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Definition: Symbionts are organisms that live in close, long-term, and physically intimate associations with a host of a different species.
Examples: Fungi are important symbionts in ecosystems.

Function: Maintains shape and provides physical protection against bursting in hypotonic environments.
Prokaryotes:
- Bacterial Cell Walls: Contain peptidoglycan, a polymer that is crucial in classification methods, specifically the Gram stain technique.
- Archael Walls: Composed of polysaccharides but lack peptidoglycan.

Derived Traits: Traits that appear in a specific lineage, differing from those of their ancestors.
Acquired Traits: Non-heritable traits gained through environmental influences and learning.

Characteristics: Abundant and complex organisms. They play essential roles in ecosystems.
- Photosynthesis: Produces oxygen as a byproduct and absorbs CO₂.
- Uses: Source of fabrics, textiles, paper, energy, medicine, and recreational substances.

Green Algae: Closest relatives to land plants; however, they did not evolve directly from them but share a common ancestor.
Monophyletic Group: A group consisting of an ancestor and all its descendants.

Eukarya: Encompasses eukaryotic organisms, including plants, protists, animals, and fungi.
Archaea: Consists of prokaryotic organisms, can live in both regular and extreme environments.
Bacteria: Prokaryotic organisms that include various types based on energy and nutrient acquisition.
- Types of Prokaryotes:
- Photoautotrophs: Perform photosynthesis.
- Chemoautotrophs: Oxidize inorganic substances.
- Photoheterotrophs: Use light for energy but obtain carbon from organic molecules produced by other organisms.
- Chemoheterotrophs: Obtain energy and carbon from organic molecules produced by other organisms.

Characteristics:
- Generally small.
- Lack a nucleus and membrane-bound organelles.
- Half exhibit taxis (movement towards positive stimuli and away from negative stimuli).

Gymnosperms: Plants with exposed seeds on sporophylls, primarily forming cones.
Angiosperms: Seed plants that produce flowers and fruits, aiding in pollination and reproduction.

Progression of life forms:
- Prokaryotic life → Oxygen production → Single-celled eukaryotes → Multicellular eukaryotes (including animals) → Colonization of land → Humans.

Hardy-Weinberg Equilibrium formula:
- $p + q = 1$
- $p² + 2pq + q² = 1$
- Where:
- $p$ : frequency of dominant allele.
- $q$ : frequency of recessive allele.
- $p²$ : proportion of homozygous dominant individuals.
- $q²$ : proportion of homozygous recessive individuals.
- $2pq$ : proportion of heterozygous individuals.

Defines species as groups of interbreeding populations that are reproductively isolated.

Habitat Isolation: Different environments.
Temporal Isolation: Different breeding times.
Behavioral Isolation: Differences in mating behaviors.
Mechanical Isolation: Anatomical differences prevent mating.
Gametic Isolation: Incompatibility of gametes.
Postzygotic Barriers:
- Reduced hybrid viability: Hybrids fail to develop properly.
- Reduced hybrid fertility: Hybrids are sterile.
- Hybrid breakdown: Subsequent generations of hybrids are inviable or sterile.

Allopatric Speciation: Occurs due to geographical isolation creating physical barriers.
Sympatric Speciation: Occurs without geographical isolation, often via polyploidy or behavioral differences.

Reinforcement: Prevent hybrid formation.
Fusion: Hybrids fuse back into one species.
Stability: Hybrids maintain presence and continue producing offspring in hybrid zones.

Stephen J. Gould's Perspective: Species tend to remain phenotypically consistent for extended periods and can undergo rapid divergence under certain conditions.