Why Is Genetic Diversity Important for Survival?

Why is genetic diversity important for survival of species on this planet? It is often said that the only constant in life is change. And it is true: our world is continually changing. The disturbance of a habitat by a storm, the illness of a community by a virus, the loss of a species of trees by a blight. These are regular events that life adapts to.

Life on earth has shown remarkable resilience, an ability to recover from even the most devastating catastrophe imaginable: a meteor strike likely followed by violent earthquake and volcanic activity at the end of the age of dinosaurs. Granted, it took a while.

On a less devastating scale, but still significant: what does it take for a species to resist attack by bacteria or viruses to recover? What will it take for species to survive the effects of globalization and climate change like weather extremes and new diseases? What prompts resilience?

Genes hold the key to survival. Over time, species have developed traits to help them flourish in their natural environment and survive in changing environments.

The more diverse genes a species has, the better its chances of resisting disease, prevailing over other stresses and adapting to changing conditions.

The definition: What is meant by genetic diversity?

Genetic diversity is the range of different inherited traits within a species.

What are genes?

In humans, genes are contained in chromosomes. Chromosomes are structures inside the cell nucleus. An adult human has roughly 15 million cells of various kinds, like brain cells, red blood cells, and liver cells and they all contain set of chromosomes.

Human set of chromosomes makes 23 pairs — always one from mother and one from father. Inside each chromosome is a tightly wrapped coil of a chemical, deoxyribonucleic acid, commonly called DNA.

DNA is actually made up of millions of even tinier chemicals called bases. These bases are of four different types:

• adenine (A)
• thymine (T)
• cytosine (C)
• guanine (G)

A gene is a short section which includes the four types of basis in a certain sequence. A chromosome can contain anywhere from hundreds to thousands of genes.

These genes provide a template of you. For example, they ultimately determine your height, body build, and eye color. An inherited trait is any gene-determined characteristic and is most often determined by more than one gene [1].

Your genes are also your body’s operating manual for governing your body processes like nutrition, respiration, and growth. A gene can be turned on and expressed or simply remain dormant.

Your internal chemistry and hence these body processes can also be influenced by external or environmental factors.

All plant and animal cells contain many thousands of different genes and typically have two copies of every gene. The two copies, alleles, may or may not be identical and one may be dominant in determining the characteristic while the other is recessive.

For example, brown eye color is dominant while blue is recessive. So, if a child is born with one gene of each color passed on, that child will be brown-eyed.

A parent does not have to have the gene expressed in order to pass it on. It is entirely possible that two blue-eyed parents will produce a brown-eyed child. It is also possible that successive children may have blue or green eyes.

How are we measuring genetic diversity?

Genetic diversity is measured by looking at how many different forms of genes exist across the genome* (the complete set of genes) among individuals in a population and how frequently they occur. 

To do this we look at how many loci have different alleles (loci is the plural of locus or site). Locus is like a physical address for a gene on a chromosome [2].

Genetic diversity of a species is high when there are many different allelic forms of all genes and when there are many different combinations expressed across the species.

Sometimes populations are examined for their heterozygosity, that is the percentage of individuals in a population that have differing alleles at a particular locus.

* The genome is the term used to describe the complete set of genetic instructions for growth and development of an organism.

Measuring the nucleotide diversity

The bases of DNA discussed above: adenine, cytosine, thymine and guanine along with their chromosomal backbone are known as nucleotides.

Genetic diversity can also be measured by examining nucleotide diversity. Nucleotide diversity is the percentage of positions within the genome where two individuals of a given species have different DNA bases. 

Deducting genetic diversity of a population

Apart from counting genes, scientists sometimes use deductive logic in determining levels of genetic diversity.

An example is the cheetah, a wild cat capable of running 70 miles per hour. Cheetah is a vulnerable species suffering from a number of pressures:

• habitat destruction,
• inability to find mates,
• capture for the exotic animal trade,
• slaughter for their pelts,
• being shot by sheep farmers and fearful humans,
• catching diseases spread by domestic cats
• suffering from climate change altering their habitat and food and water sources.

Due to these numerous problems, cheetahs have very few mating options. Looking at their genetic diversity, it is estimated that present-day cheetah population genomes have less than 10 percent heterozygosity (percentage of individuals in a population that have differing alleles at a particular locus).

In the case of cheetahs, apart from counting genes, scientists presume that many are closely related by looking to the high success rate of skin grafting.  If a skin graft from another cheetah is readily accepted, it means that both animals are genetically similar. Their immune system recognizes the graft as if it originated from the same individual.

