Investigating selection Flashcards
What is stabilising selection? Give an example of stabilisng selection in humans.
Stabilising selection is where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce. An example of stabilising selection in humans is birth weight.
What is directional selection?
occurs when environmental conditions change
individuals with a phenotypes suited to the new conditions are more likely to survive and pass on their genes
overtime the mean of the population will move towards this characteristic
A scientist was studying how an antibiotic affects the frequency of the alleles of gene A in a species of bacteria. She grew the bacteria in her laboratory and after 25 generations, she introduced the antibiotic to the bacterial culture. She sampled the culture at regular intervals throughout the experiment to monitor the frequency of two alleles of gene A. allele X and allele Y. Her results are shown in the graph on the right. Describe what the data is showing.
ii) Suggest how directional selection could have caused the results shown in the graph above.
The data shows that up until generation 25, both the alleles had the same frequency within the population. After the antibiotic was introduced in generation 25, the frequency of allele X increased whilst the frequency of allele Y decreased.
ii)Allele X may provide some sort of resistance against the antibiotic, while allele Y likely does not. Antibiotic resistant individuals were more likely to survive and reproduce, so the frequency of allele X increased.
The graph on the right shows the change in frequency of different clutch sizes (the number of eggs laid) in two populations of robins across 20 generations. Describe what the data for Habitat 1 is showing. What type of selection does this data show?
In both generations, medium-sized clutches showed the highest frequency. Between generation 1 and generation 20, there was a decrease in the range of clutch sizes with both the smallest and largest clutch sizes decreasing in frequency. There was also an increase in the frequency of medium-sized clutches. This data shows that stabilising selection took place.
ii) Habitat 2 has a greater abundance of food than Habitat 1. Suggest how this is reflected in the data shown.
6) E.g. the mean clutch size is higher in Habitat 2 than in Habitat 1. To increase the number of chicks that survive, female robins should lay as many eggs as possible - however, the amount of food available limits the number of chicks they can rear, so clutch size is restricted. As there is more food available in Habitat 2, clutch sizes can be bigger than in Habitat 1.
How could you sterilise glassware before and after an experiment?
you can sterilise the glassware in an autoclave (a machine that steams equipment at a high pressure).
When working with microorganisms, how should you minimise the contamination of work surfaces by unwanted microbes?
You should regularly disinfect work surfaces to minimise contamination. You shouldn’t place used utensils on the work surface either -any contaminated utensils should be placed in a beaker of disinfectant.
Why are aseptic techniques important when examining microbial cultures?
Aseptic techniques prevent contamination of cultures by unwanted organisms, which can affect the growth of the microorganism you’re working with. It’s also important to avoid contamination with disease-causing microbes that could make you ill.
Explain two ways you could make use of a Bunsen burner when culturing bacteria.
E.g. if you have a liquid culture in a glass container, you can use a Bunsen burner to briefly flame the neck of the container just before it’s opened and closed. This will cause air to move out of the container and prevent unwanted microorganisms from moving in. You could also work near a Bunsen flame so that microbes are drawn away from your culture by the rising hot air. (These are both examples of aseptic techniques.)
A microbiologist is researching the viability of three new antibiotics in treating E. coli infections. The diagram on the right shows a Petri dish prepared with E. coli. It has been incubated with paper discs soaked in different concentrations of each antibiotic for 48 hours.
The negative control is a paper disc soaked in sterile water. Why was it important to include a negative control?
A negative control ensures that the addition of the antibiotic is the only variable that can affect the growth of the bacteria.
Which of the antibiotics is E. coli most likely to be resistant to? Explain your answer.
E. coli is most likely to be resistant to antibiotic C because there aren’t any inhibition zones around the paper discs at either concentration of antibiotic C. This suggests that the bacteria were unaffected by the antibiotic and were able to grow normally.
Which of the antibiotics would be the most viable for treating E. coli infections? Explain your answer.
Antibiotic B would be the most viable antibiotic for treating E. coli. The discs soaked in antibiotic B have the largest inhibition zones, so antibiotic B would be the best antibiotic to treat E. coli infections as it has the strongest inhibitory effect.
