- The term used to explain how features of the environment apply a selective force on the reproduction of individual in a population
- Charles Darwin proposed natural selection as a mechanism towards the theory. It was controversial at the time as it countered the popular religious beliefs.
Darwin developed his ideas from the expedition he sailed with the HMS Beagle around the Galapagos Islands. Wallace was another naturalist who came to the same conclusion as Darwin.
- Offspring generally appear similar to their parents
- No two individuals are identical
- Organisms have the ability to produce large numbers of offspring
- Populations in nature tend to remain fairly stable in size
- There is a struggle to survive
- Better adapted individuals survive and pass on their characteristics
- Over time a number of changes may give rise to a new species
Natural Selection steps
- Mutation creates an alternative version of a gene (alleles)
- This creates genetic variation between the individuals of a species (intraspecific variation)
- When resources are scarce, the environment will select those variations (characteristics) that give an advantage. There is a selection pressure.
- Individuals with an advantageous characteristic will survive and reproduce
- Therefore they pass on their advantageous characteristics (inheritance)
- The next generation will have a higher proportion of individuals with the successful characteristics. Over time, the group of organisms becomes well adapted to their
Pesticide Resistance in insects
An insecticide applies a very strong selection pressure. If the individual insect is susceptible then it will die, but if it has resistance it will survive and reproduce, spreading the resistance through the entire population.
- E.g. Mosquitos have developed and enzyme that can break down the pyrethroids used to treat mosquito nets.
- E.g. Insect populations have become resistant to the insecticide DDT which binds to a receptor on the plasma membrane of certain cells in insects. This is due to mutations in the genes coding for cell surface receptors.
When insects become resistant it leads to pesticides accumulating in the food chain. When predators eat the insect they may get a large dose of the insecticide. This is why DDT is banned in many areas.
The use of antibiotics is a strong selection pressure on bacteria. MRSA is a very resistant bacteria that has cropped up because of over prescription of antibiotics. Medical researchers are struggling to develop new and effective drugs as the bacterial populations rapidly become resistant to them.
- In the past the world was inhabited by species that were different from those present today.
- Old species have dies out and new species have arisen.
- The new species that have appeared are often similar to the older ones found in the same place.
- One of the most complete fossil records is that of the horse.
- the presence of variety -
- the differences between individuals
Intraspecific variation – variation between members of the same species
Interspecific variation – the differences between species
Continuous variation – variation where there are two extremes and a full range of values in between
Discontinuous variation – variation where there are distinct categories and nothing in between
Causes of Variation
- Inherited or genetic variation
This includes the combination of alleles that is inherited from our parents which is completely unique to us (unless there is an identical twin).
- Environmental variation
Many characteristics are brought out by environmental changes. For example, an overfed pet can become obese and a person’s skin tone may change due to exposure to the sun.
- Combined effects
Humans have become taller as the result of a better overall diet but however well your diet, you are unlikely to grow as tall as other people if your family is short.
Not all genes are active at any one time e.g. puberty is a time when many different genes are activating.
Changes in the environment can also directly affect which genes are active.
1. Sample a population – this has to be random
2. Mean – to show variation between samples
3. Standard Deviation – to show the spread of values about the mean
4. Spearman’s Rank Correlation Coefficient – to consider the relationship of the data
5. Student’s t-test – used to compare two means
Adaptation – a characteristic that enhances survival in the habitat
Anatomical adaptations – structural features
Behavioral adaptations – the ways that behaviour is modified for survival
Physiological adaptations – affect the way that processes work (also called biochemical)
Example :Marram Grass
This is the end of the topic
Drafted by Eva (Biology)