disadvantages of sexual reproduction... 1. Only 50% of your genome is in your offspring. 2. Your good traits (assume they are good because you have survived to the point of reproduction) are victimized by recombination. 3. You are likely to incur the costs of searching for a mate, courting a mate, and guarding a mate. These activities make you more vulnerable to predators and reduce the time you can forage. 4. Cost of accumulating STDs, ectoparasite transmission 3
So you might expect that asexual reproduction would be common and sexual reproduction rare. This is not the case, so what huge advantage of sexual reproduction over-rides the disadvantages? Answer: the variation produced by recombination. Your offspring have a much better chance of surviving if there is environmental unpredictability. Will the environment be wetter, hotter, more of food type A, a new predator shows up, etc. 4
offspring number environment seen by offspring environment to which offspring best adapted survival asexual sexual asexual sexual 1 B C A - - 2 B C C - - 3 A C B - - 4 C C D + - 5 A C A - + 6 C C C + + 7 B C B - + 8 A C D - - 9 A C A - + 10 C C B + - can t predict what environment will be how often do you get a match sex wins 5
3rd greatest advantage... Many parasites are fast to mutate. For example, the influenza virus. Variation in host population leads to individuals who may be resistant. So how best to achieve sexual reproduction? A rule set *very* far back in the evolutionary history of animals based probably on trying to reconcile two incompatible successful strategies... 7
Ancient strategy 1: Put lots of investment in one or a few gametes, so the chance for survival is larger for those individuals. Fitness is increased for individuals who invest more energy per gamete. Ancient strategy 2: Put minimal energy into each gamete but make many of them. Fitness is increased for individuals who make the largest number of gametes. This strategy has the advantage that you disperse your gametes so no one environmental disaster will get them (say a predator who sweeps through the area). 8
Optimal strategy: Have your cake and eat it too! One sex puts lots of investment in gametes, the other sex makes many low-cost, high mobility gametes. A possible evolutionary trajectory was one sexual partner adding a tiny bit of nutrients to their gametes, giving a survival edge (we now call these females). By chance, the opposite sex (we now call males) of this ancestral organism did not do this. Selection ran away with more and more investment in female gametes. 9
Leads to anisogamy -- BIG egg, small mobile sperm. The HUGE consequence: females make much larger energetic investment in gametes. 1 0
Extra credit alert!! Calculate the energetic investment in making zillions of cheap sperm all the time with the cost of making one huge egg once in a while. You should pick realistic times, and look up physiological costs. This might be very difficult! 1 1
Gamete Economics (the law of supply & demand) Female gametes: Can t make them fast Can t make many So female gametes become a scarce resource. Male gametes: Many, cheap, all chasing the scarce resource.
Male reproductive success number of matings, but female reproductive success number of viable eggs produced. Drosophila experiment: female success does not improve with additional mates.
One prediction from anisogamy might be that you get an uneven sex ratio in the population (more males than females). In fact, populations self-correct back to a 1:1 ratio (with some exceptions): population with more males some males can t find mates population no longer has more males favors females who have more female offspring (who will be able to mate)
Exception: bush babies (S. African prosimian (Galago)) Mothers and daughters remain together in packs, but sons disperse. The mothers and daughters compete with each other for food. So natural selection has built in a bias to make more sons than daughters. Order Primates Suborder Strepsirrhini Infraorder Lemuriformes Infraorder Lorisiformes Family Lorisidae Family Galagidae (19 species) Suborder Haplorrhini (tarsiers, monkeys, apes)
Common scenario: Whomever makes the larger energetic investment, or incurs the greatest risk relative to reproduction becomes choosy about who they mate with. Most of the time females make the larger investment and are choosy. Sometimes there is extensive male parental care, and males become choosy. Note that the male investment must first overcome the built-in disparity relative to gamete production costs.
