Apomixis

Sterile hybrids also may reproduce asexually. One form of asexual propagation, called apomixis, includes the development and production of seeds without fertilization. Asexual propagation also includes other forms of vegetative reproduction that are discussed in Chapters 4 through 7, Chapter 14, and Appendix 4.

When species reproduce mainly by apomixis but sometimes also hybridize so that new combinations of genes are occasionally produced, they can be highly successful in

Plant Human Hybrids

Figure 15.11 A common dandelion. In addition to reproducing by ordinary vegetative and sexual means, dandelions reproduce apomictically. Apomixis is a form of sexual reproduction through which seeds are produced without fertilization.

Evolution 283

Because environment is a driver of natural selection, ecology is central to understanding this key evolutionary process. Darwin's theory of natural selection proposed that limiting factors in the environment produce differences in the rate of reproduction of individuals within populations. He reasoned that those individuals best adapted to the environment have the best chance for survival and reproduction and that the characteristics of these better-adapted individuals will increase in frequency within the population. Speciation, the origination of new species, is often influenced by ecological factors, and closely related species, derived from a common ancestor, often have different environmental requirements. In addition, biological diversity, a central concern of ecology, depends ultimately on a balance between rates of speciation and extinction.

Figure 15.11 A common dandelion. In addition to reproducing by ordinary vegetative and sexual means, dandelions reproduce apomictically. Apomixis is a form of sexual reproduction through which seeds are produced without fertilization.

nature. Dandelions and wild blackberries, for example, are among the most successful plants known. They reproduce apomictically, as well as sexually through fertilization, and also by natural vegetative propagation (Fig. 15.11).

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