Student Portal:


Investigation 2 – Concept Day








Adaptation: Investigation 2

Concept Day


In this short presentation, we will first review the concept of dominant and recessive alleles as well as genotype and phenotype. We will then look at the experimental situation that you will encounter in Investigation 2 lab.

You will see that experimental results will provide data that demonstrates that adaptation and natural selection can change the frequency that alleles occur in a species. This, in turn, can lead to the extinction of a species or its evolution into slightly modified forms.



Note: This slide reviews the concept that you were first exposed to in Investigation 1. We will continue to use the example of a “bone density” gene. Note that for the sake of clarity and illustration of basic principles, that we are assuming in this example that a single gene determines this trait in penguins. There may well be a combination of genes involved in a complex trait like bone density. Nonetheless, the basic concepts of dominant and recessive alleles and natural selection remain the same.

  • In this discussion and the experiment that you will perform in Investigation 2 lab, we will look at bone density in penguins. In our example, the blue allele is the dominant allele and is for less dense bones. The recessive allele (white) is for more dense bones. In this instance, the importance of bone density is related to how deep penguins can dive underwater to obtain food.
  • Three chromosome pairs are shown in this slide with their genotypes and phenotypes indicated below. Notice that, since the blue allele is dominant, only one of the three chromosome pairs will show a dense bone phenotype, the homozygous recessive.
  • If we were to only see a phenotype, we would have no idea what the genotype of an organism is. Both the homozygous dominant and the heterozygous dominant would have exactly the same phenotype; they would both have less dense bones. Knowing the genotype – that is the type of alleles contained in the bone density gene – we can see that while both have less dense bones, the heterozygous dominant nonetheless contains a copy of the recessive (white) allele. This is important because only the mating of two heterozygous dominant penguins or two homozygous recessive penguins can produce a homozygous recessive offspring with dense bones. We will see why this can be very important as we move on.



  • In this slide, we see two penguins of the same species. They look the same. The only difference between them is that one has dense bones (penguin B) and the other has less dense bones (penguin A). As can be seen, penguin B can dive much deeper than penguin A.
  • As shown, the genotype of penguin A may be either homozygous dominant or heterozygous dominant. Either way, it displays the less dense bone phenotype. Penguin B, on the other hand, has a homozygous recessive genotype and thus displays the dense bone phenotype and can dive relatively deep compared to penguin A.
  • There are also two species of fish pictured. One swims at the surface and the other swims deeper. The deep-diving penguin B can feed on either fish species. Penguin A, on the other hand, is restricted to shallow-swimming fish that doesn’t require deep diving to capture.



  • This slide depicts the situation at the beginning of the experiment you will perform in the Investigation 2 lab, before the selection part of the experiment takes place. It shows a hypothetical beginning genotypic and phenotypic account of the penguin population in an area. Notice that there are 6 homozygous dominant penguins, 6 homozygous recessive penguins, and 12 heterozygous dominant penguins to begin with.
  • We can determine the allele frequency of this beginning population by counting the number of dominant alleles and recessive alleles and then calculating the total number of alleles. As can be seen in this instance, there are 24 dominant (blue, less dense bone) alleles and 24 recessive (white, dense bone) alleles. This gives a total of 48 alleles.
  • Allele frequency can be determined by dividing either the number of dominant or recessive alleles by the allele total. As shown, the allelic frequency of both the dominant and recessive alleles is 0.5 (50%).



  • Without going into detail, this slide shows a schematic representation of the selection experiment that you will perform in Investigation 2 lab.
  • In essence, you will use the forceps to “feed” on the fish (sunflower seeds and kidney beans). Each penguin (A and B) will be timed. The only difference is that penguin A, the less dense bone penguin, will not be able to reach the deeper fish (kidney beans).
  • After the feeding simulation, you will determine how many of the penguins were able to capture enough fish to survive. These numbers will then be used in the next slide to determine a new allele frequency in the penguin population.



  • As can be seen, from a phenotypic standpoint, the more dense penguins did somewhat better than their less dense bone counterparts.
  • Once again, allele frequency can be determined by dividing either the number of dominant or recessive alleles by the allele total. As shown, the allelic frequency of the dominant allele is now 0.25 (25%) and that for the recessive allele is 0.75 (75%). Thus, the recessive allele was “selected for”.

Note: You will perform additional experiments of a similar nature to that described above to further analyze the impact of genotype and phenotype on allele frequency under different environmental conditions.



  • The last three slides illustrate the potential fate of the penguin population as selective pressure continues.
  • This slide once again depicts the original situation as the experiment begins. The less-dense bone penguin A (either homozygous dominant or heterozygous dominant) happily feeds on the shallow-swimming fish. The dense bone, deep-diving penguin B can feed on either the shallow- or deep-swimming fish species.



  • As an environmental change occurs that reduces the abundance of shallow-swimming fish, Penguin A is in trouble. Even if there are plenty of deep-swimming fish in the area, penguin A simply can’t dive deep enough, due to its light, less dense bones, to get to them. It could actually starve to death while seeing an abundance of the deep-swimming fish below it.
  • Penguin B, however, can go on feeding on the deep-swimming fish as before or eat the occasional shallow-swimming fish it comes across.
  • The natural selection process is, in this case, working against the dominant allele and favoring the recessive allele.



  • Finally, without enough fish to feed on at shallow depths, penguin A disappears from the penguin population, and only the dense bone, deep-swimming penguin B penguins remain.
  • When the very last penguin A individual dies, it takes with it the last copy of the dominant (blue, light bone) allele from the entire penguin population. From this point forward, only the homozygous recessive, dense bone penguins remain.
  • This scenario teaches us many valuable biological lessons. Perhaps most importantly is that genetic variety or variation in a species helps it survive. If there were only the dominant (blue) alleles in the penguin species, the change in food resources may have caused death to all individuals and the extinction of the entire species!
  • Thus, genetic variation is a very important survival mechanism of plant and animal species and many of their adaptations and behaviors are directed to maintaining or increasing genetic variation.