Introduction to Evolution and Natural Selection

All recognized life forms are categorized into three primary domains: Bacteria, Archaea, and Eukarya.
Cell categorization within these domains is as follows:
- Bacteria and Archaea consist of prokaryotic cells.
- Eukaryotes consist of eukaryotic cells.
There is a rigorous distinction between a scientific theory and a hypothesis:
- A Theory has been extensively tested and proven through repeated experimentation. While biologists often avoid the absolute term "proven" to remain open to new data, a theory is as close to a proven concept as is possible in science.
- Examples of foundational biological theories include the theory of evolution by natural selection, cell theory, and the germ theory of disease.
- The theory of evolution by natural selection has been tested thousands of times over the last $200$ years.
- A Hypothesis is defined as a tentative explanation for an observation. It usually has significantly less evidentiary support than a theory and typically focuses on a narrower scope.
- A modern example of a narrow hypothesis is the debate over the two-domain versus three-domain relationship between archaeons and bacteria.

Evolution refers to the genetic change in populations over time.
Natural selection is one of the primary mechanisms driving evolution.
Requirements for natural selection to occur within a population include:
- The population must consist of the same species that are interbreeding with one another.
- Genetic Variation: Individuals within a single population must vary in their traits, such as color, size, or physiological characteristics.
- Heritability: These variable traits must be encoded in the DNA. If a trait is genetically inherited, natural selection can act upon it.
- Example: In a population of beetles, color variations (e.g., red versus orange) are genetically encoded. Red beetles produce red offspring, and orange beetles produce orange offspring.

Differential Reproductive Success: This occurs when some individuals in a population reproduce more than others.
Biological Fitness: This is a measure of reproductive success rather than physical strength. It is defined by the number of successfully reproduced offspring an individual has.
- Example: If Beetle A has $100$ offspring and Beetle B has $75$ offspring, Beetle A is considered more fit.
Selective Pressure: Environmental factors contribute to differential reproductive success.
- Example: Blue Jays may selectively prey on lighter-colored (orange) beetles in a specific environment.
- Result: Red beetles survive longer and reproduce more frequently. Because the red trait is genetically encoded, the incidence of the red variant increases in the subsequent generations.

Research conducted by Jennifer Balsegar of the AU Department of Biology examined the evolution of pesticide resistance in insects.
The study focused on mosquitoes and their resistance to a class of pesticides known as pyrethroids.
Application of Natural Selection Principles:
- The first requirement is genetic variation within the mosquito population.
- Specific alleles (versions of a gene) confer resistance to pyrethroids.
- Mosquitoes possessing the resistance allele have a higher biological fitness when the pesticide is present because they survive and reproduce despite the chemical application.
Data trends from $2016$ to $2024$ show the frequency of the resistance allele on the Y-axis increasing over time within these populations.

Evolution is defined as a change at the population level, not the individual level.
Individual organisms do not evolve. The change occurs across generations as resistance genes are passed down and increase in frequency.
Parallel to Antibiotic Resistance: This process is identical to how bacteria like Streptococcus or Staphylococcus infections evolve resistance to antibiotics. Increased use of the selective agent (antibiotics or pesticides) leads to a harder time killing off the population as resistance becomes more common.
Socioeconomic Hypothesis: Scientists hypothesized that resistance alleles would be more frequent in areas with higher property values.
- Reasoning: Residents in wealthier areas have more disposable income to hire private companies to spray their yards with pyrethroids.
- Findings: Preliminary data from North Carolina supports this hypothesis, though it is currently considered a tentative hypothesis that requires more rigorous data from broader geographical areas to confirm the global trend.

Question: Can you tell me about the difference between a hypothesis and a theory?
Answer: The difference lies in how rigorously the concept has been tested. A theory has been tested thousands of times and is foundational, whereas a hypothesis is a tentative, narrower explanation.
Question: Which of these statements would not be a correct way to describe the process of natural selection relative to the mosquito graph?
Answer: It is incorrect to say that an individual mosquito changes or evolves. We define evolution as a change at the population level.
Question: What does the atomic number signify?
Answer: The atomic number signifies the number of protons in an atom's nucleus.