BIOB51 - Lectures 5.doc

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Biological Sciences
Maydianne Andrade

Lectures 5 & 6: Evolutionary analysis 1 1) Evolution: Pattern & Process 2) Methods of Evolutionary analysis A. Scientific method → Planned experiment → Natural experiment → Observational study B. Comparative method C. Fossil record D. Modelling (verbal or mathematical) 1. Evolution: pattern and process Pattern Process • Origin of species •Maintenance / change in traits over few • Origin / change in traits over many generations generations & across taxa •Current fitness effects • History / sequence of change •ecological time scales(generations) • geological time scales (thousands/millions years) Microevolution ( day to day) Macroevolution (across major taxonomic group) Microevolution (‘process’) Changes in gene frequencies and trait distribution within populations and species Often involves the study of changes/differences: → within populations over time (e.g. effect of drought on Darwin’s finches, p. 87-90) → Among populations across space (e.g., mosquito-fish gonopodia, fig 3.20) Time scale: relatively short period (e.g., ecological time: 1 or a few generations) Macroevolution (‘pattern’) Changes that distinguish higher taxonomic groups (e.g., genera, families, orders etc.) Involves the study of : → large-scale phenotypic changes that differentiate animal taxa (e.g., wings) → phylogenetic patterns (e.g., evolutionary history or ‘genealogy’) Time scale: long periods (e.g., geological time, large number of generations) 1. Evolution: pattern and process Method of analysis depends on time scale of interest: Pattern (Macroevolution) Process (Microevolution) B. Comparative Method A. Scientific method C. Fossil Record •Planned experiments •Natural experiments •Observational Studies A. Scientific method General procedure for answering questions 1) Observation Why? 2) Hypothesis = Educated guess (possible explanation) 3) Predictions: IF hypothesis is true, THEN 4) Test → Gather data & compare treatment groups → Experiment: independent variable (X) altered, look for effect on dependent variable (Y) by comparing treatment groups (i.e., X altered in one, but not the other),allows causation → Observation: careful measurement of (Y) as a function of natural variation in (X), nothing manipulated → Strongest tests = opposing predictions of alternative hypotheses o Strongest test is having more alternative hypothesis, one hypothesis may be constant with others, thus, should look at others → One alternative is always the null hypothesis = treatment groups are the SAME (No effect of X on Y) 5) Draw conclusions → Statistical test of data → Are these really different? → Or could this result just be due to sampling error? Actually two samples from one population? = null (x has no effect on y, not true) Statistical tests of data how likely is it that you got this result by chance? Depends on: • magnitude of difference (bigger the difference, the more likely there will be a difference) • sample size (larger sample, the more accurate) Result = P-value ~ likelihood that observed difference is due to chance (i.e., NO effect of X on Y) 0 ≤ P ≤ 1 The bigger the P value, the more likely this is due to chance Here, if P≤ 0.05, we say: X has a significant effect on Y Convention (all statistical tests) significant difference between V and W if P ≤ 0.05 ↓ (~probability of getting this difference by chance = 5/100) 2. Methods of Analysis A. Scientific method Questions: Microevolutionary questions = effects on fitness within populations •how does trait affect lifetime reproductive success? e.g., effect of shell shape on mortality from crab predation in periwinkles Results in ecological time Three types of studies: i) Planned Experiment Causation → independent variable (X) manipulated by researcher → resulting variation in dependent variable (Y) is measured ii) Natural Experiment → independent variable (X) altered naturally → resulting variation in dependent variable (Y) is measured iii) Observational Study → measurement of natural variation in proposed X & Y → are X & Y related to each other as proposed by hypothesis? correlation i) Planned Experiment Fruit flies (Tephritids) Jumping spider General Question: → Why do Tephritid flies have marked wings and a wing waving display? Possible explanation: → mimic jumping spider (fearsome predator) threat display, reduce jumping spider predation, jumping spiders retreat from other jumping spiders p. 367 -371 Alternative Explanations (Hypotheses): 1. Mimic jumping spiders. Reduce predation by jumping spiders. 2. Mimic jumping spiders. Reduce predation by other predators. 3. No mimicry. Display and coloration not an anti-predator adaptation. (null) NOTE: null hypothesis = no effect on predation Key Predictions: 1. If Mimic, jumping sp. then having wings/display = • decreased predation by jumping spiders • no change in predation by others 2. If Mimic, other predator then having wings/display = • no change in predation by jumping spiders • decreased predation by others 3. If No mimicry then having wings/display = • no change in predation rate Experiment • fly + predator in arena • is fly attacked/killed? • X = wing markings/display (+ or -) • Y = proportion of flies attacked/killed Problem: All Tephritid flies have wing markings/display Solution: Tephritid fly (Zonosemata) → Normal, intact → Normal wings removed → House-fly wings attached What might affect whether fly is attacked/killed? Experimenter varies these... 