Are you looking for a way to get the attention of your middle school science students? The funny topic of sexual selection can make for an entertaining lesson on Charles Darwin’s theory of natural selection. It certainly drives home the message that natural selection is all about reproductive fitness!
You can use the concept of sexual selection to help teach Next Generation Science Standard (NGSS) MS-LS4-4: Construct an explanation based on evidence that describes how genetic variations of traits in a population increase some individuals’ probability of surviving and reproducing in a specific environment. Although you can cover sexual selection whenever you are learning about natural selection, it’s a fitting topic to teach right around Valentine’s Day.
Sometimes we see traits in nature that at first seem difficult to explain using the theory of natural selection and the concept of “survival of the fittest”. The traits don’t seem to serve any obvious purpose for helping the individual to survive. One example is the widowbird- aptly named, as you will see. The female looks normal enough, with brown, camouflaged coloring. The male widowbird, on the other hand, has an extraordinarily long tail.
Photo by David Howard on Unsplash
How did that happen? Why would nature “select” a long, seemingly cumbersome tail like that? When I ask students what potential problems a long tail like that could cause, they have several ideas. The long tail might be heavy and make it hard for the bird to fly. A predator trying to catch the bird could grab it by the tail when it is trying to fly away. One student said that the tail looked “ugly”, but I guess that beauty is in the eye of the beholder…
One researcher who had a hunch about how natural selection could result in such extraordinarily long tail feathers decided to do a little experiment test his hypothesis. You and your students might question the ethics of this experiment, and it might seem like a bit of a mean trick, but what the researchers did was to clip tail feathers short on some birds and add them to the tails of other birds, making their tails ridiculously long. The male widowbirds were then followed to see how successful they were at winning over the females. Would you like to guess which male birds were most successful at attracting females? You've got it- the ones with the absurdly long tails! So, it was the ladies' fault- the females were selecting the tails!
This graph shows the success of the male widowbirds with and without altered tails:
Using the graph, I ask students to answer these questions:
Approximately, what is the average number of active nests of widowbirds that had their tail shortened?
Approximately, what is the average number of active nests for the birds that had their tails lengthened?
Approximately, what are the averages for the normal and control groups?
Do you see a correlation between tail length and the number of nests a male bird has?
Can you think of another explanation for this finding, aside from sexual selection, for longer tail feathers?
What might you conclude is a possible explanation for why the widowbird has evolved such long tail feathers?
This short BBC video clip from Planet Earth II, narrated by David Attenborogh, shows male widowbirds in action as they strut their stuff for females:
BBC Earth: Male Widowbird Display
The video gives a clue about the purpose that long tail serves: showing off for the females. Natural selection is about survival of the fittest, but fitness is really a measure of an individual's reproductive success. An individual with lots of offspring will pass on its traits to all of its babies. This is how a trait becomes more common in a population. In the case of sexual selection, there can be a snowball effect as females that prefer a certain trait, such as a longer tail, pass on this preference to female offspring, and male offspring inherit the trait. Over time, the trait can become more and more pronounced. Was there originally some benefit to having a slightly longer tail? Students often say that maybe it helped with balance in flying. There are different theories about how the whole thing got started.
The story of the long-tailed widowbird (aptly named?) can make for an entertaining Valentine's Day story to share with students.
For more information on the study of the widowbird's long tail, follow this link:
https://static.nsta.org/case_study_docs/case_studies/widowbirds.pdf
Sexual selection is not limited to birds. Here's a video that students like showing the display of a male peacock spider:
Peacock Spider Courtship Video
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