Read the following extract from one of the essays to see if the intentions of its introduction are carried through. It should be fairly easy to summarise each paragraph into one phrase/sentence, sometimes known as the 'topic sentence.' If you find it difficult to summarise, or ideas seem to lose focus on the question, it may lead you to question whether this piece of writing is as effective as it could be. Topics should link logically throughout the essay and remain relevant to the question throughout.
- Summarise each paragraph in the following essay, taken from the previous activity. Relate each one back to the question: Discuss the role of chance in evolution. This will reveal a lot about how the writer develops their argument.
- Note your overall impression of the way in which this writer has ordered their points. Remember: there is no 'right or wrong' approach.
- Based on the introductions in the previous activity, which one do you think this extract follows on from?
- Mutations are basically random. There is no way of predicting when or where they'll take place. The rate of mutation per gene per cell generation is said to be about 1 ´ 10-5 to 1 ´ 10-6. With such a small rate of mutation, the chances of a gene mutating are extremely small. The chances of the mutation making a difference in the organisms phenotype and considerably smaller still.
- Firstly the mutation has to take place in a stretch of DNA that actually codes for something. So if it, by chance, takes place in a stretch of junk DNA, it will have no effect on the phenotype. Should it actually take place in a gene that codes for a protein, there is still no guarantee it will have any effect since the redundancy of the genetic code means that more than one triplet codes for a particular amino acid. There are actually 6 codes that code for Leucine. There will always be a element of chance involved in every mutation that takes place.
- Further examples come from chromosome evolution. Crossing over of chromosomes usually occurs at homologous points. An imprecise crossing over only occurs due to chance. Deletions are often, but not always, lethal. Again, there is an element of chance as to where the deletion takes place. If the centromere is lost, then the entire chromosome is lost during meiosis since it can’t attach to the spindle.
- 2 or more mutations may have to be present together in order to give the organism any sort of increased fitness or even any change in phenotype. In an asexual population this would require the mutations to take place in the same line of organism. With such a low rate of mutation in any species, the chances of this happening are very small. In a sexual population it’s more likely due to the genetic material from two individuals coming together.
- Any mutation that doesn’t have any effect on fitness will never be selected for or against. The mutated genes would therefore only ever increase or decrease in frequency through random genetic drift. Genetic drift is probably the one area of evolution where chance plays a huge role. With the absence of any sort of selection, it is totally random how the frequencies of alleles change through time.
- Genetic drift can lead to random fixation of alleles. This is mainly due to a sampling error when a random sample of gametes is taken. In larger populations this generally has little effect on the overall frequency of gametes since the combination of biases in different directions leads to no overall bias. In smaller populations, sampling errors can build up in each generation. Drift can also be caused by random differences of survival and reproduction of individuals. If an allele is fixed, any other alleles can no longer reappear other than by mutation. So an entire allele can be lost in a population by chance.
- Genetic drift can also lead to a founder effect. Founders of a new population normally are a small sample of the original population. This means that the genetic variation between the new population is often low. There may also be higher frequencies of certain genes than there were in the original population. When Natural Selection then acts on the founders of their descendents, the outcome is often limited based on the variation in alleles present. This can be seen in the Afrikaners in South Africa who have a much higher proportion of porphyria variegeta(a condition involving severe reactions to barbiturates) than other populations. It is thought that all individuals with the condition are descendents of a Dutch couple.
- Certain events that can influence evolution can be considered to be down to chance. These can cover a wide range in the number of individuals they affect. The mass extinctions that are said to have taken place during the Earth’s history had huge effects on evolution. They opened up countless niches for new organisms to fill. On a smaller scale, there are events that affect particular populations. For example Northern Elephant Seals experienced a genetic bottleneck after their numbers were reduced to about 20 individuals after extensive hunting. The effect is similar to that of the founder effect. The outcome of Natural Selection is limited due to the amount of alleles present. Even when the population grew to 30,000, examination of 24 loci revealed no genetic variation.
- On an even smaller scale, accidents affecting individuals can also have an effect on evolution. If an advantageous mutation takes place in an individual that then gets run over by a bus or killed by a falling rock, any possibility of that allele spreading is immediately removed.
- Even once a mutation has taken place, it’s caused a change in a protein sequence, it’s lead to a phenotypic advantage or disadvantage and Natural Selection has started to act on it, chance still has a role to play. Natural selection can only ever work on the situation at hand. Once evolution has lead down a certain path, it is possible for there to be no way to turn around. How are ancestors evolved millions of years ago will continue to effect how we and our descendents evolve for the next million years (assuming that by chance we don’t go extinct at some point). In humans we can say that chance has lead to the receptors in the eye being the wrong way round. Now that we have a fully functioning eye, there are no advantageous intermediary steps that could lead us to redevelop an eye where the receptors were the right way round.
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