What Evolution Is (Science Masters Series)
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strikingly different from that of all other continents? W h y are there normally no mammals on oceanic islands? Could these seemingly capricious patterns of distribution be explained as the product of creation? Not easily. Darwin, however, showed that the present distribution of animals and plants is due to the history of their dispersal from their original points of origin. The longer that two continents were isolated from each other, the more different their biota became. Many organisms have
evolutionary lineages. However, Zuckerkandl and Pauling (1962) showed that many, perhaps most, molecules have a rather constant rate of change over time. Such molecules can serve as a molecular clock. Well-dated fossils with modern descendants provide us with a yardstick for calibrating a given molecular clock. It was by the molecular clock method that the branching point between chimpanzee and man was shown to be as recent as 5-8 million years ago, rather than 14-16 million years, as had been
conclusion that no two specimens in a collection from a single population were exactly identical. They all were as uniquely different from each other as are human individuals. And the animal and plant breeders, with whom Darwin was associated since his Cambridge student days, told him the same. They always knew which individuals in their herds they should select as the breeding stock for the next generation. Individuality made this possible. Since the terms "transmutationism" and
are protected against m a l a r i a l infections. T h i s advantage is lost when a c a r r i e r of the sickle cell g e n e m o v e s i n t o a m a l a r i a - f r e e region, like the U n i t e d States. T h e frequency of the sickle cell g e n e a m o n g the descendants of slaves is g r a d u a l l y b e i n g r e d u c e d o w i n g to the m o r t a l i t y of the h o m o z y g o u s c a r r i e r s w i t h o u t a n y r e c o m p e n s a t i n g advantage o f being h e t e r o z y g o u s .
often prescribes what response prevails. W h e n the selective advantage of a skeleton developed among the ancestors of the vertebrates and of the arthropods, the arthropod ancestors had the prerequisites for developing an external skeleton, and the vertebrate ancestors for acquiring an internal skeleton. The entire evolution of these two large groups of organisms has since been affected by this choice among their remote NATURAL SELECTION ancestors. It permitted vertebrates to develop such