Stickleback fish are found in both marine and freshwater habitats. The marine fish have no scales but have hardened, armorlike plates along their sides. The plates are thought to protect sticklebacks from certain predators. In the late 1980s, sticklebacks from a marine population colonized Loberg Lake, a freshwater lake in Alaska. Starting in 1990, researchers sampled fish from the lake every four years and recorded the armor-plate phenotypes of the male sticklebacks in each sample. The armor-plate phenotypes were categorized as either complete (plates extending from head to tail), partial (plates extending from head to abdomen), or low (a few plates near the head only). The results are shown in the table below. ARMOR-PLATE VARIATION IN THE STICKLEBACK POPULATION OF LOBERG LAKE Year 1990 1994 1998 2002 2006 Percent of Males in the Sample with Each Armor-Plate Phenotype Low Partial Complete 1% 2% 97% 45% 14% 41% 58% 16% 26% 76% 15% 9% 90% 6% 4% 14. Which of the following is an independent variable in the investigation?
(A) Year in which the sample was collected
(B) Salinity of the water in Loberg Lake
(C) Size of the stickleback population
(D) Percent of males with each armor-plate phenotype
15. To evaluate the reliability of the results, it would be best to know which of the following?
(A) The surface area of Loberg Lake
(B) The number of males in each sample
(C) The average daily temperature of Loberg Lake
(D) The age of the sticklebacks in each sample

14. Which of the following is an independent variable in the investigation? (A) Year in which the sample was collected

15. To evaluate the reliability of the results, it would be best to know which of the following? (B) The number of males in each sample

Explanation:

Independent (manipulated) variable: Refers to all the variables in an experiment that provoke a response in another variable. An independent variable is the one that changes or is controlled and modified in the experiment to analyze how another variable responds to it. It changes to analyze its effects on the dependent variable. Usually, the independent variable is represented by the X letter. In the exposed example, the Year in which the sample was collected is an independent variable.

Probably the year in which the sample was collected is not the cause of phenotypes differentiation between males, but it is a variable that reflects the change of armor-plate through the years. With this information, scientists can make assumptions and reach a conclusion about the phenotypic variation of the trait in sweet water through the years.  

The sample size is very important during a scientific investigation. The correct sample size depends on the investigation project. We need to study a correct amount of individuals to obtain conclusive and reproducible results or, on the contrary, the results will not be informative or robust at all. When sampling wild animals, many times it is difficult to obtain the ideal number of individuals, especially in aquatic animals that are hard to reach, so we have to manage with the amount that we are able to get. However, if we are working with a few individuals, or the samples are not comparable in size with each other, it is very important to consider this in the discussion and conclusions of your work. The risk with small-sized samples is that we might be studying individuals with extreme phenotypes that are not representative of the whole population. This is why it is important to get a certain number of animals, to analyze those that are most representative, and eliminate the outliers.

In the exposed example, per sample, we need to get approximately the same amount of male fishes to perform a reliable comparison between years and the phenotypic changes in time.