Let's continue learning where variations in populations come from.

Nonrandom mating is the process of individuals choosing a mate based on a certain characteristic. Birds are a primary example of this.

Video segment. Assistance may be required. Watch the video below about the peacock species for further explanation.

Source: Natural Selection: Tale of the Peacock, Acorvettes, YouTube

Genetic drift is the change in the frequency of an allele due to random changes as opposed to natural selection. Genetic drift occurs in small, reproductively isolated populations. Genetic drift has minimal effects on large populations. The effect of chance events lead to the following:

  1. The bottleneck effect occurs when some factor, such as a natural disaster, reduces a population to a small number which then recovers and finally expands again. This can be caused from famine, natural disaster, or loss of habitat. Just by chance, the smaller population's gene pool may have allele frequencies that are different from those of the original gene pool. When the population grows, its alleles will be different in frequency from the original population. This change is not due to fitness. The image below represents what happens during the bottleneck effect.
    2. Image shows a model of bottleneck effect. In the first bottle there is a mix of blue and red marbles. The next image shows the result of a drastic reduction in population and a few red marbles and only 2 blue marbles. The last image represents the next generation and is a bottle with many red marbles and a few blue marbles

    Source: Adapted from: Bottleneck effect 3, Martha Lee, Clikr

    Describe how this image represents the bottleneck effect in your notes.

  2. The founder effect occurs when a small group splinters off and starts a new colony in a new habitat. These founding individuals may carry alleles that differ in relative frequencies from those of the original population, just by chance. Therefore, the new gene pool may start out with allele frequencies different from those of the parent gene pool.
Image shows an equal distribution on 4 different colored dots. The second image shows 2 orange and 2 green breaking away from the original population. These are the founding population. The third image shows only green and orange dots and represents the descendants of the founding population which have a different gene frequency from the original population.

Describe how this image represents the founder effect in your notes.

Natural selection results in different survival and reproduction rates due to changing environmental conditions. This can result from climate change, food source availability, predators, parasites, diseases, or toxins. The combinations of alleles that provide "fitness" increase in the population. This is an adaptive evolutionary change.

Video segment. Assistance may be required. Watch the following video to see how the process of natural selection operates in populations of rainforest hummingbirds.

Source: Evolution Primer #4: How does Evolution Really Work?,Gravitationalist, YouTube