BIOL2007 – EVOLUTION IN SPACE AND TIME TUTORIAL ESSAYS Friday 7 March SPAIN FI
Pullenza, Patricia, Features Reporter has reference to this Academic Journal, PHwiki organized this Journal BIOL2007 – EVOLUTION IN SPACE AND TIME TUTORIAL ESSAYS Friday 7 March SPAIN FIELD COURSE ESSAYS! Weds 19 March. See web. Types of evolution: anagenesis (evolution within lineages, phyletic ev.) vs. cladogenesis (splitting of lineages, speciation) Similar distinction: microevolution vs. macroevolution. Question: Can cladogenesis, speciation, macroevolution, be explained by the same principles as as long as anagenesis, microevolution In the next few weeks we shall investigate Today: spatial evolution across the geographic range of a single species. Then: evolution of new species, or cladogenesis. Finally: higher as long as ms of evolution, macroevolution. Genetic divergence of populations Genetic divergence under selection can be classified into two major geographic modes: 1. Local Populations are in sympatry if within “cruising range”, or dispersal range. Examples: “Host races” of host-specialist parasites. Or blackbirds in addition to thrushes in London gardens.
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2. Geographic a) Parapatry Populations in contact at edges Example: divergence in melanism of peppered moth between Liverpool in addition to N. Wales b) Allopatry Populations are not in contact Example: isl in addition to populations Note: Distributions may change! Current distribution original distribution Genetic divergence in addition to speciation Speciation often involves genetic divergence over periods 100,000 10 million years. So not easy to study speciation directly, experimentally. But clue to evolution in time: we can study spatial variation in gene frequencies. Measuring dispersal If dispersal between birthplace in addition to breeding site is r in addition to om, equiv. to “drunkards walk”. Same distribution as passive diffusion: a two-dimensional normal distribution. St in addition to ard deviation, , of the dispersal distribution is the most useful measure of dispersal. A population “neighbourhood”: group of individuals who come from an area 2 wide
Dispersal is spatially limited (say 1-10 m in plants, 100m-sev. km in flying animals) distant populations share ancestry less recently than adjacent populations. Spatial variation is there as long as e related to temporal variation in gene frequencies By studying genetic variation in space, we may be able to underst in addition to the time course of genetic divergence, in addition to hence, speciation. Spatial differences in gene frequencies may represent speciation in progress Parapatric distributions in addition to hybrid zones or contact zones within species: a first step in speciation Many intermediates between slight genetic differentiation in addition to separate species occur in parapatry The remainder of the lecture will concern parapatric distributions Genetic variation across a geographic area A consistent change in gene frequency heritable phenotype, across a geographical range is known as a cline Clines occur because dispersal across a region is limited, because the whole geographical area does not as long as m a single panmictic population Population geneticists often call dispersal migration, but do not mean the kind where birds return after migration to near their parents nest! Dispersal by individuals leads to gene flow (though we usually mean genotype flow)
Causes of clines a) Clines produced by drift/migration balance R in addition to om drift : no consistent directional changes However, locally, drift may result in a temporary monotonic change. b) Extrinsic or environmental selection: imposed by the environment directly. If environments favour different genes or phenotypes, in addition to (2) these environments are sufficiently widely spaced, in addition to (3) if migration rates are not too high selection will set up a cline in gene or phenotype frequency. Examples (melanism, sickle-cell, insecticide resistance). Clines produced by selection/migration balance – EXTRINSIC selection Selection favours different alleles in different areas; dispersal limited; frequencies may diverge cline. At equilibrium, the width of a cline is proportional to dispersal divided by (selection):
( ) What does this mean 1) Width of cline should scale directly to dispersal distance; ie w ; cline wider as dispersal increases 2) Stronger selection leads to narrower cline i.e. w 1 / f (selection) So equation more or less sensible, though 1.7 comes out of the maths. Why do we want such an equation! Provides a way to underst in addition to evolution of clines. Use of cline theory Jim Bishop (1972) studied melanism in peppered moth between North Wales in addition to Liverpool Bishop obtained expected cline by computer simulation rather than by analytical theory. Used mark-release-recapture to estimate selection in addition to dispersal along the transect. Compared actual cline in melanism with predicted cline. Melanics reached further into rural N. Wales than expected. Due to selection on caterpillars
c) Clines produced by selection- migration balance INTRINSIC selection i) Heterozygous disadvantage Heterozygous disadvantage creates a kind of disruptive selection. Equilibrium gene frequency, is unstable, selection prevents polymorphism. Two peaks in mean fitness, known as adaptive peaks; fixation as long as A, in addition to fixation as long as a. a A Heterozygous disadvantage can cause clines Dispersal (or mixing) can be balanced by selection. Intrinsic selection like this will cause clines with shape similar to those caused by extrinsic selection. Constant of proportionality is different, but equations similar. Under heterozygous disadvantage, , where s’ is average selection against homozygotes. Again, stronger selection, s narrower cline; greater dispersal distance, broader cline. Moving clines But there is a big difference. Intrinsic selection does not depend on the outside environment. Depends only on “internal environment” of each population, that is, the local gene frequency. No tendency as long as a cline to remain stationary. If s t, cline will move.
