Chapter 9

1. Chapter 9 Studying Adaptation: Evolutionary Analysis of Form and Function

2. Adaptation • This next unit looks more deeply into adaptation and how natural selection increases adaptation • Research that demonstrates that traits of organisms are adaptations is known as the adaptationist program • There are many obvious explanations for why we have particular traits • We must test each explanation to see if it really arose because of adaptation

3. Adaptation • Will examine a variety of methods used by evolutionary biologists to test hypotheses about adaptations • Experiments • Observational studies • Comparative Method

4. All Hypotheses Must be Tested: the Giraffe’s Neck • Everyone knows that the giraffe evolved a long neck to be able to eat the tallest leaves, thereby escaping from competition with other herbivores • Simmons and Scheepers challenged this notion and offered an alternative explanation for the giraffe’s long neck

5. All Hypotheses Must be Tested: the Giraffe’s Neck • They found that giraffe’s most often ate leaves at shoulder height, not from the tops of trees

6. All Hypotheses Must be Tested: the Giraffe’s Neck • They also found that males with the longest necks have the largest, hardest skulls • Maybe long necks evolved for competition for females • Female necks became longer because of selection for longer male necks • Neck-as-a-weapon hypothesis

8. All Hypotheses Must be Tested: the Giraffe’s Neck • Pratt and Anderson classified social status of males • Class C were young adults • Class A were large adults • Class B were small adults • Class A males had wider, stronger heads • Studied displacement by classes and receptivity of females of classes

10. All Hypotheses Must be Tested: the Giraffe’s Neck • There is evidence for selection on longer necks for reaching high and male-male competition • When studying adaptation remember that: • Differences among populations or species are not always adaptive • Not every trait is an adaptation • Not every adaptation is perfect

12. Function of Wing Markings and Wavings of Zonosemata • Tephritid fly that has distinct dark bands on wings • Holds wings up and waves them • Display seems to mimic threat display of jumping spiders • Perhaps flies mimic jumping spiders to avoid predation • Avoid predation by other predators • Or mimic jumping spiders to avoid predation by jumping spiders

13. Function of Wing Markings and Wavings of Zonosemata • Phrase a precise question • Do wing markings and waving behavior of Zonosemata mimic threat displays of jumping spiders and deter predation? • List three alternative hypotheses • Flies do not mimic jumping spiders • Display may be used in courtship • Flies mimic jumping spiders to deter non-spider predators • Flies mimic jumping spiders to deter jumping spiders

14. Function of Wing Markings and Wavings of Zonosemata • Experimental procedure • Clipped wings of Zonosemata and house flies, exchanged wings, and glued them on opposite fly • Clipping and gluing did not affect flying or displaying • Created five experimental groups to test hypotheses

17. Function of Wing Markings and Wavings of Zonosemata • Jumping spiders retreated from flies displaying with marked wings • Other predators killed and ate test flies

18. Function of Wing Markings and Wavings of Zonosemata • Results consistent with hypothesis 3 but not 1 or 2 • Support for hypothesis that Zonosemata deters its predators by acting like one • Important experimental design • Testing control groups • All treatments handled identically • Randomization of order of treatments • Replication of treatments

19. Function of Wing Markings and Wavings of Zonosemata • Why was replication important? • Reduced distortion of results by unusual individuals or conditions (variance) • Can estimate precision of results • Study successful because many variables were tested, but each was tested independently

20. Observational Studies • Experimental studies are preferred but it is often not feasible to experiment • e.g., cannot exchange giraffe’s necks with other animal • Behavior is hard to experiment with because the experiment often alters the natural behavior • Must use observational studies sometimes • Often they are nearly as powerful as experimental studies

21. Behavioral Thermoregulation • Desert iguanas (Dipsosaurus dorsalis) are ectothermic • Must regulate body temperature behaviorally • Can only function between 15° and 45°C • Examine thermal performance curve to see adaptation to particular temperature • Body temperature affects physiological performance • Keep body temperature close to 38°C

23. Night Retreats of Garter Snakes • Do snakes make adaptive choices of where to sleep at night? • Ray Huey implanted garter snakes with radio transmitters with thermometers • Preferred body temperature is 28– 32°C • Keep body temperature near preferred during day • Exposed or under rocks

