Thus far we have seen that quite different evolutionary scenarios result, depending upon the location of a population within an adaptive

Morphological Trait 1

Figure 2.9. Modelling disruptive selection, part three. The effect of new genetic mutation in an adaptive landscape. Four new morphological variants arise randomly by mutation: one new mutation is upslope on the peak to the left (X), and one new mutation is upslope on the peak to the right (P). Two new variants are downslope, one down from the peak to the left (Y) and one down from the peak to the right (Q), and have the same adaptive value as the ancestral morphology B. Under the expectations of the theory of natural selection, organisms with new morphologies X and P should reproduce at a higher rate than organisms with morphological variants A and C in the parent population. The new morphologies Y and Q, and the organisms with ancestral morphology B, will not be favoured by selection. Thus with time, individuals with morphologies X and P should become more and more numerous, and individuals with ancestral morphological variants A, B and C less numerous, thus the end-points of the original population continue to move uphill and the population continues to split apart.

landscape, from the application of the same modelling rule that natural selection operates to move a population up the slope of an adaptive peak, from lower degrees of adaptation to higher degrees of adaptation. Now let us envision a situation where the strict application of this rule results in a population winding up with a less than optimum morphology through the action of natural selection!

Consider the adaptive landscape illustrated in Figure 2.11. A high adaptive peak exists with a smaller adaptive peak located on the slope of the taller peak. An evolving population has climbed the adaptive slope to the top of the smaller peak, the location of the local adaptive maximum. However, the higher adaptive position of the taller peak is

Figure 2.10. Modelling disruptive selection, part four. The vectors summarize the effect of disruptive natural selection in an adaptive landscape. Natural selection should always favour genetic mutational morphologies that possess higher degrees of adaptation, thus evolution should always proceed in upslope directions within the landscape, resulting in two vectors trails that separately climb two different adaptive peaks. The net result of disruptive selection is the splitting of an ancestral species population into two (or more) descendant species populations with different adaptive morphologies.

Figure 2.10. Modelling disruptive selection, part four. The vectors summarize the effect of disruptive natural selection in an adaptive landscape. Natural selection should always favour genetic mutational morphologies that possess higher degrees of adaptation, thus evolution should always proceed in upslope directions within the landscape, resulting in two vectors trails that separately climb two different adaptive peaks. The net result of disruptive selection is the splitting of an ancestral species population into two (or more) descendant species populations with different adaptive morphologies.

Figure 2.11. Modelling less-than-optimum evolution. A species population evolving under the influence of natural selection can only explore the local adaptive possibilities. The evolving population depicted by the vectors in the figure has climbed the local smaller adaptive peak and is now separated by an adaptive valley from the much higher adaptive peak located to the left. Stabilizing selection will now act to keep the population confined to the smaller peak, with a less-than-optimum adaptive value.

Figure 2.11. Modelling less-than-optimum evolution. A species population evolving under the influence of natural selection can only explore the local adaptive possibilities. The evolving population depicted by the vectors in the figure has climbed the local smaller adaptive peak and is now separated by an adaptive valley from the much higher adaptive peak located to the left. Stabilizing selection will now act to keep the population confined to the smaller peak, with a less-than-optimum adaptive value.

within sight, so to speak, so surely the population will continue to evolve to eventually conquer it, yes?

The answer is no. A population evolving under the influence of natural selection can only explore the local adaptive possibilities. That is, the higher peak is not 'in sight' at all to the population stuck on the local adaptive maximum, where the action of stabilizing selection will operate to keep it positioned.

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