Posted by: arrogantscientist | January 15, 2009

Understanding Evolution – Chance and Randomness

A common misunderstanding, or misrepresentation, of evolution made by anti-evolutionists is that humans could never have resulted from random chance. An equally wrong view would be that there is no random element to the evolutionary process.

To properly comprehend evolution, it is essential to understand how and when randomness and chance have an effect. Here I will discuss the main areas where these play a part. It is important to note however – in evolution, there is no true randomness, rather, what randomness does occur is an unpredictable result of a complicated set of contingencies.


The occurrence and character of mutations are essentially random (although, the chance of certain mutations occuring varies depending on conditions). Mutations come in many forms – from Single Nucleotide Polymorphisms to duplications of chromosomes, or even the entire genome. The effects of these mutations vary from nothing, to catastrophic, to beneficial. The improvement of a species or individual is irrelevant at this point – what happens to the propagation of these mutations in the future is, for the most part, non-random (i.e. natural selection).

Chromosomes and Sex

In total, humans have 46 chromosomes in every cell (excluding gametes). These 46 are made up of 2 copies of 23 individual chromosomes. Any one chromosome however, is not exactly the same as it’s sister – each is replete with large or small, but typically recessive, differences.
This is important when considering where the chromosomes go during sex. During meiosis and the creation of gametes (sperm and eggs), the compliment of the resulting daughter cells is halved to one copy of each chromosome (23 total). In the daughter cell where, 1a ends up in has no bearing on where 2a ends up, and so on for each of the 23 chromosomes. This shuffling process is random with all possible combinations having an equal chance of occurring.

Another process, known as Chromosomal Crossover (or “Crossing over”) ensures no two chromosomes of the same number are the identical copies of each other. For example, in a population of sperm, no two will contain a 1a with exactly the same compliment of genes as any other 1a. This is achieved by “swapping” homologous sections of chromosome while they are lined up close to each other (1a may swap material with 1b, 2a with 2b etc.).  This process is again not genuinely random, but it is almost entirely unpredictable.

These two processes conspire to ensure every sperm is sacred unique. Each egg is also unique. Now to make a new organism only one sperm and one egg are required. Assuming each sperm has an equal chance of getting to, and successfully fertilising an egg, the possibility one will reach it with its unique set of genes is minuscule. Especially when you consider the actual occurrence of sex is presumed.

Essentially, the pseudo-randomness here is what produces variation between individuals. The variation created is not guided in any way (e.g. by the needs of the organism). Natural selection can act on these variations – individuals carrying beneficial variations are more likely to be able to breed and pass on these traits to their offspring.

Genetic Drift

The other major actor at the same level as natural selection is genetic drift – a process by which traits spread across a population. Unlike the non-random “guiding force” of natural selection, there is an element of randomness to genetic drift. It is especially important for the spread of mutations that are not acted on by natural selection – ones that produce no beneficial or negative effect for the individual carrying them. The easiest way to understand genetic drift is by major disruption to a population, for example, the “bottleneck effect”. Consider a population of 100 creatures of the same species (use you imagination – penguins, fundamentalist Christians, murlocs – they all work fine). 25% carry allele A only (AA), 25% carry allele B only (BB) and 50% carry both A and B (AB). If you pick a breeding pair at random and build the population back up to 100, then there is a chance either allele A or B will not be present in this new population (i.e. if both of your pair were AA then only allele A would be present in the new population). The higher the frequency of an allele in the initial population increases the probability it will be present in the new population, but no allele is entirely safe – unless it’s present in 100% of individuals – it’s down to chance.

Genetic drift is sufficient to cause variations in separate populations of the same organism, even without natural selection.

In conclusion, Evolution is not a random process and anyone that claims it is does not understand it. It’s outcomes and processes are predictable to some extent. Unpredictability is caused by randomness and chance that act mainly at two levels – during the creation of variation, and during the spread of variation.



  1. If only they could read this without the cognitive dissonance alarm bells ringing. The whole randomness argument is nonsensical and just serves as another method for creationists to set up straw men.

    That said, I think there are plenty of additional facets that need to be adequately incorporated into the neodarwinian synthesis, such as epigenetics and horizontal gene transfer.

    Btw, good job on the article.

  2. Yeah, horizontal gene transfer has it’s place, but I generally consider higher eukaryotes, where it’s not an issue.

    Epigenetics may have a lot to answer for… but it does depend on how heritable its effects are.

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