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differences expressed among individuals within populations. Holsinger points out that selfers have fewer genotypes within populations, but greater genetic diversity among populations, than sexual outcrossers. Selfers and asexuals may thereby be less able to respond adaptively to changing environments; they also accumulate deletion mutations more rapidly. Sexual outcrossers suffer from a cost of outcrossing and may be impacted by circumstances that handicap the union of gametes produced by different individuals. These costs of outcrossing and reduced reproductive assurance lead to an over-representation of selfers and asexuals in newly-formed progeny, which may displace sexual outcrossers unless these enjoy compensating advantages in survival and reproduction.

The damage wrought by invasive species costs $122 billion per year in the United States. Successful plant and animal invasions impact ecologically and demographically the endemic flora and fauna and may have considerable evolutionary import. Norman C. Ellstrand and Kristina A. Schierenbeck (“Hybridization as a Stimulus for the Evolution of Invasiveness in Plants?” Chapter 16) note that invasions typically involve long lag eriods before they become successful, and require multiple introductions. Their explanation is that hybridization between the invaders and resident populations is a stimulus often required for successful invasion. Hybrid progenies may enjoy genetic advantages over their progenitors. Ellstrand and Schierenbeck show that, as predicted by their model, invasiveness can evolve.

Stebbins devoted two chapters (nearly 100 pages) to polyploidy. Pamela S. Solits and Douglas E. Soltis (“The Role of Genetic and Genomic Attributes in the Success of Polyploids,” Chapter 17) set forth the genetic attributes that account for the great success of polyploid plants: about 50% of all angiosperm species and nearly 95% of all ferns. Polyploids maintain higher levels of genetic variation and heterozygosity, and exhibit lesser inbreeding depression, than diploids. These may be the case because most polyploid species have arisen more than once, from genetically different diploid parents, in addition to the presence of more than two homologues. Genome rearrangement seems to be a common attribute of polyploids; and many plant species may be ancient polyploids (see the case of maize in ref. 24). Soltis and Soltis conclude that the advances of the last 50 years notwithstanding, there remains much to be known about polyploid plant species, including their general mode of formation.

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