WEIT 2012 Chap5

THE ENGINE OF EVOLUTION

What is natural selection ?

Everywhere in nature, animals seem to fit perfectly with their environment, as if they had been designed to, and early naturalists believed that this perfection was God's product. It was Charles Robert Darwin (1809-1889), who the first used the terms natural selection to describe the key process of Evolution.
In his book The Origin, he defined the natural selection as a mindless and materialistic process, gradual and non-random, by which some genotypical and phenotypical traits become either more or less common in a given population.

He came to this definition by realizing two things:
• The different species produce an excessive number of offspring. As the resources are limited, they can't all survive.
• In the diverse populations, individuals vary in many heritable traits.

From these observations, he came to the fact that in a population, the individuals who have the biological traits the more adapted to the environment, which fit the best with their environment will be more likely to survive to predators, to feed themselves, or to find a mate, etc. As a result, and from the first observation, these individuals will be able to produce more offspring than the others, and to colonize the environment in few generations. This is what he called natural selection.

Is natural selection based on chance ?

While learning Darwinism, many people understand that "everything happens by chance", but it is wrong. Natural selection, as it is called, is selective.

A species can neither decide to evolve, nor to wait and hope to evolve. Natural selection or adaptation and through it, evolution, happens because of necessity.
To evolve is a necessity, in order to maintain equilibrium in the Nature, for example between predators and preys as the Asian hornets and the Japanese honeybees (honeybees evolved in order to be able to trap the hornets and kill them). Adaptation to environment is inevitable, because the Nature needs equilibrium.

Three linked factors are involved into the process of adaptation, which can't take place if one of these conditions isn't fulfilled.

• Variability: Individuals of a same population have to show some phenotypical differences, because adaptation can't appear from nowhere. These variations are the result of mutations. Mutations occur as errors in the sequence of DNA during cell division. These mutations occur indifferently, which means that they occur regardless of whether they would be useful to the individual.
• Heritability: These variations have to be heritable, it means that a proportion of these differences needs a genetic basis. The individuals need difference in their genotypes, in order for these variations to be passed on to the next generation.
• Probability of leaving offspring: The last aspect of natural selection or adaptation is that these genetic variations must affect positively an individual's probability of leaving offspring, in order for the variation to be transmitted to the next generation.

The variations between individuals, which are the raw material of evolution ( called before mutations ), are random, as said before. But if the emergence of a mutation is due to chance, the process which leads to adaptation isn't. In order for a mutation to be passed widely to the next generation, she must confer a reproductive benefit on individuals possessing it. If a mutation is harmful or useless and don't confer any benefit, she won't be passed to the next generation, and adaptation will not take place. Richard Dawkins, a biologist, member of the Royal Society, provided the best definition of natural selection saying that: natural selection is the non-random survival of random variants.

See Appendix 1, Chap 5

Why is natural selection a very long process?

It is very slow process also because it undergoes micro changes that are often too small to be seen and it makes also impossible to prove that evolution happens without looking at fossils. If we look at the population from one generation to the next, we won't see changes.

Species are usually quite stable. When the environment doesn't change, stabilizing selection happens. For example: a species of birds has evolved into a body size that is optimum for their environment. Selection will still act, but only to cull birds that are smaller or bigger than the optimum body size. Their average body size won't change.

If there is a change in their environment, they will have to adapt by changing some of their genes, little by little, in a process called microevolution. In the end, all those changes put together will undergo macroevolution. It needs a lot of time to happen, so it can't be seen within a human lifetime. We know that it really occurred thanks to fossils, but it also explains why some people doesn't trust that evolution is still happening.
For example, the soapberry bug, a small insect that eats the seeds inside fruits using a beak, had to adapt to a new kind of fruit because it colonized other plants. Some fruits were bigger than the previous ones, some smaller. The beaks evolved in two directions: some become smaller, the other bigger, according to the plant they colonized. In a few decades, the length of the beaks had changed by up to 25 percent. This is a pretty small change, but if the beaks had increased this way during five thousand years, their would have been long enough to allow the bugs to feed in a fruit of the size of the moon !
This shows how small changes made the evolution.

It shows that natural selection is a long and continuous process.

Can selection built complexity?

