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Hybrid Zones and Clines BIOL2007 - EVOLUTION IN SPACE AND TIME Reverse also possible; allopatric distributions could result from the Types of evolution: abolition of a contact zone between anagenesis (evolution within lineages vs. cladogenesis parapatrically distributed forms. (splitting of lineages) One cannot tell much about geographical mode of origin from the current distribution. Genetic divergence and speciation Speciation involves genetic divergence; usually over a long time period. Cannot usually study speciation directly; we only have access to present-day populations. But we can study Similar to microevolution vs. macroevolution. spatial variation in gene frequencies. Can cladogenesis, macroevolution, be explained by the Dispersal is spatially limited, so distant populations same principles as for the anagenetic, microevolutionary share ancestry less recently than adjacent populations ideas we have covered so far? Spatial variation therefore related to temporal variation in In the next few weeks we shall discover… gene frequencies. Today: spatial evolution across the geographic range of a single species. By studying spatial variation, we may be able to come to Subsequently: evolution of new species, or some understanding of the time course of genetic cladogenesis. divergence and speciation. Finally: higher forms of evolution, macroevolution. Many newly formed pairs of species have parapatric or Genetic divergence of populations allopatric distributions. Genetic divergence under selection can be classified Spatial differences in gene frequencies may into two major geographic modes: represent speciation in progress Parapatric distributions and hybrid zones or contact zones within species represent a first step in speciation. 1. Sympatry Many intermediates between slight genetic Populations are “sympatric” differentiation and separate species occur in parapatry. if within "cruising range", or dispersal range. The remainder of the lecture will concern parapatric distributions. Examples: "Host races" of host-specialist parasites. Genetic variation across a geographic area Any consistent change in gene frequency heritable Or blackbirds and thrushes in London gardens. phenotype, across a geographical range -- known as a cline. Clines occur because dispersal across a region is 2. Geographic limited, because the whole geographical area does not a) Parapatry Populations in form a single panmictic population. contact at edges. (Population geneticists often call dispersal migration, but Example: divergence do not mean the kind where birds return after migration in melanism of peppered moth to near their parents nest!) between Liverpool and N. Dispersal also called gene flow, though we usually mean Wales. genotype flow). b) Allopatry Populations are not in contact. Example: island populations. Geographic distributions can change: Allopatric divergence may result in parapatric distributions via secondary contact. 1 Causes of clines Theory of clines under extrinsic selection a) Clines produced by drift/migration balance At equilibrium, the width of a cline is proportional to dispersal divided by the sq. root of selection: σ w = 1.7 ← What does this mean? s 1) Width of cline should scale directly to dispersal distance; cline wider as dispersal increases Random drift on its own will not produce consistent 2) Stronger selection leads to narrower cline directional changes in gene frequency. (For details, see i.e. w ∝ f (selection) notes on population structure and gene flow and Futuyma 2005 Ch 9: 216-222, & 1998: pp 297-298, 315- So equation seems more or less sensible, though 1.7 320, but no longer be part of the course). comes out of the maths. However, locally, drift may result in a temporary Why do we want such an equation?! Provides a way monotonic change. to understand evolution of clines. b) Clines produced by selection/migration balance - Use of cline theory EXTRINSIC selection Jim Bishop (1972) studied melanism in peppered moth between North Wales and Liverpool [OVERHEAD]. Bishop obtained expected cline by computer simulation rather than by analytical theory. Used mark-release-recapture to estimate selection and 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? Selection favours different alleles in different areas; c) Clines produced by selection- migration balance dispersal limited; frequencies may diverge → cline. -- INTRINSIC selection i) Heterozygous disadvantage Extrinsic or environmental selection is imposed by the environment directly. Heterozygous disadvantage creates a kind of disruptive t If (1) environments favour different genes or selection. Equilibrium gene frequency, is unstable, + phenotypes, (2) these environments are sufficiently s t widely