecion theory of anisogamy?
there are opposite forces of selection acting on the gametes for males and females
what are the 5 premises in supports of anisogamy development?
- each parent has an equal amount of resources to produce a gamete
- gametes can be any size depending on how the resources are used to produce it (size of gamete= resources/ number produced)
- gamete fusion is random
- size of zygote is M1 +M2
- the fitness of a zygote correlates with the size of the zygote
when is isogamy the evolutionary stable strategy?
when the fitness of the zygote is not determined by size?
when is anisogamy the ESS?
when fitness is dependent on zygote size due to the opposing selection pressures
given that fitness is correlated to zygote size, why does anisogamy arise?
if size is correlated with fitness then the fitness will depend on the size of the egg. this means that eggs will be selected for based on the size. due to the fact the egg is singular, there is a lot of competition between sperm. to stay competitive as many sperm as possible must be produced and must be fast. this means that the male divides its resources up to make many tiny sperm to increase its the competitiveness
what concepts supports the repeated evolution of anisogamy?
the fact that fitness is related to size of the zygote
why, once internal fertilisation occurs, and sperm competition decreases, does anisogamy not revert back to isogamy?
first you calculate the anisogamy ratio: ovum size/ sperm size. if this is greater than N/N-1 (N is the number of ejaculates competing for eggs) . if the anisogamy ratio i greater than the n/n-1 then anisogamy prevails. N must be very very very low for it not to be.
what is the relationship between p and the anisogamy ratio?
p is the probability of sperm competition must be higher than 4/A. if P>4/A then anisogamy i stable
give two examples of extreme sperm evolution?
diving beetle forms a long cluster of sperms and the droso species has a very long sperm which fills up the females reproductive tract
why do sexual dimorphisms between males and females arise?
for males, mating with multiple males highly increases their fitness, however, for females mating with multiple males does not increase the likelihood of them producing more young. this means that traits that increase the males chances of mating with more females will evolve.
how does sexual selection result in sexual dimorphism?
due to the fact mating with multiple males does does increase a females fitnesses and that a female can only foster one litter of offspring per mate- females need to ensure that they chose the best quality males. this means that the female will select for traits that improve the males fitness or traits that don’t improve the males fitness (selecting for traits that don’t effect the males fitness can be due to the sexy son (peacock feathers) or handicap hypothesis (stalk eyed fly)
what is the sexy son hypothesis?
The sexy son hypothesis states that females may initially choose a trait because it improves the survival of their young, but once this preference has become widespread, females must continue to choose the trait, even if it becomes harmful. Those that do not will have sons that are unattractive to most females (since the preference is widespread) and so receive few matings
how do male traits that don’t contribute to male fitness come to be selected for such as the peacock feather?
The sexy son hypothesis or the handicap principle: states that females may initially choose a trait because it improves the survival of their young, but once this preference has become widespread, females must continue to choose the trait, even if it becomes harmful. Those that do not will have sons that are unattractive to most females (since the preference is widespread) and so receive few matings