We have seen that selection was able to change things like the size of a bug beak, the colour of fur, it can also change the period of flowering of a plant, but these are small changes compared to blood clotting, the eye or even the human brain, that are all products of evolution. what selection involved for these traits ? Such complex features took a long time to evolve and most of the did so in the distant past, so we can't really know how it happened. The only alternative to selection is the creationist theory, which says that God creates the world and the humans with already their brain and other complex features.
The adaptation of a trait by natural selection has to be based on a trait already present. For the anatomical traits, evolution can be easily followed in the fossil records (when they exist). The problem is that it isn't possible for the evolution of complex biochemicals features, because they leave no trace in the fossil record. The study of biochemical evolution is still resent, but some systems are already understood.
This understanding shows that selection built these complex systems, but has there been enough time for natural selection to built them ? A scientist, Philip Gingerich, came to the fact that in laboratory and studies of colonization, the rate of change in animal size and shape was much faster than in the fossil changes. It shows that natural selection is adequate to explain the changes we see in the fossil record, even if it takes ten million years, which is pretty difficult to imagine for us.
The last point is to know whether natural selection is enough to explain the most complex organs, as for example the eye. In fact it is. The process just goes step by step: observing the other species, we can conclude that their eyes are in different steps of evolution. It begins with eyes like the ones of flatworms and changes, step by step, until the actual one, helping each species to survive and reproduce better. Each step is simple, but put together, it creates complexity.

What is artificial selection ?

Artificial selection is a process which has the same result that natural selection but through a shortest way. It is a process that consists of a transformation of a plant or an animal into an ameliorated and more adapted form in order to satisfy human's desires in a shortest time than nature selection can.
The most obvious example is the evolution of dogs. All the species have different shapes, temperaments, colours, and sizes however they descend from a single ancestral species which is the Eurasian gray wolf. Of course they evolved because of natural selection over thousand of years but they can't be as different as they are now only through this natural selection. Some dogs for example can run faster than others or are bigger than others. This is not due to natural selection but to artificial selection. For example, because people needed faster dogs in order to win some competitions, breeders changed some traits of the dogs in order to give a size that permits it to run faster than is ancestor.
It shows that artificial selection is a shortest way to obtain what humans want and what nature can make only over thousand of years.

Artificial selection is a way to go faster, but we don't know any adaptation that couldn't have been done by natural selection.

What are the advantages and disadvantages of artificial selection?

Natural selection is very advantageous for humans.
It is a process that can make life easier. For example, farmers can change the traits of their animals and plants in order to have best harvests and more productive animals in a shortest time. They also need to choose which traits they want to modify to win more time (and consequently more money). This process can also improve survival or reproductive ability by changing some traits of the plant or the animal.
However, artificial selection can have dramatic results because it is not 100% safe: it can undergo mutations.

What about evolution without selection ?

When sexual reproduction occurs, one genes of each parents isn't transmitted to the offspring, since every individual has two copies of each genes. As only one allele is transmitted to the offspring by each of the parents, the other allele is lost. This process is called random sampling.
But because several offsprings can be produced during life, each allele of the same gene have a probability of being passed to the next generation. This is why a gene can either disappear or become fixed in the population. This is called genetic drift.
Genetic drift, as natural selection, make a genetic change that allows to speak about evolution.

• As seen before, natural selection is a non random process that change the frequency of the appearence of certain alleles, regarding how useful they are. It gets rid of harmful ones and replaces them by raising the frequency of beneficial ones.
•On the other hand, genetic drift is a random process, so if the frequency of appearence of a given allele changes, it is regardless of its utility. So the evolution caused by the genetic drift is not an adaptation; it can't build an eye or a wing, things allowed by natural selection.

Genetic drift may causes an other problem. In small populations, the sampling effect can be so powerful that it can overpower natural selection, and just work in the opposite direction, making a harmful allele becoming more frequent, instead of destroying it. Genetic drift probably had a role in the evolution, most of all in the evolution of small populations. It shows that there can be evolution without natural selection, but evolution doesn't mean adaptation. Some biologists believe that it could explain some external features of organisms, as the shape of certain leaves in different tree species, especially if it doesn't affect reproduction, or if it has no selective advantage at all.

See Appendix 2, Chap 5

Is evolution still taking place in a natural way?

There are different proof that evolution didn't happened only in the past:

•First of all, there is evolution in the test tubes: in laboratories, captive populations are exposed to a new environment. The goal of the experiment is to see if there is an evolution , due to the change in environment. That's why species that have the capacity of dividing very often, like bacteria, dividing about once every twenty minutes are the best suited to this experiment. This experiment is rather natural than artificial, because humans don't choose which individuals will reproduce. The experiment shows that the bacteria adapt to every environmental changes by modifying features that they had before.

•Resistance to drugs and poisons is also an example of natural evolution: when pesticides are thrown in fields to kill bugs, some of them may have a gene that allows them to survive. The survivors, when they reproduce, will transmit it to their offspring, who will become resistant too. It is the same with viruses that evolved to resist to medicine.

Both evolution in test tube and resistance to drugs and poisons are proof that evolution still happens in nature.

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