Phenomenon
Sexual dimorphism: differences in morphology between male and female individuals of the same species. These traits seem to be disadvantageous. We provide few examples but feel free to add others.
- Ring-necked pheasants males compared with dull colored females
- Extravagant and colorful tails in male peacocks
- Big horn size in Hercules beetles
Question
How can we explain the presence of traits that clearly give the organism a disadvantage for survival?
Model Ideas
We are adding to our model of natural selection to be clear that:
- Differential survival is not enough to drive population change over time.
- Differential reproductive success is what matters.
Overview
Transition in:
In previous models we learned about what an organism needs to survive, but is individual survival enough to ensure species survival? Looking back at our natural selection model we are reminded that the continuation of a species is only possible if those who survive also mate and reproduce to pass their traits to offspring.
In this pass back through natural selection we go deeper into fitness. We pose the question: Is fitness defined only by an organism’s ability to survive? To motivate this discussion, we consider the phenomenon of the ring-necked pheasant. Male pheasants have bright coloring that makes them more visible to predators, thus less likely to survive. Students examine this phenomenon and craft explanations that (hopefully!) reinforce the primacy of reproductive productivity in defining "fitness". Students develop and deepen the natural selection model to incorporate the idea that survival only drives population change when it leads to reproductive advantage.
In simple words, the coloration of pheasants is an example of sexual selection, an advantage that individuals have over others of the same sex and species that exclusively relates to reproduction. Sexual selection drives the persistence of traits (physical or behavioral), even traits that are disadvantageous in terms of individual survival. The pheasant males are more susceptible to predation because of their coloration, however that coloration increases the probability that they will mate successfully so it persists in the population.
A short clip of the phenomena of Sexual Selection can be found in the PBS documentary: “Why Sex?” Since we don’t want to feed answers to students show them only minutes 17:32-20:13.
Transition out:
Fitness is the ability to survive, find a mate and reproduce, and as an organism reproduces the more of its traits get passed to the next generation.
1. Review of the model of natural selection and reinforce the concept of fitness. We bring back the case of the cave salamanders and ask groups to identify the fittest variation of salamander. Review their model ideas for natural selection and have a discussion about what fitness means. If you are using the doodle sheet, students can write their answers in section A.
We reviewed natural selection and discussed fitness in more detail.
2. Present the phenomenon of sexual dimorphism in ring-necked pheasants and generate questions. Present students the phenomenon of sexual dimorphism without going into much detail. It is surprising that in some species males and females can look different and the traits that differentiate them can be elaborate and exaggerated. Why can this be? We provide a few examples: horns in Hercules beetles, feathers in peacocks, and differential coloration in ring-necked pheasants –the backbone of our model. In the short clip provided the phenomenon is described very well. Students will read about the ring-necked pheasants and on their own write what they find surprising or are curious about –doodle section B. When done, students will share their notes with their group and then with the whole class. Guide them towards a consensus driving-question, which will be recorded on the board or poster.
We generated some questions about characteristics that would seem to put the organism at a disadvantage.
3. Students develop initial model ideas about why male pheasants have bright colors. Students individually will generate model ideas to why males have bright colors that puts them at risk of predators. They will record their ideas on their doodle –section C, and share them with their group. Using the talking sticks you can then have a whole class discussion. These initial model ideas are recorded on the board or poster with the driving question to make them visible to everybody. Remember that this is a speculative process and all ideas should be considered as valid. In a later segment there will be opportunities for students to revise their ideas.
We surfaced our initial ideas about the ring-necked pheasants.
4. Students analyze data to add evidence and revise the model. We provide two different data sets 1) Pheasants and their relatives, and 2) Pheasants courtship. Working in groups of four, each pair will analyze one data set, summarize their findings on their doodle sheet (section D) and share it with the whole group. They will discuss the new evidence, revise the model and return to the salamander question.
We found out more about ring-necked pheasants and used that information to refine our model.
5. Application of the model. Here is a great opportunity to asses student understanding. You can find a sample of student answers and an key to the activity on the web. Students read the comic strip Survival of the Sneakiest and answer questions on the back of the reading. An alternative is to read the comic strip all together and assign roles to few students. Then allow students to discuss the story in groups before individually writing down answers. Hand the questions after the discussion. The story explains how sneaky crickets let other males spend energy singing, and while the female is looking for the singer step in and mate with the female. This story is based on research by Robin Tinghitella at Denver University and is an example of evolution in action. Really cool research!
We applied our model to another species.
Download Resources
| Attachment | Size |
|---|---|
| All MBER-Bio Sexual Selection Materials | 5.21 MB |