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 describes 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.

• Microevolution: Observable evolutionary changes within populations over just a few generations.

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 living organisms.

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

Natural Selection

III. Requirements for Natural Selection

For natural selection to occur, the following conditions must be met:

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

• Heritability: Traits must be inheritable, meaning they can be passed down to offspring.

• 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.

IV. Mechanism and Process

Natural selection occurs as individuals with beneficial traits (those that provide a survival or reproductive advantage) are more likely to survive and pass on these advantageous traits to the next generation. Over time, these individuals become more prevalent in the population, while less favorable traits may diminish or disappear. This process drives the adaptation of species to their environment.

V. Evidence for Evolution

The evidence for evolution comes from a variety of sources that show how species have changed over time and how life on Earth is interconnected. Here are the main types of evidence:

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

• Homology: Genetic code and vestigial genes highlight shared ancestry and inherited traits.

• Convergent Evolution: Organisms independently evolve similar traits due to similar environmental pressures.

• Biogeography: Shows how species evolve and adapt to different environments, providing evidence for common ancestry and speciation.

• Fossils: Provide a chronological record of life on Earth, documenting transitions and evolutionary history.

Homology and Convergent Evolution

I. 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. The fact that this genetic code is nearly universal across all living things (from bacteria to humans) suggests a common ancestry for all life forms.

• Vestigial Genes: Vestigial genes are non-functional or partially functional genes that are inherited from a common ancestor but have lost their original function in the current organism. Example: The GULO gene in humans, which was once functional for vitamin C synthesis but is now non-functional.

II. 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 structures that allow cells to move. Although prokaryotic and eukaryotic flagella look similar and have similar functions, they are structurally different and evolved independently.

Feature	Prokaryotic Flagella	Eukaryotic Flagella
Structure	Made of protein flagellin	Made of microtubules
Movement	Rotates like a propeller using a proton (H+) gradient	Whips back and forth
Example	Escherichia coli (E. coli)	Sperm cell

Divergent Evolution and Biogeography

III. 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 species into different forms, often stemming from a common ancestor.

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

• Adaptive Radiation: This is a form of divergent evolution where a single ancestor species evolves into many forms, each adapted to a different ecological niche.

• Homologous Structures: As a result of divergent evolution, species that have evolved from a common ancestor may have homologous structures—body parts that are similar in structure but different in function. These structures provide evidence of common ancestry, even though the species have adapted to different environments.

IV. Divergent vs. Convergent Evolution

• Divergent Evolution: Results in species becoming more different from a common ancestor. Typically occurs when species adapt to different environments, leading to greater differences.

• Convergent Evolution: Results in species becoming more similar due to similar environmental pressures, as seen in the case of the wings of bats and birds—both serve the same function (flight), but evolved independently.

V. 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:

  • Isolated Species: Certain species are found only in specific geographic areas, often on islands or isolated regions. These species tend to share the closest relatives with species found in nearby areas, suggesting that they share a common ancestor and evolved in isolation over time.

Fossil Evidence and the History of Life

I. Fossils

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

• Stromatolites: Stromatolites are layered, sedimentary structures formed by the activity of cyanobacteria (blue-green algae). These are some of the oldest known fossils, dating back more than 3 billion years ago. Stromatolites provide evidence of early life on Earth and show how photosynthetic organisms like cyanobacteria contributed to the rise of oxygen in the atmosphere.

II. Summary of Evidence

1. Antibiotic resistance shows evolution occurring in real-time through natural selection.

2. Homology (e.g., the genetic code and vestigial genes) highlights shared ancestry and inherited traits.

3. Convergent evolution (e.g., flagella) demonstrates how similar traits can arise independently in unrelated organisms.

4. Biogeography shows how species evolve and adapt to different environments, providing evidence for common ancestry and speciation.

5. Fossils, including stromatolites, provide a timeline of life on Earth, documenting transitions and evolutionary history.

The Origin of Life and the Evolutionary History of Cells

I. Abiogenesis

Abiogenesis refers to the process by which life arose from non-living matter. This is the foundation for the concept of molecular evolution, 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 the influence of lightning, volcanic activity, and UV radiation, these molecules might have reacted to form the building blocks of life: amino acids, nucleotides, and simple sugars.

II. 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 set of molecules like proteins and nucleic acids. This membrane would have separated the internal environment of the protocell from the outside world, allowing for the concentration of molecules necessary for chemical reactions.

• Self-Replication: Some protocells are hypothesized to have possessed the capacity for simple replication. Protocells capable of self-replication, making chemical evolution and life possible.

• Metabolism: Protocells could also have possessed basic metabolic pathways to harness energy from their surroundings, a key step toward becoming fully functional cells. They may have been capable of simple processes such as energy conversion and molecule synthesis, although their metabolic networks would have been much simpler than those of modern cells.

III. RNA World

The RNA world hypothesis suggests that the first self-replicating molecules were RNA-based rather than DNA-based. RNA can store 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 life's initial stages. The discovery of ribozymes (RNA molecules that act as enzymes) supports this idea.

• Transition to DNA: Over time, life transitioned from RNA-based systems to a DNA-based system, as DNA is more stable than RNA, 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.

IV. Endosymbiosis

Endosymbiosis is the theory that eukaryotic cells originated through a symbiotic relationship between different prokaryotic cells. This theory explains the origin of mitochondria and chloroplasts, which are organelles found in eukaryotic cells.

• Evidence:

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

  • These organelles have their own ribosomes, and both function independently.

  • They have double (convergent evolution) membranes and are engulfed by a host cell (endosymbiosis).

• Support for Endosymbiosis: The theory is widely accepted because it explains the origin of complex organelles in eukaryotic cells evolved from prokaryotic invaders.

Eukaryotes and Multicellularity

V. 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 the endosymbiotic theory (as mentioned above), in which ancestral prokaryotic cells engulfed free-living bacteria, such as mitochondria and chloroplasts, and these engulfed bacteria evolved into organelles.

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

VI. 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: Multicellular life is thought to have evolved from colonial organisms, which were groups of single-celled organisms that lived together for mutual benefit, eventually becoming more specialized and dependent on each other.

• Cell Differentiation: Multicellular organisms have cells that perform different functions. In animals, for example, cells differentiate into muscle cells, nerve cells, and skin cells, allowing for greater complexity and specialization.