Example of male who makes more investment in gametes than female: Male Mormon crickets produce a nutrient-rich spermatophore for females, and their energetic investment is higher. Whaddya know, males are choosy relative to females! 1 7
Katydids - switch reproductive behavior according to who is making the larger investment seasonally. season food supply spermatophore mating pattern fall/winter meager small male choice spring high large female choice The spermatophore (sp) contains many nutrients for the female, so the male contributes heavily in terms of energy investment. Bad conditions: sp is precious, male who makes it is choosy. Good conditions: males can make as many sp s as needed, females become picky. 1 8
sexual selection assuming unequal energetic investment in gametes individuals of the sex that contributes more will be choosy individuals of the sex that contributes less (A) will have to compete with each other for the scarce resource of a mate who contributes more (B) natural selection will favor -A- individuals who are more competitive -B- individuals who are more discriminating 1 9
The choosy individual must be able to determine who is the more fit mate. The mate investing less = A 1. Mate A competes with other A individuals to gain access to B individuals. 2. Mate A produces a communication signal that indicates to mate B the fitness of mate A 3. Run-away selection produces signals that are themselves honest indicators of mate A s fitness Each of these patterns leads to great behavioral consequences... 2 0
1. -A- individuals compete with other -A- individuals to gain access to -B- individuals. 2. -A- individuals produce communication signals that indicate to -B- the fitness of -A- 3. Run-away selection produces signals that are themselves honest indicators of -A- s fitness process 1 leads to dominance and dominance hierarchies, competitions and scrambles (next chapter) process 2 leads to all manner of advertisement calling and advertisement displays process 3 leads to elaborate displays that are costly: ~ hard to find food, shelter ~ easy for predators to find you 2 1
Members of one sex choose mates (e.g. males compete with each other yielding one male who controls a harem) = intersexual selection Members of one sex compete with each other to get mates (e.g. each male tries to be attractive to females, ignoring other males) = intrasexual selection In both cases, sexual selection leads to sexual dimorphism 2 2
Mating success in Kenyan Baboons is proportional to dominance: In 18 Baboon groups the correlation was almost perfect, slightly less in 17 other groups, etc. Males competing with other males (intersexual) 2 3
Deceptive signaling: Although it must remain relatively rare, an evolutionarily stable strategy in some populations is an alternate phenotype of sneakers or satellites. Typically these are males that appear as females to avoid competition from other males. The deceptive males sneak copulations with females. In other cases, non-dominant males or males who believe themselves to be inferior just wait in the wings and try to copulate with females before another male drives them off. 2 4
marine isopods that live in sponges Alpha male fights off other alpha males in his territory. Best strategy when not too many females beta male pretends to be female and avoids male aggression, sneaks copulations with females in alpha s territory. Best strategy when females are plentiful and dominant male can t control them all. gamma male sneaks around in alpha s territory hoping not to be found. 2 5
Each male type of this species is genetically programmed to be an alpha, beta or gamma size. Stabilizing selection leads to all three genotypes because each has about the same fitness re: copulations. That is, the three strategies for mating with females are equally successful, and no one genotype wins out. 2 6
Both male deception and female deception driven by sexual selection: Females: Risk of choosing low grade male. Mate with male, but not sure if best choice Conditions change, now a different male better sneaky matings with other males. Must use deception to avoid male infanticide. Males: Risk of not mating at all If parental care, males may deceive female about readiness to be good parent. 2 7
run-away selection intrasexual (male competition with each other becomes extreme) 2 8
Male elephant seals are ten times larger than females!!! male elephant seals fighting for control of females 2 seal female elephant 9
Intersexual selection (either males choose females or females choose males) With run-away selection there is often no upper bound on the releasing stimulus for the choosy mate. That is, if 10 red spots are the signal of a quality mate, 20 are better and 300 are better yet. 3 0
Elaborate displays can evolve when males compete, not with each other, but for the attention of females (intersexual competition). Peacocks: the common example of extreme run-away intersexual selection. 3 1
The male has quite the burden, even when not displaying to females. Handicap hypothesis: female judges male s fitness based on his ability to survive with the handicap but note James Gould criticism of this in video! 3 2
Intersexual selection can lead to elaborate behaviorproduced static displays: Bowerbirds The more elaborate and more decorated the bower, the more likely the male will get a female. 3 3
When sexual selection drives males (or females) to perform fancy tasks, one consequence is increased brain power in the males. 3 4
Sometimes the features selected by the choosy mate are subtle: symmetry is often a surrogate for good genes and health... but this is controversial. 3 5
sperm competition External fertilization: He who makes the most and the fastest swimming sperm wins. (Of course being better at finding or getting closer to eggs at the optimum time is critical too) 3 6
Bluegill Sunfish (Discussed here relative to sperm competition, will reappear in discussion about parental care.)
Bluegill Sunfish 3 male phenotypes relative to mating: 1. Parental males: build nests, court females, take care of eggs. 2. Satellite males: female mimics, hang around nest and try to deposit sperm along with parental males. 3. Sneaker males who dart in from a distance and deposit sperm. These guys never get as close to eggs as parental males 3 8
Sunfish sperm competition Predict that sneaker males would have the lowest probability of being a parent since their sperm has to travel farther and is more subject to dilution in lake. But Their success rate is much higher than predicted. Implication: They are winning the sperm competition game. The sneaker male sperm must be better than the parental male sperm! 3 9
sperm competition with internal fertilization: postcopulatory female-choice 4 0
Some birds have sperm storage tubules. It is a first-in last-out system. Female mates with social monogamous partner... or not. Extrapair copulations are common. Female can mate with male A, but if a better male comes along, mate with male B too. The sperm from male B are more likely to fertilize egg. 4 1
Extra-pair copulation behavior should lead, via natural selection, to mate guarding behavior. How much guarding depends on cost: inability to forage vs likelihood of neighbors getting your mate. As neighbor male population goes up, foraging goes down and guarding increases. 4 2
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