Clones, good for evil space empires, bad for bananas.

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Ever since Darwin published his theory of evolution there have been those wanting to use it to whip the human race into shape.

All sorts of people, fascist dictators, politicians, scientists, social reformers, businessmen, tech bros, taxi drivers, bakers and street cleaners have believed, and still do believe, that the way forward for the human race is for someone to step up to the plate and get human reproduction organised already.

Of course, almost always, that someone is themselves.

They believe that they can make the human race great again by either preventing some people from reproducing, by getting the right people to reproduce a lot or by using shiny new toys like CRISPR to optimise the DNA of unborn babies. Or maybe all three.

As dystopian as it may sound, there is nothing really new in all this. If you look at agriculture, we’ve been trying to control the genetics of our livestock and crops using similar techniques for a long, long time.

You’d think we might have learned something from all this meddling in the genetic business of other species. But no. Some of us think we can do exactly the same thing to ourselves with no problems.

There are many reasons why this is wrong. But one of the big reasons is something we’ve seen play out in agriculture multiple times: low genetic diversity.

Low genetic diversity is a constant danger when you start splashing around in a gene pool and we need look no further than the humble banana for the proof of that.

What Is Genetic Diversity and Why Does It Matter?

First some genetics revision. I’ll be quick I promise.

Sexually reproducing organisms, like plants and animals, usually have two sets of chromosomes. One from mum and one from dad.

At specific locations on these chromosomes are genes. Within a population, individuals can have different versions of these genes.

These different versions of the same gene are called alleles.

Because one set of chromosomes is inherited from each parent, children end up with two alleles for each gene. One from mum and one from dad.

If they are the same allele the child has two copies of one allele, and they are said to be homozygous for that gene. Otherwise, they’ll have two different alleles, and they are heterozygous.

How Alleles Work to Define Our Physical Characteristics

Because an individual has two alleles, it’s how these alleles work in combination that ultimately determines our physical characteristics.

One common way alleles combine is where one allele overrides the effects of the other allele. In this case one allele is said to be dominant over another recessive allele.

The example of this we all learned in school was brown and blue eye colour. But we’ve all had to move on because it’s wrong. Eye colour is actually determined by multiple genes and so is the result of much more complex interactions.

A more disgusting, but actually real, example is whether you have dry or wet earwax. This is controlled by a single gene called ABCC11.

If you have dry earwax you have two dry earwax alleles for the ABCC11 ear wax wetness gene. But if you have one dry and one wet allele you will have wet earwax because the wet earwax allele is dominant.

Two wet earwax alleles will, of course, give you wet earwax.

The important point is that despite a small amount of people having dry ear wax, the actual dry earwax allele can still be reasonably common because of the people with both alleles.

Genetic Diversity is Just Counting Alleles

Genetic diversity is just the number of different alleles for the genes within a population.

A population with wet earwax and dry earwax people, i.e. one that has both ABCC11 alleles, has greater genetic diversity than a population of wet earwax people only.

So what? You may say. Why does genetic diversity matter?

Living organisms are very complex systems. When you change one thing other things may be affected. Things you maybe don’t expect to be affected.

People with dry earwax, for example, also do not produce underarm body odour.

Sometimes these effects can protect against disease.

How Genetic Diversity Protects Against Disease

Just like body odour, some alleles of some genes can protect individuals from disease. Or make them more susceptible. The presence of these alleles can be very important when a new disease threatens a population.

For example’s sake, let’s say that a species of virus mutated and started causing a fatal disease in humans. Something like COVID, but worse.

Also suppose that the dry earwax allele makes you resistant to this virus.

In this scenario a population that only has wet earwax alleles has a big problem. No one in that population has a dry earwax allele. They are all going to die.

On the other hand, the population with some dry earwax people not only has people with two dry earwax alleles but it also has a proportion of wet earwax people who still have one dry earwax allele.

In this population there will be plenty of survivors to bury all the dead wet earwax people.

This may sound far-fetched, but it is very real. In humans, there are multiple examples of specific alleles that protect against disease.

The classic example is the way the sickle cell anaemia allele protects against malaria. But other examples include alleles that protect individuals from HIV, trypanosomiasis (sleeping sickness) and norovirus infection (see here if you want to learn more).

There are even alleles of certain genes that can protect against COVID.

The Problem with Crop Monoculture and Vegetative Propagation

Genetic diversity is a good thing because it gives a population a natural reservoir of alleles that might come in handy if circumstances change.

But when you start artificially selecting individuals on the basis of small number of traits you quickly get a reduction in genetic diversity.

When you select a subset of a population, it will almost never have as much genetic diversity as the whole population.

This isn’t just theory. We can see it in action in some of our most important foods.

Crops like wheat, rice, maize and livestock like pigs, cows and chickens are all subject to human selective breeding programs and all have suffered reductions in genetic diversity.