spaced, and (3) if migration rates are not too high selection prevents polymorphism. Two peaks in mean ⇒ selection will set up a cline in gene or phenotype fitness, known as adaptive peaks; fixation for A, and frequency. Examples? (m, s, ir). fixation for a. Measuring dispersal Heterozygous disadvantage can cause clines? If dispersal between Dispersal (or mixing) can be balanced by selection. birthplace and breeding site is Intrinsic selection like this will cause clines with shape random, equiv. to similar to those caused by extrinsic selection. "drunkards walk". Same distribution Constant of proportionality is different, but the equations as passive will be very similar. Under heterozygous disadvantage, diffusion: a two-dimensional normal distribution. σ w = 2.8 , ... where s' is average of s & t. Dispersal measure: standard deviation, σ, of the s' dispersal distribution. Again, stronger selection, s ⇒ narrower cline; greater dispersal distance, σ ⇒ broader cline. A population "neighbourhood": group of individuals who come from an area 2σ wide. 2 Moving clines But there is a big difference: Intrinsic selection does not Specialization on the second type of seed ruins depend on the outside environment. adaptation to the first, and vice versa. Depends only on "internal environment" of each Causes stresses which multiple loci cannot easily population, that is, the local gene frequency. resolve. There are three possible outcomes: Polymorphism. A single locus or "supergene" polymorphism could evolve. Speciation. In two populations of different species, easy to evolve bimodality. Compet-ition ensures ⇒ No tendency for a cline to remain stationary. bimodal pattern of habitat use. ≠ If s t, cline will move. Selection against intermediates (or hybrids) within a species causes ii) Frequency-dependent selection reproductive isolation. Speciation may result from disruptive selection (see SPECIATION in a few days). Loss of one adaptive peak. The population may evolves towards better adaptive peak. Evolution of clines Any type of intrinsic selection -- heterozygote disadvantage, frequency-dependent selection, or disruptive/epistatic selection -- can stabilize clines. For example, warning colour: rare forms non-adaptive Under Ernst Mayr's "biological species concept, species because predators learn to avoid the commoner colour are reproductively isolated. pattern. Intrinsic and even extrinsic selection across clines gives iii) Epistatic and disruptive selection a degree of "reproductive isolation": hybrids are poorly Disruptive selection; a kind of intrinsic selection caused adapted. by the environment. Selection can favour a bimodal phenotypic distribution, Knowing about spatial evolution and cline theory is or two adaptive peaks simultaneously. therefore important for understanding speciation. e.g. Darwin's finches have available large, tough seeds, and small soft seeds which are hard to get out of their pods or off grass stems. Hybrid zones: narrow zones of contact between One type of seed selects for stout, deep beaks; the other divergent forms or even species. for narrow pincer-like beaks. Hybrid zones may include few hybrids or many, and the Disruptive selection for different adaptive peaks also hybrids themselves may consist only of F1 only, or of F1, produces epistasis F2 and every kind of backcross. Bimodal or disruptive selection implies epistasis, must be due to non-additive interactions in selection between Many species and/or races are distributed in parapatry, genes. and have narrow hybrid zones between them. Sexual selection Examples: chromosomal races of mammals, warningly Also may cause epistatic/disruptive selection: coloured butterflies, sexually selected birds [manakins genes for mate choice strongly epistatic with genes for overhead] the traits being chosen. Evolutionary result of disruptive selection Bimodal phenotypic distribution virtually impossible to maintain in randomly mating population. (mendelian inheritance → phenotypes ~ normally distributed - see QUANTITATIVE GENETICS) 3 The fire-bellied/yellow-bellied toads (Bombina) Conclusions: space and time in evolution Meet in a narrow east-west hybrid zone stretching over a large part of eastern Europe. Many definitions of species, but fundamentally, species differ genetically at multiple loci. Bombina bombina Bombina variegata If two species occur together in space, this divergence is maintained; To understand their speciation, we need to know about the events that took place in a past time. Yet for most genetic studies, we only have the present; a thin film on the surface of time. Space as a clue to time, speciation © Boris I. Tomofeev; ee http://elib.cs.berkeley.edu/ Dispersal limited, so spatial
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