Introduction

One difficult concept to grasp when learning about evolution is the fact that evolution never occurs at the level of the individual; it only reflects or describes changes that occur within an entire population over time.  In this section, we will discuss population genetics and how evolution describes the changes that occur in the collective ‘gene pool’ of a population.

Learning

When considering evolution from the perspective of a ‘population’, we can consider the collection of alleles present among all the individuals for that population as a ‘pool’ of genes.

Therefore, we can define:

• Population = a group that includes all the members of a particular species living in a defined area
• Gene Pool = all the alleles for a particular gene present in the individuals in a population

For example, consider a ‘population’ of 100 individuals. In this population, if we consider the gene that controls the ability to roll the tongue, each individual possesses two copies of this gene – therefore, the total number of alleles present (i.e. the ‘gene pool’) in this population would be 100 x 2 = 200.

Using the total gene pool size (200), we can calculate the frequency of the different alleles present within the pool.  The ability to roll one’s tongue is considered a Dominant trait (R), while the inability to roll one’s tongue is Recessive (r).  In our population, let us assume we have the following ratio of genotypes:

• 25 non-tongue rollers (genotype = rr)
• 60 heterozygous tongue-rollers (genotype = Rr)
• 15 homozygous dominant tongue-rollers (genotype = RR)

With this information, we can calculate the total number of ‘recessive’ (r) and ‘dominant’ (R) alleles present in the population:

Genotype	Number of Individuals	Number of dominant (R) alleles	Number of recessive (r) alleles
‘rr’	25	0	50
‘Rr’	60	60	60
‘RR’	15	30	0
Totals	100	90	110

From here, we can then calculate the frequency of each allele:

• Frequency of ‘R’ allele = (Total # R alleles/Total # alleles) = (90/200) = 0.45
• Frequency of ‘r’ allele = (Total # r alleles/Total # alleles) = (110/200) = 0.55

So, how is this related to understanding evolution?  Recall from our earlier definition of evolution:

“Evolution – change in heritable traits in a population across generations.”

The changes in heritable traits occur when there are shifts in the frequencies of the very alleles which determine these traits within the population.  In simple terms, the increase in the presence of a particular allele within a population will result in a higher prevalence of the trait or characteristic determined by that allele over time.

Calculating Allele Frequencies

Let’s consider another example of a population of Tongue Rollers. Assume the following:

• The population consists of 150 individuals.
• 100 of these individuals are homozygous recessive non-tongue rollers (genotype = rr).
• 30 of these individuals are heterozygous tongue-rollers (genotype = Rr).
• 20 of these individuals are homozygous dominant tongue-rollers (genotype = RR).

With this information, we can calculate the total number of ‘recessive’ (r) and ‘dominant’ (R) alleles present in the population:

Genotype	Number of Individuals	Number of dominant (R) alleles	Number of recessive (r) alleles
‘rr’	100	0	200
‘Rr’	30	30	30
‘RR’	20	40	0
Totals	150	70	230

From here, we can now calculate the frequency of each allele:

Frequency of ‘R’ allele = (Total # R alleles/Total # alleles) = (70/300) = 0.233

Frequency of ‘r’ allele = (Total # r alleles/Total # alleles) = (230/300) = 0.767

Summary

In this section, we have learned:

• A population is a group of all the members of a species living within the same region.
• A gene pool is all of the alleles for a particular gene (or set of genes) present in individuals within a population.
• The frequency of alleles within a population can be calculated as (# specific allele / Total # alleles)
• Evolution is the change in heritable traits (alleles) within a population over time.

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Sources:

“Complete with tongue roll.” by PatCastaldo is licensed under CC BY-ND 2.0

“Population Evolution.” By OpenStax Biology 2e. Retrieved from: https://openstax.org/books/biology-2e/pages/19-1-population-evolution Licensed under: CC-By: Attribution