The book, written in colloquial English, focuses on two controversial and apparently contradictory (in the light of natural selection) phenomena of biological evolution: sexual selection and altruism.
Darwinism solved the problem of "design without a designer": variation and selection alone can shape the animal world as it is, although variation is undirected and there is no selector for selection. Implicit in darwinism was the idea that evolution is due to replicators rather than organisms, that the subject of its theory is hereditary units. Natural selection is about the differential survival of replicators. Genes can be replicators whereas organisms, groups and other levels of the hierarchy cannot. Organisms are but vehicles of replicators. Genes are perpetuated insofar as they yield phenotypes that have selective advantages over competing phenotypes. Organism-centered darwinism is but an approximation of gene-centered darwinism.
Genes can also have phenotypic effects that extend beyond the bodies that house them: they can affect an "extended phenotype" (e.g., a bird's nest or a spider's web, parasites, symbiosis, etc). Pleiotropy (the phenotypic side effects) may sometime be caused by adaptation of the extended phenotype (a parasited organism may exhibit an "unintended" behavior which is in reality part of the parasite's adaptative process).
Cronin's "gene selectionism" argues that genes rather than organisms (as Darwin held) are primary units of natural selection and shows how this view can solve two notorious problems: sexual selection as displayed by the peacock and altruism as illustrated by the ant.
Cronin reviews Darwin's and Wallace's debate on the function of sexual selection. Darwin's "good taste" theory (purely aesthetic justification) could explain the extravagance of male ornament but not female choice; Wallace's "good sense" theory (search for optimal male) could explain female choice but not male ornament. Fisher proposed a compromise, by proving that good taste is good sense: choosing an attractive male is adaptive for a female because she will have attractive offsprings (success breeds success).
From the point of view of a gene, any organism carrying it is an equivalent reproductive source. In many cases siblings are more closely related (genetically speaking) that parents and offsprings. Adaptation is for the good of the replicator. Therefore, it is not surprising that sometimes organisms sacrifice themselves for improving their kin's survival. Kin selection is part of a gene reproduction strategy.
Darwin did not solve the problem of speciation (the origin of species), i.e. how a species can split into two species. Cronin briefly discusses Darwin's and Wallace's positions and her own conjectures.
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