Biology 105 - Evolution Dr. Theodore Garland, Jr. 24 Nov. 2015: "Trade-offs & Constraints"

1. Biology 105 - Evolution Dr. Theodore Garland, Jr. 24 Nov. 2015: "Trade-offs & Constraints" Reading is pages 83-89 inBergstrom, C. T., and L. A. Dugatkin. 2012. Evolution. W.W. Norton and Company.

2. "The whole organism is so tied together that when slight variations in one part occur, and are accumulated through natural selection, other parts become modified.This is a very important subject, most imperfectly understood." Darwin, 1859,The Origin of Species

3. Causes of Correlated Evolution • Genetic correlations: • Pleiotropy • e.g., genes that affect hormone levels • Linkage • Correlated selection • e.g., appropriate physiology & behavior • e.g., dietary preferences & digestive enzymes • Trade-offs • Constraints Be able to connect these terms for the final exam.

4. What is a trade-off? Trait A cannot increase without a decrease in Trait B.

5. Why do trade-offs exist? Trait 1 Trait 2 TotalAvailableEnergy 1. For a given amount of resource(e.g., energy, space), it is impossible to increase two components at once: The "Y model" Example: Size vs. number of eggs

6. Why do trade-offs exist? 2. Characteristics that enhance one sort of performance necessarily decrease another sort of performance. Gloves vs. mittens "Every solution that an arboreal animal adopts to any one of the problems posed by arboreal locomotion limits its options in dealing with others." (Cartmill, 1985, p. 75) Selective importance of different types of performance may depend on environment, e.g., gloves are fine in CA, but not in WI.

7. How is a trade-off recognized? Typically just a negativerelationshipbetween 2 traits. May not be adequate if several traits are involved! (Multivariate analyses are beyond our scope.) 0 2 4 6 8 10

8. 12 Species of Lacertid Lizards Apparent trade-off between residualspeed & stamina (residuals from regressions on snout-vent length) (Vanhooydonck et al., 2001, Evolution 55:1040-1048) Resid. log Endurance (min) -0.3 -0.2 -0.1 0 0.1 0.2 Resid. log Sprint Speed (m/s)

9. 12 Species of Lacertid Lizards What might underlie this apparent trade-off? Variation in muscle fiber type composi-tion? Resid. log Endurance (min) -0.3 -0.2 -0.1 0 0.1 0.2 Resid. log Sprint Speed (m/s)

10. 11 Species of Phrynosomatid Lizards Trade-off in hindlimb muscle fiber-type composition (iliofibularis) Bonine et al., 2001, Journal of Morphology Slow-twitch Oxidative fibers are relatively uncommon (max. 17%) 0 0.2 0.4 0.6 0.8 1

11. Among 36 Species of Lizards, no apparent Trade-off log Endurance (min) log Sprint Speed (m/s)

12. Why Trade-offs May Not Exist: Nature has more "degrees of freedom" than assumed by simple models that predict trade-offs. • For example, muscles can evolve to be larger, positions of origins and insertions can shift, legs can become longer, gaits can evolve (including bipedality). • Animals may be able to obtain and process more food, thus allowing them to increase both number and size of offspring. • Animals may alter behavior in a way that removes constraints (e.g., become nocturnal to avoid predation cost of increased foraging).

13. Why Trade-offs May Not Exist: Trade-offs may only appear at the extremes of distribution. Fig 12.3 from Garland (1994)

14. Why Trade-offs May Not Exist: "Grade Shifts" can change mean values, thus causing differences in apparent trade-offs at various phylogenetic levels. They may also involve resetting of the basic "rules" for trade-offs and constraints ... Trait B Trait A

15. What is an evolutionary constraint? Anything that limits the production of new phenotypes. Caused by: Internal factors - basic morphology, physiology, biochemistry. Will be reflected in pleiotropic gene action and hence genetic correlations. Ex.: increasing testosterone makes males better able to defend territories, but worse at taking care of offspring Ex.: turtles cannot use titanium to build their shells External factors - the selective regime dictated by prevailing environmental conditions and ecological interactions. Ex.: more foraging generally increases predation risk

