Comprehensive Study Guide for Natural and Artificial Selection

The fundamental theory of evolution describes the process of biological change over time, primarily driven by the mechanism of natural selection.
A POPULATION is defined as a specific group of organisms belonging to the same species that inhabit the same geographical area at the same point in time.
Observations of population change over time were evident to Charles Darwin during his studies.
Case Study: Life After Chernobyl
- The Grey Tit birds residing in the vicinity of the Chernobyl disaster site provide a modern example of natural selection in action.
- These birds have adapted to the high levels of radiation found in the area, demonstrating how populations can change in response to extreme environmental stressors.

Natural Selection is the process resulting in the change of characteristics within a population of organisms over many generations.
This process occurs because individuals possessing certain inherited traits are better suited to survive specific local environmental conditions.
Through the process of reproduction, these surviving individuals pass on their alleles (the genetic variants of traits) to their offspring.
Evolution by natural selection is non-random and is governed by the survival and reproductive success of the individuals within a population.

For natural selection to take place, there must be VARIATION within a species.
Variations are the differences that exist between individuals. These differences can be categorized as:
- Structural differences: Physical makeup or anatomical features.
- Functional differences: How the organism or its body parts operate.
- Physiological differences: Internal chemical or metabolic processes.
If a variation is helpful, the individual is more likely to survive, thereby increasing the probability of passing that variation to the next generation.
Over time, beneficial variations become more common and prevalent within the population.

A helpful variation eventually becomes an ADAPTATION.
An adaptation is defined as a structure, behavior, or physiological process that assists an organism in surviving and reproducing in its particular environment.
Examples of Adaptations:
- Camouflage: A structural adaptation used for avoiding detection by predators.
- Hibernation: A physiological adaptation that allows animals to survive harsh winters. It enables them to save energy by remaining inactive until food resources become available again.
- Mimicry: A strategy where a harmless species evolves to represent or resemble a harmful species to deter predators.
- Example – Monarch and Viceroy Butterflies: The Monarch butterfly is inedible to birds. The Viceroy butterfly is edible but shares a very similar appearance to the Monarch. Predators avoid eating the Viceroy because they mistake it for the inedible Monarch.

Variation is a product of an individual's genes and the genetic differences produced through the processes of meiosis and sexual reproduction.
MUTATIONS are changes in the DNA of an organism. They are critical to evolution because they are the only source of entirely new genetic information.
Characteristics of Mutations:
- They occur randomly, often during the copying of DNA or due to environmental exposure, such as UV radiation.
- For a mutation to be inherited by future generations, it must occur in a gamete (reproductive cell).
Selective Advantage:
- While some mutations are harmful (e.g., causing diseases like cancer), others are beneficial.
- A beneficial mutation provides a SELECTIVE ADVANTAGE, which is a genetic advantage that improves an organism's chances of survival, particularly in a changing environment.
- Example – Bacteria: Some bacteria can double their population size in approximately $10\,\text{minutes}$ . If a random mutation produces a new allele that provides a benefit (such as resistance), that allele can quickly spread, allowing the entire population to survive a threat.

SELECTIVE PRESSURE is an environmental condition that selects for certain characteristics in individuals while selecting against others.
Factors contributing to selective pressure include:
- Non-living (Abiotic) Factors: Such as temperature, weather, or chemistry.
- Living (Biotic) Factors: Such as predators, parasites, or competition for limited resources.
Example – Antibiotic Resistance:
- An antibiotic is a chemical selective pressure meant to kill bacteria.
- Bacteria that possess a mutation allowing them to resist the treatment will survive.
- These surviving bacteria reproduce and pass the resistance gene to the next generation, leading to a resistant population.

Natural Selection is Situational:
- Natural selection does not have a set direction or a predetermined "end goal."
- A trait that is detrimental or neutral in one environment may become highly beneficial if the environment changes.
- Evolution results in a population that is specifically adapted to its current environment through the accumulation of beneficial alleles.
Survival of the Fittest:
- This phrase describes the mechanism of natural selection where the "fittest" organisms are those that win the struggle for survival.
FITNESS:
- In biological terms, fitness refers to the relative contribution an individual makes to the next generation.
- It is measured by the number of offspring an individual produces that survive long enough to reproduce themselves.
- Greater Fitness = Producing more offspring that survive to reproductive age to pass on favorable alleles.

ARTIFICIAL SELECTION occurs when selective pressure is exerted by humans rather than the natural environment.
Humans modify species over many generations by intentionally breeding individuals that possess traits humans find desirable.
Examples of Artificial Selection:
- Crops: Breeding plants for higher yields or pest resistance.
- Livestock: Breeding cows to produce higher quantities of milk.
- Pets: The wide variety of dog breeds created through selective breeding programs.