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Ch. 15 Sec. 3. Darwin Presents His Case. I. Publication of On the Origin of Species (1859) Mechanism for evolution called natural selection Presented evidence of evolution 1. Been taking place for millions of years 2. Continues in all living things.

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ch 15 sec 3

Ch. 15 Sec. 3

Darwin Presents His Case

I. Publication of On the Origin of Species (1859)
  • Mechanism for evolution called natural selection
  • Presented evidence of evolution

1. Been taking place for millions of years

2. Continues in all living things

II. Inherited Variation and Artificial Selection
  • Members of each species vary
  • Darwin argued that this genetic variation mattered

1. Noted selective breeding

2. Artificial selection


Fig. 15-10 Page 379

In artificial selection, 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.


A. Struggle for Existence 

  • Compete for resources

a. High birth rates

b. Shortage of basic needs

i. Food

ii. Living space

2. Central to Darwin's theory of evolution

III. Evolution by Natural Selection

B. Survival of the Fittest 

1. Fitness - the ability of an individual to survive and reproduce in its specific environment

  • Result of adaptations
  • Adaptation - any inherited characteristic that increases an organism's chance of survival
2. Natural selection

a. Traits being selected contribute to an organism's fitness

b. Happens without human control

Over time, natural selection results in changes in the inherited characteristics of a population. These changes increase a species' fitness in its environment
C. Descent With Modification 
  • Natural selection

a. Organisms with different structures

b. Establish different niches

c. Occupy different habitats

2. Common descent – all living organisms are related to one another

Natural selection can create new species. These Hawaiian honeycreepers all evolved from a single ancestor, which arrived on the islands long ago. With no other birds for competition, the honeycreepers began to feed on different foods. Over many generations, their bills changed to cope with their new diets.


IV. Evidence of Evolution

Darwin argued that living things have been evolving on Earth for millions of years. Evidence for this process could be found in the fossil record, the geographical distribution of living species, homologous structures of living organisms, and similarities in early development, or embryology.


Fig. 15-13 page 382

Darwin argued that the fossil record provided evidence that living things have been evolving on Earth for millions of years.

A. The Fossil Record 

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


Fig. 15-14 page 383 The existence of similar but unrelated species was a puzzle to Darwin. Later, he realized that similar animals in different locations were the product of different lines of evolutionary descent. 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.

B. Geographic Distribution of Living Species


C. Homologous Body Structures

    • Have different mature forms
    • develop from the same embryonic tissues

Fig 15-15 page 384 The limbs of these four modern vertebrates are homologous structures. They provide evidence of a common ancestor whose bones may have resembled those of the ancient fish shown here. Notice that the same colors are used to show related structures.

Homologous structures are one type of evidence for the evolution of living things.


3. Vestigial organs - traces of homologous organs in other species

  • The appendix is a vestige of the cecum, an organ that would have been used to digest cellulose by humans‘ herbivorous ancestors
  • Coccyx, or tailbone (a remnant of a lost tail)
  • Plica semilunaris on the inside corner of the eye (a remnant of the nictitating membrane)
D. 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.
V. Summary of Darwin's Theory
  • Individual organisms differ, and some of this variation is heritable.
  • Organisms produce more offspring than can survive, and many that do survive do not reproduce.
  • Because more organisms are produced than can survive, they compete for limited resources.
Individuals best suited to their environment survive and reproduce most successfully.
  • These organisms pass their heritable traits to their offspring. Other individuals die or leave fewer offspring.
  • This process of natural selection causes species to change over time.
Species alive today are descended with modification from ancestral species that lived in the distant past.
  • This process, by which diverse species evolved from common ancestors, unites all organisms on Earth into a single tree of life.
VI. Evolutionary Theory
  • Scientific advances in many fields of biology, geology, and physics have confirmed and expanded most of Darwin’s hypotheses.
  • Evolutionary theory continues to change as new data are gathered and new ways of thinking arise.
15 3 darwin presents his case

15–3 Darwin Presents His Case

Darwin was hesitant to publish his ideas because they were so

extreme. When he learned that scientist Alfred Russel Wallace

had 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.

Darwin’s theory was based on artificial selection. In artificial

selection, nature provided the variation, and humans selected

those variations that they found useful. For example, animal

breeders used only the largest hogs, fastest horses, or cows that

produced the most milk for breeding.

Darwin thought that a similar process occurs in nature. He

called this natural selection.


Natural selection can be summed upas follows.• Individuals differ, and some of the differences can be passed on to their offspring.• More offspring are produced than can survive andreproduce.• There is competition for limited resources, or a struggle for existence.• Individuals best suited to their environment survive andreproduce most successfully. In other words, there issurvival 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.


Darwin argued that species alive today descended withmodification from species of the past. Darwin also introduced theprinciple of common descent. This principle holds that all speciescome from common ancestors. The principle of common descentlinks all organisms on Earth into a single tree of life.Darwin argued that living things have been evolving onEarth for millions of years. He presented four types of evidence in support of evolution.• The fossil record. Comparing fossils from older andyounger rock layers provides evidence that evolution hastaken place.• Geographic distribution of living species. The presenceof similar but unrelated organisms in similar environmentssuggests the action of natural selection.


• 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 rolesin descendants. If the structures are greatly reduced in size,they are called vestigial organs. For example, the appendixin humans is a vestigial organ. It carries out no functionin digestion.• Similarities in early development. The early stages, orembryos, of many animals are very similar. These similaritiesare evidence that the animals share common ancestors.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.