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June 15, 2003
REVIEWS: The ever-changing world
Ernst Mayr is Emeritus professor of Zoology at Harvard University. He is the recipient of numerous honorary awards and degrees and is regarded as one of the 20th century’s leading evolutionary biologist. In his book, What evolution is, he describes the basic principles of evolution and states that the book was written for three kinds of readers. First and foremost for anyone who wants to know about evolution. Second, for people who accept evolution but do not understand how it works. Finally, for “creationists” who want to know more about evolution, if for no other reason, “than to be able to argue against it”.
Mayr points out that mankind since ancient times has asked three questions. Who and what gave rise to our world? What will the future bring? How did we humans originate? Each civilization gave its own answers to these questions. When one studies these answers, they fall into three categories.
First, the world is of infinite duration, with no beginning and no end.
Secondly, it is a world without change but of a limited duration.
Third, it is a constantly changing or evolving world.
Aristotle believed that the world had always existed and will always exist. It has no beginning and no end. This idea was never very popular and is now regarded as scientifically untenable. But there is also a view that this is a constant world of limited duration. This embodies the Biblical concept of creation. According to this, all life forms were created as they are now and there is no place for change or improvement.
Some Christian theologians, on the basis of Biblical genealogy, have even calculated that the world was created in 4004 BC, that it is about 6000 years ago.
In the context of the belief that this is an ever changing or evolving world, the research conducted by geologists in early 17th century showed that the earth is of immense age and certainly more than 6000 years old! The discovery of extinct fossils also revealed that the belief in constancy and permanency of species is incorrect. There is undeniable evidence that numerous species have appeared and disappeared over a period of time due to a variety of reasons.
The importance of competition in extinction is demonstrated most graphically when a species gradually becomes extinct as a result of an alien species successfully colonizing its habitat. This clearly occurred in case of New Zealand when many indigenous species became extinct on the introduction of European species by the white man.
Quite different from the steady extinction of individual species are the so-called mass extinctions. These are due to physical causes. The most famous among them is the extinction of the dinosaurs 65 million years ago, probably due to the impact of an asteroid on earth.
Mayr states that evolution moves in two directions. One is the upward movement from ancestral to a derived condition, referred to as an agenesis and the other is the branching of the phylogenetic tree. The branching is called cladogenesis. For example, homo sapiens are a branch of a primate tree which also has many other apes. This is supported by DNA analysis, which shows closeness of apes to each other and particularly of chimpanzees to humans. In fact chimpanzees are closer to humans than to the gorillas and orangutans.
Most molecules have a rather constant rate of change over time. Such molecules can serve as a “molecular clock”. Using this method, fossils can be dated and the branching time between chimpanzees and humans has been calculated to be between 5-8 million years.
How does evolution occur? Darwin provided the answer by making a radical break from the widely held concept that each species is composed of a homogeneous population. Actually within each species there are small but significant differences which lead to the process of natural selection. In other words individuals in possession of certain attributes, best suited for that time and period, survive and propagate. The rest diminish in numbers and become extinct. To put it briefly, individual organism is the unit of natural selection, while a population is the unit of biological evolution. In biological evolution major groups get transformed, such as the emergence of birds from reptiles. The 145 million year old fossil of the Archaeopteryx shows this link between birds and reptiles. The further evidence of this link has come from fossils recently discovered in China.
Evolution is a gradual process. Species don’t appear suddenly and many take millions of years to emerge. Darwinian evolution depends on the continuous availability of variation in a given population. This variation is largely produced by recombination of the parental chromosomes by sexual reproduction.
In animals, which do not reproduce sexually, such as bacteria, the offspring’s are genetically identical to the parents. Each bacterium is, therefore, a clone of its parent. Variations in the bacteria occur as a result of mutations induced by external factors, such as ultraviolet light, and by exchange of genes between bacterial species by the process of conjugation, known as the lateral gene transfer.
Mayr rejects the idea of Lamarck,a predecessor of Darwin, that evolution is caused by environmental influence. He states that there is no evidence that genes are altered as a result of environmental factors and the only theory which has stood the test of time is that of natural selection as expounded by Darwin.
According to Mayr, evolution “is a fact so overwhelmingly established that it has become irrational to call it a theory”. This book stresses principles and does not get lost in details. It is therefore an ideal introduction for the students of biology, although they may not agree with all the ideas that Mayr puts forward. Science progresses as a result of discussion and debate and the book should be read with an open mind as it represents the point of view of an evolutionary biologist on a subject which still needs many explanations.
What evolution is
By Ernst Mayr
Phoenix. Available at Paramount Books, 152/O, Block 2, PECH Society, Karachi-75400
Tel: 021-4310030
Email:
paramount@cyber.net.pk
ISBN 0-75381-368-8
349pp. Rs545
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