Ontogeny recapitulates phylogeny

 Ontogeny recapitulates Phylogeny

There is a wealth of information concealed in those three little words.

Ontogeny is the development of an individual animal from a fertilized egg and on to the birth of that individual.

Phylogeny is the evolution of life from single cells which floated around in the oceans up to the animals we see around us today including ourselves.  Many of you will have come upon this concept in courses in embryology.

If we look at the early stages of a human fetus, for instance, at a certain point we see wee gill slits on the neck, at a later stage we see a wee tail, both of which are later absorbed and disappear before birth.   At least usually they do.  Occasionally an individual is born in which you can just see the trace of gills and occasionally with a wee vestigial tail.

It is interesting how this occurs.  Take as an example a species of monkey that lives in the tree tops of the jungle.  They have developed a prehensile tail which is of great help keeping them from falling to the jungle floor where they might damage themselves and where there are a range of animals that would look very favorably on monkey for breakfast.  

 

Occasionally in certain individuals, mutations occur which make their tails less effective or in an extreme case that they have no tail at all.  One way this can happen is if the gene that initiates the sequence that leads to the development of a tail, does not switch on because of some mutation.  This sort of monkey is much more likely to fall.  We usually think of natural selection developing new valuable chracteristics which help an animal exploit new environments but natural selection has much more to do with keeping and animal which has evolved to exploit a certain environment, unchanged.

Look at another example.  Take the Wolf and the Dog.  Wolves have remained pretty much the same for a few hundred of thousands of years.  They are well adapted to their wild environment and the vast majority of mutations would make them more poorly adapted to their environment.  However look at the dog.  Under the influence of the humans who feed and care for wolves and who select some of these mutations because they are valuable for humans or  just from some misguided fancy, dogs have radiated in a huge number of different forms, which based on their appearance would be considered different species if we didn't know that all can breed with each other.

So let's go back to the jungle monkey.  It is getting crowded in the jungle and some of them have to venture out into the surrounding plains to make a living.  A group evolved that make a pretty good go at plains living.  A tail is of much less value to them. In fact it costs them resources for them to grow this useless appendage. Some of them get a mutation that reduces the ability of the tail to grasp a branch,  In fact some of them have a mutation to the gene that initiates the growth of a tail such that  they are born without one.  Since they don't need the tail now, there is no selective pressure against not having a prehensile tail.  But here is the rub.

The longer they are on the plains, the more mutations they accumulate in the sequence that produces a tail.  The occurs even in monkeys that have the switch at the beginning of the tail sequence shut down.  Now if the switch gene gets another mutation that turns on the tail growing sequence, the tail that the animal will grow will be a sorry imitation of the original magnificent tail.  Hence the very rare occurrence of a wee tail in a newly born human.  (also remnant gills).

Geneticists believe from the evidence, that we shed our tails suddenly rather than over a long period so this points to some gene that started a sequence, being shut off suddenly.