What is the difference between a heritable characteristic and an acquired characteristic?

Heritable characteristics are determined by alleles in the DNA and can be passed to offspring, driving evolution. Acquired characteristics are gained during a lifetime (like a scar or learned skill) and cannot be passed on genetically.

How does natural selection differ from evolution?

Natural selection is the mechanism or process where individuals with better-adapted traits survive and reproduce. Evolution is the resulting long-term change in the heritable characteristics of the population over many generations.

Why is it incorrect to say an organism evolves 'in order to' survive?

Evolution is not a purposeful or conscious choice. It occurs because random mutations happen to provide an advantage in a specific environment, leading to higher reproductive success for those individuals.

What is the 'individual vs. population' error in evolutionary biology?

The error is claiming that an individual organism evolves during its lifetime. In reality, individuals stay genetically the same; evolution is the change in the average characteristics of the whole population over time.

What is a 'selection pressure'?

A selection pressure is an environmental factor (such as a predator, a disease, or climate change) that reduces the reproductive success of some individuals compared to others, driving natural selection.

Define 'Allele Frequency'.

Allele frequency is the proportion of a specific variant of a gene (allele) among all gene copies in a population. Evolution is essentially the change in these frequencies over time.

What are the three main sources of genetic variation in a population?

The three sources are random mutations in DNA, meiosis (which shuffles existing alleles through crossing over and independent assortment), and the random fusion of gametes during fertilization.

Why is variation necessary for natural selection to occur?

Without variation, all individuals would be genetically identical. If a selection pressure were applied, they would all be equally likely to survive or die, meaning no specific trait could be 'selected' to increase in frequency.

What does 'survival of the fittest' mean in a biological context?

It refers to individuals who are best adapted to their environment having the highest reproductive success. 'Fitness' is measured by the number of viable offspring an individual contributes to the next generation.

What happens to 'unfavourable' alleles in a population over time?

Individuals with unfavourable alleles are less likely to survive and reproduce. Consequently, these alleles are passed on less frequently and eventually decrease in frequency or are lost from the population.

Evolution by Natural Selection | Pearson Edexcel A-Level Biology

Evolution by Natural Selection

Summary

Evolution is the gradual change in the heritable characteristics of a population over time, primarily driven by the mechanism of natural selection. This process relies on existing genetic variation, environmental selection pressures, and the differential reproductive success of individuals possessing advantageous alleles.

1. Definition & Core Concepts

Evolution is defined as the change in the frequency of heritable characteristics within a population over successive generations. It is a process that occurs over long periods, often millions of years, leading to the diversity of life observed today.

Heritable characteristics are traits determined by the specific alleles (variants of genes) present in an individual's DNA. Only these genetically encoded traits can be passed to offspring and contribute to the evolutionary process.

Natural Selection is the primary mechanism of evolution, where individuals with traits better suited to their environment are more likely to survive and reproduce.

A diagram showing a population with mixed traits (red and blue) undergoing selection pressure, resulting in a final population dominated by the advantageous trait (blue).

2. Underlying Principles of Variation

Natural selection can only occur if there is variation within a population. This variation stems from differences in DNA base sequences between individuals.

The primary sources of genetic variation include random mutations (creating new alleles), meiosis (independent assortment and crossing over), and random fertilization during sexual reproduction.

Without genetic diversity, a population cannot adapt to changing environments because there would be no 'advantageous' alleles to select for.

3. Methods: The Mechanism of Selection

Variation: A population contains individuals with different alleles for a specific trait.
Selection Pressures: Environmental factors such as predation, competition for food, or disease affect the survival chances of individuals.
Differential Survival: Individuals with 'advantageous' alleles are better adapted and more likely to survive to adulthood, often called 'survival of the fittest'.
Reproduction: Survivors mate and pass their advantageous alleles to the next generation.
Allele Frequency Change: Over many generations, the frequency of the advantageous allele increases, while disadvantageous alleles may be lost.

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions

Evolution by Natural Selection

Summary

1. Definition & Core Concepts

Natural Selection is the primary mechanism of evolution, where individuals with traits better suited to their environment are more likely to survive and reproduce.

A diagram showing a population with mixed traits (red and blue) undergoing selection pressure, resulting in a final population dominated by the advantageous trait (blue).

2. Underlying Principles of Variation

Natural selection can only occur if there is variation within a population. This variation stems from differences in DNA base sequences between individuals.

Without genetic diversity, a population cannot adapt to changing environments because there would be no 'advantageous' alleles to select for.

3. Methods: The Mechanism of Selection

Variation: A population contains individuals with different alleles for a specific trait.
Selection Pressures: Environmental factors such as predation, competition for food, or disease affect the survival chances of individuals.
Differential Survival: Individuals with 'advantageous' alleles are better adapted and more likely to survive to adulthood, often called 'survival of the fittest'.
Reproduction: Survivors mate and pass their advantageous alleles to the next generation.
Allele Frequency Change: Over many generations, the frequency of the advantageous allele increases, while disadvantageous alleles may be lost.

4. Key Distinctions

It is vital to distinguish between traits that are inherited and those that are acquired during an organism's life.

Feature	Heritable Characteristics	Acquired Characteristics
Origin	Determined by alleles/DNA	Result of environment/lifestyle
Inheritance	Passed to offspring	Not passed to offspring
Evolution	Drives evolutionary change	Does not lead to evolution
Example	Natural fur color	Muscle mass from exercise

5. Exam Strategy & Tips

Avoid Teleology: Never use phrases like 'the organism evolved a trait so that it could survive.' Evolution is a passive, random process, not a purposeful response to a need.
Focus on Alleles: When explaining a scenario, always mention that advantageous alleles are passed on, not just 'traits' or 'genes'.
Population vs. Individual: Remember that individuals do not evolve; only the population evolves as allele frequencies shift over generations.
Check for Selection Pressures: Always identify the specific environmental factor (e.g., a specific predator or a limited resource) that makes one trait more advantageous than another.

6. Common Pitfalls & Misconceptions

A common mistake is assuming that 'survival of the fittest' only means being the strongest or fastest. In biological terms, 'fitness' refers to reproductive success—the ability to survive and pass on genes.

Students often forget that mutations are random. An environment does not 'cause' a mutation to appear; rather, the environment selects for mutations that already happened to exist.