Chapter 12: Evolution

Evolution

Evolution, known as the unifying theory of biology, explains organism speciation and extinction.

Speciation:

Process by which organisms develop into distinct species
- A distinct species is defined as a population that does not - or cannot - mate with other species and produce viable offspring.

Extinction:

Fossils:

Paleontology:

George Cuvier made two important paleontological observations:

Many fossils do not resemble modern animals, indicating that species have gone extinct
Younger fossils - those higher up, closer to the surface - resemble modern organisms more closely than older fossils - lower down, further from the surface - indicating that species change over time

There were two schools of thought to explain physical changes, such as extinction, speciation and geological changes:

Catastrophism:
- Favored by George Cuvier
- Large scale changes were caused by large scale natural disasters
Uniformitarianism:
- Favored by Charles Lyell
- Change takes place gradually and consistently over long periods of time
  - Examples: erosion, continental drift

Charles Darwin was a British naturalist.
He spent 5 year travelling around the world aboard the HMS Beagle, observing and collecting samples.
After thirty years of subsequent contemplation, he published his theory of evolution.

A key observation of Darwin’s travels was the finches of the Galapagos Islands.

The Galapagos Islands are a group of numerous islands.
Darwin observed that the finches of each island had a slightly different beak.
Each beak was optimized for the food-gathering features of that particular island.
It seemed that a single species of finch migrated from the mainland to the islands, but, the finch populations on each island separately evolved traits that were optimal for their particular island.
This suggests that species evolve optimized features.

Jean-Baptiste Lamarck proposed a theory explaining the mechanism behind evolution, known as “use and disuse”:

Organisms develop traits over their lifetime in response to their environment.
These traits are then passed on to their offspring.
- For example: Lamarck believed that giraffes’ long necks was caused by giraffes stretching their necks to reach higher leaves, developing longer necks, and then passing on these longer necks to their offspring.

Lamarckism is rejected, because traits obtained during an organism’s lifespan are not passed on to their offspring.

Darwin’s theory of natural selection was inspired by observing artificial selection.

Breeders would selectively breed organisms based on desired traits.
Over time, the percentage of organisms with those traits grew, due to the selective breeding.

Unlike Lamarck’s theory, which relies on acquired traits, Darwin’s theory of natural selection is based on descent with modification.

Descent with modification:

Darwin proposed that these modifications accumulated due to natural selection, or survival of the fittest:

Populations naturally have a variation of alleles. Some alleles happen to be more fit for survival in the population’s habitat than others.
Species produce more offspring than the available resources, causing the population’s organisms to compete for resources.
Due to the environmental pressure of scarce resources, the organisms with alleles more fit for their environment are more likely to survive and produce offspring.
Over time, the frequency of fitter alleles will rise, causing a population evolution.

There are multiple evidences for evolution, demonstrating a common ancestry between organisms from which the distinct organisms evolved:

Homologous Structures:
- Distinct organisms have similar structures that are used by each for a distinct purpose. This suggests that the organisms all evolved from a common ancestor with the structure, and each subsequently evolved an alternative use.
  - Example: The human arm, cat foreleg, whale flipper and bat wing have a common bone structure, but are all used for separate purposes.
Embryology:
- Related species have similar embryological stages, from which they subsequently develop distinctly
Molecular Evidence:
- Related species have similar DNA sequences.
Biogeography:
- Species are geographically distributed, suggesting that populations locally evolve based on environmental necessities.

Further evidences of evolution include:

Analogous Structures:
- Unrelated organisms sharing similar structures, suggesting that structures evolved separately due to common necessity
  - Example: insect and bat wings
Vestigial Structures:
- Unused remnants of organs, suggesting ancestral forms of the organism that had a use for the organ, from which the current organism evolved