Evolution, Cell Theory, and the Origin of Life: General Biology Study Notes

Evolution and Cell Theory

I. Evolution

Evolution is the process by which populations of organisms change over generations due to variations in traits and environmental factors. The concept was first popularized by Charles Darwin, who proposed that species evolve through natural selection. The theory of evolution explains the diversity of life on Earth and is supported by evidence from various fields, including genetics, paleontology, and comparative anatomy.

• Definition: Evolution is the process by which populations of organisms change over generations due to variations in traits and environmental factors.

• Key Mechanism: Natural selection, where individuals with advantageous traits survive and reproduce more successfully.

• Applications: Explains the diversity of life, antibiotic resistance, and adaptation to environments.

II. Cell Theory

Cell Theory is a fundamental principle in biology that describes the properties of cells and their role in living organisms.

• Tenets of Cell Theory:

  1. All living organisms are composed of one or more cells.

  2. The cell is the basic unit of structure and function in all living organisms.

  3. All cells arise from pre-existing cells through cell division.

Natural Selection and Microevolution

III. Natural Selection

Natural selection is the process by which individuals with beneficial traits are more likely to survive and reproduce, passing those traits to the next generation.

• Requirements for Natural Selection:

  1. Variation: There must be variation in traits among individuals within a population (e.g., size, color, shape).

  2. Heritability: Traits must be heritable, meaning they can be passed down to offspring.

  3. Differential Survival and Reproduction: Some traits must provide an advantage in terms of survival and reproduction, meaning individuals with certain traits are more likely to survive and reproduce than others.

• Microevolution: Small evolutionary changes within a population, such as changes in allele frequencies, observable over just a few generations.

• Example: Antibiotic resistance in bacteria is a clear example of evolution by natural selection.

Evidence for Evolution

IV. Types of Evidence

• Antibiotic Resistance: Shows evolution occurring in real-time through natural selection.

• Homology: Similarities between species due to shared ancestry (e.g., genetic code, vestigial genes).

• Convergent Evolution: Unrelated organisms evolve similar traits due to similar environmental pressures.

• Biogeography: Distribution of species across geographic areas provides evidence for common ancestry and speciation.

• Fossils: Show chronological changes in life forms and evolutionary transitions.

V. Homology

Homology refers to similarities between species due to shared ancestry. There are various forms of homology that provide strong evidence for evolution.

• Genetic Code: All living organisms use the same genetic code to translate DNA into proteins, indicating a common ancestor for all living things.

• Vestigial Genes: Genes that are non-functional or partially functional in one species but functional in another, suggesting shared ancestry (e.g., GULO gene in humans and other mammals).

VI. Convergent Evolution

Convergent evolution occurs when unrelated organisms independently evolve similar traits due to similar environmental pressures or ecological niches, rather than shared ancestry.

• Example: Prokaryotic and eukaryotic flagella are both used for movement but evolved independently and are structurally different.

Feature	Prokaryotic Flagella	Eukaryotic Flagella
Structure	Made of protein flagellin	Made of microtubules
Movement	Rotates like a propeller	Whips back and forth
Example	Escherichia coli (E. coli)	Sperm cells

VII. Divergent Evolution

Divergent evolution occurs when two or more related species become more different from each other over time, usually due to different environmental pressures or selection factors. It is a type of evolutionary process that results in the diversification of a species into different forms, often leading to new species.

• Common Ancestor: Divergent evolution starts from a common ancestor.

• Adaptive Radiation: A form of divergent evolution where a single ancestor species evolves into a wide variety of forms, each adapted to different environments (e.g., Darwin's finches).

• Homologous Structures: Structures that are similar in different species due to shared ancestry, even though the structures have adapted to different functions.

Divergent vs. Convergent Evolution

• Divergent Evolution: Results in species becoming more different from a common ancestor, typically due to adaptation to different environments.

• Convergent Evolution: Results in species becoming more similar due to similar environmental pressures, despite different ancestry.

VIII. Biogeography

Biogeography is the study of the distribution of species across geographic areas. It provides significant evidence for evolution because it shows how species have adapted to different environments over time.

• Key Patterns:

  • Endemic Species: Certain species are found only in specific geographic areas, often on islands or isolated regions, suggesting that they share a common ancestor and evolved in isolation over time.

IX. Fossils

The fossil record provides a chronological account of life on Earth, revealing how species have changed over time.

