1. Introduction: Natural Selection in Gene Pools

In the last tutorial, we learned about the Hardy-Weinberg Principle. This principle proposes a mathematical construct of a gene pool that remains at equilibrium, with no change in allele frequencies, as long as five conditions are maintained. The last of these conditions was that no alleles can be harmful or beneficial.

In any real population, where the individuals in that population are struggling to survive in the face of a changing environment, some alleles are going to code for traits that will promote individual survival, while others will code for traits that hinder survival. This, of course, was the key insight of Charles Darwin and Alfred Wallace in their theory of evolution by natural selection (something that we covered in the first tutorial in this series).

Now, we’ll look at natural selection from a population genetics perspective. 

2. Natural Selection Directly Acts on Phenotypes

What you see is phenotype

Natural selection acts on phenotypes. A phenotype is an observable trait in an organism. A phenotype could be an organism’s color, size, speed, form, or behavior. The length of an elk’s antlers, a wolf’s sense of smell, the height of a redwood tree, or the shape of an enzyme are all phenotypes

Genotypes are the underlying genes or alleles. If you inherited two alleles for cystic fibrosis (an inherited lung disease), your phenotype would involve all of the symptoms connected with cystic fibrosis (build-up of mucus in the lungs, salty sweat, problems with digestion, etc.) Your genotype would be homozygous recessive, and could be represented as “cc.” If you inherited one allele, your phenotype would be “normal,” and your genotype would be heterozygous normal, represented by “Cc.”

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[h]Genotype v. Phenotype

[q labels = “top”]

Genotype or Phenotype?
DNA sequence  _______________
Protein  _______________
Shape of an enzyme  _______________
Genes that code for an enzyme  _______________
Height  _______________
DNA that codes for a growth hormone  _______________
Shape of a bird’s beak  _______________
What natural selection selects  _______________
Cc _______________

[l]genotype

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]phenotype

[fx] No. Please try again.

[f*] Excellent!

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3. Natural Selection Increases the Frequency of Beneficial Alleles, and Decreases the Frequency of Harmful Alleles

Natural selection is frequently summarized as “survival of the fittest.” In a population genetics context, you can think of natural selection in terms of phenotypes and genotypes. Individuals who have phenotypes with some type of advantage will be selected for survival by the environment. The individuals that survive will get to pass their alleles on to the next generation. Over time, the gene pool of that population will have more copies of alleles that code for beneficial phenotypes, and less copies of alleles for harmful traits. The key idea is that through selection of phenotypes, the allele frequencies in a gene pool change. Individuals don’t evolve; populations do.

4. Natural Selection in Action: Peppered Moths, Ice Fish, and Pocket Mice

Let’s return to the peppered moth, which we discussed in an earlier tutorial in this series. Moths are eaten by birds. Birds more frequently eat the moths that are easier to see. The best camouflaged moths get eaten less. In terms of natural selection, the birds select the best camouflaged moths.

Dark and light colored moths on a light colored tree: Source: Wikipedia

What are they selecting them for? For survival and reproduction. In other words, in the same way that during the process of artificial selection, breeders select the most desired trait, the birds are selecting the best adapted trait. In response, over time, allele frequencies will shift to match this selective regime.

The biologist Sean Carroll has discussed this in relationship to a type of ice fish that lives close to Antarctica. This fish has blood that’s almost completely clear—not red like your blood, or the blood of other fish. The reason why the blood is clear is because it lacks red blood cells, which carry the iron-rich, oxygen-absorbing protein called hemoglobin. The reason why these fish lack red blood cells is to keep their blood (and body tissues) from freezing solid in the frigid water around Antarctica. Fewer blood cells means lower blood viscosity (thickness), keeping blood moving even in freezing temperatures.

Antarctic Icefish; Wikipedia

How could this have evolved? Over the past tens of millions of years, Antarctica has drifted southward to its current position, becoming colder and colder. As it did, the ancestors of these ice fish that produced less red blood cells (a phenotype), survived better, because they experienced tissue damage from freezing less often. At the same time, because cold fluids can hold more oxygen, the lack of red blood cells was not a survival disadvantage. Looking into these fish’s DNA, you can see how this happened. The alleles for hemoglobin accumulated mutations that prevented hemoglobin from being produced, These mutated alleles became fixed in the ice fish population. What would have been a deadly mutation in another species became a life-sustaining allele in the ice fish (See Sean Carroll, The Making of the Fittest, or read an excellent summary of his work here).

