Natural Selection And Evolution

I have written several articles the environment. A list of links have been provided at bottom of this article for your convenience. This article will, however address different aspects on the environment and the planet in general.

What is evolution?

In biology, evolution is the change in the characteristics of a species over several generations and relies on the process of natural selection.

  • The theory of evolution is based on the idea that all species are related and gradually change over time.
  • Evolution relies on there being genetic variation in a population which affects the physical characteristics (phenotype) of an organism.
  • Some of these characteristics may give the individual an advantage over other individuals which they can then pass on to their offspring. 

What is natural selection?

  • Charles Darwin’s theory of evolution states that evolution happens by natural selection.
  • Individuals in a species show variation in physical characteristics. This variation is because of differences in their genes.
  • Individuals with characteristics best suited to their environment are more likely to survive, finding food, avoiding predators and resisting disease. These individuals are more likely to reproduce and pass their genes on to their children.
  • Individuals that are poorly adapted to their environment are less likely to survive and reproduce. Therefore their genes are less likely to be passed on to the next generation.
  • As a consequence those individuals most suited to their environment survive and, given enough time, the species will gradually evolve.

Natural selection in action: the Peppered moth

  • Before the industrial revolution in the mid-1700s, the peppered moth was most commonly a pale whitish colour with black spots.
  • This colouring enabled them to hide from potential predators on trees with pale-coloured bark, such as birch trees.
  • The rarer dark-coloured peppered moths were easily seen against the pale bark of trees and therefore more easily seen by predators.
A pale peppered moth on an oak tree.
  • As the Industrial Revolution reached its peak, the air in industrial areas became full of soot. This stained trees and buildings black.
  • As a result, the lighter moths became much easier to spot than the darker ones, making them vulnerable to being eaten by birds.
  • The darker moths were now camouflaged against the soot-stained trees and therefore less likely to be eaten.
  • Over time this change in the environment led to the darker moths becoming more common and the pale moths rarer.
A pale peppered moth on a dark tree.

What have genes got to do with it?

  • The mechanisms of evolution operate at the genomic level. Changes in DNA sequences affect the composition and expression of our genes, the basic units of inheritance.
  • To understand how different species have evolved we have to look at the DNA sequences in their genomes.
  • Our evolutionary history is written into our genome. The human genome looks the way it does because of all the genetic changes that affected our ancestors.
  • When DNA and genes in different species look very similar, this is usually taken as evidence of them sharing ancestors.
  • For example, humans and the fruit fly, Drosophila melanogaster, share much of their DNA. 75 per cent of genes that cause diseases in humans are also found in the fruit fly.
  • DNA accumulates changes over time. Some of these changes can be beneficial, and provide a selective advantage for an organism.
  • Other changes may be harmful if they affect an important, everyday function. As a result some genes do not change much. They are said to be conserved.

Different types of evolution

Convergent evolution

  • When the same adaptations evolve independently, under similar selection pressures.
  • For example, flying insects, birds and bats have all evolved the ability to fly, but independently of each other.

Co-evolution

  • When two species or groups of species have evolved alongside each other where one adapts to changes in the other.
  • For example, flowering plants and pollinating insects such as bees.

Adaptive radiation

  • When a species splits into a number of new forms when a change in the environment makes new resources available or creates new environmental challenges.
  • For example, finches on the Galapagos Islands have developed different shaped beaks to take advantage of the different kinds of food available on different islands. 

Sketches of the heads of finches from the Galapagos Islands showing the differences in their beak shapes due to evolution.

Shutterstock

Are humans still evolving?

For much of nature, natural selection and ‘survival of the fittest’ still play a dominant role; only the strongest can survive in the wild. As little as a few hundred years ago, the same was true for humans, but what about now? 

Nowadays, with the availability of better healthcare, food, heating and hygiene the number of ‘hazards’ we experience in our lives has dramatically reduced. In scientific terms, these hazards are referred to as selection pressures. They put pressure on us to adapt in order to survive the environment we are in and reproduce. It is selection pressure that drives natural selection (‘survival of the fittest’) and it is how we evolved into the species we are today.  

The question is, now we have fewer selection pressures and more help in the form of medicine and science, will evolution stop altogether for humans? Has it stopped already?

