Lecture 23: Adaptation part 3: evolution of SEX Flashcards
Modes of propagation (2 with 2 each)
1) sexual
-Produces new genotypes
-Mating types (equivalent to sexes for things like fungi) or Male and female sexes
2)Asexual
-produces clones
-Processes like vegetative propagation, apomixis, and parthenogenesis
Sex poses a paradox (5)
-Sex is complicated, costly and dangerous as searching for a mate takes time, energy and increases exposure to STDs
-once a mate is found it takes even more energy to “woohoo”
-All of this and there is no guarantee that a mate will even prove to be fertile
-Plus if they are fertile, then you only pass on half of your genes
-despite all this, sex is still the highly predominant mode of reproduction
Maynard smith (3)
-Says that asexual females have a 2-fold transmission advantage over sexual females (offspring have 100% of their genes rather than 50%)
-Called the “cost of males”
-assumes that sexual and asexual females produce the same amount of offspring and that reproductive mode does not affect offspring survival
Long term advantage to sex
-sex provides genetic variation, giving the species a long-term advantage
short term advantages to sex (2 points and 3 advantages)
-Not mutually exclusive
-All assume that recombination and selection decrease the frequency of non-optimal genotypes and increase the frequency of superior ones
1) temporal fluctuations in selection pressures
2) heterogeneous environments (spatial variation in selection pressure)
3) Eliminating mutations
temporal fluctuations in selection pressure (3)
-Allows for fluctuation among optimal phenotypes (recall beak depth in Galapagos finches based on available food)
- Better at avoiding parasites and disease
-“Red Queen Hypothesis” – continuous adaptation is needed for a species to maintain its relative fitness among the other species and systems it is co-evolving with
Fun fact: the RQH comes from this allice and wonderland quote: “You must run as fast as you can if you want to stay where you are. If you want to get anywhere, you have to run faster still.”
heterogeneous environments (3)
-Lottery model: Offspring disperse into various patches, each trying to win the “environment lottery”. Sexual females have many different tickets whereas asexual females’ have many copies of the same ticket. In other words, for asexual females, if one looses they all lose
-Asexual species typically occur in simple environments (easy to be successful in)
-Because different genotypes differ in resource use, diverse offspring will compete with their siblings less than identical offspring
Eliminating mutations (2)
happens through process:
a) Muller’s ratchet
b) mutation threshold
Mullers ratchet (5)
-forward mutations (wildtype -> mutant) are much more frequent than reversal mutations
-Asexual offspring inherit all existing mutations and can experience new ones
-Mildly deleterious mutations accumulate in small populations over time
-Multiple mutations eventually have a drastic effect on fitness
-Mullers ratchet works like this: picture a ratchet strap, where only forward movement works due to the ratchet blocking reverse rotation, likewise mullers ratchet can only be rotated in the direction of additional harmful genes because they cannot be lost in asexual populations
Mutation threshold (3)
-Fitness is severely reduced in individuals with more mutations than a given threshold
-mutations are most rapidly eliminated when they occur with other mutations
-Before selection, sexual reproduction recombination generates a wider range of mutations per individual than asexual does
Isogamy vs Anisogamy
Isogamy
-Gametes with identical morphology
Anisogamy
-Games that are functionally divergent (like sperm vs egg)
-Large gametes are more likely to survive
-Small gametes can be produced in higher numbers
Sexual selection (3)
-Component of natural selection
-Females and males unequally invest in each offspring (Bateman’s principle)
-In animals, sexual selection is usually a result of Male-male competition and or female choice of mate
Male-Male competition (2)
Combat: can explain size dimorphism, antlers, horns etc.
sperm competition: increased group size causes increased testes size
Female choice (3)
-males often have elaborate mating displays
-Allows females to choose a male with better genes, more resources, etc,
-May exaggerate a pre-existing sensory bias
Sexual selection in animal pollinated plants (2)
-Male fitness is limited by access to mates, often through pollinator choice
-Female fitness may be limited by resources or pollen receiving ability
sexual selection and pollen packing
-Male fitness is highest when pollinators do not remove all pollen in a single visit, as it allows for staggering of flower mutation, anther mutation and pollen dispensing
