Cladograms and Allele Frequencies

Cladograms: Cladistics is a method of analysis that reconstructs an organism’s evolutionary history by inferring relationships based on shared characters. Cladistics can be used to hypothesize the sequence in which different groups of organisms evolved. The diagram that scientists use to show these evolutionary relationships among groups of organisms is called a cladogram.

Fill in the characteristics…   Humans Snake Mouse Monkey Hair Thumbs Eyes Walk Upright Total X’s

Creating a Cladogram…

Practice: Turn to page 43 in your packet. Complete pages 43-45. You may work alone or with the person next to you. (Pg. 46 is homework)

Allele Frequencies and Evolution: Review: What are alleles? Ex: Mouse fur color can be brown (BB), light brown (Bb), or white (bb). Allele frequency is how many of each specific allele is found within a population.

How Populations Change:

Natural selection can occur in 3 different ways

Directional Selection: The trait on one end becomes more common, so the graph shifts to one side. Mouse Fur Color

Directional Selection: Example After a volcanic eruption the landscape becomes darker in color, so dark fur color becomes more common.

Stabilizing Selection: The medium trait becomes more common, so the graph becomes skinnier and taller in the middle. Mouse Fur Color

Stabilizing Selection: After excessive logging, the landscape becomes dusty and desert like. Mouse fur color becomes mostly medium brown.

Disruptive Selection: The traits on both sides become more common, so the graph splits in the middle. Mouse Fur Color

Disruptive Selection: The female mice prefer male fur color to be either very light or very dark. After time, light brown and dark brown become more common.

Frequency Activity: In this activity, you will be attempting to show how giraffe necks may become longer in response to selective pressures of the environment. Assume that during a long drought, food has become scarce where the giraffes are living. Genes (symbolized by beans) control the length of the giraffe neck. Red beans represent long necks (L) White beans represent short necks (l)

Guidelines: A giraffe receiving two “long neck” alleles (2 red beans / LL) from his parents has a longer neck than the medium necked giraffe. These giraffes would be able to feed on the choice leaves higher up in the trees. A giraffe receiving one “long” allele and one “short” allele (one red and one white bean / Ll) would have a medium neck length and would only be able to feed on the old, tough lower branches that have been picked over by other giraffes. A giraffe receiving two “short” alleles (two white beans / ll) would have a shorter neck and would not be able to reach the over the lowest branches and would starve to death.

Draw this table into your notes: Generation 1 2 3 4 5 # Long Neck # Medium Neck # Short Neck

Procedure: Each pair needs to pick up a red cup containing 60 white beans and 40 red beans and an empty white/clear cup (morgue). Blindly/randomly select 25 pairs of beans from the red cup. DO NOT PEEK. Set aside these beans, keeping them in pairs. These beans represent the giraffes in the first generation. Record the results in your data table. Place all short neck combinations (2 white beans) in the morgue—NOT the red cup. Return all remaining beans to the red cup. Repeat for ANOTHER FOUR generations.

Post-Activity Questions: How did the environmental conditions affect the giraffe population? What is the allele frequency of the L allele in the 1st generation? What is the allele frequency of the l allele in the 1st generation? What is the allele frequency of the L allele in the 4th generation? What is the allele frequency of the l allele in the 4th generation?