Genetic studies have demonstrated that humans are still evolving. To investigate which genes are undergoing natural selection, researchers looked into the data produced by the International HapMap Project and the 1000 Genomes Project.

A catalogue of human genetic variation

The International HapMap and 1000 Genomes Projects both aimed to catalogue genetic variation in DNA samples taken from individual humans from across the world.

The majority of the catalogued human variation is characterised by single base changes, referred to as single nucleotide polymorphisms (SNPs). The location and frequency of these changes allows us to provide a list of regions in the human genome where genetic variation is common. Patterns of reduced variation help scientists to identify genes that may have recently been positively selected for by natural selection. 

How are genetic variants found?

Genetic variants can be found by comparing the genomes of different people and looking to see where there are differences in the DNA sequence and where the genes are located in their genomes. When genetic variants confer a particular advantage and improve our fitness they are more likely to survive and be passed onto future generations, thus becoming more common in a population. When this happens, a pattern or ‘signature’ can be found in the genomes of the population.  This is because, as a genetic variant starts to spread through a population, it doesn’t come alone but brings with it some nearby genetic ‘passengers’.  These passengers are bits of DNA that are located on either side of the advantageous variant. So, if scientists find this signature in lots of genomes in a population, it is one of the first signs that natural selection could be operating. It suggests that they all stem from a common ancestor and have therefore inherited the same pattern of genetic variation.

If the genomes of two populations are found to be very different, it could be a sign that selection has occurred in one population, but not the other. As the advantageous gene starts to become more common, it can influence which other genes are expressed and even reduce the overall level of genetic variation in the surrounding area of the genome, making it stand out.

Unfortunately, even in the absence of selection, any of these patterns can turn up by chance, especially when the whole genome is examined. To make things more complicated, events such as population expansion can mimic some of the same effects. There is no perfect way to recognise where selection has occurred, but we sometimes get a very strong hint.

Scientists have found that the majority of genes that have undergone recent evolution are associated with smell, reproduction, brain development, skin pigmentation and immunity against pathogens.

Lactose tolerance

One example of recent natural selection in humans involves the ability to tolerate the sugar, lactose, in milk. In most parts of the world, adults are unable to drink milk because their body switches off the intestinal production of lactase, an enzyme that digests the sugar in the milk, after weaning. As these people cannot digest the lactose sugar they suffer symptoms including bloating, abdominal cramps, flatulence, diarrhoea, nausea, or vomiting.

Yet, more than 70 per cent of European adults can quite happily drink milk. This is because they carry a regulatory change in the region of DNA that controls the expression of the gene that codes for lactase. This DNA change enables the lactase gene to be switched on and lactase production to continue, even after weaning. This genetic change appears to have happened between 5,000 and 10,000 years ago, which is around the same time domestication of milk-producing farm animals, such as cows, was established in Europe.

This suggests that being able to drink milk into adulthood provided a strong evolutionary advantage in Europe. This may be because sun exposure was much lower in Europe and people were in greater need of the vitamin D found in cow’s milk. Or it may be because cow’s milk provides a much safer and cleaner alternative to drinking water that may cause disease. Milk may also have prevented death from starvation when crops failed and food was scarce. Those who could not tolerate lactose would die of starvation, while those who could tolerate lactose would survive.

Whatever the reason, a strong selection pressure must have favoured those people whose lactase gene remained switched on. This variant of the lactase gene is so common in Europeans that we now consider lactose intolerance to be a health condition, rather than the natural process that it is.

Infectious disease

The strongest evolutionary pressure of all comes from infectious diseases. Millions of people die from infectious diseases each year, particularly in the poorer regions of the world. People who are able to survive infections are more likely to pass on their genes to their offspring. However, genes that provide an advantage against one disease may not provide an advantage when faced with another.

The Caspase-12 gene

When infectious diseases became more common in human populations, perhaps because populations grew in size and pathogens were able to spread more rapidly, people with a genetic advantage were more likely to survive and reproduce. As a result, these genetic advantages were selected for, allowing more people to survive and fight disease. In some cases, a genetic advantage resulted from losing the full activity of a gene.

A good example of this is the caspase-12 gene. Caspase-12 works as a part of our immune system, responding specifically to bacterial infection.

