Natural Selection Image Map

Major Changes in Selection Forces

In the long history of Earth, major catastrophes have occurred: 

  • Volcanic eruptions so massive that the entire earth was swamped in carbon dioxide and turned into a world-wide greenhouse
  • World-wide cooling that caused much of the earth to be covered in ice
  • Earth-shattering collisions with asteroids (one of which probably wiped out the dinosaurs)

Such catastrophes must have created drastic changes in selection forces. Most species became extinct and those that survived found a world of new niches and opportunities. Mutations that at one time would hinder a species from surviving in a niche, often found a welcoming environment. New species could appear so rapidly that they might not have left a fossil record of intermediate forms. 

Climate has always been important. Dinosaurs needed a jungle-like environment. Dinosaur fossils have been found near the North Pole, suggesting that millions of years ago, the earth's axis tilted different to make the Pole warm, at least part of the year.

Few species thrive in very cold climates. The simplest successful organisms are millions of species of bacteria that occupy virtually all niches on earth, including such hostile environments as Antarctica and hot volcanic beds at the bottom of the ocean (click here for news story). 

Collection of ice samples in glaciers has revealed that certain bacteria are thriving in such a hostile environment.

Niches for Early Life Forms

All scientific evidence of science supports the view that life began in the sea. When the seas receded and bare land appeared, a new range of niches became available for evolving plants and animals alike.

The oldest land fossils are of plant species, now extinct. The presence of plants created selection opportunities for land animals, which now had something to eat. And then animals that ate other animals appeared because they now had something to eat.

But what about breathing air? Air provided a wonderful opportunity for species that could use the oxygen created by the plants. Fish were and are adapted to absorbing the 0.5% of oxygen in water. But any animal that can breathe oxygen in air has access to 20% oxygen. 

Today, some living relics of the first land species of animals have been found. These are so-called "lung fish."  They are true fish, relying on gills and life in water. But if they become stranded when their ponds and creeks dry up their little lungs can still keep them alive. Also, their pectoral fins are primitive limbs and they can squirm their way back to water. 

Scientists think that relatives of these lung fishes gave rise to the first true amphibians, like frogs and salamanders, because the bone pattern in the limbs of fossils or early lungfish show a strong resemblance to the limbs of reptiles and mammals. Individuals that happened to have better limb bones and more efficient little lungs are thought to have accumulated in their populations because they had a selective advantage of being able to thrive on land.

Amphibians were the first land animals, being able to thrive in both water and on land. Reptiles soon appeared because they could leave the sea entirely (although some reptile species were sufficiently adapted to life in the sea that they still exist today - like the turtle). 


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