Unformatted text preview: 10/13/2009 Stories of Adaptation Repeated themes more compelling... The trouble with "just-so" stories "justExamples seem to be compelling evidence of adaptation, but we: b 1. Hand-picked examples. 2. Didn't show that a trait works as we suggest gg it does. 3. Didn't show that a trait's benefits exceeds its costs. 4. Didn't show differential reproduction by different genotypes. Is Predator Avoidance an Adaptation?
Mobbing as a Case Study Mobbing: A behavior whereby members of a group attacks a predator Hypothesis: Mobbing reduces predation on animals and/or young. Is Mobbing an Adaptation? So... mobbing is functional. But has it evolved under natural selection where it is favored? (versus being something that any species can learn to do).
adaptation a behavior that serves a function AND evolved under natural selection 1 10/13/2009 The Comparative Approach If mobbing is an adaptation, then species that live under strong threat of predation should show mobbing, p g whereas species that do not should not show mobbing. First Line of Evidence
Related species, differing risk herring gull nest IN OPEN predation risk HIGH MOBBING observed kittiwake gull nest ON CLIFFS predation risk LOW NO MOBBING STRONG support for adaptation hypothesis Second Line of Evidence
Related species, similar risk herring gull blackblack-headed gull laughing gull all nest IN OPEN predation risk HIGH for all MOBBING observed in all WEAK test of adaptation hypothesis! WHY?
herring g gull laughing g gull blackblack-headed g gull ancestral gull species All species may mob because common ancestor did so. Just a single data point! Third Line of Evidence
Unrelated species, similar risk nests on ground herring gull nests in river banks bank swallow ground squirrel lives on open plains predation risk HIGH for all MOBBING observed in all STRONG test of adaptation hypothesis! 2 10/13/2009 These last comparisons constitute an example of convergent evolution evolution, wherein distantly related species independently evolve a similar adaptation. Contrast this with divergent evolution evolution, wherein closely related species evolve differences due to differences in the nature of selection. kittiwake gull herring gull ground squirrel blackblack-headed gull relatively recent common ancestor These species comparisons imply a common ancestor ancestor, And something about recency of ancestry ancestry. relatively distant common ancestor Phylogeny
the evolutionary history of a group A phylogeny usually takes form of a tree. old Tree of Life logo Figure 6.7 Issues in Phylogenetic Analysis
1. How many times did a behavior evolve?
We said that song learning evolved 3 times. How do we know? Perhaps it evolved once and was lost in some groups. Figure 2.14 3 10/13/2009 To decide, we count up the # gains and # losses required in each case.
Trait gained Trait lost To decide, we count up the # gains and # losses required in each case.
Trait gained Trait lost OneOne-origin hypothesis requires 1 gain but 3 losses, = 4 total evolutionary events. ThreeThree-origin hypothesis requires 3 gains and 0 losses, = 3 total evolutionary events. We choose the explanation requiring the fewest # events! Nesting habit in gulls two scenarios Principle of parsimony The best explanation is that explanation requiring the fewest number of evolutionary events. t How many times did cliff-nesting evolve? cliff- Example. powered flight Powered flight occurs in just three types of animals: insects, bats, and birds.
Figure 6.5 parsimonious to conclude that cliff-nesting arose once in gulls (model A), and that ground-nesting is ancestral. also parsimonious to conclude that ancestor to all gulls engaged in mobbing and further that mobbing lost when cliff-nesting evolved. What is the most parsimonious estimate of the number of times th t powered fli ht evolved? 1? b f ti that d flight l d? Or, 2? Or, 3? 4 10/13/2009 echinoderms arthropods arthropods 4 changes just 2 changes Possibly there was one origin, but 3 losses. A total of 4 changes... in arthropods and chordates. more parsimonious conclusion: flight evolved independently But how many times within chordates? Once? Twice? mammals mammals lizards more parsimonious! One hypothesis: a total of 1 origin but 3 changes. Alternative hypothesis: a total of 2 origins and 2 changes. Flight in bats evolved after origin of mammals, further evidence of its independent evolution. Summarizing, S mm i i Altogether, powered flight arose 3 times, times once in insects, once in birds and once in bats.
