Darwin’s Theory of Change Over Time

Evolution•Evolution, or change over time, is the process by which modern organisms have descended from ancient organisms.•The theory of evolution can explain Earth’s biodiversity.•A scientific theory is an explanation of natural events that is supported by evidence and can be tested with new evidence.

Voyage of the Beagle•Charles Darwin: The individual who contributed more to our understanding of evolution than anyone.•H.M.S. Beagle set sail from England in 1831 for a voyage around the world.•During Darwin’s travels, he made numerous observations and collected evidence that led him to propose a revolutionary hypothesis about the way life changes over time.

Section 15-1 Darwin’s Voyage

Darwin’s Observations•Darwin observed many organisms. He saw that many plants and animals were very well suited to their environment.•Darwin collected fossils, or the preserves of ancient organisms.•Some of the fossils were unlike any creatures he had ever seen.•Darwin wondered why the species in the fossils had disappeared.

Darwin’s Observations•Darwin’s observation on the Galapagos Islands (group of small islands west of South America) influenced him the most. •The islands are near one another but have different climates.•Darwin saw that the characteristics of many animals and plants varied noticeably among the different islands.•He wondered whether animals on different islands had once belonged to the same species.•According to this hypothesis, these separate species would have evolved from an original ancestor species after becoming isolated from one another.

Galapagos Islands

Pinta IslandIntermediate shell

Pinta

Isabela IslandDome-shaped shell

Hood IslandSaddle-backed shellHoodFloreana

Santa Fe

Santa Cruz

James

Marchena

Fernandina

Isabela

Tower

Giant Tortoises of the Galápagos Islands

Ideas That Shaped Darwin’s Thinking

•In Darwin’s day, most Europeans thought that Earth and all of its life forms had existed for only a few thousand years. •They also thought that species did not change.•Some scientists of Darwin’s time began challenging these ideas. •These scientists influenced the development of Darwin’s theory of evolution.

An Ancient, Changing Earth•James Hutton and Charles Lyell helped scientists recognize that Earth is many million years old, and that it changed over time.• These ideas helped Darwin realize that life might change as well.•Knowing that Earth was very old convinced Darwin that there had been enough time for life to evolve.

Sea level

Sedimentary rocks form in horizontal layers.

When part of Earth’s crust is compressed, a bend in a rock forms, tilting the rock layers.

As the surface erodes due to water, wind, waves, or glaciers, the older rock surface is exposed.

New sediment is then deposited above the exposed older rock surface.

Movement of Earth’s CrustSea level

Geological Change

Lamarck’s Evolution Hypothesis

•Jean-Baptiste Lamarck was on of the first scientist to see that evolution occurred.•He also recognized that organisms adapt to their environment.•Lamarck proposed that by selective use or disuse of organs, organisms acquired or lost certain traits during their life time. These traits could then be passed on to their offspring.•Over time, this process led to change in species.•Scientists now know that some of Lamarck’s ideas were wrong. (can’t control what traits are passed on to the next generation) However, his general ideas about evolution and adaptation are correct.

Lamarck’s Theory of Evolution

Population Growth•Economist Thomas Malthus also influenced Darwin.•Malthus thought that if the human population kept growing, sooner or later there would be insufficient living space and food for everyone.•Darwin thought this was true for all organisms.

Darwin Presents His Case•Darwin was hesitant to publish his ideas because they were so extreme.•When he learned that scientists Alfred Russell Wallace has the same ideas, Darwin published On the Origin of Species in 1859.•In the book, Darwin supplied evidence that evolution has occurred. He also explained his ideas about how evolution occurs.

Inherited Variation and Artificial Selection

•Darwin’s theory was based on artificial selection. In artificial selection, nature provided the variation, and humans selected those variations that they found useful. •Example, animal breeders used only the largest hogs, fastest horses, or cows that produced the most milk for breeding.

Artificial Selection

Evolution by Natural Selection•Darwin thought that a similar process occurs in nature. He called this natural selection. This process can be summed up as follows.

– Individuals differ, and some of the differences can be passed on to their offspring.

– More offspring are produced than can survive and reproduce.

– There is competition for limited resources, or a struggle for existence.

Natural Selection

Evolution by Natural Selection Continued– Individuals best suited to their environment survive and

reproduce most successfully. In other words, there is survival of the fittest.

• Fitness is the ability to survive and reproduce in a given environment. It results from adaptations.

• Adaptations are inherited traits that increase an organism’s chance of survival. Only the fittest organisms pass on their traits. Because of this, a species changes over time.

Evolution by Natural Selection Continued

•Darwin argued that species alive today descended with modification from species of the past.•Darwin also introduced the principle of common descent.

Evidence of Evolution•Darwin argued that living things have been evolving on Earth for millions of years. •He presented four types of evidence in support of evolution.

– The fossil record: Comparing fossils from older and younger rock layers provides evidence that evolution has taken place.

– Geographic distribution of living species: The presence of similar but unrelated organisms in similar environments suggests the action of natural selection.

Fossil Record

Beaver

NORTH AMERICA

Muskrat

Capybara SOUTH AMERICA

Coypu

Geographic Distribution of Living Species

Beaver

Muskrat

Beaver andMuskrat

Coypu

Capybara

Coypu andCapybara

Evidence of Evolution Continued

– Homologous structures of living organisms: Homologous structures have different mature forms but develop from the same embryonic tissues. They provide strong evidence that organisms have descended, with modifications, from common ancestors.

• Some homologous structures no longer serve major roles in descendants. If the structures are greatly reduced in size, they are called vestigial organs. For example, the appendix in humans is a vestigial organ. It caries out no function in digestion.

Evidence of Evolution Continued

– Similarities in early development: The early stages, or embryos, of many animals are very similar. These similarities are evidence that the animals share common ancestors.

Concept Map

includes

Evidence of Evolution

Physical remains of organisms

Common ancestral species Similar genes Similar genes

which is composed of which indicates which implies which implies

The fossil recordGeographic

distribution of living species

Homologous body structures

Similaritiesin early

development

Strengths and Weaknesses of Evolutionary Theory

•Scientific advances have upheld most of Darwin’s hypotheses. However, evolutionary theory continues to change as new data are gathered and new ways of thinking arise.