Biology (0610) | Variation

🧬 Chapter 18: Variation and Selection Study Notes 🧬

Hello Biologists! This chapter is all about what makes you unique—Variation! We’ll explore why individuals look different, how those differences arise, and how the environment then "selects" the best-suited organisms to survive and pass on their traits. This is the foundation of evolution, so pay close attention!

18.1 Understanding Variation

What is Variation?

Variation simply means the differences between individuals of the same species.
Think about your classmates: you are all humans (the same species), but you have different heights, eye colours, and blood groups. This is variation in action!

Two Main Types of Variation

Variation can be categorised based on how the characteristics are measured or displayed.

1. Continuous Variation

• Definition: Variation that results in a range of phenotypes between two extremes. There are no clear-cut categories; individuals can fall anywhere along a spectrum.
• Examples: Body length, body mass (weight), hand span, height, leaf size.
• Causes: Usually caused by the combined effect of both genes AND the environment.
• Visualisation: If you plotted data for continuous variation on a graph, it usually forms a smooth bell-shaped curve (a normal distribution).

2. Discontinuous Variation

• Definition: Variation that results in a limited number of phenotypes with no intermediate types. You either have the trait, or you don't.
• Examples: ABO blood groups (A, B, AB, or O), eye colour, earlobe attachment (attached or unattached), seed shape in peas.
• Causes: Usually caused by genes only. The environment has little or no effect.

Quick Review:
Continuous = Spectrum (Genes + Environment)
Discontinuous = Categories (Genes Only)

Sources of Genetic Variation (Extended/Supplement Content)

Where does all this amazing difference come from? Genetic variation is introduced into a population through several key processes.

1. Mutation: The ultimate source of new alleles.
2. Meiosis: Creates genetically different gametes (sperm and egg) through crossing over and random assortment of chromosomes.
3. Random Fertilisation: Any sperm can fuse with any egg, creating a huge variety of possible gene combinations in the offspring.
4. Random Mating: Organisms within a population mate randomly, ensuring a mix of different alleles in the gene pool.

Mutation: The Source of New Alles

• Definition: A genetic change—a random change in the structure of DNA or in the number of chromosomes.
• Key Role: Mutation is the way in which new alleles are formed. Without mutation, all variation would eventually run out!
• Gene Mutation (Supplement): A random change in the base sequence of DNA. This small change can lead to a new protein being made, or no protein at all.
• Increasing Mutation Rate: Factors known to increase the rate of mutation include ionising radiation (like X-rays or UV light) and certain chemicals.

18.2 Adaptive Features

What is an Adaptive Feature?

An adaptive feature is an inherited feature of an organism that helps it survive and reproduce in its specific environment.
If a feature helps you live long enough to have babies, it's adaptive!

Interpreting Adaptive Features

When you look at an organism, you can usually identify features that help it cope with its surroundings:

• A lion's sharp claws and teeth are adaptive features for predation.
• A cactus's thick stem and lack of leaves are adaptive features for surviving dry conditions.
• The white fur of a polar bear is adaptive for camouflage in snow.

Adaptations to Water Availability (Extended/Supplement Content)

Plants must adapt strongly to the amount of water available in their environment:

1. Xerophytes (Xeros = Dry)

• These are plants adapted to survive in very dry environments (deserts, high winds).
• Adaptive Features Explained:
  • Thick waxy cuticle: Reduces water loss through transpiration (evaporation).
  • Small or rolled leaves/spines: Reduces the surface area available for water loss.
  • Deep or widespread roots: Maximise water absorption from a large area or deep underground.
  • Fewer stomata: Reduces the number of pores through which water vapour escapes.

2. Hydrophytes (Hydro = Water)

• These are plants adapted to live either submerged in water or floating on the surface.
• Adaptive Features Explained:
  • Stomata only on the upper surface (for floating leaves): Allows gas exchange with the air, as the lower surface is in water.
  • Large air spaces in stems/leaves: Provides buoyancy (helps them float) and enables gases to move around the submerged parts easily.
  • Little or no lignified tissue/xylem: Water is abundant, so they don't need much support or large transport vessels.

18.3 Selection: Natural and Artificial

Natural Selection

Natural selection is the process where organisms better adapted to their environment tend to survive longer and produce more offspring. It's often called “survival of the fittest.”

The Five Steps of Natural Selection: (The mechanism of evolution)

1. Genetic Variation: Individuals within a population show genetic differences (due to mutation, meiosis, random fertilisation). (e.g., some rabbits run faster than others.)
2. Production of Many Offspring: Organisms produce more young than the environment can support.
3. Struggle for Survival: Because resources (food, water, space) are limited, there is competition and predation.
4. Greater Chance of Reproduction: Individuals with better adaptive features (the "fittest") are more likely to survive this struggle and breed successfully. (e.g., the faster rabbits evade the fox and survive.)
5. Passing on Alleles: These successful individuals pass on their advantageous alleles (genes) to the next generation, making the adaptation more common over time.

Adaptation (Supplement)
Adaptation is the result of natural selection. It is the process by which populations become more suited to their environment over many generations as advantageous alleles are passed on.

Example: Antibiotic Resistance (Supplement)

This is a crucial modern example of natural selection in action:

1. Variation: A population of bacteria has variation; a few individuals might have a random mutation making them slightly resistant to an antibiotic.
2. Selection Pressure: When the antibiotic is used, it kills the majority of the non-resistant bacteria.
3. Survival and Reproduction: The few resistant bacteria survive the treatment (the selection pressure) and reproduce rapidly.
4. Inheritance: They pass on their resistant alleles, and soon the population is dominated by the resistant strain.

Artificial Selection (Selective Breeding)

Unlike natural selection, where the environment chooses the traits, in artificial selection, humans choose the traits.

The Three Steps of Selective Breeding:

1. Selection: Humans select organisms (plants or animals) that possess desirable features (e.g., cows that produce lots of milk, crops that yield larger fruit).
2. Crossing: These selected individuals are deliberately crossed (bred) together to produce the next generation.
3. Offspring Selection: The offspring that show the desirable features most strongly are selected for further breeding. This process is repeated over many generations.

Example: Producing dairy cows, creating new varieties of crops like wheat or maize, or breeding specific dog breeds like Labradors.

Differences Between Natural and Artificial Selection (Supplement)

It is important to outline the differences: