Introduction

Biology: IB Diploma Higher Level, topic 3.3.1

Principles of Natural Selection

Natural selection is the process that leads to evolution in a population. In order to understand how this process functions, it is important to learn the key principles of natural selection.

Random mutation

Mutations are changes in the gene sequence of DNA.
Random mutations can arise spontaneously.
New mutations can result in new alleles of a gene.

Variation in mutations

Most mutations are harmful because they alter the normal functioning of a gene.
But by chance, some mutations might confer a beneficial trait.
An individual with a beneficial mutation will be more likely to survive and reproduce than an individual without the mutation.

Inheritance

When an individual reproduces, their offspring will inherit half of their genes.
When an individual with an advantageous mutation reproduces, the allele is inherited by their offspring.

Change in allele frequency

Individuals in the next generation who have the advantageous mutation are also more likely to reproduce and pass on the allele.
Over many generations, the new allele will increase in frequency in the population.
The change in allele frequency over time is called evolution.

Natural Selection and Evolution

Natural selection is the process where the frequency of alleles in a population changes over time. Natural selection is a process that gives rise to evolution.

Selective advantage

Genetic variation exists between individuals in a population.
Some individuals will be more likely to survive (e.g. by being better at fighting disease) than others.
Individuals that are better at surviving than others have a selective advantage.

Producing offspring

Individuals with a selective advantage are more likely to survive to reproduce than others.
This means that the genes of an individual with a selective advantage are more likely to be passed onto offspring than the genes of an individual without a selective advantage.

Increasing allele frequencies

Individuals with a selective advantage are more likely to pass on their beneficial alleles than other individuals.
The next generation is more likely to have alleles that provide a selective advantage than alleles that do not.
This generation is also more likely to survive to reproduce and pass on their genes.
This causes the alleles that provide a selective advantage to increase in frequency in the population.

Natural selection

The process where the frequency of beneficial alleles increases over time is called natural selection.
Natural selection controls the frequency of alleles in a population.
If a harmful allele develops in an individual, this individual is less likely to survive and the harmful allele will decrease in frequency. This is also natural selection.

Evolution

The process of natural selection gives rise to evolution.
Evolution is defined as a change in allele frequencies over time.
E.g. The evolution of humans is the change in allele frequencies that has taken place over millions of years. This change has been driven by natural selection.

Test yourself on Introduction

Which of the following statements about natural selection are true?
What is the definition of evolution?
Which of the following statements about natural selection are true?
What is the name of the process where the frequency of beneficial alleles increases over time?

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