There are a few topics which you can pretty much guarantee will be tested somewhere in the two iGCSE Biology papers. There will be a genetics problem to solve (see later post) and in almost every year there is a question about the process of natural selection. These questions tend to be based around either the evolution of antibiotic-resistant strains of bacteria or an animal example based around some adaptation.
Questions on evolution are usually worth four or five marks and I would suggest you always answer them with bullet points. Mark schemes for these questions are often similar and once you have revised the topic, some time spent with past questions and mark schemes would be time well spent.
Imagine you are set a question about cheetah and high speed running. (Everyone knows cheetah can run for short distances at up to 70 mph: so can the gazelle of course – that’s a coincidence isn’t it?) How did modern-day cheetah evolve to run so fast?
Key ideas to include in your answer:
1) Variation in cheetah population: in any population of cheetah at any point in their evolutionary history, some cheetah will just happen to be able to run a little faster than others. This continuous variation could be due to environmental factors (diet, access to gyms etc.) or it could be due to the combinations of genes they happen to have inherited from their parents, or more likely to a bit of both. Environmental causes of variation are not inherited of course but the genetic ones can be and that’s the key to natural selection.
2) Competition: variation by itself cannot lead to natural selection. If all cheetah survived to breed however slowly they ran, then high-speed cheetah would never have evolved. In my example, cheetah are competing with other cheetah for access to prey species. Gazelle run pretty quick too (I wonder why?) so cheetah who are slower than average will get less food. Conversely if you are a cheetah who just happens due to random genetic variation to be a little quicker than your neighbours, you will get more food, be more healthy and more likely to survive to adulthood.
3) If your particular combination of genes makes you more likely to survive, then you are more likely to breed and pass these genes onto future generations of cheetah. This process is called Natural Selection and it results in certain alleles becoming more frequent in a population over time. In this example, the alleles that produce aerodynamic, long-limbed and muscle-bound cheetah become more frequent over time while alleles building lethargic, over-weight and peaceful cheetah tend not to be passed on as well to future generations.
4) This produces a gradual change in the population over time. Selection is a cumulative process: small changes from one generation to the next can add up to big changes over thousands of generations.
NB This answer does not contain the word mutation and this is quite deliberate on my part. You want to make absolutely clear in your answer that at no point in the history of cheetah, did two slow-running cheetah parents give birth to a “mutant” cheetah with Usain Bolt like qualities. Mutation is a random change in the DNA of an organism and much of the genetic variation described above comes not from new mutations appearing but from the shuffling up of alleles into new combinations in meiosis. These new combinations of alleles can produce new phenotypes and these are the features on which selection can act.
But…… If you are writing about the evolution of antibiotic resistance in bacteria, this can be due to a single mutation. One altered gene can produce an enzyme that will breakdown antibiotic molecules or pump them out of a bacterial cell, so in this example it would be correct to talk about a random mutation occurring in the bacterial population. The key thing here is that these mutations have been occurring randomly for billions of years in bacteria. The mutation is totally independent of the use of antibiotics. All that has happened differently in the past 50 years is that for most of evolutionary history these random mutations would have been harmful to the survival chances of the bacterium unfortunate enough to acquire them. Now in an environment particularly in hospitals flush with antibiotics, these once harmful mutations can give the bacteria a massive selective advantage. Hence the evolution of strains of bacteria in hospitals resistant to a variety of different antibiotics e.g. MRSA and C. difficile
This is an essential topic to get your head around for the exam. Please comment on the blog post if you have any questions or contact me via Twitter.