Here’s a final example. In southern Arizona, the rock pocket mouse lives on two types of landscapes: dark colored lava, and lighter colored sand.

Rock pocket mice placed on surfaces to show good (top) and poor (bottom) camouflage. Source: http://www.pnas.org/content/100/9/5268.long. Used with permission of Professor Michael Nachman.

The mice on the dark colored lava are darker colored, while those that live on sand are lighter colored. The phenotype is selected by the environment (in this case, owls who prey on the mice). And as a result, the frequencies of coat-color alleles have shifted in each population. Alleles for dark coloration are in higher frequency in the mice living on dark lava; alleles for lighter coloration are in higher frequency in the mice living on lighter sand (source: The Genetic Basis of Adaptive Melanism in Pocket Mice). 

Natural Selection Can Unfold in Three Ways

The type of natural selection described above moves the average phenotype in a population in one consistent direction. In the case of the peppered moth, the darkening of tree trunks set the stage for birds who ate moths to select moths with darker coloration. Over time, as pollution continued to darken the forests, the number of moths with the dark phenotype increased, and the number of moths with a lighter phenotype decreased. This, in turn, changed the underlying allele frequencies. In the case of the ice fish, you can imagine that over the millions of years during which this adaptation took place, the amount of red blood cells/unit volume of blood steadily decreased, as did the amount of hemoglobin.

This type of natural selection is called directional selection. If you were to graph the frequency of the some trait (size, color, enzyme activity, etc.), it would look like what’s shown below. The average moves in a specific direction. For example, as the giraffe was evolving, the average length of the neck and legs was increasing. Or, in the case of the peppered moth, the average amount of pigment increased. For blue whales, it’s increasing size. For jackrabbits, it’s heat tolerance. For rattlesnakes, it’s venom toxicity. For cheetahs, it’s running speed.

directional selection

Selection doesn’t have to push in one direction. There can also be selection for the average, and away from the extremes. This is called stabilizing selection, and, if graphed, it looks like this:

Think about the birthweight of babies. Babies that are too small have various problems that lowers their survival rate. But babies that are too large (think back to the times before Cesarian births could be safely and successfully performed) can be difficult for their mother’s to deliver, also lowering survival rate. So, if the trait is birthweight, then nature is selecting against both birthweights that are too low and those that are too high. The result is stabilization around a mean (which in babies of European descent is about 3.5 kilograms (7.7 lbs), with all but 5% of babies born between 2.5 and 5 kilograms).wikipedia

Finally, there can be selection against the mean, and for the extremes. This type of selection is said to play an important role in dividing up gene pools as a prelude to speciation. When graphed, disruptive selection looks like this:

Got it? The key ideas from this section are

  1. The presence of harmful or beneficial changes is a violation of one of the key Hardy-Weinberg principles, and results in changes in allele frequencies.
  2. During natural selection, the environment selects for or against certain phenotypes.
  3. If a phenotype is selected for, then the individuals with that phenotype will survive and reproduced at higher rates. The alleles that code for that phenotype will increase in frequency in that gene pool.
  4. If a phenotype is selected against, the individuals with the phenotype will survive at lower rates. The alleles that code for that phenotype will decrease in frequency in that gene pool.
  5. Selection comes in three main varieties
    1. Directional selection selects for one extreme, and against the other. This moves the average phenotype within a population in one direction.
    2. Stabilizing selection selects against both extremes, and for the mean. This narrows the range of variation within a population.
    3. Disruptive selection selects against the mean, and for the extremes. This produces a bimodal distribution of phenotypes (like the hump of an Asian camel).

If you feel that you understand these concepts well, take this brief quiz. If not, carefully re-read the material above, and then take the quiz.

Once you’ve mastered all the concepts, you should go on to the next tutorial in the series (link is below).