In a study carried out by researchers at the Wellcome Trust Sanger Institute in 2005, it was suggested that the caspase-12 gene was gradually inactivated in the human population because the active gene can result in a poorer response to bacterial infection. People with fully functional caspase-12 were at a much higher risk of a fatal bacterial infection (sepsis) if bacteria entered the bloodstream, than people with the inactive version of the gene.

Before improved hygiene and antibiotics, survival of severe sepsis would have been a strong selective force for the inactive gene, which would have been greatly favoured. Today, people with two copies of the inactive gene are eight times more likely to escape severe sepsis if suffering with an infectious disease and three times more likely to survive.

But the study leaves us with a key question. If it is so good to have the inactive gene, why did our ancestors have an active form in the first place? It may be because in some areas of the world having the active gene carries an equal advantage to carrying the inactive gene in other areas of the world. What is clear however, is that all organisms are dynamic and will continue to adapt to their unique environments to continue being successful. In short, we are still evolving.

HIV susceptibility

HIV is a modern-day driving force for human evolution. In certain parts of South Africa, nearly half of women are infected with the virus. In a study in Durban, Dr Philip Goulder and colleagues from the University of Oxford found that women with a certain combination of variants in a human leukocyte antigen (HLA-B27) were better at clearing HIV infection than those with the HLA-A or HLA-C genetic subtypes. HLAs, produced by the major histocompatibility complex (MHC), are by far the most variable region of the human genome, and are an essential part of the immune system. Infected mothers with HIV-protective HLA-B genes were more likely to survive HIV infection and pass on these genes to their children.

It has been proposed that the relatively low level of HIV in Western Europe is aided by a common variation in a co-receptor for the HIV virus particle (CCR5). This variant protects people almost completely against HIV and is found in 13 per cent of Europeans. However it is extremely rare in other populations around the world, including Africans. The origin of the variant in humans dates back thousands of years ago, well before the AIDS epidemic which only dates from the late 1970s. It is therefore likely that this variant may have been selected because it protects against other viral or bacterial infections.

What is inheritance?

Inheritance is the process by which genetic information is passed on from parent to child. This is why members of the same family tend to have similar characteristics. 

  • We actually have two genomes each
  • We get one copy of our genome from each of our parents
  • Inheritance describes how genetic material is passed on from parent to child.

How is genetic material inherited?

  • Most of our cells contain two sets of 23 chromosomes (they are diploid).
  • An exception to this rule are the sex cells (egg and sperm), also known as gametes, which only have one set of chromosomes each (they are haploid).
  • However, in sexual reproduction the sperm cell combines with the egg cell to form the first cell of the new organism in a process called fertilisation.
  • This cell (the fertilised egg) has two sets of 23 chromosomes (diploid) and the complete set of instructions needed to make more cells, and eventually a whole person.
  • Each of the cells in the new person contains genetic material from the two parents.
  • This passing down of genetic material is evident if you examine the characteristics of members of the same family, from average height to hair and eye colour to nose and ear shape, as they are usually similar.
  • If there is a mutation in the genetic material, this can also be passed on from parent to child
  • This is why diseases can run in families. 

How is sex determined?

  • The sex of an individual is determined by the sex chromosomes called the X chromosome and the Y chromosome.
  • Females have two X chromosomes (XX).
  • Males have an X chromosome and a Y chromosome (XY).
  • Female gametes (eggs) therefore always carry an X chromosome.
  • Male gametes (sperm) can carry either an X or a Y.
  • When an egg joins with a sperm containing an X chromosome, the result is a girl.
  • When an egg joins with a sperm containing a Y chromosome, the result is a boy.

What is a genotype?

  • The genotype is a description of the unique genetic makeup of an individual. It can be used to describe an entire genome or just an individual gene and its alleles.
  • The genotype of an individual influences their phenotype.
  • For example, if we are talking about the genotype for eye colour we may say an individual has one brown eye allele (B) and one blue eye allele (b).
  • As a result, the individuals phenotype will be brown eyes.
  • This is because the allele for brown eyes is dominant, while the allele for blue eyes is recessive (see image below).

Illustration showing the inheritance of dominant and recessive alleles for eye colour.

