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The genetic diversity between organisms is accounted for by variation in DNA. Even a tiny difference in DNA can result in a lot of variation. What’s more, even identical DNA can still result in variation due to certain genes being active and others not. Is it visible that these two have largely identical DNA?
Nope. Yet when compared to bacteria or an imaginary alien, those two above definitely look more closely related. So the similarities between humans’ DNA and chimps’ DNA cause the visible and functional (phenotypical) similarities such as limbs, eyes, mouth, etc. while the differences in DNA result in the phenotypical differences such as hair thickness, facial features and so forth.
This knowledge has been used for a very long time by humans to manipulate wild plants and animals in order to make them somehow useful. People selectively breed plants and animals to change their genetic profile, so that certain traits are preferred over others.
Plants are selectively bred for higher yields, while animals are bred for many different purposes, such as food production, competitive events, aesthetic features, etc.
Selective breeding always leads to a decrease in variation. Overall, variation may increase as many different breeds are created – for example with dogs. Yet within one breed, variation is very low. There are ethical issues to be considered when selectively breeding animals. Dogs bred purely for cosmetic reasons, like chihuahuas, can suffer from illnesses associated with their size, bone structure or facial features.
The Founder Effect
Suppose a boat traveled from one island to another. In the process, several lizards were transferred from the first island to the other. The lizards breed and settle down to form a new lizard population on their new island. This is called the founder effect. The small number of founding lizards formed the genetic base on which the whole population was built. This genetic base is significantly smaller than that of the original lizard population on the first island.
Therefore, the genetic diversity of the new population is lower than that of the original population.
The only difference between the founder effect and genetic bottlenecks is the way in which the new genetic pool is formed. In the founder effect the new pool is formed when a few individuals from a population become geographically isolated, while in genetic bottlenecks the new gene pool is formed when only a few individuals from a population survive a mass disaster, or are the only ones to breed.
The effect is the same: the genetic variation of the new population is decreased compared to the original population.