CHAPTER 22DESCENT WITH

MODIFICATION: A DARWINIAN VIEW OF LIFE

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EVOLUTION

Evolution is defined as change over time in the genetic composition of a population (not an individual)Most attention given to Charles Darwin’s On the Origin of Species by Means of Natural Selection (November 24, 1859)

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CONCEPT 22.1: Pre-Darwinian Views

A. Creationism (Scala Naturae)Aristotle’s philosophyAristotle opposed any concept of evolution Viewed species as fixed and unchangingAll living forms could be arranged on a ladder of increasing complexity with each perfect, permanent species having its own rungStrict belief in the Book of Genesis and creation

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B. Natural TheologyViewed the adaptations of organisms as evidence that the Creator had designed each species for a purpose

Major follower—Carolus Linnaeus (1707-1778)--developed hierarchy of taxonomic categories based morphology and anatomy--did not believe in evolution--taxonomic system became the focal point of Darwin’s arguments for evolution

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C. CatastrophismGeorges Cuvier, French anatomist and paleontologist and father of paleontology (study of fossils)Based on fossils found in sedimentary rock strataIdea that the boundaries between strata were due to local catastrophic events (fires, floods, volcano eruptions, droughts) that destroyed the species then present

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Cuvier proposed the theory of catastrophism to reconcile fossil evidence and his anti-evolutionary backgroundProposed that catastrophes were localized and regions were repopulated by species immigrating from unaffected areasOpposed idea of gradual evolutionary change

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D. GradualismJames Hutton, geologistChange is cumulative product of slow continuous natural processes identical to those currently operatingCompetitive with Cuvier’s theory

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E. UniformitarianismCharles LyellExpansion on gradualismGeological processes had not changed throughout Earth’s history

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F. Darwin was strongly influenced by Gradualism and Uniformitarianism and reasoned that:

Earth must be older than 6000 years.Slow and subtle processes can act on living organisms producing substantial change over a long period of time.

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G. LamarckismJean Baptiste Lamarck (1744-1829)Mechanism:1. Use and Disuse

--body organs used become stronger/larger--those not used deteriorated

2. Inheritance of Acquired Characteristics --changes acquired during a lifetime are

passed on to offspring

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Lamarck

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II. Concept 22.2: Charles DarwinA. The Voyage of the Beagle

Sailed from England in December of 1831 to chart South American coastlineDarwin Noted:

1. Geographical distribution of species2. On the Galapagos Islands he collected

several different kinds of finches that were similar but seem to be different species

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Voyage of the Beagle (December 1831-October 1836)

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B. The Book1. Darwin perceived the origin of new species and

adaptation as closely related processes2. Formulated theory by early 1840’s3. 1844 wrote essay on origin of species and

natural selection but did not publish4. June 1858 received Alfred Wallace’s manuscript

on evolution and natural selection

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Wallace

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5. Both papers (Darwin and Wallace) presented by Lyell to Linnaean Society of London on July 1, 1858

6. Darwin published Origin of the Species in 18597. Two main ideas of the book:

a. Species evolved from ancestral species; not specially created

b. Mechanism for evolutionary change—natural selection

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C. Principle of Common Descent1. In the 1st edition, he used term “descent with

modification” instead of evolution.2. Modifications occur as organisms moved into

new habitats or as habitats changed3. Viewed history of life as a tree with most

branches evolutionary dead ends4. Common ancestors at each fork

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Phylogenetic Tree

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Logic of Darwin’s Theory of Natural Selection1. All species have potential fertility to show

exponential growth in a population if all reproduced successfully.

2. Populations tend to remain stable in size, except for seasonal fluctuations.

3. Resources are limited.4. Individuals of population vary extensively.5. Much of this variation is heritable.

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Three conclusions:

1. Production of more organisms than can survive leads to a struggle for survival.

2. Survival isn’t random, but depends on heredity.3. Unequal ability to survive and reproduce will

lead to gradual change in a population with favorable characteristics accumulating over generations.

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D. Natural Selection 1. Natural selection is the differential success in reproduction that results from the interaction between individuals that vary in heritable traits and their environment.2. Thomas Robert Malthus’s Essay on the Principles of Population (1798) outlined a “struggle for existence” and heavily influenced Darwin’s views on “over reproduction.”3. Natural selection is made possible by:

a. Variationb. Over reproduction

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4. Important Facts About Natural Selection:A population (group of interbreeding individuals of the same species in the same geographic area) is the smallest unit that can evolve.Natural selection can amplify or diminish only heritable variations.Natural selection is always operating, but which traits are favored depends on the environment.

5. Example of Natural Selection: antibiotic resistance in bacteria

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III. Concept 22.3: Scientific Evidence to Support Evolution 1. Artificial Selection

Striking differences can occur in a very short time if one selects which organisms reproduce.

2. BiogeographyGeographical distribution of speciesPangeaEndemic species

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3. Fossil RecordMissing links gradually being filled inShows chronological appearance of

vertebrates (fish, amphibians, reptiles, birds, mammals)

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Artificial Selection

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4. TaxonomyLinnaeus’ taxonomic scheme reflected the genealogy of the tree of life

5. Comparative Anatomya. Anatomical similarities between species in same

taxonomic group show evidence of common descent

b. Based on homologous and vestigial structures

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homologous structures-represent variations on a structural theme that was present in their common ancestor

-Ex: skeleton elements of mammalian forelimbsvestigial organs-remnants of structures that served important functions in the organism’s ancestors

-Ex: pelvic and leg bones in snakes; human appendix

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Homologous Structures

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convergent evolution—refers to the development of structures which have a similar function but develop differently and appear unrelatedLeads to analogous structures meaning they have similar functions but not common ancestryEx: sugar glider and flying squirrel

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6. Comparative EmbryologyDefined as the comparison of early stages of animal developmentClosely related organisms go through similar stagesEx: all vertebrate embryos have pharyngeal pouches and a tail at some time“Ontogeny recapitulates phylogeny.” Embryonic development (ontogeny) is a replay of the evolutionary history of the species (phylogeny).

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Comparative Embryology

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Evolutionary tree

Evolutionary trees are diagrams that reflect evolutionary relationships among groups of organisms

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7. Molecular BiologyHereditary background is reflected in genes and protein products.Since the genetic code is shared by all organisms, it is likely that all species descended from a common ancestorCloser the relationship, the higher the percentage of common DNA.

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Molecular Biology

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You should now be able to:

1. Describe the contributions to evolutionary theory made by Linnaeus, Cuvier, Lyell, Lamarck, Malthus, and Wallace

2. Describe Lamarck’s theories, and explain why they have been rejected

3. Explain what Darwin meant by “descent with modification”

4. List and explain Darwin’s four observations and two inferences

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5. Explain why an individual organism cannot evolve6. Describe at least four lines of evidence for

evolution by natural selection

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