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EVOLUTION in SPACE and TIME Cladogenesis species X anagenesis BIOL2007 species Y EVOLUTION IN SPACE AND TIME cladogenesis Kanchon Dasmahapatra species Z Time Phylogeny with anagenesis and cladogenesis anagenesis : evolution within lineages microevolution vs. vs. cladogenesis : splitting of lineages, speciation macroevolution Genetic divergence of populations Genetic divergence of populations GEOGRAPHIC LOCAL: sympatric divergence allopatric divergence e.g. island populations e.g. Rhagoletis hostraces parapatric divergence Note: Distributions may change! Current distribution ≠ original distribution 1 species X anagenesis Spatial differences in gene frequencies may represent speciation in progress Parapatric distributions and hybrid zones or contact zones species Y within species: a first step in speciation? rate of Many intermediates between slight genetic differentiation cladogenesis hybridisation and separate species occur in parapatry The remainder of the lecture will concern parapatric species Z distributions Time Phylogeny with anagenesis and cladogenesis Genetic variation across a geographic area Measuring dispersal A consistent change in gene frequency heritable phenotype, If dispersal between across a geographical range is known as a cline birthplace and breeding site is Clines occur because dispersal across a region is limited. random. Dispersal by individuals leads to gene flow Same distribution as passive diffusion: a two-dimensional normal distribution . Standard deviation, σσσ, of the dispersal distribution is the most useful measure of dispersal . A population "neighbourhood" : group of individuals who come from an area 2σ wide 2 1 A Dispersal rate, σ Causes of clines: selection/migration balance 0 .6 0.5 0 .5 extrinsic or environment selection 0 .4 0 .3 Gene frequency Gene σ = 0 0 .2 0 .1 0 a 0 -10 -5 0 5 10 -10 -5 0 5 10 1 A 0.5 0 .0 7 0 .0 6 0 .0 5 0 .0 4 Gene frequency Gene 0 .0 3 0 .0 2 Width of a cline a 0 .0 1 0 0 -10 -5 0 5 10 -10 -5 0 5 10 is proportional 1 A to dispersal 0.5 0 .0 3 5 divided by 0 .0 3 0 .0 2 5 selection/migration balance 0 .0 2 Gene frequency Gene 0 .0 1 5 √(selection): 0 .0 1 a 0 .0 0 5 0 0 -10 -5 0 5 10 -10 -5 0 5 10 1 σ A w=1.7 s 0.5 0 .0 0 4 5 0 .0 0 4 0 .0 0 3 5 0 .0 0 3 extrinsic or environment selection 0 .0 0 2 5 0 .0 0 2 Genefrequency 0 .0 0 1 5 σ = infintity 0 .0 0 1 a 0 .0 0 0 5 0 0 -10 -5 0 5 10 -10 -5 0 5 10 Distance Distance environment A environment a Causes of clines: Cline in peppered moth melanism Cline in sickle cell anaemia selection for carbonaria selection against carbonaria selection against typica selection for typica 3 Heterozygote disadvantage can cause clines European shrew chromosome races Two stable situations: fixed for A OR fixed for a Oxford race Different populations fixed for either A or a Hermitage Oxford Ancestral Hermitage fertile unviable fertile Cline where populations mix. Dispersal balanced by Oxford fertile fertile Ancestral fertile selection against heterozygotes. Hermitage race Under heterozygous disadvantage, 100% σ 75% w=2.83 , ... where s' is average selection s' against homozygotes. Hermitage homo 50% hybrid genotypes Frequency ancestral homo 25% stronger selection, s ⇒ narrower cline; Oxford homo 0% greater dispersal distance, σ ⇒ broader cline. 239 230 224 217 212 205 199 197 190 185 180 175 174 168 166 157 146 Distance / km Causes of clines: selection/migration balance Causes of clines: selection/migration balance Intrinsic selection Intrinsic selection FREQUENCY-DEPENDENT SELECTION Moving clines Intrinsic selection does not depend on the outside environment. ⇒ No tendency for a cline to remain stationary. If s ≠ t, cline will move. e.g. warning colour: rare forms nonadaptive because predators learn commoner colour pattern. Intrinsic selection again 4 Heliconius erato hybrid zone Causes of clines: drift/migration balance 1.0 0.8 allele 2001 yb 0.6 40 km 0.4 1982 Random drift : no consistent directional changes 0.2 Frequency of of Frequency However, locally, drift may result in a temporary 0.0 monotonic change. -100 0 100 200 300 Distance (km E of Panama City) MJ Blum 2002. Evolution 56, 1992-1998 Causes of clines: drift/migration balance Hybrid zones time Narrow zones of contact between divergent forms or even species. “Multiple narrow clines” Within hybrid zones : few hybrids or many hybrids themselves may consist only of F1 only, or of F1, F2 and every kind of backcross. Many species and/or races are distributed in parapatry, and have narrow hybrid zones between them. STAGE II of Sewall Wright’s SHIFTING BALANCE THEORY 5 Anartia fatima amathea hybrid zone Tarapoto mutliple hybrid zones N A. fatima A. amathea Tarapoto Over 20 species of Ithomiine and Heliconius butterflies show clear subspecies differences 75 km Dasmahapatra et al. 2002 Evolution The fire-bellied/yellow-bellied toads (Bombina) Meet in a narrow east-west hybrid zone stretching over a large part of eastern Europe. Bombinabombina Bombinavariegata 6 The Bombina hybrid zone Hybrid zones, then, are places where narrow clines at multiple Clines at a few major loci loci occur together. No differences in allozymes, chromosomes etc. Hybrids viable and fertile. Narrow clines due to strong selection. Hybrid zones, cline theory and speciation Conclusions Hybridising forms/species indicate speciation in progress Species differ genetically at multiple loci. Reproductive isolation incomplete. To understand their speciation, we need to know about Study genes leading to speciation. species X divergence that took place in the past. Yet for most genetic studies, we only have the present; a thin film on the surface of time. species Y Spatially separated populations may be “incipient species”. rate of hybridisation species Z Time 7 Spatial evolution and cline theory: Spatial evolution and cline theory: understanding “extrinsic” or environmental adaptation “intrinsic” selection Migration can swamp local adaptation. Intrinsic selected genes (heterozygous disadvantage, frequencydependent selection) will also evolve spatially to But wherever the cline width, w, is substantially smaller than form clines. environmental patch width, adaptation can occur in parapatry, in spite of gene flow. These clines can move. Differently selected forms can evolve in parapatry. Hybrid zones: regions where multiple narrow clines occur. Can be used to understand gene flow and selection. Reading http://www.ucl.ac.uk/taxome/kanchon/biol2007/lectures/kanchon /Hz/Hz.html FUTUYMA, DJ 2005. Evolution. Ch 6, Ch 9: 3269 FUTUYMA, DJ 1998. Evolutionary Biology. Geographic variation and clines: chapter 9 (pp. 257262). Cline theory: chapter 13 (pp. 381383). Speciation and natural selection in Hybrid zones: chapter 15 (pp. 454456, 464468). anciently asexual bdelloid rotifers Dr Tim Barraclough (Imperial College, London) 5 pm Wednesday, 25th February 2009 Medical Sciences AV Hill Lecture Theatre, UCL 8.
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