Tuesday, May 28, 2013

Evolution 01

One of my favorite subjects and the source of constant wonder is evolution. It is also a subject so rich and complex that it cannot possibly be encompassed altogether in a single article... hence the "01" in the title of today's article.

It is also a subject that triggers a lot of angst from the fundamentalist religious, mostly American Protestants and Muslims from any country, because their religious dogma, their narrative of humanity's emergence, does not stand compared to the powerful explanation that derived from Charles Darwin's findings.

This post, however, only aims at explaining the core mechanisms of evolution through natural selection (ETNS). There are 3 central elements to ETNS:
  1. mutation
  2. competition
  3. heredity

Mutation:

The code that defines much of what we are and how we develop is DNA. A metaphor for DNA is a book. A book is made of chapters, which are made of sentences, which are made of words, which are made of letters. Without going into the details here, a mutation is generally the addition, deletion, or replacement of one letter by another. Sometimes, the change is benign. Sometimes it makes things very wrong. Look at the following example (largely borrowed from Robert Sapolsky's course of Human Behavioral Biology available on Youtube):
Original phrase: I will now give you your money back
Benign mutation: I will naw give you your money back
Severe mutation: I will not give you your money back

It happens for a variety of reasons. Sometimes because of cosmic radiation (we are traversed by neutrinos and other energetic particles/waves all the time) that alters the DNA of sperms and eggs. There's also a high suspicion that keeping a mobile phone in a man's pocket is bad news for sperms as these cells are particularly sensible to radiation.

Sometimes it happens because of exposure to chemical compounds. For instance "agent orange" (warning: graphic photos), DDT, and depleted uranium (used in Iraq wars) are causes of stillbirths and babies malformations.

Sometimes, it happens a simple accident of cell division/replication during the mechanisms that generate sperms and eggs.


Competition:

Competition happens through a variety of ways. The general idea though, is to survive and reproduce.

When an organism is alive, it is subjected to environmental pressure in many forms: hot or cold weather, scarce food, wet or dry environment (maybe changing with seasons), presence of predators, acidity, violence from other members of the same specie, etc. If one individual of a species has received a mutation, it might translate to a slight advantage or hindrance to the chances of survival... or in a more realistic way, it might simply be a slight advantage to reproduce before his competitors (the members of his own species) or to reproduce more. Because more offspring means your family becomes a larger part of the total population and your DNA has gained some "market shares".

One thing that is often misunderstood about ETNS is that, even though you have to compete with your predators, the main competition is about competing with other members of your own species. There's a joke reminding us of that (and it exists with various animals):
2 campers are surprised at their camp by a grizzly bear. They start running and the bear runs after them. Holding their shoes in their hands, one of the campers stops to put his shoes on. So his friend says: "This is useless, we can never outrun the bear". But our camper, now wearing shoes replies: "I'm not trying to outrun him. I'm trying to outrun you".

Reproduction is exactly what the name implies. If you (or a plant) reproduce earlier than others, if you have more offspring... then you become the majority and you are more successful from an ETNS point of view.

A note of caution: the notion of "success" from the point of view of ETNS is distinct from the notion of success within human society and one should be very careful when using these notions. If you have plenty of children, this may have implications on your financial situation, your schedule, your relationship with your spouse and friends... you might as well be a rapist who impregnated many of your victims. This is undoubtedly bad from any human society's point of view, while this is successful evolutionarily.

Heredity:

Heredity is the conservation of DNA characteristics (including new mutations) through reproduction. If you have blue eyes and so does your partner, then this characteristic will be passed on to your children. If you are a red rose, then your pollen will give birth to red roses, not yellow ones.

This mechanism is like the validation of the previous step of competition. Successful individuals with more offspring see their success validated by the transmission of their characteristics to their little ones. A famous example of this was with Darwin Finches. After a rigorous winter, the individuals with shorter beaks, less able to dig for food had died while finches with longer beaks had survived. Their babies had (in average) longer beaks than the previous generations.

The same thing is observed with insects' resistance to pesticides. The individuals able to survive the pesticides are those which (by chance of a mutation) have a resistance plus a few lucky ones. When these resistant individuals reproduce, the new generation of that species will have a resistant gene in greater proportion than the previous generation. That's why using pesticides is either a lost cause or a never-ending battle between the insects ability to evolve a resistance and our ability to create new products that insects are not yet resistant to.


Conclusion:

One of the frequent mistakes about ETNS is the presumption that it is a random mechanism. It is not. The random aspect of it is the apparition of random mutations. But once individuals are born with certain mutations, it is hardly a matter of randomness anymore. The fitter individuals (to one specific environment) will have an edge on the others and over several generations, their mutation that makes them fitter will become more and more common as they are more able to feed themselves, more able to fight, more able to outrun the others, more able to reproduce early... That's how, in a polluted England where the bark of trees turned from white to black due to particles in the air, some rare black individuals of a butterfly species became the majority while the white majority of butterflies was easily spotted and targeted by birds. This spreading (and reciprocally disappearance) of genes is what we call genetic drift.

If you're interested in the subject and have questions, feel free to ask. But in any case, there will be more articles coming down the line about evolution and natural selection.

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