Quantitative genetics leftovers • Examples of heritabilities Humans h2 height 0.65 serum immunoglobin level 0.45 Cows adult body weight 0.65 butterfat 0.40 milk yield 0.35 Pigs weight gain per day 0.70 litter size 0.05
Adaptation: Key questions: Are all traits adaptations? Other possible hypotheses to explain current traits? How can we test hypotheses about selection that might have taken place in the past?
Concealed estrous in humans. Adaptive? Most primates advertise ovulation: why don’t human females know? (Female pygmy chimp with genital swelling)
Case study: laryngeal nerve Laryngeal nerve anatomy 1. down the neck 4. to larynx 2. behind the aorta 3. up neck Is it adaptive? For giraffes?
Historical constraint: the laryngeal nerve For fish, no problem But the basic anatomy has been modified since then.
Case study: sutures in the human skull • Are they adaptive? • If so, what is the selective force? • Was this the historical cause? • adaptation • exaptation
A metaphor: spandrels The basilica of St. Mark’s was designed to have many spandrel’s to decorate . . .
Case: spotted hyenas Male
Case: the female hyena “penis” • High levels of fetal androgens • Clitoris is enlarged • Birth canal exits through clitoris • First born often dies in birth (75%) • Mothers often die in birth (8%) • Clitoris rips, allowing passage of other cubs Adaptive? How and why?
Case: Testes size in bats • Testes size varies between species • Does increased male-male competition lead to larger testes? • Hypothesis: larger groups, more mating, more sperm competition, so larger testes • How to test?
First approach: plot group size vs. testes mass Is the regression significant? Figure 10.12
Phylogenetic non-independence Group Testes Species Size Mass A 5 8 B 5 8 C 5 8 D 20 16 E 20 16 F 20 16 Figure 9.12
Phylogenetic non-independence Figure 9.12
Strategy: Independent contrasts Figure 9.13
Strategy: Independent contrasts Figure 9.13
Bat testes: independent contrasts Figure 9.14
Bat testes: the sequel (2005) • Testes are expensive • Brains are expensive • Can bats afford both? • Approach: independent contrasts. • Compare mating system, testes, brains.
Case: Male sterility in flowers Some flowers are female Some flowers are hermaphrodites Female flowers produce 1.5 x as many seeds as hermaphrodites Which would natural selection favour?
Mechanism of male sterility • Known • Mutation to mitochondrial gene (ATPase) • Produces toxic compound • Transgenic yeast, E. coli: dead • Unknown • Why kill anthers and pollen, not ovules, not entire plant?
Selection? • Pollen: no mitochondria • Ovules: have mitochondria
Case: Apert’s syndrome Symptoms: fused fingers, facial abnormalities, cranial sutures close early Cause: mutation to fibroblast growth factor receptor 2 (FGFR2). Mutation in male germ line: increases with age
Apert’s syndrome puzzle Cause: mutation at one nucleotide: TCG –> TGG. Puzzle: This mutation is common; other mutations are not (eg TCG –> TAG). Hypothesis: TCG – TGG is favoured by selection.
Case: genome size in plants Two species of sunflowers in the southwest 17 pairs of chromosomes, but genome size – 11 or 7 pg Helianthus anomalus (dunes) and H. annuus (plains)
Why the larger genome? • Adapted to different habitat? Baack et al 2005
Sources Baack et al 2005. Hybridization and genome size evolution: timing and magnitude of nuclear DNA content increases in Helianthus homoploid hybrid species. New Phytologist 167:623-630. Crow, J. 2006. Age and sex effects on human mutation rates: an old problem with new complexities. J. Radiation Research 47:B75-B82. Diamond, J. 1992, The third chimpanzee. Harper-Collins. Goriely, A et al. 2005. Gain-of-function amino acid substitutions drive positive selection of FGFR2 mutations in human spermatogonia. PNAS 102:6051-6056. Pitnick et al., 2006. Mating system and brain size in bats. Proceedings of the Royal Society: Biology.
Questions 1. Imagine that you are examining shrubs that grow on two isolated islands, A and B. The shrubs appear to be very similar, and you perform test pollinations to confirm that they can mate with one another. DNA markers suggest that they are very closely related to each other. On island A, the shrub grows at higher elevations that are cooler and moister. On island B, the shrub grows at lower elevations which are hotter and dryer. Island B has lizards, while island A does not. You discover that the leaves of B shrubs contain many more toxic compounds than island A shrubs, and suspect that this is due to the herbivorous lizards. What alternate hypotheses should you consider, and what experiments could you perform to test your hypotheses? 2. Many plant species contain toxic compounds, and many of these compounds have been demonstrated to deter insect attacks or prevent attacking insects from growing. Why aren't plants more poisonous so that they are able to prevent all herbivory? Consider three hypotheses, and describe ways that you might put these to the test. 3. Like bats, primate species differ in the relative size of testes (compared to the total body mass). Describe how you would test an adaptive hypothesis. (Remember, not all tests are experimental!)
Questions 4. The persistence of female plants in Silene is puzzling, since they have 75% of the fitness of hermaphrodite plants. Although they produce 1.5x as many seeds, they do not pass their genes on via pollen, which should be half of the reproductive success of hermaphroditic plants. Explain why female plants would be favored from the point of view of mitochondria. What would happen to a mutation to a nuclear gene that counteracted the ability of mitochondria to eliminate pollen production? 5. On many Pacific islands, bird species are going extinct because human travel has introduced new predators to the islands. Brown tree snakes were introduced to Guam in 1952. Birds previously had no snake predators on the island: a dozen species have gone extinct since the snake's introduction. Why didn't the birds evolve to defend themselves against the snake? Was there no heritable variation in response to snakes, or simply insufficient time for selection to act on this variation? Describe how you might study this question using bird species from Pacific islands where the brown tree snake has not yet eliminated the birds.