Darwin: Death and Genetic Selection

Charles Darwin wrote a treatise called On the Origin of Species, in which he proposed the concept of evolution. His theory that higher organisms evolved out of lower ones revolved around natural selection, or the idea that weaker organisms would be less likely to reproduce, thus giving way to better adapted ones. This provides death with a very clear ecological function: serving to clear away past generations to make way for more advanced future ones.

Of course some people have taken this concept and run with it, perhaps making light of death. The so called Darwin Awards commemorate people who did the stupidest things, but fortunately are removed from the gene pool prior to procreation. This is a somewhat lighthearted glimpse at the very dire theory of social Darwinism, which mutates the original theory to justify the removal of "lesser" humans to allow for general progress. Such perceptions not only justify some people's discrimination against the disabled, but it also was the basis of Hitler's quest for the perfect race.

The fusion of Darwin's theories and Gregor Mendel's observations of pea plants has yielded modern evolutionary and genetic theory, founded on natural selection and independent assortment. Everyone has two genes for a given trait, one from each parent, which can involve any combination of the possible alleles for the two traits. Often, one of these traits will be dominant, so that if a person has a heterozygous genotype (has one of each of two alleles), only the dominant one will be expressed in the phenotype. For example, J could be the dominant allele for brown eyes. Then, a child of a brown-eyed Jj parent and a brown-eyed Jj parent will have a 1/4 chance of being JJ (pure brown-eyed), 1/2 chance of being Jj (heterozygous but expressing brown eyes, the dominant trait), and a 1/4 change of being jj (homozygous recessive, and thus not having brown eyes).

Hardy and Weinberg produced a theory to understand the distribution of alleles in a population. Explained further below, it basically said the relative number of each allele would not change given that:

no mutations occur
there is no net migration
the population is large enough for probability to apply
mating is random
all alleles are equally viable

The proportions do change when any of these conditions are not met. However, another implication is that harmful alleles would never be completely eliminated from a population because they become increasingly shielded inside of heterozygous individuals where they will not be selected out.

To better understand this concept, try the demonstration below.
In this model, anyone who has the bb genetic composition dies, and every couple has three children. Unmated inidividuals do not have children.

It is best not to change any of the individual genetic compositions, but simply to keep clicking "submit query", watching how the proportion of recessive alleles decreases but never reaches zero.

Each click represents another generation. The "q" column represents the recessive gene. If this was a gene for a fatal condition, such as haemophilia, you can see that althought the gene reduces in frequency in the population it always remains in the gene pool.

Person 1 Person 2
Person 3 Person 4
Person 5 Person 6
Person 7 Person 8
Person 9 Person 10

Initial conditions p distribution: .5 q distribution: .5 p^2 = .25 (homozygous dominant) q^2 = .25 (homozygous recessive) 2pq = .5 (hetereozygous)

The Hardy-Weinberg principle says that p^2 + 2pq +q^2 = 1 and that p + q = 1, where p is the fraction of alleles in a population that are dominant, q is the fraction of alleles in a population that are recessive, p^2 is the fraction of homozygous dominant (BB) genotypes, q^2 is homozygous recessive (bb), and 2pq is the number of heterezygous (Bb) genotypes.

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