ii) Frequency-dependent selection e.g. warning colour: rare as long as ms non-adaptive because predators learn commoner colour pattern. Intrinsic selection again MJ Blum 2002. Evolution 56, 1992-1998 iii) Epistatic in addition to disruptive selection Disruptive selection; a kind of intrinsic selection caused by the environment Selection can favour a bimodal phenotypic distribution, or two adaptive peaks simultaneously e.g. Darwin’s finches have available large, tough seeds, in addition to small soft seeds which are hard to get out of their pods or off grass stems Large seeds select as long as stout, deep beaks; small seeds as long as narrow pincer-like beaks
Evolutionary result of disruptive selection Bimodal phenotypic distribution virtually impossible to maintain in r in addition to omly mating population. Causes stresses which multiple loci cannot easily resolve. There are three possible outcomes: Polymorphism. A single locus or “supergene” polymorphism could evolve. Speciation. Selection against intermediates (or hybrids) within a species causes reproductive isolation. (see SPECIATION in a few days). Loss of one adaptive peak. The population evolves towards better adaptive peak. Hybrid zones Narrow zones of contact between divergent as long as ms or even species. Multiple narrow clines Hybrid zones : few hybrids or many hybrids themselves may consist only of F1 only, or of F1, F2 in addition to every kind of backcross. Many species in addition to /or races are distributed in parapatry, in addition to have narrow hybrid zones between them. Examples: chromosomal races of mammals warningly coloured butterflies sexually selected birds
The fire-bellied/yellow-bellied toads (Bombina) Meet in a narrow east-west hybrid zone stretching over a large part of eastern Europe. Bombina bombina Bombina variegata
The Bombina hybrid zone Hybrid zones, then, are places where narrow clines at multiple loci occur together. The use of gametic or linkage disequilibrium to measure selection in addition to gene flow in hybrid zones A useful equation, but only gives ratio of gene flow to selection. To solve, we could find some other way. Barton, used linkage disequilibrium. In Bombina, R (~ D/Dmax) = 0.22, So = 0.99 km / gen w = 6.05 km wide, so s = 0.21 a, b A, B D = pAB – pApB Conclusions: space in addition to time in evolution Species differ genetically at multiple loci. If two species occur together in space (sympatry), this divergence does not break down by definition; To underst in addition to their speciation, we need to know about divergence that took place in the past. Yet as long as most genetic studies, we only have the present; a thin film on the surface of time.
FURTHER READING FUTUYMA, DJ 2005. Evolution. Ch 6, Ch 9: 326-9 FUTUYMA, DJ 1998. Evolutionary Biology. Geographic variation in addition to clines: chapter 9 (pp. 257-262). Cline theory: chapter 13 (pp. 381-383). Hybrid zones: chapter 15 (pp. 454-456, 464-468).
Pullenza, Patricia Features Reporter
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