Evolution is the foundation for all of biology. Evolution refers to the change in a species over time. It explains the great diversity and hereditary relationships between all organisms that have ever existed. The father of modern evolutionary thought, Charles Darwin, proposed the theory of natural selection. This theory is based upon the premise that naturally occurring variations exist within a population and that changing environmental conditions select for those individuals that are best suited for the changed environment.

Here are the five points used by Darwin that are most important in your understanding of natural selection.

1. Organisms produce more offspring than can survive.
2. Because of this overproduction, some of the offspring will die before they can reproduce.
3. Among the offspring there exist variations which affect the individual's characteristics.
4. Nature selects for those characteristics that will allow the individual to be most successful.
5. Because traits are heritable, those organisms that are successful will live to pass on their advantageous traits to the next generation.

Molecules and Cells

• Chemical Evolution

The primordial Earth was a massive sphere of volcanic eruptions, solar energy, and mixed gases - but was oxygen-deficient. Chemical evolution is the process that gave rise to the earliest forms of life from the original conditions. Various combinations of these early gases formed the monomers of today's macromolecules. The passing of time yielded the macromolecules themselves (proteins, lipids, carbohydrates, and nucleic acids.) These macromolecules joined to form precells capable of metabolism, growth, and reproduction. Heterotrophic prokaryotic cells developed from these precells. Once the available nutrients began to dwindle, nature selected for those organisms that could provide their own nutrients - autotrophs. A vast majority of these autrotrophic organisms are now known as cyanobacteria. With the advent of these photosynthetic organisms, the atmosphere became enriched with oxygen. This step eventually led to the evolution of aerobic eukaryotic cells.

• The Membrane Invagination Hypothesis and the Endosymbiont Hypothesis

These two hypotheses attempt to explain the evolution of eukaryotic cells from prokaryotic cells.

The membrane invagination hypothesis assumes that the prokaryotic cell membrane folded in on itself so that a double membrane was formed. This double membrane is common in the organelles of eukaryotic cells.

The endosymbiont hypothesis holds that smaller prokaryotic cells embedded themselves into larger prokaryotic cells, forming a dependent relationship. The small prokaryotic cells became the organelles of the newly formed eukaryotic cells.

• C₄ and CAM Pathways

Both of these pathways are alternate forms of CO₂ fixation that plants have evolved for use in hot, dry environments. C₄ plants use the CO₂ more efficiently than regular plants - C₃ plants. CAM (Crassulacean Acid Metabolism) plants only open their stomates at night and can hold onto the CO₂ for later fixation.

Heredity and Evolution

• Evidence for Evolution

1. Artificial selection
2. Biodiversity
3. Biogeography
4. Fossils
5. Homologies
• Comparative anatomy
• Comparative cell physiology
• Comparative embryology
• Comparative biochemistry and molecular biology
6. Vestiges
7. Convergent evolution

• Pathways of Evolution

1. Divergent evolution
2. Convergent evolution
3. Coevolution
4. Adaptive radiation
5. Parallel evolution
6. Extinction

• Mutation

Mutation provides the variation necessary for natural selection to act upon. This results in speciation, extinction, and evolution.

• The Move onto Land

PLANTS

Advantages of living in water

1. Water provides support
2. Light, O₂, H₂, and C are available in open water (but are not as plentiful as on land)
3. Fertilization made easy - water as a transport medium
4. No need for vascular tissues

Disadvantages of living in water

1. Varying light intensities depending on depth
2. Nutrients run out
3. Nutrients are richest in estuaries - aren't many areas with these conditions
4. O₂ harder to extract from water than from air; less O₂ in water than in air

Advantages of living on land

1. Abundant light, CO₂, O₂
2. Available space - less competition

Disadvantages of living on land

1. Need support
2. New methods of fertilization and embryo development necessary - without water
3. Water loss is a concern
4. Temperature extremes

Adaptations to life on land

1. Protective waxy cuticle (not in some mosses)
2. Stomates (not in liverworts)
3. Multicellular reproductive organs (gametangia)

archegonia - for developing egg

antheridia - for maturation of pollen

4. Zygote stays in female gametangia for protection
5. Lignin in cell walls for support
6. Vascular system in tracheophytes
7. Seed for protection of embryo, including dormancy period
8. Pollen grains protect male gametes in waterproof casing
9. Extensive root sytems for water absorption (in higher plants)
10. CAM and C₄ plants
11. Fruits protect seeds and aid dispersal

ANIMALS

Advantages of living in water

1. Support - bouyancy
2. No internal fertilization necessary (not in aquatic or marine reptiles or mammals)
3. Some organisms are sessile - water current provides nutrients (not possible on land)
4. Water acts as a temperature buffer

Disadvantages of living in water

1. Harder to obtain O₂ from water than air
2. Motion required to keep water moving over the gills

Advantages of living on land

1. High concentration of O₂

Disadvantages of living on land

1. Water loss is a concern
2. New methods of fertilization necessary - without water
3. Significant temperature fluctuations
4. Greater effect of gravity

Adaptations to life on land

1. Internal fertilization provides protection for zygote
2. Amniote egg - self-contained, watertight eggshell
3. Waterproof skin, hair, scales, feathers - prevent water loss and insulation against temperature variations
4. Exo- and endoskeletons for support along with muscles
5. Temperature regulation (i.e. hibernation, shivering, sweating etc.)
6. Animals became highly motile in order to seek out the best food

Organisms and Populations

• Paths of Speciation

1. Phyletic speciation - one species evolves to the next
2. Allopatric speciation - organisms in significantly different habitats, physical separation
3. Parapatric speciation - organisms live in similar habitats, but differing enough to allow speciation
4. Sympatric speciation - organisms live in the same habitat, but something allows for speciation
5. Hybridization speciation - two different species of organisms mating