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Mechanisms of Evolution

Mechanisms of Evolution

The Gene Pool

Ready For a Dip?

It's time to break out your swim trunks and floaties, because we're going for a dip in the gene pool.

Okay, so we can't really swim in a gene pool. Just pretend we can sit by its edge, under a warm summer sun, and dangle our feet in it. While we lounge and sip fruity drinks, we can talk about what's in here and what happens when someone does a cannon ball into its pristine waters.

Every population of a species has a variety of genes, and each gene can have multiple alleles. All the available genetic information within a population is known as a gene pool. When life is thriving in a population and there is genetic equilibrium, then these alleles are not changing in frequency. There is no selection. There is no evolution. Everything is calm. Like the pool…before the cannon ball.

Larger populations are more likely to be unaffected by a large disruption. Think of it like a larger body of water. A stone's throw won't make much of a difference in the ocean, but you'd notice it in a puddle.

When small changes occur within a population, evolution happens on a small scale. This is microevolution. Larger changes, bigger ripples in the tide pool, are known as macroevolution. The two ideas are not mutually exclusive. When lots of microevolution happens over a larger period of time, what you see in the rear view mirror is macroevolution. For example, we know life began in the water, and eventually moved onto land. Microevolution caused allele frequencies to change in a water-living species. Macroevolution is when that species diverged to a new group that was able to hang out on the sand.

If life at the gene pool is at genetic equilibrium and allele frequencies are unchanging, the Hardy-Weinberg principle applies. In the next sections, we will talk in depth about the mechanisms (the cannon balls, if you will) that disrupt a population's equilibrium and drive evolution. These mechanisms include mutations, gene flow, genetic drift, sexual selection, and Darwin's natural selection. If any mechanism of evolution is occurring, the Hardy-Weinberg principle is null and void. Therefore, we can use the principle to help us identify if a population is evolving.

Under good ole' HW, you can hold these truths to be self-evident
  1. The alleles of the gene under investigation will have unchanging frequency
  2. After a generation of random mating, the allele frequencies will remain the same
The second point can be proven by the Hardy-Weinberg equation:

p2 + 2pq + q2 = 1

(percent homozygous p) + (percent heterozygous pq) + (percent homozygous q) = 100% of the population

Here, for one gene locus, one allele of the gene is p and another allele of the gene is q. Keep a few things in mind:
  1. 1. Although it is possible to have multiple alleles for a gene, this equation only works if there are two alleles for the gene of interest. It's more complicated if more options exist.
  2. The organism must be diploid as a reproducing adult. A diploid carries two copies of each gene. Each copy could be either the "p" or "q" allele.
Look at an example to test the good ole' HW equation.

How fast does an inchworm inch if an inchworm inches all day?

Say you have a gene that codes for inchworm speed. To make life easier, we'll stick with p's and q's. There are two alleles of this gene. One allele makes inchworms quick. We'll call this q. One allele makes the inchworms oh so painfully slow. We'll call this p.

Remember that gametes (sex cells) are haploid. They will only have one allele or variant of each gene. Currently, if you were to look at 100 random inchworm gametes in the population, 75 of them would have the p allele. This allele is more frequent in the population. That means the other 25 gametes would have the q allele. Mating is random in Hardy-Weinberg equilibrium, and the contribution of either a p or a q is an independent event.
  Female Gamets
p q
Male Gamets p pp (p2) Pq (pq)
q Pq (pq) qq (q2)
If a male and a female each have the diploid genotype pq, then they can produce gametes with either the p allele or the q allele. This Punnett square shows the possible combinations. In parenthesis is the frequency of the alleles described by the Hardy-Weinberg equation.

The Hardy-Weinberg equation describes the probability of each of the following combinations: homozygous p, homozygous q, and heterozygous (pq). The probability of having a gamete with the p variant of the gene fusing with a gamete with the q variant is p times q (pq). This value gets multiplied by 2, because there are 2 ways to get fertilization with one p and one q. The female could be p while the male is q, or vice versa. Similarly, we can determine the probability of getting a gamete with two p's (p2) or two q's (q2) by multiplying the frequencies of the alleles together.
  Eggs in population
p q
Sperm in population p f(pp) = p2 f(pq) = pq
q f(pq) = pq f(qq) = q2
Relative allele frequencies (f) of p and q in the inchworm population can be used to calculate the proportion of each individual genotype in the population. Because the p allele has a frequency of 75%, compared to an allele frequency of 25% for q, more of the population will have the genotype pp, or p2. A smaller amount of the population will have the genotype qq, or q2. This is visually demonstrated by the relatively gigantic size of the pp square and the itty bitty baby qq square.

For our quick (q) and painfully slow (p) alleles, the math goes as follows:

p2 + 2pq + q2 = 1
(pp): 0.75 × 0.75 = 0.56
2(pq): 2(0.75 × 0.25) = 2(0.19) = 0.38
(qq): 0.25 × 0.25 = 0.06

Notice these numbers add up to 1 (0.56 + 0.38 + 0.06).

By these calculations, 56% of the population has the genotype pp, 6% is qq, and 38% is pq. Let's next look at what happens when this generation goes through another random mating. What are the allele frequencies of the population now?

You are calculating the frequencies of alleles if the organisms from this generation randomly contributed gametes. (You know, take this one, and put it with that one. Then this one with that one over there…and so on.)

Step 1: Divide the percent of pq individuals in half, because only half of the alleles from these individuals are p and the other half are q.
0.38/2 (p alleles = 0.19, q alleles = 0.19)
Step 2: Add this to the percent of p
0.56 + 0.19 = 0.75 or 75%
Step 3: Add this to the percent of q
0.06 + 0.19 = 0.25 or 25%

The frequency of the p allele in the second generation is 75%. The frequency of the q allele is 25%.

Do you recognize these digits? Yup. The same ones we started out with.

If you went into the population and calculated the allele frequencies of the second generation and they didn't match, then what happened? A cannon ball. We're being metaphorical. You can bet that one (or more) of the five mechanisms of evolution just created a splash in the gene pool.

Brain Snack

Curl up with your Snuggy and watch the evolution of "organisms" in different gene pools in a computer simulation.

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