• Stromatolites: Layered, sedimentary structures formed by the activity of cyanobacteria (blue-green algae), some of the oldest known fossils, dating back more than 3 billion years. Stromatolites provide evidence for early life and photosynthetic organisms.

Origin of Life and the Evolution of Cells

X. Abiogenesis

Abiogenesis refers to the process by which life arose from non-living matter. This is the foundation for the concept of the origin of life, marking the transition from simple molecules to living organisms.

• Early Earth Conditions: The Earth's atmosphere around 4–5 billion years ago was very different from today, containing water vapor, ammonia, and hydrogen. Under these conditions, energy from lightning, volcanic activity, and UV radiation could drive the formation of the building blocks of life: amino acids, nucleotides, and simple sugars.

XI. Protocells

Protocells are simple, membrane-bound structures that could have been precursors to the first living cells. They are considered an important step in the transition from non-living to living matter.

• Structure: Thought to consist of a lipid bilayer membrane that encloses basic sets of molecules, allowing for the concentration of molecules necessary for chemical reactions.

• Self-Replication: Some protocells may have possessed the basic metabolic pathways to harness energy from their surroundings, a key step toward becoming fully functional cells.

XII. RNA World Hypothesis

The RNA world hypothesis suggests that the first self-replicating molecules were RNA-based rather than DNA-based. RNA can store genetic information and can catalyze chemical reactions (like enzymes), which makes it a strong candidate for the origin of life.

• RNA's Dual Role: RNA can serve as both a genetic material (like DNA) and as a catalyst (like enzymes), making it versatile and potentially able to perform the functions necessary for early life.

• Transition to DNA: Over time, life transitioned from RNA-based systems to a DNA-based system, as DNA is more stable than RNA and better suited for long-term storage of genetic information.

• Evidence: Many of the molecular machinery of modern cells (like the ribosome, which synthesizes proteins) are based on RNA, suggesting that RNA may have played a central role in early cellular life.

XIII. Endosymbiosis

Endosymbiosis is the theory that eukaryotic cells evolved when one prokaryotic cell engulfed another, and the two cells formed a symbiotic relationship. This theory explains the origin of mitochondria and chloroplasts in eukaryotic cells.

• Evidence:

  • Mitochondria and chloroplasts have their own DNA, which is distinct from the nuclear DNA of eukaryotic cells and more similar to that of bacteria.

  • They have double membranes (suggesting engulfment by another cell).

  • They reproduce independently within the host cell, similar to bacteria.

XIV. Eukaryotes

Eukaryotic cells are more complex than prokaryotic cells and have several defining features:

• Key Characteristics:

  • Nucleus: Eukaryotic cells have a membrane-bound nucleus that contains the genetic material (DNA), whereas prokaryotes have no such structure.

  • Membrane-bound Organelles: Eukaryotes possess a variety of organelles (e.g., mitochondria, chloroplasts, endoplasmic reticulum) that perform specialized functions within the cell.

  • Larger Size: Eukaryotic cells tend to be much larger and more complex than prokaryotic cells.

• Origin: Eukaryotic cells are believed to have evolved through endosymbiosis, as mentioned above.

• Key Evolutionary Step: The transition from prokaryotes to eukaryotes represents a major event in the history of life, as eukaryotes have the ability to compartmentalize activities within cellular functions, enabling more sophisticated processes, such as multicellularity.

XV. Multicellularity

Multicellularity is the organization of cells into more complex structures, where cells work together as a coordinated unit, leading to the evolution of multicellular organisms.

• First Multicellular Life: The earliest multicellular organisms likely evolved from colonial protists, which were groups of single-celled organisms that lived together for mutual benefit.

• Cell Differentiation: Multicellular organisms have cells that perform different functions. In animals, for example, there are muscle cells, nerve cells, and blood cells, each specialized for a particular role.

Summary Table: Evidence for Evolution

Type of Evidence	Description	Example
Antibiotic Resistance	Real-time evolution through natural selection	Bacteria evolving resistance to antibiotics
Homology	Shared ancestry shown by genetic code and vestigial genes	GULO gene in humans and other mammals
Convergent Evolution	Similar traits in unrelated organisms	Flagella in prokaryotes and eukaryotes
Biogeography	Distribution of species across environments	Endemic species on islands
Fossils	Chronological record of life and evolutionary transitions	Stromatolites, transitional fossils

Additional info: Some explanations and examples have been expanded for clarity and completeness, based on standard General Biology curriculum.