MadSci Network: Evolution
Query:

Re: What factors would encourage reverse evolution?

Date: Thu Feb 17 08:29:03 2000
Posted By: James Cotton, Graduate Student
Area of science: Evolution
ID: 948463886.Ev
Message:

Dear Brad,

To be honest, I’m not entirely certain what you’re getting at with your question, but I’ll try and help as best I can. I’ve done a few internet searches, and come up with somebody claiming that:

"The Law of Reverse Evolution (or Backward Evolution as it is sometimes referred to in the science of Behavioral Genetics) simply states that when there are no upward forces (such as intelligence, competency, courtesy, rationality, etc.) acting to control the minimum for participation in a given activity (like driving, for example), then the average intellect of the group engaged in the activity slowly falls the lowest common denominator, or zero, (and this is critical!) whichever is lower". (Link to source)

The same source claims that this ‘operates counter to Darwin's law of Natural Selection’.
 

I assume you mean something like this.

This, of course is not any kind of scientific law, but just a common kind of saying. Certainly behavioural geneticists believe no such thing. There is however, something similar that happens in evolution, and is of interest to biologists.

For example, the coccyx, which is the little bony bump at the base of your spine, is a remnant of the tail that most other old-world (African and Asian) primates possess. During the evolution of the great apes and the hominids [humans and their cousins, like Homo erectus (Java or Peking man)] man left a life in trees and became associated with grasslands, which were spreading at the time, thanks to the evolution of large mammalian herbivores, relatives of the horses, cows, elephants etc that we see today. Bipedal grassland-dwelling apes had no use for a tail, so it gradually shrunk to leave just the coccyx humans have. Other examples include the appendix (a remnant of the large caecum of other mammals), and the unusual hair-like feathers of some flightless birds, which are much reduced from the complex feathers of other birds.

I must emphasise, however, that this is IN NO WAY counter to natural selection, and indeed is a great example of the power of natural selection. The reduction of the tail in humans occurred because apes with even slightly shorter tails, which occurred occasionally due to small genetic mutations, on average left very slightly more descendants than the other apes, and these descendants in turn tended to have slightly shorter tails (on average, you tend to resemble your parents more than you resemble other people). Over a long period of time, the tail length of the ape population as a whole will fall. We suppose that shorter-tailed apes had more children because they don’t have an unwieldy tail that served no useful function as it had in arboreal (tree-living) apes, or simply because they don’t have to invest precious energy in building and maintaining useless tail flesh. I have just described exactly the way evolution by natural selection works.

I hope this helps! Another possible meaning for reverse evolution is evolution working to make an organism, or a particular part of an organism, more like its ancestors. This could, in principle, happen whenever an organism that more closely resembles some ancestral organism in some way is more successful in a particular environment that other organisms in a species, just as I explained before – it will leave more offspring, and so the population will become more ‘ancestor-like’. It is, however, pretty unlikely this will happen, as natural selection can only act upon mutants that happen to appear. It is unlikely, particularly at the molecular level – the level of DNA – that a mutation would appear that returned any of the organisms genes to be identical to an ancestral gene, simply because there are so many billions and billions of different mutations that could occur. This kind of reverse evolution would be more likely If there were some constraints on what sort of organisms occur, and if these constraints were quite strict. These are quite complicated ideas, though, and I’m hesitant to go into too much depth – basically, the way animals, in particular, but also every other sort of living thing, are built from the instructions in their DNA depends upon the interaction of thousands or even millions of different protein molecules, translations of millions and millions of different DNA genes. The vast majority of possible mutations never occur (organisms having them die very early in development) because the new mutant protein cannot interact properly with others. If this sort of thing is very common, then organisms that look very much like their ancestors could appear because only a few thousands of different shapes are possible from mutations for that particular organism, and one, or a few of these, could be ‘ancestral’, making it appear that evolution has reversed.

I hope this quick answer is of some help – feel free to ask more questions about any of this if you need more information. Sorry it took a while to answer,

Yours,

James

James Cotton.
 
 

If you’d like to read more about how evolution and natural selection work, I can recommend ‘The Blind Watchmaker’ by Richard Dawkins, or, slightly drier and more difficult, but excellent, ‘Evolution’ by Mark Ridley.


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