Adding to the Evolution’s Definition
Evolution is the process in which an organism changes with time. However, a more apt evolution definition is the change in a population’s allele frequencies over time. The keyword is population.
Why?
Because population evolve, not individuals.
Huh?
Evolution requires variation amount individuals within a population and reproduction. Reproduction is the mechanism that allows organisms to pass traits on to the next generation. Sexual reproduction, the process most plants and animals use to reproduce, usually requires at least two individuals. Therefore, a single individual does not evolve.
What about the allele frequency part of the definition?
Well, to answer that question, I first need to define an allele.
What is an allele?
An allele is a variation of a gene. For example, all humans have a pigmented muscle in their eye called the iris. The iris’ job is to constrict and dilate the pupil so the eye can regulate the amount of light that hits the retina (the retina contains the neurons that send the visual information to your brain). We all have genes that code for our iris’ color, but the iris’ color varies from dark brown to pale blue.
If we all have the genes for eye color, why is there so much color variation in the human population?
Great question.
Thank you.
The variation in eye color comes from the combinations of alleles. Each gene comprised two alleles; one from mom the other from dad. There are three primary alleles that code for eye color: a brown allele, a green allele, and a blue allele. If you receive a brown allele from each parent, your eyes will be brown. If you have green eyes, you received a green allele from your parents. And, if you have blue eyes, well, you guessed it, you received blue alleles from your parents.
What color eyes will I have if my mom gave me a brown allele, and my dad gave me a blue allele?
Another good question.
Thank you. I ate my Wheaties this morning.
If you receive a brown allele from mom and a blue allele from dad, your eyes will be a shade of brown.
Why?
You must of had two bowls of Wheaties this morning.
I had seven.
Anyway, to answer why your eyes will be brown requires a greater understanding of genetics, which you will learn in the next unit. However, to understand evolution, you need to know that each gene contains two alleles (one from mom and the other from dad) and that alleles are the genetic basis (genotype) of the variation of traits (phenotypes).
Chromosomes House the Alleles
You have 23 pairs of chromosomes in each cell’s nucleus. Each pair contains a chromosome from mom and a chromosome from dad. Both chromosomes in the pair contain the same genes, but they include a different combination of alleles. For example, the chromosome pair below contains two genes. Each gene comprises an allele pair; therefore, there is a total of 4 alleles.
The gene at the top of the chromosome codes for eye color, and the gene at the bottom codes for hair color, and each gene has a pair of alleles. Mom’s chromosome contains a green eye allele and a brown hair allele, and dad’s chromosome contains a blue eye allele and a brown hair allele. Both chromosomes have the same genes (eye color and hair color), but the allele combination is different. The difference in allele combinations is what gives a species its diversity.
Calculating Allele Frequencies
Let’s say you are studying wolf fur color in Yellowstone National Park.
Why would I do that?
There are may reasons why, but let’s say that you want to know if fur color affects a wolf’s fitness.
Ah, so that is my driving question.
What’s with the Darth Vader memes and gifs?
Are you sure you are not a seven-year-old boy?
You should stop.
Assume there are 100 wolves in Yellowstone and the fur color comes in two variations: gray and black. Forty wolves have gray fur, and 60 have black fur. Assume one gene code for fur color. For simplicity, the gray wolves have two gray alleles, and the black wolves have two black alleles. Therefore, there is a total of 200 alleles for fur color in the population.
1 gene = 2 alleles
100 wolves = 100 genes for fur color
100 genes for fur color x 2 alleles = 200 alleles for fur color
All the alleles in a population is a gene pool. Therefore, there are 200 alleles for fur color in the gene pool.
Allele frequency is a quantitative measurement of the percentage of alleles in a gene pool. Scientists measure evolution by calculating the change in allele frequencies over time. Let’s use wolf fur color to show the evolution of a different wolf population between 2010 and 2020.
In 2010 there are 10 wolves, 6 with black fur and 4 with gray fur. To calculate the allele frequency, we need to calculate the total number of alleles in the wolf gene pool.
10 wolves = 10 genes for fur color
10 genes for fur color x 2 alleles = 20 alleles for fur color
Now we need to calculate how many gray and black alleles there are in the gene pool.
6 black wolves = 6 genes
6 genes x 2 alleles = 12 black fur alleles
4 gray wolves = 4 genes
4 genes x 2 alleles = 8 gray fur
Therefore, in 2010, the wolves’ gene pool comprises 12 black fur alleles and 8 gray fur alleles. The total number alleles for fur color in the wolves’ gene pool is 20. Now let’s calculate the allele frequency.
Formula: Number of Alleles for a Fur Color / Total Number of Alleles in the Gene Pool = Allele Frequency
12 black fur alleles / 20 alleles = 0.60 or 60%
8 gray fur alleles / 20 alleles = 0.40 or 40%
In statistics, percentages range from 0.0 (0%) to 1.0 (100%). Therefore, an allele frequency of 0.57 is 57%. Either answer, 0.57 or 57%, is correct, and you are free to use the notation that makes the most sense to you. However, you must only use one notation on your work – i.e., 0.57 or 57%. If you write 57%, you cannot use 0.48 to answer the next problem, because it will have a value of 0.48%.
In 2020 there are 16 wolves, 12 with black fur and 4 with gray fur. Calculate the allele frequencies of the 2020 wolf population.
12 black wolves = 12 genes
12 genes x 2 alleles = 24 black fur alleles
4 gray wolves = 4 genes
4 genes x 2 alleles = 8 gray fur
Therefore, in 2020, the wolves’ gene pool comprises 24 black fur alleles and 8 gray fur alleles. The total number of alleles in the wolves’ gene pool is 32. Now let’s calculate the allele frequency.
Formula: Number of Alleles for a Fur Color / Total Number of Alleles in the Gene Pool = Allele Frequency
24 black fur alleles / 32 alleles = 0.75 or 75%
8 gray fur alleles / 32 alleles = 0.25 or 25%
Since the allele frequencies of the wolf population changed between 2010 and 2020, evolution happened. If the allele frequencies remained the same, then no evolution would have occurred. Natural selection selects which traits have higher fitness, which indirectly changes the allele frequencies. Therefore, the trait with an increase in allele frequency has higher fitness than the trait with a decrease in allele frequency.
And, I’ll leave you with the evolution of a stormtrooper.
