Variation And Natural Selection

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Notes

Variation

**Variation** refers to differences between individuals of the same species, giving rise to different **phenotypes**.
**Continuous variation** shows a range of phenotypes with no distinct categories (e.g., height, body mass).
Continuous variation produces a smooth **bell-shaped curve** when plotted as a frequency histogram.
**Discontinuous variation** results in a limited number of distinct phenotypes (e.g., blood group, pea seed shape).
Discontinuous variation produces a **step-shaped bar graph**.
**Mutation** is a genetic change involving alterations to **DNA**, leading to new **alleles** and phenotypic variation.

Natural Selection

Natural selection is the process where **better adapted** individuals survive and reproduce more successfully.
Individuals in a species show **variation** due to genetic differences.
Overproduction of offspring leads to **competition** for resources and a **struggle for survival**.
Individuals with advantageous **alleles** are more likely to survive and reproduce.
Advantageous alleles are **passed on** to offspring, increasing in frequency over generations.
Example: **Peppered moths** – environmental change made the dark allele advantageous, leading to increased frequency.

Natural Selection: Antibiotic Resistance (Extended)

Variation in bacteria includes some with **resistance alleles** to an antibiotic.
Exposure to antibiotic kills non-resistant bacteria; resistant ones survive and **reproduce**.
The resistance allele is **passed on**, becoming more common in the population.
Over time, the entire population becomes an **antibiotic-resistant strain**.
Key steps: variation → competition → survival of fittest → inheritance of advantageous alleles.

Artificial Selection (Selective Breeding)

**Selective breeding** is the process where humans choose individuals with **desirable characteristics** to breed.
Selected individuals are **crossed** together, and offspring with the desired traits are chosen.
This process is **repeated over many generations** to enhance the traits.
Examples: dog breeds with calm temperaments, crop varieties with high yield or pest resistance.
Selective breeding has produced a wide range of **domestic varieties** from wild ancestors.

Monohybrid cross showing discontinuous variation in pea seed shape (round vs wrinkled).

Energy pyramid illustrating trophic levels and energy transfer.

Practice questions

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1.Which of the following is an example of discontinuous variation?
Easy
- AHeight in humans
- BBody mass in dogs
- CBlood group in humans
- DLeaf length in plants
2.Continuous variation produces a limited number of distinct phenotypes.
Easy
True or false?
3.Define variation.
Easy
4.Complete the sentence about natural selection:
Medium
Individuals that are better $____ to$ their environment have a higher chance of survival and more chances to reproduce.
5.Place the following steps of natural selection in the correct order:
Medium
- Advantageous alleles become more common in the population
- Individuals in a species show variation
- Better adapted individuals survive and reproduce
- Struggle for survival due to competition
6.What is the source of new alleles in a population?
Medium
7.Match each type of variation with its correct description:
Medium
- Continuous variation
- Discontinuous variation
- Shows a range of phenotypes
- Results in a limited number of phenotypes
8.In a population of bacteria, 5% initially carry a resistance allele. After exposure to an antibiotic, 80% of the surviving bacteria carry the resistance allele. If the survival rate of non-resistant bacteria is 0.1%, what is the survival rate of resistant bacteria? (Assume the population was large and the antibiotic killed all non-resistant bacteria that did not survive.)
Hard
- A10
- B7
- C8
- D9

Slides

Variation

Variation refers to differences between individuals of the same species.
It gives rise to different phenotypes.
Variation can be continuous or discontinuous.

What is variation?

Differences between individuals of different species
Differences between individuals of the same species
Similarities between individuals of the same species
Changes in an individual over its lifetime

Continuous vs Discontinuous Variation

Continuous variation shows a range of phenotypes with no clear categories (e.g., height).
Discontinuous variation results in a limited number of distinct phenotypes (e.g., blood group).
Continuous variation produces a bell-shaped curve; discontinuous produces a bar chart.

Which of these are examples of discontinuous variation?

Blood group
Height
Pea seed shape (round or wrinkled)
Body mass

Mutation

Mutations are changes to DNA.
They can create new alleles.
New alleles can lead to phenotypic variation.

Mutations are changes $to ____$ that can create $new ____$ .

Natural Selection

Individuals in a species show variation due to genetic differences.
Reproduction produces many offspring, leading to competition.
Better-adapted individuals survive and reproduce more.
Advantageous alleles become more common over generations.

What is the first step in natural selection?

Advantageous alleles become common
Individuals show variation
Competition for resources
Survival of the fittest

Antibiotic Resistance in Bacteria

Some bacteria have alleles for antibiotic resistance.
When exposed to antibiotic, resistant bacteria survive and reproduce.
The resistance allele is passed on and becomes more common.
Eventually the whole population becomes resistant.

In bacteria, ____ selection can lead $to ____$ resistance.

Artificial Selection (Selective Breeding)

Humans select individuals with desirable characteristics.
These individuals are bred together.
Offspring with the desired traits are chosen and bred.
Repeated over many generations to produce varieties.

Which of these are steps in artificial selection?

Humans select individuals with desired traits
Individuals compete for survival
Offspring with desired traits are bred together
The process is repeated over many generations

Examples of Selective Breeding

Dog breeds with calm temperaments and attractive traits.
Crop varieties resistant to pests and with high yield.
All descended from wild ancestors through artificial selection.

What is the main difference between natural and artificial selection?

Natural selection is faster
Artificial selection is driven by humans
Natural selection produces new species
Artificial selection only occurs in plants

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