For example, we’ve lost about 75% of genetic diversity in our plant crops over the last century. A serious problem for our food supply, especially in the context of climate change.

Understanding Polyploidy in Food Crops

The partial loss of genetic diversity is a problem, but plants also give us spectacular examples of what can happen if you remove all diversity from your gene pool. In plants this is tied up with something known as polyploidy.

Occasionally, an organism may end up with more than two sets of chromosomes. A condition called polyploidy.

In humans, this is always a very bad thing, but some plants can tolerate extra sets of chromosomes. In plants, polyploidy is fairly common.

Wheat for example is hexaploid (six sets of chromosomes), potatoes are tetraploid (four sets) and strawberries win the chromosome show down with a massive eight sets of chromosomes (octaploid).

Polyploidy, apart from making our Punnett squares much more complicated, often leads to sterility. Often this sterility is caused by losing the ability to make seeds.

For an individual this could be a catastrophic turn of events. But, in some plant species, sterile ployploid plants can produce asexually.

In fact, asexual reproduction for a plant can be a good short-term strategy. As long as conditions are good, asexual reproduction can result in explosive growth of the new variant.

If humans are around, for example, the loss of seeds can be a good thing. Lazy humans like seedless fruit so farmers tend to look after these varieties.

The problem, of course, is that there is zero genetic diversity in a population where everyone is an exact copy of each other.

And this can be a very bad long-term strategy.

Bananas: A Living Lesson in Genetic Catastrophe

Just how bad zero genetic diversity can be is obvious if you have a look at bananas.

There are 300+ different varieties of cultivated banana and mostly they are sterile seedless triploid crosses of two wild banana species, Musa balbisiana and Musa acuminata (though there may be more).

Although wild bananas can, and are eaten, they are smaller and have large seeds. Which is the point of the fruit after all.

Humans, though, are not so keen on all those big seeds. What we like are bananas like the Cavendish.

The Cavendish, the variety that accounts for 40% of the worldwide banana stock, is not only seedless, but has a higher yield, a thick skin and a longer shelf-life.

In short, it is a banana that is well-suited to our tastes and our global logistics networks.

Yet, before the 1960s, no one ate Cavendish bananas. They ate another variety called Gros Michel (or Big Mike). This variety was similar to our Cavendish but had a firmer texture and, according to the experts, had a more intense banana flavour¹.

The Gros Michel was also seedless and thus a clonal population and in the 1950’s it was effectively wiped out by a soil-borne fungus that caused Panama disease.

Because the Gros Michel had no genetic diversity it had no ability to resist the disease and there was little that we could do to save it.

Luckily, we had the Cavendish variety waiting in the wings. Not only was it similar to the Gros Michel in most ways but, crucially it was immune to Panama disease. So, disaster avoided.

The problem, of course, is that we are back to exactly the point that we were with the Gros Michel. A worldwide banana market dependent on a vulnerable clonal population.

It is almost inevitable that we will lose the Cavendish as well. Fungal diseases like Black Sigatoka and Fusarium Wilt Tropical Race 4 threaten and the Cavendish has no resistance to these diseases.

Unless we manage to pull off some genetic engineering wizardry we are going to have to go back to the drawing board and develop a new variety of resistant banana.

If it is another clonal variety then we’ll have to do it all over again in the future because low genetic diversity does not make for a robust, resilient species.

What Is the Banana Trying To Tell Us?

The banana is an extreme example of the dangers of low genetic diversity. It illustrates, graphically, the importance of genetic diversity for the long-term survival of any species.

The fate of the Gros Michel also shows that genetic diversity is especially important when rapidly evolving pathogens are after you. Which, by the way, is always.

Genetic diversity helps make sure that there will be some survivors to start rebuilding when a novel pathogen comes along.

This may sound overly dramatic but remember that in some parts of Europe up to 60% of the population died during the Black Death. Yeah, we have better medical care now, but it’s like they say, prevention is better than the cure.

Conclusion

The privileged are going to do what the privileged always do. They’re going to overestimate their talent and underestimate their luck.

After Darwin published his theory it took about five minutes for the privileged classes in England to realise they had the perfect base for a pseudo-science to support their pretensions.

The poor were poor, the reasoning went, not because of limited opportunity, but because they had bad genes. It was like gravity or the conservation of mass, a scientific law.

Of course, once you’ve embraced this idea it is a very short step indeed to start thinking about eugenics. Thinking about ways that allegedly bad genes can be bred out to improve human beings. You know like we do with cows and wheat.

But if we start treating ourselves like we treat our food we’ll inevitably reduce our genetic diversity. We will make our species weaker in the face of a constantly changing world.

Because it had no genetic diversity, nothing could save the Gros Michel when things went wrong. Probably nothing will save the Cavendish either. Do we want to go the same way?