Evolution Through Natural Selection (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Evolution Theory

• Evolution can be defined as:

A change in the inherited characteristics of a population over time

• Evolution occurs by a process known as natural selection

Natural selection

• Charles Darwin, as a result of observations on a round-the-world expedition, backed by years of experimentation and discussion and linked to developing knowledge of geology and fossils, proposed the theory of evolution by natural selection

• Darwin’s theory of natural selection states that:

  • individuals in a species show a wide range of variation caused by differences in genes

  • individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce

  • alleles that code for advantageous characteristics are therefore passed to offspring at a higher rate than those that code for characteristics less suited to survival

  • over many generations these beneficial characteristics become more common in the population and the species changes, or evolves

• The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago

• Natural selection can lead to the development of new species by a process known as speciation

  • Speciation is said to have occurred when two populations have phenotypes that are so different that they can no longer breed together to produce fertile offspring

Evolution diagram

The theory of evolution states that all living things have evolved from simple life forms

Speciation

• Alfred Wallace did much pioneering work on speciation but more evidence over time has led to our current understanding of the theory of speciation

• Speciation is a process that results in the formation of a new species

• When populations of the same species become so different that they are unable to interbreed and produce fertile offspring, they are considered different species and speciation has occurred

• Speciation can occur as a result of a combination of isolation (when populations of the same species become separated) and natural selection:

  • Populations of the same species can become isolated from one another due to the formation of a physical barrier (eg. a new river or mountain range) – this is known as geographic isolation

  • The environment will be different on either side of this physical barrier (eg. different climates or different food available)

  • The environmental differences on either side will provide different selection pressures and natural selection will cause a different set of characteristics to become more common in the two isolated populations

  • Over many generations, individuals from the two populations will have become so distinct (genetically, behaviorally, physically) that they will no longer be able to interbreed and produce fertile offspring

  • The two populations are now separate species

The process of speciation