Mechanisms Of Speciation
Mechanisms of SpeciationSome species show little geographic variation. while others vary quite a bit.1
Geographic variation may be gradual.Geographic Variationin Poisonous Clovers. or abrupt.Terrestrial adults of six species of ambystomatid salamanders.What is a species?Because speciation is often a gradual process,it may be difficult to recognize boundariesbetween species.2
For All Species (Spp.) Concepts: Spp. consist of groups of “actual” or “potential” interbreeding pop’s. Spp. are a fundamental unit of evolution (bridging bothmacroevolution and microevolution). Spp. share a distinguishing characteristic, which is evolutionaryindependence. This occurs when microevolutionary forces(mutation, selection, migration & drift) operate on each spp.separately. Forms a boundary for the spread of alleles. Different spp. follow independent evolutionary trajectories.Biological Species Concept (BSC) Species are groups of actually or potentiallyinterbreeding individuals that arereproductively isolated from other suchgroups (Mayr). Used by the Endangered Species Act (forbetter or worse). What about non-overlapping pop’s, fossilrecord, & microbes?3
Difficulties with the BiologicalSpecies Concept Asexual reproductionMany geographically isolated populationsVariation in reproductive isolationHybridization and introgressionEndosymbiont-caused isolationAsexual reproductionMany geographicallyisolated populations4
Variation in reproductive isolationDegree of sexual isolation between populationsof Desmognathus ochrophaeusHybridization andIntrogressionHybrids may form if separated populations rejoin withoutsufficient genetic differences having accumulated.Hybridization andIntrogressionThe movement of a gene from one species into the gene pool ofanother by backcrossing an interspecific hybrid with one of its parents.5
Endosymbiont-causedisolationCytoplasmic incompatibility in the waspsNasonia vitripennis and N. giraulti.aka son-killerPhylogenetic Species Concept (PSC) The smallest aggregation of populations orlineages diagnosable by a uniquecombination of character states (Nixon &Wheeler). Monophyletic groups derived from a singlecommon ancestor. Not standardized, which traits are mostimportant?6
Evolutionary Species Concept (ESC) A single lineage of ancestor-descendantpopulations which maintains its identityfrom other such lineages and which has itsown evolutionary tendencies (Simpson). Required for fossil record analysis. Must be applied carefully and consistently,i.e., “Cryptic species”.10 Cryptic Species revealed in the neotropicalskipper butterfly Astraptes fulgerator.Diagnosing Species in Practice Morphology Behavior Molecular data7
What is speciation? Isolation: Physical separation between pop’s May or may not be required! Divergence: In habitat use and/or mating tactics Via Selection, Mutation, and/or Drift Completion: aka Reproductive Isolation 2 Contact via Reinforcement Hybridization eventsMajor Questions in Speciation Research What is the geographic context of speciation? What are the roles that evolutionary forces(selection, drift, gene flow) play in speciation? Are few genes or many involved in speciation? How long does speciation take? Why do some lineages speciate more rapidly thanothers?8
Under the Biological Species Concept, the keyto understanding the formation of new specieslies in understanding the evolution ofreproductive barriers.Intrinsic vs. extrinsic barriers(Rem: Mass vs. Weight example)aka Spatial isolationaka Assortative matingHybrid low viabilityHybrid infertilityHybrid zygote abnormalityPrezygotic Barrier:Temporal IsolationWestern Spotted Skunk breeds in late summerEastern Spotted Skunk breeds in late winter9
Prezygotic Barrier:Behavioral IsolationPrezygotic Barrier:Mechanical IsolationThe posterior lobe of the genital arch in males of threeclosely related species of Drosophila.Prezygotic Barrier:Gametic Isolation10
Postzygotic Barrier: Hybrid InfertilityHorseDonkeyMuleSuccessive stages in each of four models of speciationdiffering in geographic settings.(Founder effect)Allopatric speciation – Geographic isolation disruptsgene flow.Peripatric Speciation – Founder effectSympatric Speciation – No physical separation required. Polyploids especially in plants. Disruptive selection createsdivergence in ecological traits. Selection for assortative mating.Parapatric Speciation – Divergence along a gradient. Requires a hybrid zone.11
Allopatric Speciation: VicarianceThe Isthmus of Panama formed from 15 Mya to 3 Mya.Speciation in snapping shrimp across theIsthmus of Panama7 morphospecies w/o reproClosest clades show “FinalBreak” at 3 Mya(‘) cryptic speciesAllopatric speciation is common in islandarchipelagoes via Dispersal & Colonization12
