Evolution

Credit: Dept. of Zoology, CambridgeOpens in new window

Evolution is the accumulation of genetic changes in species or in populations. It is an ongoing process. In fact, as you read this sentence something is evolving somewhere on this planet. It could even be a newly dreaded disease of humans that is being created.

For those of you who study biology, genetics is generally part of the curriculum. We realize that organisms and traits can change dramatically by what we do to influence them.

A good example of this is the carcinogenic effects of smoking. Most people who don’t smoke don’t get lung cancer. Many who do smoke do get lung cancer. Why?

Because the substances in cigarette smoke trigger the cancer by interfering with the genes of the cell, and thus the proper functioning of the cells. The genes mutate.

There are four major mechanisms that give rise to evolution:

1. Mutation
2. Genetic drift
3. Migration
4. Natural selection

Mutation

Mutation is the direct change in the DNA of an individual. Mutations can be beneficial, neutral or deleterious. Mutations can be caused by a mutagen, something that causes a mutation.

Mutagens can be physical, chemical, or biological. An example of a physical mutagen is UV light. A chemical mutagen can be something like Benzo a-pyrine (found in cigarettes). An example of a biological mutagen can be a virus, which when infecting an organism may cause cancer.

It is imperative here to give you an example of how we describe mutation. From this you will be able to understand how a gene change can cause a problem.

The proper sequence of bases encode for specific proteins. Proteins are composed of amino acids. In humans there are 20 amino acids. A sequence of three nucleotides encode for one amino acid. If one base is either removed or replaced by a mutagen (something that causes a mutation), a whole sequence can be changed, thus causing a mutation which may be neutral, serious, or harmless.

As mentioned earlier, DNA is composed of nucleotides, each containing one of the four bases. The sequence of the bases makes up the genes. Therefore, if there is a change in the sequence, a mutation can occur. Let me show how this works by giving you a hypothetical sequence of bases on a chromosome. I will just use 15 bases as an example. Chromosomes have millions of bases.

Notice that I divided the above sequence into three bases. In strand #1, the first sequence is A-T-C. If A is replaced by a G, you may get a different amino acid. If, as in strand #2, you add a base G (see big G below) after the first C, the frame will be totally changed.

This type of change is called a frameshift mutation because the whole frame of the molecule will be changed.

Notice that instead of the sequence as in strand #1, the whole sequence of bases has been changed in strand #2. Instead of ATC CCC GAT GCA TTA you will get ATC GCC CGA TGC ATT-A etc. By adding or deleting or changing a base, a disaster can occur. Thus organisms can mutate and those that were once harmless can become more virulent (dangerous) or even vice versa.

Genetic Drift

Genetic drift is a change in allele frequencies mainly by chance from one generation to another. This is more likely to occur in small populations. A good example of this phenomenon is the Bottleneck effectOpens in new window. In this case, a natual population may be reduced in size by earthquakes, floods, droughts, lava flows, and even human habitat destruction.

In these cases the gene pool (available genes in the present population) may be weeded out or totally eliminated. This may cause mating organisms to lose all their genetic variability.

Put in plain English this means that if you marry and mate with a close relative you can keep “bad” genes, thus causing some defect. This is why people or some animals that are close genetically, such as brothers and sisters, may have defective children if they reproduce. These defects can be physical or mental.

The quintessential example of this occurring is with the African Cheetah. These animals are known to have low genetic variability, probably due to an even that occurred around 10,000 years ago. The event was probably some isolating mechanism.

Migration

Migration of populations that have different allele frequencies is referred to as migration. This may cause changes in the genes, thus allowing organisms to either adapt or not survive.

Migration of organisms into new environments may cause a change in gene frequencies. Organisms will mate with individuals from other locations and this may help to change the gene pool. This may eventually help to cause a mutation.

It should be pointed out that mutations are relatively rare, but nevertheless they do occur. It is possible that global climate change may enhance alleles to change in an adverse way. Organisms that live in a stable environment probably don’t change much.

I believe this is the reason why the Ebola and Corona (Covid-19) viruses suddenly appeared on the scene. Ebola is a dealy member of the deadliest virus family, called filoviruses.

Ebola made itself famous by appearing in Zaire, a country in Africa. Scientists are not 100% sure where it originally came from, but there is suspicion that it originated in monkeys. Remember that Africa has a large rainforest. Africa, by the way, is the least densely populated continent on the planet.

After learning about Ebola, and of course teaching, I began to put 2 and 2 together. Remember what I said about Africa being the least densely populated. Now add the fact that there are not enough jobs to go around for everyone, especially in countries in Africa where there is lots of political corruption and poverty. This poverty will cause unemployed humans to earn money any way they can, even if it means cutting, chopping and poaching everything they can. This also means that they (humans) will venture out anywhere.

Many of the areas in the rainforest are far from human habitations. Organisms such as monkeys, bats, insects, etc. were living in harmony with each other. It is possible that viruses such as Ebola live naturally with these animals or plants. But when a new organism, such as humans, encroaches on their territory, the viruses may jump species – such as from monkeys to humans – and thus cause disease. They may even mutate to be more virulent.

Currently there are four known strains of Ebola: Ebola Zaire, Ebola Ivory Coast, Ebola Sudan and Ebola Reston (this is the strain made famous in Richard Preston’s book, The Hot Zone). It may even be possible that as you read this another strain is evolving.

Natural Selection

This is a term used to describe a force that changes gene frequencies. There are three main types of natural selection:

1. Directional
2. Disruptive (Diversifying)
3. Stabilizing

To make this easier to understand, let’s mention two terms Phenotypes and Genotypes. Phenotype is the term used to describe the way an organism looks. We can state that XX is the genotype of a female human and XY is the male genotype. X and Y refer to one of the 46 chromosomes that humans have.

1.   Directional selection

In directional selecton one extreme persists. Let’s say that you have black, white and grey butterflies, where grey is the average color. In directional selection black or white will predominate, with the other colored butterflies being almost eliminated or greatly reduced.

2.   Disruptive selection

In disruptive selection the two extreme phenotypes will predominate. Thus black and white butterflies will predominate while the grey colored ones are reduced in number.

3.   Stabilizing Selection

In stabilizing selection the average phenotype will predominate. In this case the grey butterflies will predominate, while black and white butterflies are reduced.

It should be pointed out that there are many examples of natural selection. Now if evolution occurs, it is possible that organisms can adapt to changing environment either benignly (harmlessly) or by turning into a virulent (bad) organism. It could be that in order to survive they may have to cause a disease.

When we see and hear about deadly viruses like Ebola, we can see what may happen when organisms and non-organisms like viruses go wild. Again, I am not saying that this will happen, but may happen. The fact is that with the knowledge gained through the study of genetics and breakthroughs in biotechnology we can see that virtually anything can happen.

Figure X-4. Diagram showing the differences between directional, disruptive and stabilizing selection.