changing population characteristics Flashcards
natural selection
All organisms are exposed to selection pressures. These are factors that affect the organism’s chances of survival or reproductive success (the ability to produce fertile offspring).
Organisms that are best adapted to their environment are more likely to survive and reproduce. As a result of natural selection these adaptations will become more common in the population.
Organisms that are poorly adapted are less likely to survive and reproduce. Therefore their characteristics are not passed on to the next generation. As a result, less of the population will display these characteristics.
Natural selection follows a number of steps:
1)Organisms within a species show variation in their characteristics that are caused by differences in their genes (genetic variation).
For example, they may have different alleles of a gene for a particular characteristic. New alleles can arise by mutation.
2)Organisms whose characteristics are best adapted to a selection pressure such as predation, competition (for mates and resources) or disease, have an increased chance of surviving and successfully reproducing. Less well-adapted organisms die or fail to reproduce.
This process is known as ‘survival of the fittest’.
3)Successful organisms pass the allele encoding the advantageous characteristic onto their offspring. Conversely, organisms that possess the non-advantageous allele are less likely to successfully pass it on.
4)This process is repeated for every generation. Over time, the proportion of individuals with the advantageous adaptation increases. Therefore the frequency of the allele that codes for this particular characteristic increases in the population’s gene pool.
5)Over very long periods of time, many, many generations and often involving multiple genes, this process can lead to the evolution of a new species.
antibiotic resistant bacteria
Methicillin-resistant Staphylococcus aureus (MRSA) has developed resistance to many antibiotics. Bacteria reproduce very rapidly and so evolve in a relatively short time. When bacteria replicate, their DNA can be altered and this usually results in the bacteria dying.
However, a mutation in some S. aureus arose that provided resistance to methicillin.
When the bacteria were exposed to this antibiotic, resistant individuals survived and reproduced, passing the allele for resistance on to their offspring. Non-resistant individuals died. Over time the number of resistant individuals in the population increased.
peppered moth
Dramatic changes in the moth’s environment in the 19th century caused changes in allele frequency in peppered moths (Biston betularia). Before the industrial revolution, most peppered moths in Britain were pale coloured. This provided camouflage against light-coloured tree bark, increasing their chance of survival. Those that were dark were easily spotted by birds and eaten. The different colourings are due to different alleles.
During the industrial revolution many trees became darker - partly due to being covered in soot, and partly due to the loss of lichen cover caused by increased atmospheric pollutants. The dark moths were now better adapted, as they were more highly camouflaged. More dark peppered moths survived and reproduced, increasing the frequency of dark moths (and the ‘dark’ allele) in the population. After a few years the number of dark peppered moths close to industrial towns and cities became much higher than pale peppered moths.
Since the Clean Air Act of 1956 steps have been taken to improve air quality in towns and cities, and to reduce the levels of pollution released from factories. The bark on the vast majority of trees in the UK is once again lighter coloured, and therefore the frequency of the pale allele
in the moth gene pool has increased.
sheep blowflies
Sheep blowflies (Lucilia cuprina) lay their eggs in faecal matter around a sheep’s tail - the larvae then hatch and cause sores. This condition is known as ‘flystrike’, and if left untreated is normally fatal.
In the 1950s in Australia, the pesticide diazinon (an organophosphate pesticide) was used to kill the blow flies and prevent the condition.
Within six years, blowflies had developed a high level of resistance to diazinon. Individual insects with resistance survived exposure to the insecticide, and passed on this characteristic through their alleles, allowing a resistant population to evolve.
Io investigate how this evolution occurred so quickly, scientists extracted DNA from a sample of 70-
year-old blowflies kept at
the Australian National Insect Collection. Two Australian sheep blowflies were studied, Lucilia cuprina and the closely related Lucilia sericata. The researchers compared the blowflies’ resistance
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genes before and after the introduction of the pesticide. Diazinon resistance was not found in the DNA of the 70-year-old flies, whereas it is present in the modern species. However, when they performed the same investigation with malathion (another organophosphate pesticide), they found resistance alleles in both the old and modern blowflies, showing there was pre-existing resistance to this chemical.
The scientists concluded that pre-adaptation contributed to the development of diazinon-resistance. Pre-adaptation is when an organism’s existing trait is advantageous for a new situation. The alteration in the DNA that caused the pre-existing resistance allowed the flies to rapidly develop resistance to organophosphate chemicals in general, and ultimately a specific diazinon-resistance allele.
The existence of pre-adaptation in an organism may help researchers predict potential insecticide resistance in the future.
flavobacterium
Most evolution occurs as a negative result of selection pressures.
However, some organisms have evolved due to opportunities that have arisen in their environment. For example, scientists have found a new strain of Flavobacterium living in waste water from factories that produce nylon 6. Nylon 6 is used to make objects like toothbrushes and violin strings. This strain of bacteria has evolved to digest nylon and is therefore beneficial to humans as they help to clear up factory waste.
These bacteria use enzymes to digest the nylon known as nylonases.
They are unlike any enzymes found in other strains of Flavobacterium, and they do not help the bacteria to digest any other known material.
It is beneficial to the bacteria as it provides them with another source of nutrients.
Most scientists believe that the gene mutation that occurred to produce these enzymes was a result of a gene duplication, combined with a frameshift mutation (an insertion or deletion of DNA bases that causes the genetic code to be read incorrectly).