1. Wing markings 2. Waving display 3. Type of predator Must control for these... 1. Hunger level of predator 2. Behaviour of individual fly 3. Behaviour of individual predator 4. Wing operation Tephritid flies a. Intact (marks + wave, no operation) b. Wing removed, replaced (marks + wave +operation) c. +House-fly wing (wave +operation, no marks) House flies d. +Marked wing (marks +operation,no wave) e. Intact (no marks, no wave, no operation) (fig. 10.6) Present 5 prey types to a predator in random order Predators(eats others): •jumping spiders (n = 20; 11 species) •starved for 2 days •Other predators (starved) Jumping spiders/other predators in a box and different flies were put in at random order. Results: 1. Other predators: All flies eaten quickly 2. Jumping spiders: Flies with wave + marks = rare attacks/kills; Flies with no wave, no marks = often attacked/killed (fig. 10.7) Statistical test example Null Hypothesis: No mimicry Prediction: Predation rate: Predation rate: Intact fly = with house-fly wings Outcome Intact fly with house-fly wings J. spider Kills/attacks flies 5/20 (25%) 18/20 (90%) J. spider Retreats from flies 15/20 (75%) 2/20 (10%) Statistical test: P < 0.01  Significant difference •Reject the null hypothesis •(wing markings/waving has no effect on predation) •Support the alternative hypothesis, that it reduces predation by jumping spiders Conclusion: → Wing-wave display and markings mimic jumping spider displays and reduce predation by jumping spiders Some key points: 1. Alternative hypotheses strengthen test. •The Null hypothesis (‘X’ has no effect on ‘Y’) must always be considered. 2. Must control for possible confounding factors •control treatments •randomization •standardized test conditions 3. Sample size is important. ii) Natural Experiment 1. e.g., Soapberry bugs X = host plant (old or new) Y = beak length 2. Ancestors of tibetans move to mountains ~2700 years ago X = altitude (sea level or >3,000m) Y = % with allele for increased high-altitude oxygen saturation 78% difference Han chinese 9% Tibetans87% Danes 0% 3. General question: Why do Galapagos finches have such diverse beaks? Possible macroevolutionary (pattern) [looking at different species in a single genus] explanation: o A single lineage of birds underwent an adaptive radiation with different lines evolving to specialize on different types of food The modern synthesis: Macroevolutionary patterns arise from microevolutionary processes Hypothesis (process): Natural selection favours birds with beaks that can exploit the available food (these will be less likely to starve) Prediction: A shift in type of food available should cause a matching shift in the beak characteristics of surviving/reproducing birds (tested within specieswhat distinguishes from macrosevolutionary) Large ground finch (Geospiza Magnirostris) Medium ground finch (Geospiza fortis) → Seed eaters → Larger G. Fortis(physically) eat larger seeds, smaller G. fortis eat smaller seeds Test (Natural Event): 1977: Drought on the island of Daphne Major → Marked and tagged all the birds → 130mm rainfall normal, 24 mm rainfall in 1977 → Drought = manipulation of X → In drought year, the seeds became large/hard and normally they’re soft/small Results: Effect on beak depth in Geospiza fortis 1976, All Daphne birds → n=751 1978, Post-drought → n=90 Most birds disappear (starve) Survival is NOT random. Birds with deeper beaks are more likely to survive The evolution of deeper beaks → off spring of survivors have a shift in bigger beak sizes, shows evolutionary distribution change Effect on beak depth in offspring of surviving Geospiza fortis Trait Before Drought (1976) Offspring of survivors (1978) Beak depth (mm) 9.21 9.70 Beak length (mm) 10.63 10.95 P < 0.05 Body size also increased (larger birds with larger beaks were more likely to survive & reproduce) 30 years of data 1984-85. ↑ • Unusually WET weather = fewer large seeds, more small soft seeds (need less seeds for metabolism compared to bigger birds) •Selection for smaller G. fortis with smaller beaks ↑beak size 1977 Drought year ↓beak size 1984-85. Wet year Fig 3.15 Evolution is DYNAMIC, and not directional Populations can change repeatedly (or change in one direction, then back again) Fig 3.15 Medium ground finch (G. fortis) Population of Large ground finch (G. magnirostris) reaches high numbers on Daphne Major (~1995)  bully, competition for medium ground finch Severe drought after island colonized by Geospiza magnirostris (large ground finch) Fig 3.15 Evolution has no ‘GOAL’ ↓beak size 2003-04. Drought years With G. magnirostris competitors •Selection for much smaller G. fortis due to combined effect of drought and interspecific competition Types of selection: → Other competitors are there and seed size Summary: • Drought = many large, hard seeds •Larger birds with deeper beaks can better exploit available food •Deeper beaks, large body size become more common in the population •Shift in conditions (wet versus dry; with or without competitors) shifts the direction of evolutionary change Conclusion: Natural selection favours beak characteristics that allow exploitation of available food type What does this ‘natural experiment’ suggest about evolution? 1. Microevolution is occurr
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