Phylogeny of Mammalia from Tree of Life lizards turtles turtles birds birds echinoderms cnidarians cnidarians flatworms flatworms chordates annelids annelids sponges sponges chordates 5 10/13/2009 Issues in Phylogenetic Analysis 2. What behavior is ancestral? Example: Cooperative Breeding in Jays cooperative breeding helper at nest brings food to offspring not its own and defends nest from predators; a form of "altruism" singular c b c.b. 1 pair and usually 1 helper rear young plural c.b. >1 female in territorial group rear young with helpers Which mode of breeding is ancestral? Example. Evolution of eusociality* in Lasioglossum sweat bees non-cooperative breeding singular cooperative breeding plural cooperative breeding 1. non-cooperative breeding has most recent nonorigin 2. plural cooperative breeding is ancestral! *eusociality advanced form of sociality involving eusociality: reproductive division of labor and cooperative brood care. Issues in Phylogenetic Analysis
2. What behavior is ancestral? Caching behavior in corvid birds Solitary behavior ancestral. Eusociality lost numerous times. Some species specialized for caching (= 100% reliance on stored caches) 6 10/13/2009 Some ancestral types known, But some ambiguous. g >2 independent origins for specialized caching. Independent evolution of caching behavior supported by differences in caching morphology: Both Clark's nutcracker and pinyon jays have specialized pouches for carrying seeds seeds. But Clark's nutcracker have sublingual pouch, while pinyon jay has expandable esophagous. Example. Prey detection by lizards
What parsimonious conclusions can be drawn? Issues in Phylogenetic Analysis
3. Do traits show evolutionary trends? Use olfaction Use vision 1. Use of vision is ancestral in this group (A) 2. Use of olfaction arose 3 times (B, C, D). 3. Use of olfaction was lost once (E). Cowbird species vary in host breadth. Was common ancestor a specialist or a generalist? Is there an evolutionary trend in specialization? Issues in Phylogenetic Analysis
4. How does natural selection shape behavior? 7 10/13/2009 lekking Mating system in which males gather at non-resourcebased display ground (lek and form territories; lek) Females enter lek and choose mates. Male mating success highly skewed skewed. Hypothesis Lekking promotes evolution of male courtship traits and kk l f l h d thus generates sexual dimorphism dimorphism. Male is much bigger than female in sage grouse. medfly sage grouse Data on birds below suggest that lekking is associated with size dimorphism. Size Dimorphism Mating System lekking non-lekking + 69 13 18 13 lekking nonnon-lekking hypothetical phylogeny What if... lekking birds tended to be closely related to each other?
Then what looks like many independent data points is actually many fewer. BUT... data don't consider phylogeny. What if... In fact, lekking species do indeed tend to share a common ancestor. When data are reduced: Size Dimorphism Mating System lekking non-lekking + 11 6 9 9 Phylogenetic analysis allows us to control for ancestry when using species comparisons to test hypotheses about adaptations. pattern more or less disappears!! At present, few complete cases for adaptation in behavior using phylogenetic analysis. 8 10/13/2009 Issues in Phylogenetic Analysis
5. Can behavior be used to build phylogenies? Case of the bowerbirds
Bowerbirds so-called because males build sostructures called bowers with which to attract females. Shown is the satin bowerbird Answer seems `no' because behavior varies so much, even within a species. manakin and bowerbird video
Satin bowerbird Spotted bowerbird Two species of `avenue builders' Bowers, decorations, dancing and vocalizations of males vary among bowerbird species. Bowerbirds are monophyletic (one origin for the group). A `maypole builder' doesn't build bowers Bowerbuilding either arose twice or arose once and lost once. Authors favor single origin for bowerbuilding. 9 10/13/2009 Related species of bowerbirds have similar types of bowers. Bowers could be used to build phylogeny. Maypole bower builders are monophyletic, and avenue bower builders are monophyletic. 10 ...
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- Spring '09
- Evolution, phylogenetic analysis, Gull