Types of natural selection
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Explainer Video
Summary
Types of natural selection
In a nutshell
Natural selection is the process by which populations of species adapt to their environment. Directional selection is when the advantageous allele is for an extreme phenotype, while stabilising selection is for a median phenotype. Both these types of selection can be applied to a variety of adaptations. The adaptations could range from behavioural, physiological and even anatomical.
Natural selection
Definition
Natural selection refers to the adaptation of a species population to environmental change. The process of natural selection is outlined below.
| 1. | Genetic mutations arise that can sometimes be beneficial. |
| 2. | Individuals with the mutated alleles may be better suited to their environment. |
| 3. | This means they can survive and reproduce at a higher frequency. |
| 4. | Limited resources cause individuals with unfavourable alleles to reproduce at a lower frequency. |
| 5. | Over time, mutated alleles becomes more common within the gene pool, leading to evolution. |
Types of natural selection
There are various types of natural selection and each impacts allele frequency differently. The two types of natural selection are directional selection and stabilising selection.
Directional selection | Stabilising selection |
| Directional selection occurs when natural selection favours an allele for an extreme phenotype. This leads to the allele frequency shifting over time toward that extreme phenotype. | Stabilising selection occurs when natural selection favours an allele for a median phenotype. This leads to the allele frequency stabilising rather than shifting to either extreme phenotype. |
 A. Optimum fur length, B. Selection pressure, C/D. Individuals with fur length greater than , 1. The mean fur length in a stable climate is , 2. The temperature decreases, this causes a selection pressure, 3. Over a number of generations there is a shift in the mean fur length, 4. The mean fur length is now . |  A. Selection pressure, B. Optimum fur length. 1. There is a wide range of fur lengths, 2. In a stable climate the individuals with very short or very long hair die. |
ExampleSome bacteria in a population have alleles that make them resistant to an antibiotic. When the population is exposed to the antibiotic, those without resistance are killed. Resistant bacteria survive and reproduce, this passes on antibiotic-resistance genes. Eventually, most bacteria will carry the antibiotic-resistant allele. | ExampleAt birth, humans have a variety of weights. Babies that are very small and have a low birth weight are less likely to survive as they struggle to maintain their body temperature. It is difficult to give birth to large babies so they are also less likely to survive. Medium-sized babies are therefore most likely to survive so weight shifts towards the middle of the range. |
Testing antibiotics
You can test the effect of different antibiotics on the growth of bacteria.
Procedure
| 1. | Using a sterile pipette, transfer the bacteria to an agar plate. Spread the bacteria over the agar plate using a plastic spreader. |  |
| 2. | Place paper discs soaked in antibiotics equal space apart on the agar plate. Include one disc soaked only in water as a negative control | |
| 3. | Tape the lid onto the agar plate, invert and incubate at for . |  |
| 4. | Any areas where the bacteria cannot grow, as it has been inhibited by the antibiotic, will appear clear. This is known as a zone of inhibition. The larger the zone of inhibition, the greater the effect of the antibiotic on the bacteria. You can calculate the zone of inhibition using this formula: . |  |
FAQs - Frequently Asked Questions
What is stabilising selection?
Stabilising selection occurs when natural selection favours an allele for a median phenotype.
What is directional selection?
Directional selection occurs when natural selection favours an allele for an extreme phenotype.
What is natural selection?
Natural selection refers to the adaptation of a species population to environmental change.