Presentation on theme: "2. You have collected and genotyped 200 individuals from a population of the fruit fly, Drosophila pseudoobscura. Your sequencing study revealed 20 individuals."— Presentation transcript:
1 2. You have collected and genotyped 200 individuals from a population of the fruit fly, Drosophila pseudoobscura. Your sequencing study revealed 20 individuals with genotype AA, 70 individuals with genotype Aa, and 110 individuals with genotype aa.Additional information:𝐺=2 𝑖=1 𝑚 𝑥 𝑖 𝑙𝑛 𝑥 𝑖 𝑥 𝑖A table of 𝜒 2 values can be found on the last page of your exam.A (20pts). What are your a priori expectations for the number of individuals with each genotype if the population is in Hardy-Weinberg equilibrium?B (20pts). Based on the genotype counts you observe, is there evidence that this population of fruit flies deviates from your a priori expectations under Hardy-Weinberg equilibrium?
3 How many species are there? All organismsJust animalsIn short, an awful lot!
4 What is a species?Species – The members of a group of populations that interbreed or potentiallyinterbreed with each other under natural conditions.
5 Speciation: some definitions Speciation – Evolution of reproductive isolation within an ancestral species,resulting in two or more descendant species.Reproductive isolating mechanisms:1. Prezygotic isolating mechanisms – Prevent or reduce the likelihood of theformation of hybrid zygotes, often through positive assortative mating.2. Postzygotic isolating mechanisms – Factors that reduce the fitness of hybridzygotes by reducing their survival or reproductive rates.
6 Prezygotic isolating mechanisms 1. Potential mates do not meetA. Temporal isolation – Mating occurs at different timesB. Habitat isolation – Mating occurs in different areas2. Potential mates meet, but do not mate (Behavioral isolation)3. Copulation occurs but no transfer of male gametes takes place (Mechanical isolation)4. Gamete transfer occurs, but egg is not fertilized (Gametic incompatibility)
7 Prezygotic isolating mechanisms: an example Three morphologically indistinguishablespeciesMales attempt to initiate a duet of lowfrequency songs produced by abdominalvibrationsMating does not occur unless the femalerespondsFemales respond much more frequently tosongs of their own speciesGreen Lacewing
8 Postzygotic isolating mechanisms Zygote dies (zygotic mortality soon after fertilization)Hybrid has reduced viability (hybrid inviability)Hybrid has reduced fertility (hybrid sterility)
11 Modes of speciationAllopatric speciation – The evolution of reproductive isolation between populations that are geographically separated.2. Parapatric speciation – The evolution of reproductive isolation between populations that are continuously distributed in space.3. Sympatric speciation – The evolution of reproductive isolation within a single randomly mating population.
12 Allopatric speciation The evolution of genetic reproductive barriers between populations that are geographically separatedAllopatric speciation can occur in two ways:Vicariant speciationPeripatric or “Founder” speciationTimeTime
13 The process of allopatric speciation: Step 1 Begin with a single species whose populations are physically connected. Gene flow substantial.
14 The process of allopatric speciation: Step 2 Next, one or more populations becomes isolated from the main population either through vicariance or through a colonization event. Gene flow stopsTime
15 The process of allopatric speciation: Step 3 Finally, genetic differences accumulate through drift or selection, resulting in divergent populations which can no longer interbreed
16 The process of allopatric speciation: an example from sticklebacks > 13,000 years agoICEThree-spined sticklebackGasterosteus aculeatusIcePacific OceanDrawing of marine sticklebacks. Credit: David Kingsley, Stanford University (based on Cuvier & Valenciennes, 1829).
17 The process of allopatric speciation: an example from sticklebacks ≈ 13,000 years agoIceThree-spined sticklebackGasterosteus aculeatusPacific OceanDrawing of marine sticklebacks. Credit: David Kingsley, Stanford University (based on Cuvier & Valenciennes, 1829).
18 The process of allopatric speciation: an example from sticklebacks Since the glaciers retreated and the rivers became impassible (≈13,000 years ago), substantial morphological evolution has occurredThis evolution is hypothesized to be the result of natural selection for life in freshwaterSticklebacks with armor stainedDrawings of sticklebacks showing armor in redCredit: David Kingsley, Stanford University (based on Cuvier & Valenciennes, 1829).
