Rates of Evolution

From the examples given, it seems that the allelic composition of a population could change rapidly, within a few generations, but that is not typically the case. Most popula­tions are relatively well adapted to their habitat, or they would not exist. Very few muta­tions produce a new phenotype so superior that it immediately outcompetes all other members of the population.

It is difficult to identify the presence of particular alleles in a population unless they result in an easily identifiable effect on the phenotype. Consequently, most studies of evolution concern the changes in gross structures such as flowers, leaves, fruits, shoots, and trichomes. But these complex structures are the product of the developmental interaction of many genes. Any new mutation results in a more adaptive structure only if the effects of the new allele fit into the already existing highly integrated mechanism of morphogenesis without causing serious disruptive effects. As systems become more intricate, the probabil­ity decreases that any random change is beneficial.

Evolutionary changes that result in the loss of a structure or metabolism can come about quickly, however, and for the same reason: complexity. If a feature becomes selec­tively disadvantageous, many of the mutations that disrupt its development become selec­tively advantageous. Because disruptive mutations outnumber constructive mutations, loss can occur relatively rapidly. For example, the ancestors of cacti lived in a habitat that became progressively drier; large thin leaves were advantageous because they carried out photosynthesis but disadvantageous because too much water was lost by transpiration (Fig.)

Mutations that disrupted formation of the lamina were advantageous, and cacti lost their leaves in perhaps as little as 10 million years, whereas the evolutionary formation of leaves in seed plants had required over 200 million years. Leaves could not be lost too quickly, however, because the plants would be left with virtually no photosynthetic surface area Mutations that caused the complete absence of leaves could not be selectively advan­tageous until other mutations had occurred that permitted the stem to remain green and photosynthetic, that prevented the early formation of an opaque bark, and that slowed the metabolism of the plant to a level compatible with reduced photosynthesis. The loss of leaves could occur only simultaneously with or after these modifications of the stem.

 FIGURE :(a) The ancestors of the cactus family were large woody trees with rather ordinary dicot leaves. The cactus genus Pereskia still contains members quite similar to the ancestors, as shown here Apparently few genes had been modified by the time Pereskia appeared. (b) This Gymnocalycium is also a cactus, but its phenotype is significantly different from the ancestral condition; apparently all critical genes involved in leaf production have mutated so much that they are nonfunctional or absent. Genes involved in stem elongation now produce short stems, and this species contains genes for succulence that were not present in the ancestral Pereskia-like species; these new genes probably are highly mutated forms of "extra" genes from a tetraploid ancestor or arose by other methods of gene duplication.