Chapter 9: Studying Adaptation: Evolutionary analysis of form and function
Giraffe neck length • Giraffes famous for their long necks. Classical explanation is that long necks evolved to enable giraffes to reach higher browse. • Long neck is an adaptation: a trait or set of traits that increase the fitness of an organism.
Giraffe neck length • Is explanation for giraffes neck true? • How do we demonstrate a trait is an adaptation?
Giraffe neck length • To demonstrate that a trait is an adaptation must: • determine what trait is for • show that individuals with trait contribute more genes to next generation than those without it.
Giraffe neck length • Simmons and Scheepers (1996) questioned conventional explanation for giraffe neck length. • Observations of giraffes feeding showed they spend most time in dry season feeding at heights well below maximum neck length.
Giraffe neck length • Simmons and Scheepers alternative explanation: giraffes neck evolved as a weapon. • Bulls use their necks as clubs in combat over mates.
Giraffe neck length • Males have necks 30-40cm longer and 1.7 times heavier than females of same age. • Males skulls are armored and 3.5 times heavier than females.
Giraffe neck length • Males with heavier necks consistently win in interactions with other males. • Females also more likely to mate with males with larger necks.
Giraffe neck length • Long and heavier-necked males intimidate other males and obtain more matings. Thus, trait increases reproductive success of possessor. • But why do females have long necks?
Giraffe neck length • Cannot uncritically accept hypotheses about adaptive significance of traits. Must be tested rigorously. • Also should bear in mind certain caveats about adaptation.
Caveats about adaptation • Not all differences among populations are adaptive. Giraffe populations have different coat patterns. May or may not be adaptive.
Caveats about adaptation • Not every trait is an adaptation. Giraffes can feed high in trees, but does not necessarily mean that this is why they have long necks. • Not all adaptations are perfect. Long neck makes drinking very difficult.
Why do tephritid flies wave their wings? • Testing adaptive explanations with experiments. • Tephritid fly Zonosemata vittigera has distinctive dark bands on its wings. When disturbed holds wings straight up and waves them up and down.
Tephritid fly displays • Display appears to mimic threat display of jumping spiders. • Suggested (i) mimicking jumping spider may deter other predators (ii) mimicry may deter jumping spiders.
Tephritid fly displays • Greene et al. (1987) set out to test ideas. • Hypotheses: • 1. Flies do not mimic spiders. Display has other function. • 2. Flies mimic spiders to deter non-spider predators. • 3. Flies mimic spiders to deter spiders.
Tephritid fly displays • Experimental design tested hypotheses by using flies capable of giving all or only part of the display. • Five groups of flies.
Tephritid fly displays • Predictions for how predators (both spider and non-spider) will respond to display clearly distinguished between competing hypotheses.
Tephritid fly displays • Experiment: Flies from each treatment group presented in random order to starved predators in test arena. • Recorded predators response for 5 minutes.
Tephritid fly displays • Results clear cut. • Non-spider predators ignored display and captured flies of all 5 groups with equal probability. • Spiders generally retreated from flies with barred wings that gave wing waving display.
Tephritid fly displays • Greene at al. (1987) experiment well designed. • 1. There were effective controls. Cutting and gluing control (B) ensures that group C flies failure to deter attack not due to gluing. • 2. All treatments handled alike. One arena used.
Tephritid fly displays • 3. Randomization of presentation of flies eliminated any effects of presenting flies in a set order. • 4. Experiment replicated with multiple individual predators used.
Advantages of replicated experiments • Advantage of replicated experiments. • Reduce effects of chance events. • Allows researchers to estimate how precise their estimates are by measuring amount of variation in data. • Can apply statistical analysis to results.
Observational studies • Not all hypotheses about adaptation can be easily tested experimentally. • Behavioral thermoregulation: Most animals are ectothermic and depend on external sources of heat. Try to maintain body temperature within narrow limits by behavioral means.
Do garter snakes make adaptive choices in burrow selection • Huey et al. (1989) studied thermoregulation of garter snakes. • Snakes prefer to maintain body temperature between 28 and 32 degrees C. • Monitored snakes’ temperatures using implanted transmitters.
Garter snake choices • Snakes spent most of time beneath rocks or basking.
Garter snake choices • Size of rock important to thermoregulatory strategy. • Snakes under thin rocks would get too cold at night and too hot during day. • Thick rocks would offer protection, but generally are a bit too cool.
Garter snake choices • Medium rocks have variation in temperature and snake can move around and stay within optimal temperature range.
Garter snake choices • Huey et al. (1989) predicted snakes would preferentially choose medium rocks and avoid thin rocks.
Garter snake choices • All three rock sizes equally common. Snakes avoided thin rocks choosing medium or thick ones to spend the night beneath. • Medium rocks used twice as often as thick rocks and about nine times as often as thin rocks.
Trade-offs and constraints in selection • Begoniainvolucrata is monoecious. There are separate male and female flowers on same plant. • Pollinated by bees. • Male flowers offer bee a reward in form of pollen. Female flowers offer no reward.
Trade-offs and constraints in selection • Bees make more and longer visits to male flowers. • Female flowers closely resemble male flowers. Rate at which female flowers attract males determines fitness. • Fitness depends on close resemblance to males.
Trade-offs and constraints in selection • Agren and Schemske (1991) examined two hypotheses about mode of selection in these begonias. • 1. Bees visit female flowers that most resemble male flowers. Selection is stabilizing: best phenotype for females is mean male phenotype.
Trade-offs and constraints in selection • 2. Females that look like most rewarding male flowers will be visited more often. If bees prefer larger male flowers then selection is directional with larger female flowers favored.
Trade-offs and constraints in selection • Used arrays of artificial flowers of 3 different sizes. Recorded frequency of bee visits.
Trade-offs and constraints in selection • Larger flowers attracted more bees. Selection is directional
Trade-offs and constraints in selection • Given that larger flowers attract more bees close resemblance in size of female to male flowers appears maladaptive. Why are they not larger? • Trade-off between number and size of flowers in infloresences. The larger the flowers, the fewer there are.
Trade-offs and constraints in selection • There is a limited amount of energy that can be devoted to flower production. Plants can produce many small flowers or fewer large ones.