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Darwin’s Theory of Evolution

Biology 1 2009-2010

Copyright Pearson Prentice Hall

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15-1 The Puzzle of Life's Diversity

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Evolution is the process by which modern organisms have descended from ancient organisms.

A scientific theory is a well-supported testable explanation of phenomena that have occurred in the natural world.



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Voyage of the Beagle


In 1831, Darwin set sail from England aboard the H.M.S. Beagle for a voyage around the world.

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During his travels, Darwin made numerous observations and collected evidence that led him to propose a hypothesis about the way life changes over time.

That hypothesis has become the theory of evolution (aka theory of natural selection).

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Darwin's Observations


• He observed many plants and animals were well suited to the environments they inhabited.

•He was impressed by the ways in which organisms survived and produced offspring.

•Darwin was puzzled by where different species lived and did not live.

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Living Organisms and Fossils

Darwin collected the preserved remains of ancient organisms, called fossils.

Some of those fossils resembled organisms that were still alive.

Others looked completely unlike any creature he had ever seen.

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The Galápagos Islands

Darwin observed that the Galápagos Islands were close together but had very different climates.

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The Journey Home

Darwin observed that the characteristics of many animals and plants varied noticeably among the different islands of the Galápagos.

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The Journey Home

Darwin wondered if animals living on different islands had once been members of the same species.

These separate species would have evolved from an original South American ancestor species.

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An Ancient, Changing Earth

Hutton and Lyell helped scientists recognize that Earth is many millions of years old, and the processes that changed Earth in the past are the same processes that operate in the present

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An Ancient, Changing Earth

This understanding of geology influenced Darwin:

• If the Earth could change over time, life might change as well.

• It would have taken many years for life to change in the way Lyell suggested.

This would have been possible only if the Earth were extremely old.

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Lamarck's Evolution Hypotheses


Jean-Baptiste Lamarck recognized that:

• living things have changed over time.

• all species were descended from other species.

• organisms were adapted to their environments.

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Lamarck proposed that by selective use or disuse of organs, organisms acquired or lost certain traits during their lifetime. These traits could then be passed on to their offspring. Over time, this process led to change in a species.

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Lamarck's HypothesisA male fiddler crab uses its front claw to ward off predators and to attract mates.

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Lamarck's HypothesisBecause the front claw is used repeatedly, it becomes larger.

This characteristic (large claw) is passed onto its offspring.

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Evaluating Lamarck's Hypotheses

Lamarck’s hypotheses of evolution are incorrect in several ways.

Lamarck did not know:

• how traits are inherited.

• that an organism’s behavior has no effect on its heritable characteristics.

However, he paved the way for the work of later biologists.

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Publication of On the Origin of Species

In 1859, Darwin published his book, On the Origin of Species.

• proposed a mechanism for evolution called natural selection.

• presented evidence that evolution has been taking place for millions of years—and continues in all living things.

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Inherited Variation and Artificial Selection

Darwin noted that plant and animal breeders would breed only the largest hogs, the fastest horses, or the cows that produced the most milk. Darwin termed this process artificial selection.

Artificial selection is the selection by humans for breeding of useful traits from the natural variation among different organisms.

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Evolution by Natural Selection

Survival of the Fittest

The ability of an individual to survive and reproduce in its specific environment is fitness.

Darwin proposed that fitness is the result of adaptations.

An adaptation is any inherited characteristic that increases an organism's chance of survival.

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Because of its similarities to artificial selection, Darwin referred to the survival of the fittest as natural selection.

In natural selection, the traits being selected contribute to an organism's fitness in its environment.

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Over time, natural selection results in changes in the inherited characteristics of a population. These changes increase a species' fitness in its environment.

Each living species has descended, with changes, from other species over time.

Darwin referred to this principle as descent with modification.

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Evidence of Evolution


Darwin argued that living things have been evolving on Earth for millions of years. Evidence for this process include:

•the fossil record•geographical distribution•homologous structures•similarities in early development, or embryology.

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The Fossil Record

Darwin saw fossils as a record of the history of life on Earth.

By comparing fossils from older rock layers with fossils from younger layers, scientists could document that life on Earth has changed over time.

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Geographical Distribution

Similar, But Unrelated Species

and

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Homologous Body Structures

Structures that have different mature forms but develop from the same embryonic tissues are called homologous structures.

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Not all homologous structures serve important functions.

The organs of many animals are so reduced in size that they are just vestiges, or traces, of homologous organs in other species.

These organs are called vestigial organs.

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Similarities in Embryology

The early stages, or embryos, of many animals with backbones are very similar.

The same groups of embryonic cells develop in the same order and in similar patterns to produce the tissues and organs of all vertebrates.

Chick embryo (LM) Human embryo

Pharyngealpouches

Post-analtail

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