Evolution and Natural Selection

An analogy, inspired by Rettie (1950), maydevelop a proper feeling of perspective.

Let us compress the Earth's history into thescope of a normal calendar year of 365 days.To do this, imagine a picture of our planettaken once each year, and these pictures runas frames in a motion picture projector at therate of 144 frames per second, six times theusual speed. Each second that our imaginarymovie is shown, 144 years of the Earth'shistory flashes by. To show the entire 4.7billion- year history, we must keep theprojector running continuously, 24 hours a day,from midnight on New Year's Eve until the nextNew Year's Eve.

Throughout J anuary, February, March,and much of April, the history of Earth isunrecorded, and our film is blank. On April12, for a few seconds only, one corner ofthe screen is visible, and shows us apicture of the oldest known rocks.

On April 20, life first appears, but only for afew seconds and only in a small corner of thescreen. May, J une, J uly, and August go by, andthe film is nearly as blank as before, with onlyfleeting glimpses here and there of bleak anddesolate scenery, still containing no multicellularorganisms.

The picture improves during September andOctober, but only after November 16, representingthe beginning of the Cambrian period some 600 millionyears ago, are we watching anything even approachinga continuously visible motion picture.

The Devonian period, or "Age of Fishes”,begins on December 1. The "Age of Reptiles," orMesozoic era begins on the morning of December14th and for 12 1/2 days the dinosaurs dominatethe scene. The extinction of the dinosaursushers in the Age of Mammals on December 26at about 11 P.M.

The 3.75 million- year old genus Homo, towhich we belong, first appears at 4:45 on NewYear's Eve, and the species Homo sapiens atabout 23 minutes to midnight.

The entire history of human civilization,since the start of agriculture, is shown onthe screen in less than a minute!

Jean-Baptiste Lamarck• 1744-1829

• French biologist known for his idea that acquired traits are inheritable.

Alfred Russell Wallace• a contemporary of Darwin• born 1823 in England• died 1913• 1848-53 Rio Amazonas & Rio

Negro• 1854-62 in Malay

Archipelago documenting >125,000 species (>1,000 new sp.)

• independently discovered a theory of natural selection

• read Thomas Malthus• toured US & Canada 1886-87

Charles Darwin• Charles Darwin 1809-1882

• H.M.S. Beagle 1831-1836

• convinced two colleagues to present his and Wallace's work at Linnaean Society meeting.

• “On the Origin of Species by Means of Natural Selection , or the Preservation of Favored Races in the Struggle For Life” published in 1859

Voyage of HMS Beagle

Robert Fitzroy

• Captain of the HMS Beagle

• surveyor

The voyage of the Beagle

Major Influences

• geology• experiences in the Galapagos Islands• Thomas Malthus (English Economist)-

wrote an essay on principles of population

Darwin’s Observations and Inferences

Observation 1: Left unchecked, the number of organisms of each species will increase exponentially, generation to generation.

Observation 2: In nature, populations tend to remain stable in size.

Observation 3: Environmental resources are limited.

Inference I

Production of more individuals than can be supported by the environment leads to a struggle for existence among individuals, with only a fraction of offspring surviving in each generation.

more observations• Observation 4: Individuals of a

population vary extensively in their characteristics with no two individuals being exactly alike.

• Observation 5: Much of this variation between individuals is heritable.

Inference IISurvival in the struggle for existence

is not random, but depends in part on the heritable characteristics of individuals. Individuals who inherit characteristics most fit for their environment are likely to leave more offspring than less fit individuals.

Inference III

The unequal ability of individuals to survive and reproduce leads to a gradual change in a population, with favorable characteristics accumulating over generations (natural selection).

Linking Evolution and Genetics

A major problem in Darwin’s theory was the lack of a mechanism to explain natural selection.How could favorable variations be transmitted to later generations?

With the rediscovery of Mendel’s work and its vast extension in the first half of the 20th century, the missing link in evolutionary theory was forged.

Evolution

"descent with modification"; change in genetically determined characteristics of populations.

Natural Selection

The sustained differential contribution of offspring by different genotypes.

“sustained differential”

eliminates the random element; natural selection is deterministic.

“contribution of offspring” emphasis on differential

reproduction rather than survival.

“survival of the fittest” -vs- “fitness”

“different genotypes”

the true unit that is selected for or against.

Natural Selection Sequence

1. Variation occurs within populations.

2. Sexual reproduction is the fundamental source of variation.

3. Variation must be heritable or genetically based.

4. Organisms produce more offspring than can possibly survive and resources are limiting, therefore competition among individuals within the population occurs.

5. Individuals show differential mortality and differential reproductive success.

6. Superior survivors/reproducers increase their genotypic frequency within the population.

7. Populations become adapted to the environment.

8. The environment is constantly changing.

Stabilizing Selection

• extreme phenotypes are selected against

Directional Selection

one extreme phenotype or the other is selected against.

Disruptive Selection

average phenotype is selected against.

example: two colored morphs (brown & green) in a single population of insects.

Biological Species Concept• A species is a population of

organisms that actively or potentially interbreed, producing viable offspring and which remain reproductively isolated from other such populations.

Isolating Mechanisms

geographic isolation

temporal isolation

behavioral isolation

mechanical isolation

Geographic Isolation

Galapagos Islands

Evidences for Natural Selection• fossil record• biogeography• comparative anatomy• comparative embryology• comparative biochemistry• comparative behavior• comparative physiology

Fossil Record

Homologous Structures

Comparative Embryology

Convergent Evolution