Role of Extinction

In the last several decades, scientists have questioned the role of extinction in evolution. Of the millions of species that have existed on this planet, more than 99 percent are extinct. Historically, biologists regarded extinction as a natural outcome of competition between newly evolved, adaptively superior species and their older, more primitive ancestors. Recently, however, paleontologists have discovered that many different, unrelated species living in large ecosystems tend to become extinct at nearly the same time. The cause is always some sort of climate change or catastrophic event that produces conditions too severe for most organisms to endure. Moreover, new species evolve after the wave of extinction removes many of the species that previously occupied a region for millions of years. Thus extinction does not result from evolution, but actually causes it.

Scientists have identified several instances of mass extinction, when species apparently died out on a huge scale. The greatest of these episodes occurred during the end of the Permian Period, some 245 million years ago. At that time, according to estimates, more than 95 percent of species – nearly all life on the planet – died out. Another extensively studied extinction took place at the boundary of the Cretaceous Period and the Tertiary Period, roughly 65 million years ago, when the dinosaurs disappeared. In all, more than 20 global mass extinctions have been identified. Some scientists theorize that such events may even be cyclical, occurring at regular intervals.

In the view of many scientists, mass extinctions can be explained by changes in climate – episodes of global warming or cooling that destroy sensitive ecosystems, such as tropical or marine habitats. Other theories have centered on abrupt changes in the levels of the world's oceans, for example, or on the effect of changing salinity on early sea life. Another theory blames catastrophic events for mass extinction. Strong evidence, for example, supports the theory that a meteorite some 10 km in diameter struck the Earth 65 million years ago. The dust cloud from the collision, according to this impact theory, shrouded the Earth for months, blocking the sunlight that plants need to survive. Without plants to eat, the dinosaurs and many other species of land animals were wiped out.

Extinction as a cause of evolution rather than the result of it is perhaps best demonstrated in terms of our own ancestors – ancient mammals. During the time of the dinosaurs, mammals constituted only a small percentage of the animals that roamed the planet. The demise of dinosaurs provided an opportunity for mammals to expand their numbers and ultimately to become the dominant land animal. Without the catastrophe that took place 65 million years ago, mammals may have remained in the shadow of the dinosaurs.

Exercises

1. Continue making the complex plan of the text "Developing of Evolutionary Theory".

2. Find evidence for or against the statements:

1. Modern synthesis appeared due to investigation into many sciences.

2. According to Stebbins plants display evolutionary patterns distinct from those of animals.

3. Comparing DNA and proteins from different organisms opened up entirely new aproach to study evolution.

4. Populations varied poorly at the molecular level.

5. Mutation is responsible for the variation at the molecular level.

6. Sociobiology is rather a recent science.

7. The altruistic behaviour evolved by natural selection.

8. The theory of punctuated equilibria challenges the central idea of evolution by gradual change.

9. When conditions are severe the equilibrium state becomes "punctuated" by one or more speciation events.

10. Extinction is a natural outcome of competition between newly evolved, adaptively superior species and their older, more primitive ancestors.

11. Changes in climate, catastrophic events can explain mass extinction.