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When Is a Flipper a Wing? All living things are related. Some relationships are easy to see— your pet cat may not roar like a lion, but it clearly resembles one. Other relationships are less obvious. Section 15-3. Interest Grabber.

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interest grabber
When Is a Flipper a Wing?

All living things are related. Some relationships are easy to see—your pet cat may not roar like a lion, but it clearly resembles one.Other relationships are less obvious.

Section 15-3

Interest Grabber
interest grabber continued
1. On a sheet of paper, construct a table that has five columns and six rows. In the columns, write the following heads: Animal Group, Example, Legs, Fins, and Tail. Then, place the following animal groups in their own row: Mammal, Bird, Fish, Amphibian, Reptile, and Insect.

2. Give one example for each group, and then fill in the information for that example. For Legs, write in the number of legs that each animal has. Do animals with fins have legs? Do animals with wings have legs? If so, how many?

3. Can you tell from your table if a fish is more closely related to a bird or to an amphibian? Explain your answer.

Interest Grabber continued
darwin s publication

After Darwin’s journeys he wrote and published a book called On the Origin of Species

  • In this book he proposed his thoughts on how evolution happened called natural selection
Darwin’s Publication
objective summarize darwin s theory of evolution by natural selection
Influences on Darwin

Thomas Malthus—(Essay on the Principle of Population, 1798)

plants and animals produce more offspring than can survive.

human overpopulation would lead to decreasing living standards if left unchecked.

Charles Lyell (1797-1875)

Assumed natural forces in past same as today (erosion, sedimentation, etc.)

Uniformitarianism (idea originally proposed by James Hutton but popularized by Lyell)

"In October 1838, that is, fifteen months after I had begun my systematic inquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long- continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The results of this would be the formation of a new species. Here, then I had at last got a theory by which to work".Charles Darwin, from his autobiography. (1876)

Objective: Summarize Darwin’s theory of evolution by natural selection

"I am well aware that this doctrine of natural selection . . . is open to the same objections which were at first urged against Sir Charles Lyell’s noble views on ‘the modern changes of the earth, as illustrative of geology;’ but we now very seldom hear the action, for instance, of the coast waves, called a trifling and insignificant cause, when applied to the excavation of gigantic valleys or the formation of the longest lines of inland cliffs. Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being; and as modern geology has almost banished such views as the excavation of a great valley by a single diluvial wave, so will natural selection, if it be a true principle, banish the belief of the continued creation of new organic beings, or of any great and sudden modification in their structure.” Charles Darwin, from the Origin of Species, 1859

evolution by natural selection

Darwin proposed that as populations grew that there would be a struggle for existence

  • A key factor in the above would be survival of the fittest
    • Successful adaptions enable organisms to be better suited for environment
    • Those adaptations are then passed on to next generations
    • Those that are adapted best for their environment will have more offspring and those that are not will either die or have few offspring
Evolution by Natural Selection
Natural selection

Some members of a species have characteristics that enable them to survive, produce more offspring than others, and pass these characteristics on.

(Species—a group of similar organisms capable of reproducing with one another).

Fitness—the relative ability of an organism to survive and transmit its genes to the next generation.

Adaptation—a characteristic that increases the chances an organism will survive and reproduce in its environment.

3 Types:




Note: It is species that evolve, not individuals.

Gene pool—the entire collection of genes among a population of organisms.

Population genetics—the study of gene pools and their changes.

natural selection

Populations tend to be constant

Naturally occurring variations—small differences between members of a species


If more offspring are born, yet population is constant, then there is competition for food, water, light, etc.

Some variations are helpful and others aren’t. Natural selection eliminates those that aren’t.

Good variations are adaptations

Natural Selection
  • If variations are selected, then organisms may change—given enough time this may result in a new species.
causes of variation
Recombination of genes during mating.

Genotype—the genetic makeup of an individual.

Phenotype—the visible characteristics of an organism





Chiasma (site of crossing-over)

What is crossing over?

Causes of Variation
descent with modification

Darwin proposed that over long periods of time organisms developed different structures to adapt to different environments

  • As a result species today look different from their ancestors
  • Common Descent
    • All species-living and extinct-were derived from same ancestors
Descent with Modification
the fossil record
The Fossil Record


geographic distribution of living species

Section 15-3




Beaver andMuskrat



Coypu andCapybara





Geographic Distributionof Living Species


observations experiments that support natural selection
Intermediate fossil forms

Archaeopteryx indicates that birds probably evolved from reptiles.

Changing populations

Artificial selection (domestication)

Antibiotic resistant bacteria

Anol Lizards in the Bahamas, Etc.

Homologous structures—2 organisms w/parts that have similar organization that is functionally unnecessary (meaning the similarity is unnecessary but the trait in question may be, and usually is, functional).

Indicates a common ancestor

May or may not have similar function today

Whale fin & our forelimb

Observations/Experiments that Support Natural Selection
Analogous structures (for contrast with homologous structures—analogous structures are not evidence for common descent)—2 organisms w/parts that have similar function, but different internal structure.

Did not come from a recent common ancestor

Whale vs. shark fin

Insect vs. bird wing

Vestigial structures—parts that are no longer useful.


Snakes with nerves to their “legs”

Birds have genes that code for a reptilian mouth and tail.

5 main assumptions of darwin
Overpopulation—Tendency to have more offspring than can survive

Variations—within a species there are variations—hair, color, height, etc.

Inheritance of variations

Not all survive. Struggle for existence. Competition for resources.

Survival of the fittest, the best adapted.

5 Main Assumptions of Darwin
homologous body structures

Section 15-3





Homologous Body Structures

Typical primitive fish

concept map

The fossil record

Geographic distribution of living species

Homologous body structures

Similaritiesin early development

Physical remains of organisms

Common ancestral species

Similar genes

Similar genes

Evidence of Evolution


which is composed of

which indicates

which implies

which implies

Concept Map