25. Night Retreats of Garter Snakes • How do they choose good retreats at night? • Thickness of rock determines microhabitat temperature • Thin rocks heat alot during day and cool alot during night • Thick rocks heat and cool slowly • Medium rocks heat and cool just enough • Garter snakes should choose medium rocks

26. Night Retreats of Garter Snakes • Huey placed snake models under different rocks, in burrows, and on surface • Tested temperature fluctuations • Found that snakes choose medium rocks to heat and cool near preferred temperature range

30. The Comparative Method • Purpose of the comparative method is to remove the effects of evolutionary history from an analysis (phylogenetic independence) • The reasons why you need to remove effects of phylogeny from ecological or behavioral analyses are best demonstrated through examples

31. The Comparative Method • Why do some bat species have bigger testes? • Some bats have larger testes for their body size than others • Hosken hypothesized that bigger testes evolved for sperm competition • Female bats may mate with more than one male so the more sperm deposited by a male, the better chance he has of fertilizing the eggs • Bigger testes mean more sperm

32. The Comparative Method • Hosken reasoned that bat species that live in larger groups would have greater sperm competition • Therefore, they should evolve larger testes • Hosken collected data on roost group size and testes size and found a significant correlation

34. The Comparative Method • Hosken realized that this correlation may be misleading

35. The Comparative Method • Joe Felsenstein developed a way to evaluate cross-species correlation among traits • Start with a phylogeny • Look at where sister species diverge • Does the species that evolves larger group sizes also evolve larger testes? • Plot pairs of sister species connected • Drag closest point to origin • Erase origin points and examine independent contrasts

37. Significant positive correlation • Sperm competition and testes size

38. Complex Adaptations in Current Research • Will now examine how researchers use the methods mentioned above to investigate hypotheses about complex topics • Experiments • Observational studies • Comparative Method

39. Phenotypic Plasticity • We have assumed that phenotypes are fixed • In reality, many phenotypes are plastic, i.e., individuals with identical genotypes may have different phenotypes if they live in different environments • Phenotypic plasticity is a trait that can evolve • It may or may not be adaptive

40. Phenotypic Plasticity • Water flea, Daphnia magna, lives in lakes • Usually reproduces asexually • Good for studies of phenotypic plasticity because genotype is known • Luc de Meester studied plasticity in phototactic behavior • Selected 10 genotypes and used clones of each to test • In graduated cylinder, illuminated from above and recorded which direction they swam

41. Phenotypic Plasticity • Each lake population has genetic variation • Tested Daphnia by using water from lakes where fish occurred and where they were absent • Hypothesized that when fish are present negative phototaxis is more adaptive • Phototactic behavior was phenotypically plastic

42. Phenotypic Plasticity • Genetic variation in plasticity • Some populations were more plastic than others • Genotype-by-environment interaction

43. Evolution of Adaptive Traits • Every adaptive trait evolves from something else • How did the mammalian ear evolve? • Mammalian ear has three bones (ossicles) • Malleus, incus, and stapes • Other vertebrates do not have all three • Ear bones transmit energy from tympanic membrane to oval window in inner ear

45. Evolution of Adaptive Traits • Why do we have three bones instead of one? • Increases sensitivity of hearing • To figure out where the bones came from we must: • Establish the ancestral condition • Understand the transformational sequence • How and why they changed over time

46. Trade-Offs and Constraints • Organisms cannot optimize all features at once • Many factors limit adaptive evolution • Trade-offs • Functional constraints • Lack of genetic variation

47. Trade-Offs and Constraints • Begonia involucrata is a monoecious plant pollinated by bees • Male flowers offer pollen to bees • Female flowers offer nothing • Female flowers resemble male flowers to trick bees into visiting them • What mode of selection do bees impose on flower size?

49. Trade-Offs and Constraints • Schemske and Agren’s two hypotheses • The more closely female flowers resemble males, the more often they are visited by bees • Stabilizing selection toward mean male phenotype • The more closely female flowers resemble the most rewarding males, the more often they are visited by bees • If large male flowers offer bigger rewards, directional selection for larger flowers

50. Trade-Offs and Constraints • Schemske and Agren made artificial flowers put equal numbers of three sizes in a forest • The larger the flower, the more often bees visited it • Why aren’t female flowers always large? • Maladaptive • Maybe not enough genetic variation