Illustration to show the inheritance of dominant and recessive alleles for eye colour.
Image credit: Genome Research Limited

What is a phenotype?

  • The phenotype is a description of the physical characteristics of an organism. For example, if we are talking about eye colour the phenotype of an individual may mean blue, brown or green eyes.
  • Most phenotypes are influenced by an individual’s genotype, although environment can also play a role (nature versus nurture). 

What is Mendelian inheritance?

  • The simplest form of inheritance was uncovered from the work of an Austrian monk called Gregor Mendel in 1865.
  • From years of experiments using the common pea plant, Gregor Mendel was able to describe the way in which genetic characteristics are passed down from generation to generation.
  • Gregor used peas in his experiments primarily because he could easily control their fertilisation, by transferring pollen from plant to plant with a tiny paintbrush.
  • Sometimes he transferred pollen to and from flowers on the same plant (self-fertilisation) or from another plant’s flowers (cross fertilisation).
  • In one experiment he cross fertilised smooth, yellow pea plants with wrinkly, green peas:
    • Every single pea resulting from this first cross, the first generation (F1), was smooth and yellow.
    • However, when two smooth, yellow peas from this first generation were crossed to produce a second generation (F2), the result was 75 percent smooth, yellow peas and 25 percent wrinkly, green peas (3:1).
    • This outcome shows that the genes for smooth, yellow peas are dominant while the genes for wrinkly, green peas are recessive.
  • The results from this and further experiments led Gregor Mendel to come up with three key principles of inheritance:
  1. The inheritance of each trait is determined by ‘factors’ (now known as genes) that are passed onto descendants.
  2.  Individuals inherit one ‘factor’ from each parent for each trait.
  3. A trait may not show up in an individual but can still be passed onto the next generation.
  • Genetic traits that follow these principles of inheritance are called Mendelian.

Gregor Mendel 

What is genetic variation?

Genetic variation is a term used to describe the variation in the DNA sequence in each of our genomes. Genetic variation is what makes us all unique, whether in terms of hair colour, skin colour or even the shape of our faces. 

  • Individuals of a species have similar characteristics but they are rarely identical, the difference between them is called variation.
  • Genetic variation is a result of subtle differences in our DNA.
  • Single nucleotide polymorphisms (SNPs, pronounced ‘snips’) are the most common type of genetic variation amongst people.
  • Each single nucleotide polymorphism represents a difference in a single DNA base, A, C, G or T, in a person’s DNA. On average they occur once in every 300 bases and are often found in the DNA between genes.
  • Genetic variation results in different forms, or alleles, of genes. For example, if we look at eye colour, people with blue eyes have one allele of the gene for eye colour, whereas people with brown eyes will have a different allele of the gene.
  • Eye colour, skin tone and face shape are all determined by our genes so any variation that occurs will be due to the genes inherited from our parents.
  • In contrast, although weight is partly influenced by our genetics, it is strongly influenced by our environment. For example, how much we eat and how often we exercise. 
  • Genetic variation can also explain some differences in disease susceptibility and how people react to drugs.
  • Genetic variation is important in evolution. Evolution relies on genetic variation that is passed down from one generation to the next. Favourable characteristics are ‘selected’ for, survive and are passed on. This is known as natural selection.

Conclusion:

Natural selection and evolution are the order of things. They are the forces that have driven our natural history, and are responsible for the diversity of life on this planet. This is how we came about. However, Homo Sapiens as the dominant species have been artificially changing these forces. We have been using technology and our ability to reason to gain an unfair advantage and to change our environment. Many people have reasoned that our increased Carbon Dioxide footprint has accelerated global warming to such an extent that many species cannot adapt quick enough to survive. We have also hunted countless species to extinction. The definition of cancer is a disease caused by an uncontrolled division of abnormal cells in a part of the body. The human species, through the instrument of culture, has become the dominant force of planetary ecological change. Our adaptations have become maladaptive. Moreover, the human species as a whole now displays all four major characteristics of a malignant process: rapid, uncontrolled growth; invasion and destruction of adjacent normal tissues (ecosystems); metastasis (distant colonization); and dedifferentiation (loss of distinctiveness in individual components). We have become a malignant ecopathologic process. 