19 The process of allopatric speciation: an example from sticklebacks If females of one type are now given a choice between the two types of males, they prefer their sympatric typeThis suggests that we are observing the initial stages of allopatric speciationNo way
20 Parapatric speciation The evolution of reproductive isolation between populations that are continuously distributed in spaceTime
21 The process of parapatric speciation: Step 1 Begin with a single species whose populations are physically connected. Gene flow is substantial, although it decreases with distance
22 Deep water selects for large fish Shallow water selects for small fish The process of parapatric speciation: Step 2Spatially variable selection drives local adaptationDeep water selects for large fishShallow water selects for small fish
23 The process of parapatric speciation: Step 3 Selection favors the evolution of pre-zygotic isolating mechanisms because such mechanisms reduce the production of less fit offspring with intermediate phenotype gene flow reduced or eliminatedNo wayNo way
24 The process of parapatric speciation: an example from ‘Evolution canyon’ IsraelNorth facing slope(Cool and wet)South facing slope(Hot and dry)100m400m
25 Parapatric speciation: an example from ‘Evolution canyon’ Israel Genetic differences for adaptive traits persist onthe different sides of the canyon despite the potentialfor substantial gene flowDrosophilamelanogasterDispersal distance of Drosophila (≈ 2km)100m400mSouth facing slopeNorth facing slopeDecreased desiccation resistanceDecreased tolerance for thermal stressIncreased desiccation resistanceIncreased tolerance for thermal stress
26 Parapatric speciation: an example from ‘Evolution canyon’ Israel How are these adaptive differences maintained?Strong assortative mating existsSouth side males prefer south sidefemales and vice versaNorth side males prefer North side
27 Parapatric speciation: an example from ‘Evolution canyon’ Israel Studies of neutral loci suggest that substantial genetic differentiation existsbetween south side and north side populationsThese studies estimate that the number of migrants per generation, Nm, issomewhere betweenThis result demonstrates that genetic differentiation is occurring despite thepotential for significant gene flow between north and south side populations.Taken together, these results suggest that these flies may be in the processof speciation.
28 Sympatric speciationThe evolution of reproductive isolation within a single randomly mating populationSympatric speciation can occur in two ways:Instantaneous post-zygotic isolationAdaptive or ecological speciation2x2x2x2x2x4xTime2x2x2x4xTime
29 The process of instantaneous sympatric speciation: polyploidy 2x2xParental diploids (2x)MeiosisDiploid pollen (2x)Diploid ovule (2x)4xTetraploid offspring (4x)
30 The process of instantaneous sympatric speciation: polyploidy 2x4xHaploid pollen (1x)Diploid ovule (2x)Triploid offspring3xInfertileInviableAs a result, polyploidy generates immediate reproductive isolation and sympatric speciation
31 Heuchera grossulariifolia on The process of instantaneous sympatric speciation: an example from Heuchera grossulariifoliaAutotetraploid plants haveformed multiple timesTriploid hybrids have low fitness andlow fertilityThe two ploidies have divergedgreatly in morphology andphenologyHeuchera grossulariifolia onthe Salmon RiverSalmon River 2x4x2xSalmon River 4x# open flowersDate
32 The process of adaptive sympatric speciation: Step 1 Begin with a species composed of a single panmictic population. Individuals with similar phenotypes compete more intensely than individuals with disparate phenotypesWSize
33 The process of adaptive sympatric speciation: Step 2 Because individuals with extreme phenotypes compete less, extreme phenotypes are favored by natural selection and begin to increase in frequency within the populationTime
34 The process of adaptive sympatric speciation: Step 3 Because extreme individuals which mate with extreme individuals of the other type produce intermediate and unfit offspring, assortative mating is favored by natural selection, leading to the elimination of intermediate phenotypesNo wayNo wayTime
35 The process of adaptive sympatric speciation: an example from apple maggot fly Mmmmm… Yummy!Rhagoletis pomonella(apple maggot fly)
36 An example of host race formation Native HawthorneAppleIntroduced ≈ 1647Flies originally used native Hawthorne species as hostsPopulations (species?) of flies now use introduced appleThese plants grow sympatrically
37 Testing for host race formation Flies on apples emerge earlierApple flies prefer to mate with appleflies rather than Hawthorne flies andvice versaApple flies have higher fitness onapples than on Hawthorne andSubstantial genetic divergence existsbetween apple and Hawthorne flypopulationsAppears to be a case of sympatric speciation ‘in action’
39 How can we infer the process by which speciation occured? We have covered examples that illustrate the process of speciationDemonstrating that current species pairs formed through a particularmechanism, however, is extremely difficult and very contentiousLake 1Lake 2Lake XHow did speciation occur?
40 Some hypotheses… H1: Two sympatric speciation events H2: Speciation in allopatry followed by recolonizationH3: Speciation in allopatry followed by dispersal and extinctionLake 1Lake 2Lake XHow did speciation occur?
41 How can we infer the process by which speciation occured? Using phylogenies to infer the mechanism of speciationPossible phylogeny 1Possible phylogeny 2L1L1L2L2L1L2L1L2How did speciation occur?
42 Phylogeny 1 suggests sympatrically Lake 1Lake 2Lake XSpeciationSpeciation
43 Phylogeny 2 suggests allopatrically Step 1: Both lakes colonized, adaptation to lakes causes formation of new species in allopatryLake 1Lake 2Lake XAdaptationAdaptation
44 Phylogeny 2 suggests allopatrically Step 2: After a significant amount of time, lakes are re-colonizedLake 1Lake 2Lake X
45 Summary of SpeciationSpecies are maintained through pre-zygotic and post-zygotic isolating mechanismSpeciation can occur in sympatry, parapatry, or allopatryStudying the process of speciation is relatively easy, and we have good evidence foreach processIn contrast, inferring how existing species actually formed is very difficult andcontentious