# Selection on Beak Length Stereolithography (.STL) Model Files and Lesson Plan

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Purchase access to the 3D stereolithography (.STL) files and accompanying lesson plan for the model:

Natural Selection on Beak Length

Hits the standard: HS-LS3-3

Apply concepts of statistics and probability to explain the variation and distribution of expressed traits in a population.

Lesson Brief:
Traits within a population vary around the average value. Variation within a population is necessary for natural selection to occur as it allows for different reproductive success, or genetic fitness, based upon the traits. In this activity we will examine how a morphological trait, beak length, may be selected for depending upon the environmental conditions.
This activity may be paired with the HS-LS3-3 Lesson Variation in a Population by assigning different beak lengths to the different bins in the resulting distribution from that activity.

Background:
Within species, some traits will partly determine the reproductive success of individuals. Different traits therefore allow for different reproductive success, one key criteria for natural selection to occur. The length of the beak within a bird species shows some small variation around the mean for the species. Changes in the lengths of beaks of finches has been studied on islands. In years when seeds are sparse and finches must access very tough seeds to gain enough protein to survive and reproduce. In these years, finches with shorter, stronger beaks may be at an advantage. However, in years with abundant and diverse seeds available, longer beaks that can pull seeds from cracks and holes may be advantageous and outweigh the advantage of cracking open one type of seed that is a lower proportion of what is available.
How much will the force exerted at the tip of the beak differ with different beak lengths?

Activity:
Assemble the bird model by sliding a beak into the lower jaw model and one into the bird head model. Insert a straightened paperclip sideways through the holes in the head to act as a hinge. Adjust the beak length to a mid-point to start. Wrap an elastic band around the base of the beak just behind the raised section. This will act as the jaw muscles pulling the beak closed. Measure the force exerted at the end of the beak. This can be done by:
a) Opening the beak and allowing it to deform modeling clay. Measure the distance that the clay was deformed by.
b) Pulling the beak open a set distance. Measure the force required with a spring scale.
c) Devise a way to use a triple beam balance by noting how much weight must be applied to the other side to balance the scale.

Repeat this measurement with different beak lengths.

Plot a graph of beak length (X) versus force exerted (Y).

What causes this relationship? Could birds with different beak lengths differ in their fitness when the availability of hard to access versus hard to crack seeds are present?

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