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. Darwins 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 Darwins 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,
Weismanns 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, Kimuras 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 IIs 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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