B1.8 Evolution
Summary
Particular genes or accidental changes in the genes of plants or animals may give them characteristics which enable them to survive better. Over time this may result in entirely new species. There are different theories of evolution. Darwin’s theory is the most widely accepted.
Darwin
- Darwin’s 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.
- These first organisms were very simple single-celled organisms, similar to bacteria.
- The theory of evolution by natural selection was only gradually accepted because:
- the theory challenged the idea that God made all the animals and plants that live on Earth
- there was insufficient evidence at the time the theory was published to convince many scientists
- the mechanism of inheritance and variation was not known until 50 years after the theory was published.
Conflicting theories on evolution
- Scientists may produce different hypotheses to explain similar observations.
Before Darwin, Lamarck used a different theory.
- He proposed a theory of acquired characteristics.
- He argued that an individual’s characteristics will change over its lifetime due to amount of use.
- Giraffes necks get longer due to stretching for leaves.
- Then he suggested that these characteristics could be inherited.
- We now know that in the vast majority of cases this type of reproduction cannot occur.
- Changes in the body do not change the genes.
Classification
- Studying the similarities and differences between organisms allows us to classify living organisms into animals, plants and microorganisms.
- This helps us to understand evolutionary and ecological relationships.
- Models allow us to suggest relationships between organisms.
- Evolutionary trees (models) are used to represent the relationships between organisms.
- This tree indicates how long ago the ancestors of ape species diverged from each other.
- It indicates that humans share a common ancestor with chimpanzees more recently than with any other ape species.
Darwin’s theory of natural selection
- Organisms produce large numbers of offspring
- Individual organisms within a particular species may show a wide range of variation because of differences in their genes
- There is a struggle for all organisms to exist (eg. Predation)
- Individuals with characteristics most suited to the environment are more likely to survive to breed successfully
- The genes which have enabled these individuals to survive are then passed on to the next generation.
- Gradually, this can result in changes in the characteristics of a species.
Mutations
- How do mutations occur?
- Errors occur when the DNA is replicated prior to cell division.
- Errors may occur when chromosomes are separated during cell division.
- What do mutations do?
- Genes control the synthesis of proteins.
- Therefore a change in a gene or a new sequence of genes can result in different proteins being synthesised.
- This can change a characteristic.
- Most mutations are harmful or fatal.
- Occasionally, some mutations are useful.
- When new forms of a gene result from mutation, and they give rise to useful characteristics, there may be more rapid change in a species if the environment changes.