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Figure 5.2 Allopatric speciation models, occurring either symmetrically (a), where the parent species is divided into two roughly equal halves by a geographic barrier, or asymmetrically (b), where a small peripheral population is isolated by a barrier. In the first case, two new species may arise; in the second, the parent species may continue unaltered, and the peripheral population may evolve rapidly into a new species.

(a)

Figure 5.3 Two models of speciation and lineage evolution. (a) Phyletic gradualism, where evolution takes place in the lineages, and speciation is a side effect of that evolution. (b) Punctuated equilibrium, where most evolution is associated with speciation events, and lineages show little evolution (stasis).

2 In the punctuated equilibrium model (Fig. 5.3b), with rectangular branches, almost no evolution takes place within species lineages (they show stasis), and evolution is concentrated in the speciation events that coincide with major sideways shifts.

These two models of evolution seem so distinctive, both in the shape of phylogenies, and in their interpretation, that it should be possible to test between them by observations from the fossil record.

Testing punctuated equilibrium: problems

Eldredge and Gould (1972) argued that many test cases of the pattern of evolution at the species level could be studied from the fossil record. These should have the following features:

1 Abundant specimens.

2 Fossils with living representatives, so that species can be identified clearly.

3 Information on geographic variation, so that rapid speciation events (punctuations) could be distinguished from migrations in or out of the area.

4 Good stratigraphic control, in terms of long continuous sequences of rocks

Sedimentary Level of lake sequence

Morphology of molluskan species tuff beds

Suregai tuff—

50 m fall

r ^rr snalis

Morphology of molluskan species r ^rr

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