In this chapter, Carroll introduces the role of probability into evolutionary theory, although, he accidentally displays how probability is often misused. It may be true that the chance of being eaten by a shark is 1 in 300 million – if 300 million people actually exposed themselves to shark attack each year. In reality, it is only those who enter shark populated water that are actually at risk – therefore, the real risk to those putting themselves in harm’s way is actually higher. This may seem to be a minor point, but it serves as a reminder of the misuse of probabilities by creationists. They often build straw man arguments to claim that evolution is mathematically impossible – usually involving inane positions that pretend 300 amino acids randomly coming together to form a functional protein.
Carroll does however convincingly demonstrate using models and real studies that evolution can produce new traits in a population in a relatively short time. He cites the change in the peppered moth population from light coloured to dark coloured in response to environmental pollution darkening the trees. This has probably happened on at least 4 separate occasions, as there are 4 known different mutations that cause the dark phenotype. He also gives a more recent example of pigeon populations developing a white rump. This gives them an advantage when it comes to avoiding attack by falcons; perhaps by momentarily confusing the falcon as the pigeon rolls to avoid the attack.
The rate of change in the population is proportional to the selection coefficient, and his example of mouse populations becoming all black in less than 2000 years is discussed here. He makes the point that not all individuals survive and that this is determined by the advantage/disadvantage a particular gene confers on its owner. There is also some discussion about whether natural selection acts on small or large differences between organisms. He comes down on the side of small changes. However, I would argue that it acts on both, as small changes in genes can actually cause a big difference in a characteristic. An example would be the genes controlling the sizes of tomato plants.
He introduces the idea of plasticity in species at the start of the chapter (and its role in Darwin forming his ideas of evolution) and ends with listing some of the possible types of mutation that occur – insertions, deletions, inversions, duplications, cut and paste, single nucleotide substitutions etc and will discuss some of these later in relation to evolving new functions. He also attacks the notion that all mutations are harmful. Some are actually beneficial (see above and here) and some a neutral and have no effect on function. This latter type of mutation can be used to study the effect of selective pressures on the rates of gene changes – by providing a reference point to the effect of random cumulative mutation.
Carroll does however convincingly demonstrate using models and real studies that evolution can produce new traits in a population in a relatively short time. He cites the change in the peppered moth population from light coloured to dark coloured in response to environmental pollution darkening the trees. This has probably happened on at least 4 separate occasions, as there are 4 known different mutations that cause the dark phenotype. He also gives a more recent example of pigeon populations developing a white rump. This gives them an advantage when it comes to avoiding attack by falcons; perhaps by momentarily confusing the falcon as the pigeon rolls to avoid the attack.
The rate of change in the population is proportional to the selection coefficient, and his example of mouse populations becoming all black in less than 2000 years is discussed here. He makes the point that not all individuals survive and that this is determined by the advantage/disadvantage a particular gene confers on its owner. There is also some discussion about whether natural selection acts on small or large differences between organisms. He comes down on the side of small changes. However, I would argue that it acts on both, as small changes in genes can actually cause a big difference in a characteristic. An example would be the genes controlling the sizes of tomato plants.
He introduces the idea of plasticity in species at the start of the chapter (and its role in Darwin forming his ideas of evolution) and ends with listing some of the possible types of mutation that occur – insertions, deletions, inversions, duplications, cut and paste, single nucleotide substitutions etc and will discuss some of these later in relation to evolving new functions. He also attacks the notion that all mutations are harmful. Some are actually beneficial (see above and here) and some a neutral and have no effect on function. This latter type of mutation can be used to study the effect of selective pressures on the rates of gene changes – by providing a reference point to the effect of random cumulative mutation.
4 comments:
I thought the maths was very useful in showing how simple it all is and not that unlikely.
It also meant I didn't have to worry too much about biology :-)
One thing that could have been emphasised too was the fact that sex speeds up the process of spreading a gene through a population. In asexual reproduction, only your direct decendants possess a trait. With sex, you can recombine with unrelated individuals and transmit the trait.
Another thing pf course is that multiple traits can be selected at the same time. It is naot a case that an animal must develop longer legs then shock absorbing tendons. Bot can happen concurrently.
I enjoyed this chapter - possibly because, when it comes to maths and statistics I'm, as it were, a toddler rather than a baby when it comes to how much I've learned; and, as you say, the central point is convincingly explained...
It's interesting you picked up on the shark attack thing. I did eyebrow exercises myself a bit here, since, in terms of what the book is about, the impression I have (perhaps entirely wrongly?) is that the immediate environment and direct evolutionary pressures are something we can't avoid, whereas you can (at least if you live where I do) exclude youself from any risk at all of shark attack if you're so minded; it's much harder (again, at least here) to entirely avoid contact with dogs, which would obviously make them more "dangerous".
As you say, it's a minor point however you slice it...
Mark_W
in terms of what the book is about, the impression I have (perhaps entirely wrongly?) is that the immediate environment and direct evolutionary pressures are something we can't avoid
Exactly, organisms generally have little say in avoiding selective pressures - we are however different in that respect - we are better able to control our environment - food, medicine, clothing etc... That is not to say that other animals dont have some ability to control their environment - eg beavers bulding dams
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