The preferred way of potato reproduction in agriculture is cloning existing potatoes. Growing potatoes by cloning is cost-efficient, and it increases the potato yield, however, because cloned potatoes share the same DNA between each other they are vulnerable to the same potato diseases. Therefore, the spread of a certain potato disease can potentially wipe out all cloned potatoes. In other words, if there is not enough genetic variation among the individuals of a species, under the same environmental factors, all of them or none of them are fit to survive.
Why Are Viruses Resilient?
One of the biggest contributing factors to the Great Famine, which claimed 1 million lives between 1845 and 1849 in Ireland, was the potato disease that infected potato plants throughout the Island of Ireland. The potato disease could successfully spread and devastate a potato population was because most potato-plant DNAs were identical or too similar to each other as a result of cloning.
Unlike potatoes, the only way to reproduce for many one-celled, such as viruses, is cloning. In other words, cells with identical DNAs multiply to become more cells with identical DNAs. Then why are the virus species that we killed off are numbered?
The generation time of viruses can be as short as seconds. This means in a comparatively short span of time minor mutations in the virus emerge and build up. This causes genetic variation among the virus species. However, generation time for more complex organisms is long, for instance for humans up to 33 years. This makes it highly likely that the population of cloned species goes extinct before minor mutations build up to cause meaningful genetic variations. Therefore a more complex way to reproduce is needed for more complex organisms.
Random Factors in Reproduction
Your genes play a significant role in making you who you are. The color of your hair, the color of your eyes, your height, the shape of your nose, and the dimples on your cheeks are all mostly by your genes.
At first, it might seem like children from the same parents should look alike. After all, siblings get their genes from the same parents, and the DNA of humans remains the same throughout their lives.
Siblings do not look exactly alike because everyone, including their parents, has two copies of most of their genes, which may be different. Parents pass one of their two copies of each of their genes to their children. Which copy a child gets is random, and this is the core reason you do not look like your brother.
For certain gene types, one of the two genes suppresses the other to manifest itself, for some other types of genes, both genes manifest themselves.
Reproduction and Survival of Species
Since this is all a bit abstract, let us give an example of eye colors.
Imagine a man who has a blue-eye gene and a brown-eye gene that he each inherited from his parents. Because the brown-eye gene suppresses the blue-eye gene, this man will have brown eyes. Physically, there will be no indication that the man carries a blue-eye gene.
In our example, this man goes on to have children with a brown-eyed woman who, just like him, inherited a blue-eye and a brown-eye gene from her parents.
The first child of this couple inherits by chance the brown eye gene both from the mother and father and has brown eyes. The first child’s future offspring can only inherit a brown eye from him.
The second child of this imaginary couple inherits by chance a brown eye gene from one of the parents and a blue-eye gene from the other. This child also has brown eyes, but because she carries a suppressed blue-eye gene, her future offspring can inherit a blue-eye gene from her.
Their third child of this couple inherits by chance two blue-eye genes, one from each parent. The third child will have blue eyes, although all other members of the core family have brown eyes.
Most of the other genes are not passed on from parents to their children any differently than eye color genes. The random components of the mechanism are enough to result in children with, similar but overall different genetic information.
The genetic variation among siblings, as well as the general population, is crucial. The genetic variation between siblings increases the chances that at least one of the children survives in case of an adverse change in the environmental factor. Looking at the bigger picture, because the adverse changes in the environment do not affect everyone the same way, genetic variation in a population increases the chances that enough people survive to repopulate.
Siblings, and as a result all humans, look different because variation in DNA makes species resilient and agile.