Evidence for Evolution and the Mechanisms

Evidence: 15-3Mechanism: 16-3

People have been selecting domesticated plants and animals for thousands of years. These activities have amounted to large, long-term, practical experiments that clearly demonstrate that species can change dramatically through selective breeding.

Humans select from among the naturally occurring genetic variations in a species. From a single ancestral plant, breeders selecting for enlarged flower buds, leaf buds, leaves, or stems have produced all these plants.

Artificial Selection (pg. 379)

• In artificial selection, nature provided the variation and humans selected those variations that they found useful.

The various Ensatina salamanders of the Pacific coast all descended from a common ancestral population. As the species spread southward from Oregon and Washington, subpopulations adapted to their local environments on either side of the San Joaquin Valley. From one population to the next, in a circular pattern, these salamanders are still able to interbreed successfully. However, where the circle closes -- in the black zone on the map in Southern California -- the salamanders no longer interbreed successfully. The variation within a single species has produced differences as large as those between two separate species.

Credits: Illustration by Randy Schmieder. Reprinted from Life on the Edge: A Guide To California's Endangered Natural Resources by Carl G. Thelander. Copyright © 1994 by Ten Speed Press, Berkeley, CA.

Natural Selection (page 381)

• “Survival of the fittest”• Why is this good? They survive because of their

good traits, reproduce and pass on those good traits to their offspring so that they too can survive!

• Natural selection results in changes in the inherited characteristics of a population. These changes increase a species fitness in its environment so it can survive and reproduce.

Here, the beaver and the capybara are similar species that inhabit similar environments of North America and South America. The South American coypu also shares many characteristics with the North American muskrat.

The existence of similar but unrelated species

Geographic Distribution (page 383)

• Organisms descend from a common ancestor with modification (change).

• This is how animals may be similar in different geographical areas. They move to new areas and adapt for that specific habitat.

Frogs, birds, rabbits and lizards all have different forelimbs, reflecting their different lifestyles. But those different forelimbs all share the same set of bones - the humerus, the radius, and the ulna. These are the same bones seen in fossils of the extinct transitional animal, Eusthenopteron, which demonstrates their common ancestry.

Homologous Structures (page 384)

• Organisms descended from common ancestors have similar genes. This accounts for why different organisms have similar structures, like bones, in the previous example.

These three animals are skinks, a type of lizard. In some species of skinks, legs have become useless. They are so reduced that they no longer function in walking. In humans, the appendix is an example of a useless organ because it carries out no function in digestion.

Vestigial Structures (page 384)

• Structures that organisms inherited from ancestors but no longer have a purpose.

• They are no longer used because the organism lives in an environment where the adaptation is not needed.

Mechanisms of Evolution

Chapter 16 section 3

The Isthmus of Panama only arose some 3 million years ago. Nancy Knowlton of the Smithsonian Tropical Research Institute has been studying how this geological phenomenon produced a speciation event: Populations of snapping shrimp divided by the Isthmus have diverged into separate species.Credits: Courtesy of The Smithsonian Institution/Carl Hansen and Nancy Knowlton

Geographical Isolation (page 405)

• When a population of organisms becomes divided by geography (river, mountain, isthmus, etc.) this is called Geographical Isolation.

• This allows these organisms to adapt to their specific environments.

• After a period of time, they will have become so different (different species) that they can no longer interbreed.

John Endler of the University of California has conducted experiments with the guppies of Trinidad that clearly show selection at work. The scenario: Female guppies prefer colorful males for mating purposes. Predatory fish also "prefer" colorful males, but for a less complimentary purpose — a source of food that is easy to spot. Some portions of the streams where guppies live have fewer predators than others and in these locations the males are more colorful (top frame). Not surprisingly, males in locations where there are more predators tend to be less colorful (bottom frame).

When Dr. Endler transferred predatory fish to the regions with brightly colored male guppies, selection acted rapidly to produce a population of duller males.

Behavioral Isolation (page 404)

• Occurs when two populations are capable of interbreeding have differences in courtship rituals or other reproductive strategies that involve behavior.

Temporal Isolation (page 404)

• When two or more species reproduce at different times.

• If organisms cannot breed together then different gene pools will form forcing different species to emerge.