Successive stages in each of four models of speciationdiffering in geographic settings.Reproductive IsolationByproduct (Mayr) vs.Reinforcement (Dobzhansky)Reinforcement – type of selection that leads to assortative matingand prezygotic isolation.Reproductive isolation revisited: Both prezygotic and postzygotic barriersincrease gradually over time for either model. Reinforcement speeds up prezygotic barriersin sympatric sister species through assortativemating.13
Pre- or postzygotic reproductive isolation betweenpairs of populations & species of DrosophilaFaster to full isolationi.e., barrier is complete( time)Level of prezygotic isolation between allopatricand sympatric pairs of Drosophila populationsFaster to full isolationi.e., barrier is complete( time)Sympatric speciation via hybridizationGoatsbeards(Tragopogon)14
Sympatric speciation via hybridizationSpokaneSunflower speciationTetraploid hybrid species(broader range than parentals)Diploid parental speciesBlack-bellied Seedcrackers (Pyrenestes) Live in marshes in W. Africa Eat seeds, primarily of two plant species One seed type is small, the other type is large Bill dimorphism reflects the effects ofdisruptive selectionLeads to Assortative Matingand Sympatric Speciation.15
Sympatric speciation in the three-spined sticklebackHybridization – fitness of hybrids determines hybrid zone and eventual outcome.SagebrushHow many genes are involved in speciation?Proportion of males with motilesperm in nonhybrid D. simulansand in backcross hybrids.X has a major effect on motility!(i.e., prezygotic barrier)16
Sequence divergence of mtDNA betweenpairs of closely related songbirdsTwo meanings of the “rate of speciation” How long does it take? Biological speciation interval Time for speciation Must grow exponentially No extinctions17
Estimates of time required for the speciation processin various groups of organismsFactors promoting rapid speciation Many speciesOpportunities for geographic isolationLimited mobilityShort generation timeSexual selectionEcological specializationEcological Specialization18
Allopatric Speciation: Vicariance The Isthmus of Panama formed from 15 Mya to 3 Mya. Speciation in snapping shrimp across the Isthmus of Panama 7 morphospecies w/o repro Closest clades show “Final Break” at 3 Mya (‘) cryptic species Allopatric speciation is common in island archipelagoes via Dispersal & Colonization
3. evolution 11 3.1 the ideas 11 3.2 level of selection 13 3.3 more than genes? 13 4. speciation 14 4.1 species concepts 16 4.2 isolation of populations 17 4.3 types of speciation 18 4.3.1 allopatric speciation 18 4.3.2 sympatric speciation 19 4.4 phylogenetic relationship 20 5. insects as models in evolution 21 5.1 plant insect interactions 21 6.
single studies of speciation are unlikely to achieve this result, we suggest that a comprehen-sive understanding of the speciation process requires demonstrating (a) axes of differentiation, (b) speciation phenotypes (i.e., traits whose divergence contributes directly or indirectly to a de-
Mechanisms of Speciation Ch. 16 Ch. 16 Ch. 15 Human Evolution Ch. 20 Ch. 20 Ch. 19 . Course Policies and Specific Expectations . Mechanisms of Speciation I Chapter 16 Q A Session/Concept Review Reading Quiz 7 Nov 17 (T) Mechanisms of Speciation II Niche Project Day Nov 19 (TH) Human Evolution Chapt
SYMPATRIC SPECIATION IN CICHLID FISHES FROM NIGARAGUAN LAKES HOW CAN REPRODUCTIVE ISOLATION DEVELOP IN THE ABSENSE OF BARRIERS TO GENE FLOW? Sympatric speciation in animals is a controversial mechanism. Host-race spe
A related process – parallel speciation – lends powerful support to any hypothesis of ecological speciation, because it demonstrates that a particular selective environment can repeat-edly generate the same phenotype during speciation. In parallel speciation, a single species invading several new places gives
these parameters in phylogenetic diversification models 17. If speciation rates are controlled by energy—perhaps owing to accelerated chemical reactions, life histories or mutation rates 18,19—then we should observe a footprint of rapid speciation in the distribution of recent speciation times for tropical taxa.
8 MAE 342 –Dynamics of Machines 15 Torfason’s Classification of Mechanisms Snap-Action Mechanisms Linear Actuators Fine Adjustments Clamping Mechanisms Locational Devices Ratchets and Escapements Indexing Mechanisms Swinging or Rocking Mechanisms Reciprocating Mechanisms Reversing Mec
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