16. What is an evolutionary constraint? Thus, genetic and developmental processes can restrict the types of phenotypes that arise in the future and, given that the environment (and hence selective regime) changes over time, yesterday’s adaptation may be tomorrow’s constraint. • Genetic correlations are usually viewed as something that places constraints on what selection can do. • For example, if two traits are negatively genetically correlated, then if selection favors increasing both of them, the rate of evolution will be slow. • However, if the genetic correlation is in the same direction as what selection is "trying" to do, then it can facilitate evolution. • Ex.: activity level and predatory aggression during the evolution of carnivory (see end of previous lecture).

17. Evolutionary constraints are often invoked: All optimality models assume some sort of constraint. Otherwise, they would always predict "more is better."

18. Examples of evolutionary constraints? Slugs have no skeleton, and this probably constrains how large they can grow and what they can do.

19. Examples of evolutionary constraints? http://evolution.berkeley.edu/evolibrary/article/_0/constraint_01 Arthropods do not get very large. Arthropods have been evolutionarily successful in many ways, including longevity in geological time, number of individuals, number of species, habitat occupancy (e.g., aquatic, terrestrial, flying), and variety (amount of "morphospace" occupied). However, arthropods do not get as large as vertebrates. This probably has to do with jointed appendages and molting, which were inherited from their ancestors, and how these characteristic interacts with the fundamental laws of physics.

20. Examples of evolutionary constraints? • http://evolution.berkeley.edu/evolibrary/article/_0/constraint_01 • Molting: Molting is more hazardous for larger animals. Have you eaten soft-shell crabs? • Exoskeleton strength: The exoskeleton may not be strong enough to support larger animals.Also remember the 1950s science fiction movies about giant ants, etc.? • Respiration: Many arthropods can only get enough oxygen to support small bodies due to the nature of their resipratory system.

21. Examples of evolutionary constraints? • No snakes are herbivorous • No birds are viviparous • Blackburn, D. G., and H. E. Evans. 1986. Why are there no viviparous birds? Am. Nat. 128:165-190. • All Anolis lizards lay a single egg per clutch • But maybe this is best viewed as an alternate strategy because they also have relatively high brood frequency. • Marsupial immune system constrains evolution of placental exchange. • Incompatibility between sexual and parental behavior in some birds. Testosterone increases mate-searching behavior at the expense of taking care of existing offspring.

22. How can constraint hypotheses be tested? Thorough understanding of how organisms work Look for organisms that "break the rules" Physiological or biochemical manipulations, e.g., hormone therapy Direct genetic manipulations (e.g., transgenics) Perform a selection experiment Example: Weber, K. E. 1990. Selection on wing allometry inDrosophila melanogaster. Genetics 126:975-989. (But must have some variation expressed in order to select ...)

23. An example of the use of selection experiments to test hypotheses about constraints on evolution: • Selection on wing allometry in Drosophila melanogaster.

24. Testing alternative hypotheses about (tight) allometric relationships: • Genetic, developmental, physiological, and/or biomechanical "constraints" keep organisms clustered about a line. • Natural selection keeps them there.

25. Weber, K. E. 1990. Selection on wing allometry in Drosophila melanogaster. Genetics 126:975-989.

31. Biology 105 - Evolution Dr. Theodore Garland, Jr. 24 Nov. 2015: "Sexual Selection" Reading is Chapter 17 inBergstrom, C. T., and L. A. Dugatkin. 2012. Evolution. W.W. Norton and Company.

32. Darwin defined Sexual Selection as: • "the advantage which certain individuals have over others of the same sex and species solely in respect to reproduction" • Darwin, C. R. 1871. The descent of man, and selection in relation to sex. J. Murray, London. • = Variation in fitness, within one sex, solely with respect to reproduction.

