Alternative way to view the shifting balance theory: consider a surface with troughs and pits in it. Put several marbles on the surface. If marble is near pit it falls in selection ~ gravity . Shake surface and balls will roll up out of pits against gravity and make their way to new pit. Shaking ~ drift. See figures 8.8-8.11, pgs. 215-219. Before discussing the shifting balance view of evolution we considered selection as if it were acting on a single locus. This is a gross oversimplification because many loci are linked along the chromosome. Who's to say that selection is acting the same way on both loci? Things get much more interesting (but more complicated) when we face the reality of linked loci. Consider the cross between the two two-locus genotypes: The offspring can be AB/AB, AB/ab or ab/ab. Other two locus genotypes are possible: or . But these can only be produced in the cross if there is recombination between the two loci. We can thus refer to four two-locus gametes AB and ab are the
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