Chapter 22Part 2

Descent with Modification

A Darwinian View of Life

• As the 19th century dawned, it was generally believed that species had remained unchanged since their creation

• However, a few doubts about the permanence of species were beginning to arise

Concept 22.2: Descent with modification by natural selection explains the adaptations of organisms and the unity and diversity of life

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Darwin’s Research

• As a boy and into adulthood, Charles Darwin had a consuming interest in nature

• Darwin first studied medicine (unsuccessfully), and then theology at Cambridge University

• After graduating, he took an unpaid position as naturalist and companion to Captain Robert FitzRoy for a 5-year around the world voyage on the Beagle

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The Voyage of the Beagle

• During his travels on the Beagle, Darwin collected specimens of South American plants and animals

• He observed adaptations of plants and animals that inhabited many diverse environments

• Darwin was influenced by Lyell’s Principles of Geology and thought that the earth was more than 6000 years old

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Darwin

• His interest in geographic distribution of species was kindled by a stop at the Galápagos Islands near the equator west of South America

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Charles Darwin

• Father of the modern theory of evolution.

• Theory - Descent with Modification.

Darwin's Background

• Trained as a Naturalist (after trying religion and medicine).

Voyage of the Beagle

Result

• Darwin's training and travel opportunities allowed him to formulate and support his ideas on Natural Selection.

Fig. 22-5

NORTHAMERICA

EUROPE

AFRICA

AUSTRALIA

GREATBRITAIN

SOUTHAMERICA

ATLANTICOCEAN

PACIFICOCEAN Cape of

Good Hope

Tierra del FuegoCape Horn

TasmaniaNewZealand

Ande

sEquator

TheGalápagosIslands

Pinta

MarchenaGenovesa

SantiagoDaphneIslands

PinzónFernandina

IsabelaSan

Cristobal

SantaFe

SantaCruz

Florenza Española

Darwin’s Focus on Adaptation

• In reassessing his observations, Darwin perceived adaptation to the environment and the origin of new species as closely related processes

• From studies made years after Darwin’s voyage, biologists have concluded that this is indeed what happened to the Galápagos finches

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Galapagos Finches

• In 1844, Darwin wrote an essay on the origin of species and natural selection but did not introduce his theory publicly, anticipating an uproar

• In June 1858, Darwin received a manuscript from Alfred Russell Wallace, who had developed a theory of natural selection similar to Darwin’s

• Darwin quickly finished The Origin of Species and published it the next year

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Alfred Wallace - 1858

• Paper on Natural Selection identical to Darwin's ideas.

Result - July 1, 1858

• Dual presentation of the Wallace-Darwin ideas to the Linnaean Society of London.

Darwin - 1859

• Publication of "The Origin of Species”

Comment

• Darwin best remembered for the theory because of his overwhelming evidence and because he published.

The Origin of Species

• Darwin developed two main ideas:–Descent with modification explains life’s unity and diversity–Natural selection is a cause of adaptive evolution

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Descent with Modification

• Darwin never used the word evolution in the first edition of The Origin of Species

• The phrase descent with modification summarized Darwin’s perception of the unity of life

• The phrase refers to the view that all organisms are related through descent from an ancestor that lived in the remote past

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Darwinian View

• History of life is like a tree with branches over time from a common source.

• Current diversity of life is caused by the forks from common ancestors.

• In the Darwinian view, the history of life is like a tree with branches representing life’s diversity

• Darwin’s theory meshed well with the hierarchy of Linnaeus

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Example

“The Origin of Species”

• Documented the occurrence of evolution.

• Suggested that the mechanism for evolution was Natural Selection.

Artificial Selection, Natural Selection, and Adaptation

• Darwin noted that humans have modified other species by selecting and breeding individuals with desired traits, a process called artificial selection

• Darwin then described four observations of nature and from these drew two inferences

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Fig. 22-9

Kale

Kohlrabi

Brussels sprouts

Leaves

Stem

Wild mustard

Flowersand stems

Broccoli

Cauliflower

Flowerclusters

Cabbage

Terminalbud

Lateralbuds

Observations:

Observation 1 – Members of a population often vary greatly in their traits.

.

Observation 2

• Traits are inherited from parents to offspring.

Observation 3

All species are capable of producing more offspring than their environment can support.

Observation 4

• Owing to lack of food or other resources, many offspring do not survive.

Inference 1

• Individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment tend to leave more offspring than other individuals.

Inference 2

• This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations.

Nature

• Determines which characteristics are favorable.

• Determines who survives.• Result - “Natural Selection”

• Darwin was influenced by Thomas Malthus who noted the potential for human population to increase faster than food supplies and other resources

• If some heritable traits are advantageous, these will accumulate in the population, and this will increase the frequency of individuals with adaptations

• This process explains the match between organisms and their environment

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Natural Selection: A Summary

• Individuals with certain heritable characteristics survive and reproduce at a higher rate than other individuals

• Natural selection increases the adaptation of organisms to their environment over time

• If an environment changes over time, natural selection may result in adaptation to these new conditions and may give rise to new species

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Natural Selection in action

Fig. 22-12b

(b) A stick mantid in Africa

• Note that individuals do not evolve; populations evolve over time• Natural selection can only

increase or decrease heritable traits in a population• Adaptations vary with different

environmentsCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Artificial Selection

• When man determines the characteristics that survive and reproduce.

• Result - the various breeds of animals and plants we’ve developed.

Example - Mustard Plant

Original Cultivars (califlower, brussel sprout, broccoli)

Example - dogs

Evolution Success Measured By

• Survival • Reproduction• Whoever lives long enough and

has kids is the “winner” in evolution.

Requirements

• In order for Natural Selection to work, you must have:–Inheritable Variations within a population.–Long periods of time (according to Darwin).

Comment

• Acquired characteristics may allow a species to evolve "outside" of Natural Selection.

• Ex: culture, learning

Evidences for Evolution

• Direct observation of evolutionary changes.

• Fossils• Homology• Convergent Evolution• Biogeography• Molecular

Discussion

• Why is it that individual organisms cannot be said to evolve?

• Though an individual may become modified during its lifetime through interactions with its environment, this does not represent evolution. Evolution can be measured only as a change in proportions of heritable variations from generation to generation.

How does Darwin’s theory of evolution relate to:

• Overproduction of populations?

• Limited resources?

• Heritable variation?

• Species have the potential to produce more offspring than survive (over-reproduction), leading to a struggle for resources, which are limited. Populations exhibit a range of heritable variations, some of which confer advantages to their bearers that make them more likely to leave more offspring than less well-suited individuals. Over time this natural selection can result in a greater proportion of favorable traits in a population (evolutionary adaptation).

Next time….

Quiz

and

How Evolution is supported by an overwhelming amount of

scientific evidence