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The World of Darwin
The Passage of Time
Genes and Circumstances
Goldfish and eagles are survivors that have been around on earth form millions of years.

During this time generations of eagles and fish have seen the earth undergo some remarkable changes. Over the course of geological history, land heaves and groans, mountains are lifted up and washed down again, Meadows become sea floors and rivers gouge huge trenches into the earth's crust. Never still, the earth and its environment are constantly changing.

What was once a friendly habitat for goldfish many slowly change and become warmer, more salty, and less comfortable. What is now a fertile valley filled with prey for an eagle may one day dry out and render food scarce for the carnivore. Surviving in a changing world is difficult if an organism cannot change along with it.

Change and change alike As circumstances change, the combinations of genes that produce the most successful phenotype also change. Evolution is the gradual adaptation to new environments by selection of gene combinations that give the greatest reproductive success.

Populations evolve
But the individual eagle and the individual fish cannot change their combination of genes. They are stuck throughout their lives with the combination of genes donated to them by their parents.

During the process of sexual reproduction, these combinations of genes are shuffled and redistributed every generation. Among many goldfish progeny, therefore, it is likely that one of more of them will have just the right combination of genes to survive in a habitat that is becoming warmer and more salty.

These offspring will survive, enjoy reproductive success, and pass on more of their genes than those individuals that are less well adapted. Subsequent generations of these fish will possess more and more genes for survival in warm salty water than did their ancestors.

Slowly, the population of fish will become more fully adapted to its new environment.

Evolutionary change comes about at the level of populations, not at the level of individuals.

Sometimes the changes cannot take place fast enough and a whole population dies out. Species become extinct when the environment changes more rapidly than the natural processes that allow them to adapt.

Many times in the history of the earth, cataclysmic alterations of either the local or global environments have been too severe for most animals and plants to survive. Mass extinctions have swept away millions of species that were very successful in their habitats only moments before.

Chance favored a few survivors who suddenly found themselves in a world full of new opportunities. Free of restraints, they moved into new habitats and radiated outward to take advantage of every new possibility.

A new species Because new combinations of genes worked better in one locality than in another, populations split, diverged and gradually became separate species. Small groups became isolated from larger populations, found themselves with only a subset of all the possible genes available, and either enjoyed reproductive success with these genes or failed and vanished.

Formation of new species of organisms is favored by rapidly changing environments and isolation of small populations into new founding groups.

Eagles and goldfish represent collections of genes that, in just the right combinations, produce successful members of two different species.

Their success is due largely to genes whose products help the to survive long enough to reproduce, and thus pass on their genes. These genes are the raw material for success and for a successful species. It is at the level of these genes and the successes they bring that the mechanism of evolutionary change begins, and marches through the dimension of time.

These fish are perfectly adapted to living in cool water.

A mutation strikes one of the fish.
The change enables this fish to live better in warmer water.

The water temperature increases.
The mutant fish, better adapted, lays more eggs, and leaves more survivors.
The population of fish in the warm water now has more of the mutant fish than the original fish.
Figure legend: Mutation and Adaptation: Random mutations enable individual fish to live in warmer water.

If the environment changes and the water increases in temperature, the fish with these new genes lay more eggs and leave more offspring.

Slowly the population of fish becomes more tolerant of the new conditions.

© 2001, Professor John Blamire