Evolution by natural selection

In a nutshell

Charles Darwin came up with the theory of natural selection and evolution. Evolution can then lead to the development of a new species. Fossils and antibiotic resistance can both provide evidence of evolution.

Natural selection

Charles Darwin developed the theory of evolution by natural selection. Individuals within a population show genetic variation because of differences in their alleles. These differences can arise by mutations.

Disease, predation and competition act as selection pressures as some organisms will have specific characteristics that make them better suited to their environment and this will make them better competitors.

These organisms will be more likely to survive and reproduce, so these useful genes are more likely to be passed on to their offspring. Over time, these useful genes will become more common in the population. The individual has therefore evolved.

Steps of natural selection

1.	Individuals in a population show a wide range of variation in characteristics due to differences in genes.
2.	Individuals in a population have to compete for resources such as food or shelter. They may also have to adapt to a change in their environment.
3.	Individuals with characteristics that are best suited to the environment or that help the individual compete more successfully are more likely to survive and reproduce. Their genes are passed onto the next generation.
4.	Individuals with characteristics that are not well suited to the environment or do not help the individual compete are poorly adapted. They are less likely to survive and therefore less likely to reproduce. Their genes are less likely to be passed down to the next generation.
5.	The genes responsible for useful characteristics are inherited by the offspring.
6.	Over many generations, these genes become more common in the population so the characteristic becomes more common in the population.

Evolution leads to new species

Natural selection changes the phenotype (the visual characteristics) of an organism. Over time, this phenotype can change so much that a new species is made. A new species is made when different populations of the same species change so much that they can no longer breed with each other to produce fertile offspring.

Evidence for evolution

Genetics

Genetics have shown that characteristics can be passed on through an organism's genes and that genes can impact the phenotype of the organism to make them better adapted to their environment.

Antibiotic resistance

Antibiotics are drugs that kill bacteria, however, bacteria can become resistant to these drugs through natural selection and evolution.

Biology; Natural selection and genetic modification; KS4 Year 10; Evolution by natural selection

1.	Variation in the bacterial population means some bacteria develop mutations in their DNA. These mutations may make them resistant to a particular antibiotic.
2.	Other bacteria in the population will not have this mutation so they will still be susceptible to antibiotics.
3.	Antibiotics are added to the population.
4.	The bacteria with the resistance gene survive the antibiotic treatment.
5.	The bacteria without the resistance gene do not survive the antibiotic treatment.
6.	This means that the resistant bacteria can reproduce more as they are not being killed. They will then pass on this resistance gene to their offspring. The antibiotic resistance gene will then become more common in the population over time. Evolution happens much faster in bacteria than other organisms as they reproduce more quickly.

Fossils

Fossils are often found in rocks and they are the remnants of organisms that lived a very long time ago. Scientists use fossils to provide evidence of evolution as comparing different fossils can show much much organisms have changed (evolved) over time.

However, early organisms were soft-bodied and soft tissue decays completely so the fossil record is incomplete. Some fossils also formed millions of years ago so they may have been destroyed by geological activity like the movement of tectonic plates.

Biology; Natural selection and genetic modification; KS4 Year 10; Evolution by natural selection

Biology; Natural selection and genetic modification; KS4 Year 10; Evolution by natural selection