Another sign of being related is asymmetrical skull development. A survey of museum specimens has confirmed asymmetrical skulls prevalent among cheetahs [3].

Still, all may not be lost as life is indeed resilient. Cheetahs survived a similar contraction (bottleneck) of their numbers between 6,000 and 20,000 years ago and it may be that their genetic diversity is still slowly rebounding [4].

Does genetic diversity increase biodiversity?

Biodiversity is the variety of life in the world.

Biodiversity is increased by genetic change which increases a species’ ability to adapt. Conversely, a population cannot evolve to adapt to environmental change without genetic diversity [5].

In our world of diminishing biodiversity and dramatic environmental changes, the maintenance of high genetic diversity has become a conservation priority [6].

How is genetic diversity created?

Genetic diversity is created when different members of a species breed and pass on copies of their respective genes.

The more variety of physical traits between the parents, the greater the possibility of diverse genetic characteristics being passed on to their offspring during fertilization.

The chance of inheriting certain genes appears to be random.

What are the factors affecting genetic diversity?

The number of different genes available affect genetic diversity. A large gene pool with a variety of different genes allows for more differences.

Consider the mating of a dark-skinned, brown-eyed stocky native human from the tropics in Ecuador with a lithe fair-skinned, blue eyed native of Sweden. Should their offspring then mate with the offspring of a red-haired Irishman and an Aleutian or perhaps an Asian.

The gene pool available to successive generations is much more diverse that if the Ecuadoran had simply mated with another from her own tribe who would likely share many of the same genes she has.

In the past, before easy travel and globalization, many societies were isolated. Still, history shows us that most tribes understood that mates should be selected from other tribes and developed rituals to accomplish this.

It is also common in the animal world for offspring to leave their tribe in search of a mate.

There have of course been situations where this has been difficult if not impossible and the resulting population from close breeding is more likely to pass on an otherwise uncommon allele.

In humans, this is called the founder’s effect, named for those who found a new remote colony. But it can happen anywhere breeding takes place within a small community.

In these situations, children are more likely to inherit two copies of a recessive gene that leads to genetic disease. An example is the Amish community where members are forbidden to marry outside the faith. Many Amish have extra fingers or toes, a symptom of the genetically inherited disease, Ellis-van Creveld syndrome [7].

Habitat fragmentation decreases genetic diversity of species

While there have always been alterations in the natural world that split populations, like rivers moving, mountains forming, and glaciations, species have continued to thrive. In some instances, this has led to the development into new species once separated into distinct environments.

The fragmentation of species is hastening dramatically today as the human population explodes. In the plant and animal worlds, genetic diversity is threatened by the disruption of habitat due to human expansion. Urban sprawl and development are shrinking and isolating populations across the world.

Even in the human population, sometimes alleles simply disappear when they are not manifested over time. In the plant world, a similar phenomenon is taking place, only at a very rapid pace due to human intervention.

We are losing crop diversity as agriculture has become industrialized and agribusiness focuses on cost-efficiency by mass producing just a few varieties of crops.

Wild animal populations are suffering from habitat fragmentation, and from being hunted and captured. They are no longer able to find mates outside their inner circle. Nor is it infrequent for exotic wild animals to be captured and raised in zoos.

Attempts to breed animals held in captivity have had varying success with one of the problems being the small number of potential mates. Zookeepers make deliberate attempts to rotate individual animals into and out of a population in order to bring in new genes.

When a species can only reproduce within a small or isolated population of organisms, individuals of that species may be forced to breed with close relatives which simply creates a more uniform and smaller gene pool for the species. Inbreeding, as the phenomenon is called, makes species weaker and more susceptible to diseases.

Apart from humans, where incest is considered illegal in most countries and where even marriage among cousins is often prohibited, the concerns related to inbreeding are very well illustrated in the case of some of our favorite pets.

What is the importance of genetic diversity for a species?

Genetic diversity of plants, animals and other living organisms is what enables them to survive and thrive in this world. The capacity of species to adapt to new circumstances, whether this is resource scarcity, a changing environment or other disturbances to their natural environment, depends on genetic diversity.

The greater the variation in genes, the more likely is that individuals in a population will possess the differentiated genes which are needed to adapt to an environment. The theory of natural selection suggests that it is this variety of genes that allows species to evolve, adapt and propagate successfully.

