Types Of Selection Flashcards
what is directional selection?
- selection may favour individuals that vary in one direction from the mean of the population
- changes the characteristics of the population
what is stabilising selection?
- selection may favour average individuals
- preserves the characteristics of a population
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
- depending on which characteristics are favoured selection will produce a number of different results
what is a polygene?
-more than one gene
which genes are most likely to be influenced by the environment?
- polygenes
- the effect of the environment on polygenes produces individuals in a population that vary about the mean which produces a normal distribution curve
how does directional selection occur with resistance to penicillin?
- a spontaneous mutation occurred in the allele of a gene in a bacterium that enabled it to make a new protein which could break down penicillin before it was able to kill the bacterium
- the mutation in the bacterium gave it the advantage of being able to use penicillinase to break down the antibiotic and so survive while the rest of the bacterial was killed
- the surviving bacteria was able to divide by binary fission to build up a small population of penicillin resistant bacteria
- membranes of this small penicillin resistant population were more able to survive and multiply in the presence of penicillin than members of the non resistant population
- the population of penicillin resistant bacteria increased at the expense of the non resistant population so the frequency of the alele that enabled the production of penicillicase increased in the population
- the populations normal distribution curve is shifted in the direction of a population having greater resistance to penicillin
what does directional selection result in?
-phenotypes at one extreme being selected for and those at the other extreme selected against
what is directional selection?
- if the environmental conditions change the phenotypes best suited to the change are the most likely to survive
- some individuals which fall to the left or the right of the mean will possess a phenotype more suited to the new conditions
- these individuals will be more likely to survive and breed which will contribute more offspring to the next generation than other individuals
- over time the mean will then move in the direction of these individuals
what is stabilising selection?
- if environmental conditions remain stable it is the individuals with phenotypes closest that are favoured and more likely to pass their alleles on to the next generation
- individuals with phenotypes at the extremes are less likely to pass on their alleles
- stabilising selection tends to eliminate the phenotypes at the extremes such as human birth weights
which adaptions do natural selection cause?
- anatomical
- physiological
- behavioural
what is an example of anatomical selection?
-shorter ears and thicker fur in artic foxes compared to foxes in warmer climates
what is an example of physiological selection?
-oxidising fat rather than carbohydrate in kangaroo rats to produce additional water in a dry desert environment
what is an example of behavioural selection?
-the autumn migration of swallows from the UK to Africa to avoid food shortages in the UK winter
what does stabilising selection result in?
-phenotypes around the mean of the population being selected for and those at both extremes being selected against
why does huma birth weight show stabilising selection?
-infant mortality rises at the extremes below 2.5kg and above 4kg showing these weights cause a greater risk of infant health so the birth weights are narrow between 2.5 and 4kg
