Genetic Drift Simulation

What is Genetic Drift?

To begin with, let's examine a simple model of a population of fictional organisms called driftworms. In the following examples, the driftworms have only one gene, which controls skin color. Worms reproduce asexually and are connected to their parents by lines. In the population of five worms below, each worm gives rise to exactly one worm in the next generation. There are five alleles (skin colors) at generation 0 and the same five alleles at generation 4.

Note that the model above starts with a diverse population (5 worms, 5 alleles). What would the model look like if there were no diversity to begin with?

With no diversity in generation 0 and no forces of evolution acting on the population, the model above begins and ends with all worms in the population having the same allele.

The forces of evolution

In the above examples, the populations of worms are not evolving--neither the genotypes nor phenotypes are changing. For evolution to occur, there must be mutation, selection, or random genetic drift. These are the three major forces of evolution. The cause changes in genotypes and phenotypes over time. They also determine the amount and kind of variation seen in a population at a given time. This simulation focuses on drift (mutation and selection are covered in later simulations).

Random genetic drift

When genetic drift is introduced into the model, the results are different:

Note that in generation 2, the pink worm produces 1 offspring, the 3 green worms produced none, and the dark blue worm produced 4.

The role of chance

In real life, some individuals have more offspring than others--purely by chance. The survival and reproductions of organisms is subject to unpredictable accidents. It doesn't matter how good your driftworm genes are if you get squished by a shoe before producing offspring.

1. An ant gets stepped on.


2. A rabbit gets swept up by a tornado.


3. An elephant drinks up a protozoa living in a puddle.


4. A plane crashes killing a Nobel Laureate.

None of the above events has anything to do with the dead organism's genotype or phenotype these events occurred purely by chance.

Fixation of an allele

In a population model with genetic drift, alleles will eventually become "fixed". When an allele is fixed, all members of the population have that allele. In the graphic below, note that the dark blue allele fixed after 4 generations.

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Natural Selection

Natural selection is the process where heritable traits that make it more likely for an organism to survive long enough to reproduce become more common over successive generations of a population. It is a key mechanism of evolution. The natural variation within a population of animals, plants, bacteria, etc. means that some individuals will survive better than others in their current environment.

Fitness The concept of fitness is central to natural selection. Broadly, individuals which are more "fit" have better potential for survival, as in the well-known phrase survival of the fittest. Modern evolutionary theory defines fitness not by how long an organism lives, but by how successful it is at reproducing. If an organism lives half as long as others of its species, but has twice as many offspring surviving to adulthood, its genes will become more common in the adult population of the next generation. This is known as differential reproduction. Though natural selection acts on individuals, the effects of chance mean that fitness can only really be defined "on average" for the individuals within a population. The fitness of a particular genotype corresponds to the average effect on all individuals with that genotype.

Types of selection: The unit of selection can be the individual or it can be another level within the hierarchy of biological organisation, such as genes, cells, and kin groups.

Natural selection occurs at every life stage of an individual. An individual organism must survive until adulthood before it can reproduce, and selection of those that reach this stage is called viability selection.

In many species, adults must compete with each other for mates via sexual selection, and success in this competition determines who will parent the next generation.

When individuals can reproduce more than once, a longer survival in the reproductive phase increases the number of offspring, called survival selection.

The fecundity of both females and males (for example, giant sperm in certain species of Drosophila) can be limited via fecundity selection. The viability of produced gametes can differ, while intragenomic conflicts such as meiotic drive between the haploid gametes can result in gametic or genic selection.

The union of some combinations of eggs and sperm might be more compatible than others; this is termed compatibility selection.

It is also useful to distinguish between ecological selection and the narrower term sexual selection. Ecological selection covers any mechanism of selection as a result of the environment (including relatives, e.g. kin selection, competition, and infanticide), while sexual selection refers specifically to competition for mates. Sexual selection can be intrasexual, as in cases of competition among individuals of the same sex in a population, or intersexual, as in cases where one sex controls reproductive access by choosing among a population of available mates. Most commonly, intrasexual selection involves male-male competition and intersexual selection involves female choice of suitable males, due to the generally greater investment of resources for a female than a male in a single offspring organism.