Species

322 Species

model. In the second case (B), they allowed stochastic extinction, and the surpris-

ing result here is that the clustering, while tighter, still spreads out evenly. In the

third case (C), however, they allowed logistic diversification, equivalent to “niche

packing” according to carrying capacity, and now we see the scatter of clusters that

we do find in genotypic space in asexual lineages. A fourth case (D) allowed devel-

opmental entrenchment, in which the longer the trait (in our case, gene sequence) has

been in the lineage, the less likely it is to go extinct, and the clustering is even more

diverse and tightly packed. Since A and B are stochastic, while the other two cases

answer to selection for niche occupancy and internal cohesion respectively, there is

little help to be garnered from simple stochasticity, and we must consider the other

two cases separately.

However, as this is only a simulation, other realistic factors not included here,

such as differential extinction due to habitat fracturing, may still form clusters in a

stochastic manner. There can be, and must certainly often be, other reasons why the

carpet is not smooth but patchy. Extinction can cause there to be patches in genome

space. Some varieties die out. While this can be because of selection, often it will

be due to plain old genetic drift, and contingency. A lineage of asexual genomes can

stochastically drift due to random biases in mutational direction, for instance, while

earlier forms can go extinct simply because of random drift or random termination

of that clone. Moreover, if a genome evolves in a habitat that is sensitive to sudden

Snapshots of morphospace 1 Separation Clustering

11

1

11

1

1

0.5 0.5

0.5

0.5

0.5 0.5

0.5

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0 0

0

0

(^0000)
(^000)
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05
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35
510
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A
C
B
D

Figure 13.3 Pie and Weitz’ simulation of speciation.

four assumptions. A. Speciation without extinction. B. Speciation and stochastic extinction.
C. Speciation, extinction, and logistic diversification. D. A−C plus developmental entrenchment.
The column “Separation” indicates the mean pairwise distance between taxa (solid line) and
the mean pairwise distance squared (dotted line). The column “Clustering” indicates cluster
size distribution over time. Note that under developmental entrenchment, mean separation
decreases after initial increase.