Variation & Natural Selection
Section: Biology | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
What is Variation?
- Variation refers to differences in characteristics between organisms of the same species
Importance of Variation
- Makes each individual unique, except identical twins
- Allows species to adapt to changing environments
- Drives evolution through natural selection
Types of Variation
- Variation can be caused by genes, environment, or a combination of both factors
Genetic Variation
- Caused by differences in genes inherited from parents
- these characteristics cannot be changed by the environment
- Examples: eye colour (brown, blue, green), blood type (A, B, AB, O), natural hair colour, earlobe shape (attached or free), ability to roll the tongue
Environmental Variation
- Caused by factors in the environment where an organism lives
- these characteristics can change during an organism's lifetime
- Examples: accent or language spoken, scars from injuries, muscle development from exercise, sun tan, body piercings or tattoos
Combined Genetic and Environmental Variation
- Some characteristics are influenced by both genes and environment working together
| Characteristic | Genetic Factor | Environmental Factor |
|---|---|---|
| Height | Genes set potential maximum height | Diet affects whether potential is reached |
| Weight | Genes influence body type and metabolism | Diet and exercise determine actual weight |
| Skin colour | Genes determine natural skin tone | Sun exposure causes tanning |
| Intelligence | Genes influence brain potential | Education and stimulation develop abilities |
Genetic-only, environmental-only, and combined causes of variation
Continuous vs Discontinuous Variation
- Variation can be classified into two types based on how characteristics are distributed in a population
- the pattern of variation depends on the number of genes involved and environmental influences
Discontinuous Variation
- Shows distinct categories, with no in-between values
- usually controlled by one or a few genes, and not affected by the environment
| Characteristic | Possible Categories |
|---|---|
| Blood type | A, B, AB, or O (no in-between) |
| Earlobe attachment | Attached or free |
| Tongue rolling | Can or cannot roll |
| Sex | Male or female |
- Discontinuous variation is displayed on bar charts, because the data falls into separate categories
Continuous Variation
- Shows a range of values between two extremes
- usually controlled by many genes (polygenic), and often affected by the environment
- Examples: height (can be any value in a range, e.g. 145 cm, 146 cm, 147 cm), weight, foot size, hand span, skin colour
- Continuous variation is displayed on line graphs or histograms, because the data can take any value within a range
How discontinuous and continuous variation are displayed on graphs
Natural Selection
- Natural selection is the process by which organisms with characteristics best suited to their environment are more likely to survive, reproduce, and pass on their genes to the next generation
- also known as "survival of the fittest" — drives evolution over many generations
How Natural Selection Works
- Natural selection is a step-by-step process that occurs over many generations
The Process of Natural Selection
- Step 1 — Variation exists in a population, due to genetic differences
- Step 2 — Competition for resources occurs: food, water, mates, space
- Step 3 — Selection pressure from environmental factors: predators, disease, climate
- Step 4 — Advantageous characteristics help some organisms survive better
- Step 5 — Survivors reproduce and pass on their genes
- Step 6 — Offspring inherit the advantageous characteristics
- Step 7 — Over many generations, the frequency of advantageous characteristics increases
- Summary: variation → competition → selection → reproduction → evolution
How variation, selection and reproduction combine to change a population over generations
Examples of Natural Selection
- Natural selection can be observed in nature and in laboratory settings
- these classic examples demonstrate how populations change in response to environmental pressures
Peppered Moths
- The peppered moth example from England demonstrates rapid evolution in response to environmental change
- before the Industrial Revolution: light-coloured moths were camouflaged on lichen-covered trees; dark moths were easily seen and eaten
- during the Industrial Revolution: pollution darkened the trees; dark moths became camouflaged, and light moths were eaten instead
- after pollution controls: trees became lighter again, so light moths survived better once more
- The moth population colour changed from mostly light to mostly dark during pollution, then back to mostly light after cleanup
- this shows natural selection in action
Antibiotic Resistance in Bacteria
- Bacterial antibiotic resistance is a modern example of rapid evolution through natural selection
- a bacterial population has variation — some bacteria are resistant to antibiotics
- antibiotic use creates selection pressure
- non-resistant bacteria die, while resistant ones survive and reproduce rapidly
- the new population is mostly resistant, so the antibiotic becomes ineffective
- Completing full antibiotic courses is important, to prevent resistant bacteria from surviving and multiplying
Worked Example: Giraffe Neck Evolution
- How did giraffes evolve long necks through natural selection?
- Step 1: ancestral giraffes had variation in neck length, due to genetic differences
- Step 2: competition for food (leaves on trees) created selection pressure
- Step 3: longer-necked giraffes reached more food and survived better
- Step 4: longer-necked giraffes reproduced more successfully
- Step 5: offspring inherited genes for longer necks
- Answer: over millions of years and many generations, the average neck length in the population increased — this demonstrates evolution through natural selection
Evaluating Evidence for Natural Selection
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