33. Mating success will often trade-off with other components of overall Darwinian fitness (lifetime reproductive success), such as survivorship. Sexual selection will tend to increase traits that positively affect mating success up to the point that their "cost" in terms of other fitness components (e.g., survivorship, foraging ability) outweighs their benefit. These trade-offs are somewhat unusual because they involve interactions of multiple individuals.

34. Darwin explained many male secondary sexual characters (e.g., antlers of deer, tail feathers of peacocks, giraffe necks?[http://en.wikipedia.org/wiki/Sexual_selection], sauropod necks?) as features that enable individuals to father more offspring, by conferring superiority in: 1. Competition or combat for mates (intrasexual selection) 2. Mate choice (intersexual selection); not necessarily conscious Both may occur in some species.

35. Why did Darwin come up with this as something separate from natural selection? • Natural selection may explain how organisms become better adapted to their environment, but it does not explain some costly traits, such as the tail of the peacock or being eaten alive during sex in some spiders and praying mantids:

36. 1. Competition or combat for mates (intrasexual selection) Sperm is cheap, and males can make lots of it. So, their reproductive success tends to be "female limited." 2. Mate choice (intersexual selection); not necessarily conscious Eggs and raising offspring are expensive. So, females tend to be "resource limited."

37. Anolis lizards Competition for Mates: The special structures and behaviors displayed by males are often associated with aggressive attempts to control access to females.

38. Movie of Mountain Dew ad: Mt. Dew Ad.mov https://www.youtube.com/watch?v=Tt6C6P3bJq8

39. Sometimes very large males evolve. In elephant seals, males can weigh 6 X as much as females.

40. Among species of pinnipeds, sexual dimorphism increases with harem size: Stearns and Hoekstra, 2000, p. 183

41. In fact, males are larger than females in most vertebrates. Given the general differences in behavior and ecology, this may also result from natural selection. Alternatively, it could result from primary sex differences, such as the higher testosterone levels of males, i.e., a proximate explanation. Or, at least in ectotherms, it could result because females spend relatively more energy on making babies than on growing their own bodies, another proximate explanation. On the other hand, in many ectotherms, such as lizards, clutch size is positively correlated with body size, so natural selection should generally favor larger females. Where the balance should be, including any effects of sexual selection, is difficult to predict, but "reverse sexual dimorphism" (females larger) is uncommon.

42. Galapagos Marine Iguanas: Males are larger than females. Males fight for territories. Mating success is highly skewed.

43. Primate Species: Sexual Selection Females likely to copulate with > 1 partner per estrus Testes Mass (g) Femalesmate withonly 1 male Body Mass (kg)

44. Males (or females) often show strange morphology: Horn of plenty. The Australian dung beetle Onthophagus neostenocerus (left) and the Central American dung beetle Onthophagus crinitis panamensis (right). The position of horns on the beetle's body influences the size of nearby organs such as the wings or eyes. Harvey, P. H., and C. J. Godfray. 2001. A horn for an eye. Science 291:1505-1506.

45. And these structures can be costly: Fig. 2 from: Emlen, D. J. 2001. Costs and the diversification of exaggerated animal structures. Science 291:1534-1536. "Functional costs of beetle horns. In each species, the relative size of horns was negatively correlated with the relative size of the nearest neighboring structure (antennae, eyes, or wings), and these developmental tradeoffs were present only in the sex-expressing enlarged horns." Thus, the presence of a horn comes at the expense of important nearby structures.Example of "Y model" of resource allocation.

46. Males often show strange behaviors:

47. Mate Choice: In many species, males possess characteristics that are hard to reconcile with natural selection, and do not seem beneficial for male-male competition.

48. Movie of Peacock: Tale of The Peacock.mov (4 min) http://www.pbs.org/wgbh/evolution/library/01/6/l_016_09.html

49. Darwin suggested that those traits evolved because they help increase a male’s chance of mating. This suggests females have a vote in the matter and that they preferentially mate with some males over others. Even Darwin’s most ardent supporters thought this was a ludicrous idea. However, we now know that the concept is indeed supported, including in peacocks and in human beings ...