We have violated the laws of natural selection by maintaining non viable life through technology, in a misconception that we are improving life. I have been a Registered nurse for 19 years and an ICU nurse for 9 of those years. I have see life prolonged in a hopeless attempt to alter our evolutionary and aging process. We are in some cases causing needless suffering in an attempt to prolong the lives of patients with terminal illnesses. In some cases this is being done against the patients expressed wishes. We are also through the miracles of modern medicine and nutrition we are extending the lifespan of the human race. In many cases those additional years are not ones of joy and abundance, but of disease, weakness and senility. In the animal kingdom those individuals would be attacked and killed by predators, thereby conserving resources. In the animal kingdom on the strongest individuals in the species survive to reproduce. In the human race weak and feeble individuals, and others with genetics disorders (such as Down’s Syndrome, Sickle Cell Anemia) continue to breed, thereby continuing in perpetuity these disorders. If natural selection was truly allowed to work in the human race these disorders would be virtually non existent. Don’t get me wrong I am not saying we should kill people off or castrate these individuals. I am strictly speaking in a evolutionary manner.

One of the more optimistic theories in ecology has been the Gaia hypothesis, which holds that the entire planet is a self-interacting system that balances itself and regulates the status of life everywhere. Gaia keeps life in a healthy state, just as the mechanism of homeostasis in the human body works to keep every cell healthy and in balance. But bodies die from cancer, and one wonders if the human race is a cancer that the planet cannot correct for. If all of life constitutes a single “superorganism,” how much adaptability is left in that organism? Like a malignancy, the basic problem with Homo sapiens is that we want to take over everything. We want to expand without regard for other life forms. Eventually, like cancer, our destruction may be caused by our very success. In its insatiable urge to dominate, a cancer cell drives away all competition, but in so doing it wrecks the body’s ecology and therefore is doomed. Gaia, or the total ecosystem if you prefer, has accommodated human life as one species among many, but it is hard to see any planetary mechanism that can check the spread of humans in so many destructive areas. Of course, the fact that we may pollute ourselves out of existence can be accommodated ultimately. should the human race reach its end game, other life forms will persist and carry on without us. Has the planet reached its limit as an ecosystem that can include us? If not, how close are we? The human body is so masterful at self-correction, I wonder if there is a similar self-correcting mechanism in the human mind that can be called upon, or that will assert itself in the coming generations.

Enter the covid-19 virus, Is this our tool of natural selection? It is a simple strand of genetic material that is kicking our asses. It seems to be geared towards the elderly, sick, obese and feeble individuals. It could very well be nature’s way to relieve the pressure that the human race is putting on the planet. In fact, acting to bring about a natural selection cleansing of our species. So my question is should we just allow it to run its course? By stopping the lock downs and opening up our world economies. Yes we can still fight it and work on vaccines, but should we allow herd immunity to take its course? You may ask the question, has modern medicine and our civilization helped to keep alive weaker gene pools? Please realize that I am not in any way proposing that we let people die. I am simply posing topics a for discussion and debate. What do you think?

Resources:

yourgenome.org, “Are humans still evolving?”; yourgenome.org, ” What is Evolution?”; pubmed.ncbi.nlm.nih.gov, “Has the human species become a cancer on the planet? A theoretical view of population growth as a sign of pathology,” By W. M. Hern; huffpost.com, “Is the Human Race a Planetary Cancer?, By Deepak Chopra;

Environmental Postings
https://common-sense-in-america.com/2020/10/18/why-2-2-doesnt-4-and-an-apple-is-not-an-orange/
https://common-sense-in-america.com/2020/10/17/forest-fires-and-other-natural-disasters-and-their-effect-on-the-environment/
https://common-sense-in-america.com/2020/10/08/sharks-sharks-everywhere-oh-my/
https://common-sense-in-america.com/2020/10/06/natural-selection-and-evolution/
https://common-sense-in-america.com/2020/09/16/our-western-fires/
https://common-sense-in-america.com/2020/09/12/the-creation-of-our-planet-is-a-miracle-and-should-be-cherished/
https://common-sense-in-america.com/2020/06/19/global-warming-and-other-environmental-issues/
https://common-sense-in-america.com/2020/06/12/protecting-our-heritage-saving-endangered-species/

7 thoughts on “Natural Selection And Evolution

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s