18.5 — types of selection Flashcards
what are selection pressures?
enviromental factors that affect an organism’s survival and reproductive success
what are some selection pressures?
- predation
- competition for resources
- climate change
- disease
why is the over-production of offspring is important in the process of natural selection?
because it leads to competition for limited resources
what happens when there is competition for limited resources?
those best adapted to their environment are more likely to survive, reproduce and pass on their genes — resulting in selection
what are the steps in how variation can drive evolution?
- it generates a range of phenotypes within a population, enhancing the likelihood that some individuals will have advantageous alleles
- individuals with these traits are more likely to survive and reproduce under changing conditions, transmitting the advantageous alleles alleles to their offspring
- natural selection occurs
what are the three types of natural selection?
directional, stabilising and disruptive
what does a graph look like representing directional selection?
look up
what does a graph look like representing stabilising selection?
look up
what does a graph look like representing disruptive selection?
look up
how does directional selection affect phenotypes?
selects one extreme phenotype over other phenotypes
how does stabilising selection affect phenotypes?
selects for the average phenotype and selects against extreme phenotypes
how does disruptive selection affect phenotypes?
selects for extreme phenotypes and selects against the intermediate phenotypes, especially when an enviromental takes two or more distinct forms
what is the effect of directional selection on allele frequency?
increase of allele frequency for one extreme phenotype
what type of phenotypes are selected in directional selection?
one extreme phenotype
what is the effect on the normal distribution curve in directional selection?
shifts the curve in the direction of the favoured extreme
what is an example of directional selection?
antibiotic resistance in bacteria
how does stabilising selection effect allele frequency?
it increases allele frequency for the average phenotype, decreases allele frequency for the extremes
what phenotypes are selected for stabilising selection?
average phenotypes
what is the effect of stabilising selection on the normal distribution curve?
narrows the curve
what is an example of stabilising selection?
human birth weights
what is the effect of disruptive selection on allele frequency?
increases the allele frequency for multiple extreme phenotypes, decreases allele frequency for intermediates
what phenotypes are selected in disruptive selection?
very different extreme phenotypes
what is the effect of disruptive selection on the normal distribution curve?
the curve shifts into multiple peaks either side of where the average phenotype peak was
what is an example of disruptive selection?
bird peaks adapting to become larger and smaller when there are two different food sources