Genetic diversity helps maintain the health and vigor of a population to resist infectious diseases, pests and other stresses. And it better equips a species to survive in a changing environment.

Consider that we may be facing a markedly hotter world. Darker eyes have more pigment to protect against sun damage and ultraviolet radiation than blue or green eyes [8]. Likewise, darker skin is less prone to damage from the sun’s ultraviolet rays [9]. Having these traits available may prove to be an advantage for survival.

Having a high homozygosity (percentage of two identical alleles of a particular gene) is problematic. Many deleterious alleles are recessive and a different dominant allele would mask it so that the disease does not manifest. However, individuals with low genetic diversity are more likely to inherit the deleterious recessive alleles and suffer of the disease.

Another upside of having two different genes at the locus is the “heterozygote advantage”. There are some traits like, for example, the allele for sickle cell anemia which actually protects against malaria in heterozygotes but causes a deadly disease in homozygotes [10].

Examples of species with low genetic diversity and consequences

Large populations tend to have high levels of genetic diversity. However, as populations shrink, they lose much of their diversity. The result is that the remaining individuals are more genetically similar to one another.

This becomes a problem if survival traits have been lost and if genetic combinations causing diseases are expressed with marked frequency.

The potato famine

The absence of genetic diversity was the reason behind one of history’s biggest famines. The causes of the Potato Famine in Ireland which took place in the 19th century can be traced back to the susceptibility of the new potato plant to a specific disease.

Because new potato plants are not a result of reproduction – they are instead created from one parent plant – they exhibit very low genetic diversity.

The potato’s low genetic diversity meant that the virus spread to the vast majority of the potato crop which was a staple food for the Irish population, leaving one million people to starve to death. Absence or low genetic diversity is also in part what makes agricultural monocultures more susceptible to disease.

Atlantic wild salmon endangered by salmon hatcheries

Atlantic wild salmon may be losing the traits needed to survive in ocean waters.

Raising fish for restocking as a solution to dwindling populations either from overfishing or in the case of Sweden to mitigate the impact of hydropower plants, appears to be creating a different problem: lower genetic diversity.

The rivers of Sweden feed into the Baltic Sea, providing it with 90 percent of its juvenile wild salmon. A recent study of Atlantic salmon populations across thirteen rivers in Sweden, five of which are home to salmon raised in hatcheries, show that in contrast to one hundred years ago before the stocking measures began the fish are more genetically similar.

One might expect those raised in the hatcheries to be, since they are selected for fast growth rather than speed and prowess in the wild and are breeding within a defined population. However, it appears that when they breed with the wild salmon, the fish are passing on their inferior genes. This is jeopardizing the survival abilities of the salmon entire population [11].

Platypus population from the King Island

A study of platypus population on King Island in the Bass Strait off the northwestern coast of Tasmania, Australia, found that low levels of genetic diversity are affecting reproductive success, survival, and parasite resistance of these animals.

The low genetic diversity in an important immune response system is especially of deep concern. Scientists are worried that this could have devastating consequences for the species if the fungal mucormycosis from the Tasmanian mainland reached the island population. The fungal mucormycosis is a disease caused by the fungus Mucor amphibiorum, which causes infection prone skin lesions and can be deadly to platypuses. 

Without genetic variation a population does not have the arsenal to help it respond to changing environmental variables.

In 1492 when explorers brought a host of new diseases to the Americas including smallpox, measles and flu the Native American population experienced a “massive demographic collapse” [12].

It is estimated that the new diseases killed 90% of the Native American population [13]. If a population does not have the genes to resist a disease and the disease is so virulent it threatens to wipe out the entire population before the species has an opportunity to evolve a resistance, the population then may face extinction.

Examples of species with high genetic diversity

The most genetically diverse species of the eukaryotes, organisms with DNA in their chromosomes within the nucleus of their cells, including plants, animals and fungi, is a mushroom that lives on decaying wood. The split gill mushroom.

The split gill mushroom can have a nucleotide diversity of twenty percent. This means that two different mushrooms can have different DNA bases at 20 out of every 100 positions in their genomes [14].

Before its discovery the record was held by a one-millimeter long bacteria-eating worm, the C. brennari. Factors attributed to its genetic diversity are the fact that it can procreate by having sex with itself so has no problems finding a mate.

This worm lives in the tropics where is plenty of bacteria in rotting vegetation and such environment can support large populations. Large populations increase the chance of novel adaptation mutations to emerge and take hold [15].

Variety may not only be the spice of life, but its very sustenance.