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[h]Natural Selection in Gene Pools
[!!!!!!] question 1 +++++++++[/!!!!!!]
[q topic= “natural_selection_in_gene_pools”]When you’re looking at an organism, you’re directly looking at its
[c] genotype
[c*] phenotype
[f] No. The genotype is the underlying genes. What you see when you look at an organism is the expression of those genes.
[f] Correct. When you look at an organism, you’re looking at its phenotype.
[!!!!!!] question 2 +++++++++[/!!!!!!]
[q topic= “natural_selection_in_gene_pools”]Natural selection acts on an organism’s
[c] genotype
[c*] phenotype
[f] No. The genotype is the underlying genes. Natural selection (the environment) can’t see the underlying genes. It can only see the expression of those genes.
[f] Exactly. Natural selection (the environment) can only act upon an organism’s phenotype.
[!!!!!!] question 3 +++++++++[/!!!!!!]
[q topic= “natural_selection_in_gene_pools”]What evolves? Populations or individuals?
[c*] populations
[c] individuals
[f] Yes. One way to look at evolution is as a shift in the allele frequencies in a population’s gene pool. Individuals get selected to survive and reproduce. Populations evolve.
[f] No. One way to look at evolution is as a shift in the allele frequencies in population’s gene pool. Individuals get selected to survive and reproduce. What evolves?
[!!!!!!] question 4 +++++++++[/!!!!!!]
[q topic= “natural_selection_in_gene_pools”]In the rock pocket mouse, allele “b” codes for light coloration. Allele “B” codes for darker coloration. This is shown in the table below. In a desert environment with light colored sand, what would be selected?

[c] the BB or Bb genotypes
[c] dark fur
[c] the bb genotype
[c*] light fur
[f] No. The environment doesn’t select genotypes. Next time, choose a phenotype.
[f] No. In a light colored environment, a mouse with dark fur would be easily seen by predators. Think of selection as the role of the breeder, who selects which individuals get to survive and reproduce. What are the owls selecting for reproduction?
[f] No, but you seem to be thinking about this in a smart way. Just remember that it’s the phenotype, not the genotype, that gets selected.
[f] Exactly. Mice with light fur will be better camouflaged. As a result, that trait will be selected for survival and reproduction by the environment.
[!!!!!!] question 5 +++++++++[/!!!!!!]
[q topic= “natural_selection_in_gene_pools”]In the rock pocket mouse, allele “b” codes for light coloration. Allele “B” codes for darker coloration, as is shown in the table below. If a population of mice is living on dark, volcanic rock, which of the following would most likely be correct.

[c] Owls will directly select against dark fur.
[c] Owls will directly select against the “B” allele.”
[c] Owls will directly select against the “bb” genotype.
[c*] Owls will directly select against light fur.
[f] No. In a dark colored environment, the owls will be catching and eating more of the mice with light fur. In other words, predation by owls results in selection FOR the dark furred phenotype. This is because the dark furred mice will be eaten less, and will survive more, enabling them to reproduce.
[f] No. A predator is only going to see the phenotype, not the genotype. The owls can’t see the ‘B’ allele. What phenotype can they see?
[f] No. A predator is only going to see the phenotype, not the genotype. The owls can’t see a genotype. What phenotype can they see? That’s the phenotype they’ll be selecting against.
[f] Correct. In a dark colored environment, owls will see and eat mice with light fur. In other words, the owls are selecting against the light furred phenotype.
[!!!!!!] question 6 +++++++++[/!!!!!!]
[q topic= “natural_selection_in_gene_pools”]Natural selection changes
[c] Phenotypes in individuals
[c] Genotypes in individuals
[c*] Allele frequencies in populations
[f] No. Nature can select a phenotype for survival and reproduction. But the selection process doesn’t change that phenotype. Over time, what changes?
[f] No. The environment doesn’t change an organism’s genotype. Evolutionary change doesn’t happen on an individual level. What’s the level of biological organization that’s larger than the individual (and the level on which evolutionary change occurs?)
[f] Correct. By selecting phenotypes, natural selection changes the allele frequencies of a population.
[!!!!!!] question 8 +++++++++[/!!!!!!]
[q topic= “natural_selection_in_gene_pools”]In a gene pool, a dominant allele has a frequency of 0.2 (20%) and the recessive allele has a frequency of 0.8 (80%). If neither allele is for a phenotype that is being acted upon by the environment, then after 10 generations, you would expect that
[c*] the allele frequencies will remain about the same.
[c] The recessive allele, because it’s already so much more common, will become fixed at 1.0 or 100%
[c] The dominant allele, because it dominates the recessive allele, will steadily increase in frequency.
[f] Yes. Unless some force like natural selection is acting upon a population, the frequency of the alleles that make up its gene pool should remain about the same.
[f] No. There’s no built in tendency for frequent alleles to become more frequent. Unless some force like natural selection is acting upon a population, the frequency of the alleles that make up its gene pool should remain about the same.
[f] No. Unless some force like natural selection is acting upon a population, the frequency of the alleles that make up its gene pool should remain about the same. Dominant alleles are only dominant insofar as they can mask a recessive allele when both appear in the same individual. On a population level, there’s nothing about dominant alleles that will make them increase in frequency.
[/qwiz]

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