Ayala, Francisco J. “The Evolution of Life: An Overview.”

According to Francisco Ayala, the evolution of organisms, that is, their descent with modification from a common origin, is at the core of biology. Though evolution is universally accepted by biologists, its mechanisms are still actively investigated and debated by scientists. Darwin’s explanation was essentially correct but incomplete, awaiting the discoveries and power of genetics and molecular biology. Ayala then distinguishes between two questions: whether and how evolution happened.

Ayala briefly traces historical sources and then focuses on Darwin, who proposed natural selection to account for the adaptive organization of living creatures and their apparent purpose or design. Missing in Darwin’s work was a theory of inheritance that would account for the preservation of variations on which selection could act. Mendelian genetics eventually provided the “missing link.” In addition, Weismann’s germ-plasma theory helped counter the Lamarckian alternative to Darwin and contributed to the neo-Darwinian theory that emerged out of the nineteenth century. Further progress came from Dobzhansky in the 1930s. In 1953, Watson and Crick discovered the structure of DNA. In 1968, Kimura’s work on “molecular clocks” made possible a reconstruction of the evolutionary history of life with its many branchings. Finally, the recent techniques of DNA cloning and sequencing have provided additional knowledge about evolution.

Next Ayala discusses three related issues: the fact of evolution, the details of evolutionary history in which lineages split, and the mechanisms by which evolution occurs. The first, that organisms are related by common descent with modification, is both fundamental to evolution and heavily supported by the evidence. The second and third are mixed, with some conclusions well established and others less so. Before delving into the details, Ayala briefly comments on the mix of responses to evolution from the religious communities. It can seem incompatible to those holding to a literal interpretation of Genesis, the immortality of the soul, or humans created in the image of God. To others, God is seen as operating through intermediate, natural causes, including those involved in evolution. Here Ayala cites Pope John Paul II’s recent comments on evolutionary biology. Ayala then turns to a detailed exposition of the evidence for evolution, drawing on paleontology, comparative anatomy, biogeography, embryology, biochemistry, molecular genetics, and other fields. He focuses on the question of speciation, including models such as adaptive radiation for how reproductive isolation arises. After giving a reconstruction of evolutionary history, Ayala concludes his essay by discussing gradual and punctuated evolution, DNA and protein evolution, the molecular clock of evolution, and human evolution.

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