Variation and Selection (B17) Study Notes

Hey there, future Biologists! This chapter is all about understanding why we are all unique and how these differences drive evolution and change in populations. Variation is the spice of life, and selection is the force that decides who gets to pass on their spice! Don't worry if this seems tricky at first; we'll break down Darwin's ideas and modern breeding techniques into simple steps.


B17.1 Understanding Variation

Variation simply means the differences between individuals of the same species. If every student in your class was exactly the same height, wore the same clothes, and had the same blood type, there would be no variation! These differences are essential for species survival.


Types of Variation (Core)

We classify variation based on how the characteristics look or measure:

  • Continuous Variation: This results in a range of phenotypes (observable features) between two extremes, with many intermediate values.
  • Think of it like a gradient. There are no clear categories.
  • Examples include: Body length (height), mass, leaf width, or human skin colour.
  • When plotted on a graph, continuous variation usually gives a bell-shaped curve.
  • Discontinuous Variation: This results in a limited number of phenotypes with no intermediate types.
  • Think of it like distinct boxes. You are either in one box or another.
  • This type of variation is usually controlled by one or two genes.
  • Examples include: ABO blood groups (A, B, AB, or O), tongue rolling ability, or certain flower colours.

Quick Review Box:
Continuous = Range (e.g., height, length)
Discontinuous = Distinct Categories (e.g., blood group)

Sources of New Variation (Core)

Where do these differences come from?

1. Inheritance: Offspring inherit a mix of alleles (alternative forms of a gene) from their parents during sexual reproduction. This mixing creates new combinations of existing traits.

2. Mutation: This is the ultimate source of all new variation.

  • A mutation is a spontaneous genetic change in the structure of the DNA or the number of chromosomes.
  • Mutation is the way in which new alleles are formed.
  • Did you know? Most mutations are harmful or have no effect, but sometimes, a mutation can produce an allele that gives an individual an advantage in their environment.

Key Takeaway: Variation is the difference within a species, sorted into smooth continuous traits (like size) or separate discontinuous traits (like blood type). New variation starts with mutation.

B17.2 Selection

Selection is the process by which certain organisms are favoured to survive and reproduce, based on their characteristics (phenotype).


1. Natural Selection (Core)

Natural selection is the process where organisms better adapted to their environment tend to survive and produce more offspring. This leads to evolutionary change over many generations. This is often summarized as "Survival of the fittest."


The process of Natural Selection can be broken down into five key steps:

Step 1: Genetic Variation within Populations

Individuals in a population are not identical. They show a wide range of inherited characteristics (variation). This is caused by mutation and inherited alleles.

Step 2: Production of Many Offspring

Organisms usually produce more offspring than the environment can support. (Imagine a rabbit population suddenly having thousands of babies!)

Step 3: Struggle for Survival (Competition)

Because resources (food, water, space, mates) are limited, there is a fierce struggle for survival. Individuals must compete with each other and cope with environmental pressures (like predators, disease, or harsh weather).

Step 4: Survival of the Fittest

Individuals that possess beneficial traits (alleles) that make them better adapted to the environment have a greater chance of reproduction than others. They survive longer and breed successfully.

Step 5: Allele Transfer to the Next Generation

The adapted individuals pass on their beneficial alleles to their offspring. Over many generations, the frequency of these advantageous alleles increases in the population, and the population slowly changes.

Memory Aid for Natural Selection Steps:
Variation
Offspring (many produced)
Struggle for survival
Survival and Reproduction (Selection)
Inheritance

Example of Natural Selection: Antibiotic Resistance (Supplement/Extended Content)

This is a critical modern example illustrating natural selection in action. We use antibiotics to kill bacteria that cause disease.

1. Variation: Within a large population of bacteria, there is natural genetic variation. Due to random mutation, a few bacteria possess an allele that makes them resistant to a particular antibiotic.

2. Selection Pressure: When a patient uses the antibiotic, it acts as a strong selective pressure. It kills the vast majority of the non-resistant bacteria.

3. Survival and Reproduction: Only the few resistant bacteria survive the treatment. Since the competition is now gone, these survivors reproduce rapidly (because bacteria divide very quickly).

4. Inheritance: These survivors pass on the resistance allele to their offspring, leading to a new, large strain of bacteria that is now completely resistant to the original antibiotic. The medicine is no longer effective.

Common Mistake Alert: Antibiotics do not cause the mutation for resistance; they only select for bacteria that already have the advantageous mutation.

2. Selective Breeding (Artificial Selection) (Core)

Unlike natural selection, where the environment chooses who survives, selective breeding (or artificial selection) is controlled by humans to produce organisms with desirable traits.

This process is used to improve:

  • Crop plants (e.g., maximizing yield, resistance to disease, better taste).
  • Domesticated animals (e.g., cows producing more milk, dogs with desired temperaments).

The process of Selective Breeding works as follows:

Step 1: Selection of Individuals

Humans examine a population and select individuals with desirable features (e.g., the cow that produces the most milk, or the plant that yields the largest fruit).

Step 2: Controlled Crossing

These selected individuals are crossed (mated) to produce the next generation.

Step 3: Selection of Offspring

The offspring are monitored. Only those offspring that show the desired features are chosen to breed in the next generation.

Step 4: Repetition

This process must be repeated over many generations. By consistently selecting and breeding only the best individuals, the desirable alleles become much more common in the population, leading to a permanent improvement in the strain.

Think of it like this: Natural selection is like finding gold naturally occurring in a riverbed. Selective breeding is like mining for gold, refining it, and building a specific golden statue with it.

Key Takeaway: Natural selection is driven by environmental pressure (survival of the fittest). Selective breeding is driven by human choice to produce useful traits over many generations.