9.4 - Types of selection Flashcards
What is selection
- The process by which organisms that are better adapted to their environment tend to survive and breed, while those that are less well adapted tend not to
- every organism is subjected to it, based on its suitability for surviving the conditions that exist at the time
- different environmental conditions favour different characteristics in the population
What are the 2 types of selection
- Directional selection
- stabilising selection
What is directional selection
Selection may favour individuals that vary in one direction from the mean of the population. This is called directional selection and changes the characteristics of the population
What is stabilising selection
selection may favour average individuals. This is called stabilising selection and preserves the characteristics of a population
Describe the process of directional selection
- If the environmental conditions change, the phenotypes that are best suited to the new conditions are most likely to survive
- Some individuals, which fall either left/right of the mean, will posses a phenotype more suited to the new conditions
- these individuals will be more likely to survive and breed
- therefore they will contribute more offspring (and the alleles these offspring possess) to the next generation than other individuals
- over time, the mean will then move in the direction of these individuals
Describe what a distribution curve is
- Most characteristics are influenced by more than 1 gene (polygenes)
- these types of characteristics are more influenced by the environment than ones determined by a single gene.
- the effect of the environment on polygenes produces individuals in a population that vary about the mean
- when you plot this variation on a graph you get a normal distribution curve
Explain one example of directional selection
Antibiotic resistance to penicillin
- spontaneous mutation occurs in the allele of a gene in a bacterium that enabled it to make a new protein. The new protein was an enzyme that broke down the antibiotic penicillin before it was able to kill the bacterium. The enzyme was given the name penicillinase
- The bacterium happened to be, by chance, in a situation where penicillin was being used to treat an individual. This meant that the mutation gave the bacterium an advantage in being able to use penicillinase to break down the antibiotic and so survive while the rest of the population of bacteria was killed by it
- the bacterium that survived was able to divide by binary fission to build up a small population of penicillin-resistance bacteria
- members of this small penicillin-resistance population were more able to survive, and therefore multiply, in the presence of penicillin than members of the non-resistant group
- The population of the penicillin-resistant bacteria increased at the expense of the non-resistant population. Consequently the frequency of the allele that enabled the production of penicillinase increased in the population
- The population’s normal distribution curve shifted in the direction of a population having greater resistance to penicillin
In the example of bacteria resisting antibiotics, why did the bacteria mutate once in the presence of antibiotics
- it didn’t = mutations occur randomly and are very rare
- because there was so many bacteria around = the total number of mutations is large
- most of these mutations will be harmful or give no advantage to the bacterium
- very rarely = a mutation can be advantageous in certain circumstances
—> therefore: directional selection results in phenotypes at one extreme of the population being selected for and those at the other extreme being selected against
Describe the process of stabilising selection
- If environmental conditions remain stable, it is the individuals with phenotypes closest to the mean that are favoured
- these individuals are more likely to pass their alleles on to the next generation
- those individuals with phenotypes at the extreme are less likely to pass on their alleles
- stabilising selection therefore tends to eliminate the phenotypes at the extremes
Explain one example of stabilising selection
Human birth weights
- stabilising selection results in phenotypes around the mean of the population being selected for and those at both extremes being selected against
- The body mass at birth of babies born at a hospital was measured over a 12-year period. %of births in the population (y-axis on the left) is plotted against birth mass of the infants as a histogram
- over the same period, the infant mortality rate was also recorded. The infant mortality rate is measured on a logarithmic scale (y-axis on the right) and plotted against infant body mass at birth as a line graph
- by looking at the line for infant mortality, the line climbs steeply where the birth weight is below 2.5kg and again where it is above 4.0kg. In other words, there is a much greater risk of infant death when the birth weight is outside the range 2.5-4.0kg.
- this illustrates stabilising selection because the mortality rate is greater at the 2 extremes. The infants with the highest and lowest birth masses are more likely to die (are being selected against) while those around the mean are less likely to die (are being selected for/favoured). The population’s characteristics are being preserved rather than changed
- stabilising selection therefore results in phenotypes around the mean of the population being selected for and those at both extremes being selected against
What are the different adaptations an organism may gain as a result of natural selection
- Anatomical
- Physiological
- Behavioural
What is an anatomical adaptation
Such as shorter ears and thicker fur in artic foxes compared to foxes in warmer climates
What is a physiological adaptation
For example oxidising fat rather than carbohydrate in kangaroo rats to produce additional water in a dry desert environment
What is a Behavioural adaptation
Such as the autumn migration of swallows from the UK to Africa to avoid food shortages in the UK winter
