Chapter 22 Notes: Introduction to Evolution
· Our planet is home to a huge variety of organisms! (Scientists estimate 5-20 million species of organisms alive today!)
· Even more amazing is evidence of organisms that once lived on earth, but are now extinct.
· Several hundred million species have come and gone during 4.5 billion years life is believed to have existed on earth
· So…where have they gone… why have they disappeared?
EVOLUTION: the process by which modern organisms have descended from ancient organisms.
(“CHANGE OVER TIME”)
· Central Idea: organisms alive today have been produced by a long process of change over time.
· FITNESS: refers to traits and behaviors of organisms that enable them to survive and reproduce
· COMMON DESCENT: species share common ancestors
· ADAPTATION: any inherited characteristic that enhances an organism’s ability to survive and reproduce
~based on variations randomly inherited
NOTES: LAMARK VS. DARWIN & THEORY OF NATURAL SELECTION
LAMARK’S EXPLANATION OF EVOLUTION
Assumption #1:
Law of Use and Disuse: an organism can change certain body parts during its lifetime and pass these changes on.
Assumption #2:
Inheritance of Acquired Characteristics: acquired traits can be passed on to offspring therefore, the population changes.
Criticism of this argument: Implies that an organism can sense its needs and change to meet them.
Criticism of this argument:
Suggests that changes in BODY CELLS (somatic cells) can be inherited.
2 factors that helped DARWIN formulate his explanation
1. Overproduction of offspring occurs in nature (Malthus); competition for food, space, mates
2. Artificial selection (selective breeding)- Darwin wondered if a similar force existed in nature.
Charles Darwin’s Explanation on Evolution
(six major ideas of Darwin)
SUMMARY: LAMARK: Individuals can change
DARWIN: Evolution acts on populations
HOW DO WE KNOW THAT EVOLUTION HAS OCCURRED (and is still happening!!!)???
Lines of evidence for evolution:
1. Biogeography:
*closely related species tend to be found in the same geographic region, whereas the same ecological niches in distant regions are occupied by very different (though sometimes similar-looking) species
*this is what 1st suggested evolution to Darwin
-Example: Australia is home to a group of mammals (marsupials) that are distinct from another group of mammals (eutherians) that live elsewhere on Earth
àAustralia: sugar glider is superficially similar to flying squirrel (N. America)
à sugar glider has characteristics that define it as distinctly marsupial
*These external superficial similarities are an example of CONVERGENT EVOLUTION (not homologous structures & therefore, not evidence of evolution)
2. FOSSILS:
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-fossil record supports evidence from biochemistry which places prokaryotes as ancestors to eukaryotes
-When looking at the fossil record, we see progressive changes based on the order they were buried in sedimentary rock:
*Few fossils of one species à many fossils of that species
*Simple organisms à complex organisms
- Keep in mind:
- Age of Earth (about 4.5 billion years old)
* Rates of motion of tectonic plates
* Radioactive dating
- The Earth is dynamic!
3. Applied Genetics: “Artificial Selection”
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4. Homologies:
-BIOCHEMICAL
*
*
-ANATOMICAL
* *
5. Vestigial Organs:
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Warning!!!! ANALOGOUS STRUCTURES:
-both fly, but butterflies don't have bones so support their wing whereas birds do; DNA instructions are different
Darwin’s Theory of Natural Selection- example
1. ORGANISMS HAVE MORE OFFSPRING THAN CAN SURVIVE AND…
2. RANDOM, HERITABLE VARIATION EXISTS AMONG THESE ORGANISMS (EX. MUTATIONS), SO…
3. BECAUSE THERE IS A STRUGGLE FOR EXISTENCE (COMPETITION), THEN…
4. THE ORGANISMS WITH FAVORABLE VARIATIONS (THE “FITTEST”) SURVIVE AND…
5. GENES FOR FAVORABLE FEATURES ARE PASSED TO OFFSPRING THROUGH REPRODUCTION, AND THEREFORE, THE POPULATION EVOLVES!!!
AND, if populations are geographically isolated, changes will accumulate to the point of reproductive isolationà new species
…a herd of giraffes would produce hundreds of descendents over several generations.
…by genetic “chance”, some giraffes were born with longer necks.
…giraffes were competing for food source (vegetation).
…a long neck enabled giraffes to reach an “untapped” food source…tree tops! (short-necked giraffes ran out of food and starved)
The giraffes with longer necks survive and pass their genes (for longer necks) on to their offspring…the process continues, and whole population EVOLVE