Genetics of Speciation 
What genetic changes are responsible for species differences?

How many & what kinds of loci change?
    Observed genetic differences between species may accumulate
       Before: intraspecific polymorphism
       During: what happens during cladogenesis?
       After: subsequent anagenesis

Speciation is difficult to study:
       Speciation is a lengthy process (10 6s of generations)
       Cannot be observed directly from beginning to end
       Therefore: we study successive stages in existing organisms

Protein changes can provide basis of new species
       Ex.: Mammals do not digest plant material directly:
              Bacteria break down cellulose, lysozyme breaks down bacteria.
             Ruminant Artiodactyls (cows & deer) have efficient lysozyme,
                     most other creatures don't
             Leaf-eating macaques (Macaca, Cercopithecidae)
                  have evolved an artiodactyl-like lysozyme, in parallel with cows (Stewart, 1984)
                  New mutations permit effective exploitation of new feeding niche

Speciation in the Drosophila willistoni complex (Ayala 1975)

       Protein electrophoresis can measure genetic differentiation at many protein loci

       Genetic Identity (I) = prob. that two alleles drawn from separate individuals are identical
       Genetic Distance (D) = -ln (I)   1 - I  (if D is small)
                                               ~ average number of mutations / locus
Genetic differentiation in Drosophila
Stage D I % Loci Fixed Organismal Differences
Populations 0.03 0.97 1% Geographic polymorphism
Subspecies 0.23 0.80 5% Polytypic races
Sibling Species 0.58 0.56 >30% Reproductively isolated, 
morphologically identical
Species >1.0 0.35 >62% Morphologically distinct

       Ayala suggests the following patterns:
              Critical changes for habitat & ethological shifts occur in first 5%
              Many loci become fixed during isolation (next 25%)
              Remaining differences accumulate after speciation

Is this result general? Some case studies

Speciation in Primates
       King and Wilson (1975) compared proteins of chimps & humans
          Pan and Homo show D = 0.62 :
              Genetic differences are similar to those between sibling species of Drosophila
                [one major chromosome rearrangement]
                but morphological, cultural differences place them in separate families
            Genetic & organismal evolution are "uncoupled"
                    morphological differences can't be predicted from genetic differences

       Li et al. (1987) compared DNA sequence differences among Great Apes
# substitutions / 100 base pairs = % sequence difference
Species Family Ho Pa Go Po Rh Common Name
Homo Hominidae -

Pan Pongidae 1.45 -

Gorilla Pongidae 1.51 1.57 -

Pongo Pongidae 2.98 2.94 3.04 -
Rhesus Cercopithecidae 7.51 7.55 7.39 7.10 - Old-World monkeys

       Humans, chimps, & gorillas are all about equally different (ca. 1.5%)
           Orangs are about twice as different from any of these (ca. 3.0%)
               Old-World Monkeys are more than twice as different again (ca. 7.4%)

       Genetic differences are proportional to time of separation,
         not to morphological differentiation
         Homework: calculate UPGMA tree for these data

Speciation in freshwater fishes (Avise 1987)

       Minnows (Cyprinidae) vs. sunfish (Centrarchidae) [Perciformes]
              minnows are speciose (many species / taxon)
              sunfish are species depauperate (few species / taxon)
              both families are of equivalent geological age

       If speciation is always accompanied by a large "genetic revolution"
         then a typical pair of minnow species ought on average to be more different
                     than a typical pair of sunfish species

       Data: average pairwise D between species is identical between families
            no"genetic revolution" at speciation

Studies of protein & DNA variation may not tell us much about speciation

       Species differences typically do not result from modification of structural proteins
            difference bx chimp & human is not "chimp stuff" vs "human stuff"

       Speciation may result from changes in gene regulation
              When & where are protein loci turned on & off?
                     recall 5' promoter & enhancer sequences
              How much product is produced?

    See material on Origin of Evolutionary Novelty for some alternatives

Text material © 2010 by Steven M. Carr