Why Sex? Sexual Reproduction and Genetic Diversity

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Objective

SWBAT to explain how sexual reproduction results in offspring with genetic variation.

Big Idea

Students discuss the results of sexual reproduction: offspring that are genetically different from both of their parents.

NGSS

1 minutes

This is lesson 2 in a 3 lesson series on reproduction. 

Lesson 1 - Asexual Reproduction

Lesson 2 - Sexual Reroduction

Lesson 3 - Investigating Reproductive Strategies

 

The first two lessons provide the background knowledge for lesson 3, which addresses the following:

NGSS standard:

MS-LS3-2. Develop and use a model to describe why asexual reproduction results in offspring with identical genetic information and sexual reproduction results in offspring with genetic variation. 

Cross Cutting Concept:

Cause and Effect

Cause and effect relationships may be used to predict phenomena in natural systems. 

Science and Engineering Practices:

Developing and Using Models

Develop and use a model to describe phenomena.

Engage

10 minutes

In this section of lesson I Engage students by showing them a video on Floral Arrangements and discussing the following questions:

  • What nonliving force do plants rely on most often for pollination?
  • What are some of the ways in which plants encourage or trick animals into carrying their pollen to other plants?
  • What proportion of each parent plant's genetic material does each offspring plant have?

Next, I show the The Red Queen, a PBS Learning video about the Mexican Poeciliid fish and discuss the following:

  • What are the differences between the two species of minnows featured in the video?
  • Which species -- the asexual or the sexual reproducers -- tends to be more heavily parasitized by the worm that causes black-spot disease?
  • How are the sexual reproducers able to evolve defenses against parasites more quickly and more effectively than their asexual counterparts?

Explore

20 minutes

Students in this section explore the The Mating Game PBS Learning media web activity in pairs.  The objective of this activity is for students to see the elaborate and at times costly reproductive strategies of various species. (MS-LS3-2 Develop and use a model to describe why asexual reproduction results in offspring with identical genetic information and sexual reproduction results in offspring with genetic variation/SP2 Developing and Using Models) 

Part 1

It is recommended that students play two rounds of the game and then choose four of the species highlighted in the activity's Dating and Mating Gallery. Ask students to take notes on the information provided about each species, focusing on the reproductive similarities and differences among them. Have pairs of students present their findings to the class while you record the various reproductive strategies on the board.

Part 2

Ask the class to rank the reproductive strategies on the board in order of relative difficulty, or "expense," to the animal. For example, the tube sponge's strategy of casting out clouds of sperm or egg cells into the open water is relatively less expensive than the bowerbird's efforts to attract a mate by building an elaborate bower. Ask students:

  • What are some of the things that animals can't do when they're focusing so much time and energy on finding or attracting a mate?
  • What proportion of each parent's genetic material would the offspring of any of these species have?

Part 3

Ask students to consider why some species might have evolved reproductive strategies that require a lot of energy and that allow individual organisms to pass only half of their genes on to their offspring. Ask students what benefits they think sexual selection might have.

Explain

10 minutes

In this section of lesson I show students the following video that introduces students to sexual reproduction.  Concepts covered in the video are reproductive cells, internal/external fertilization, and genetic diversity of offspring. 


Students complete the Sexual Reproduction handout after watching video.

Elaborate

10 minutes

In this section of lesson students elaborate on what they have learned by reading two background essays.

1.  Background Essay Floral Arrangements

2.  Background Essay Red Queen Hypothesis

Both of these essays elaborate on the information they learned during the engage section of the lesson.

To help students interact with texts I have students complete Writing in the Margins.

Writing in the margins engages readers in the reading task and allows them to document their thinking while reading. Both writing in the margins and drawing in the margins engages students in actively thinking about the texts they read. The power of this strategy is not the actual act of writing and drawing in the margins; instead, it is the thinking processes that students must undergo in order to produce such ideas. 

1.  The Writing in the Margin strategy that I have students use for Background Essay Floral Arrangements is the Summarize strategy.  The text discusses various pollination strategies therefore summarizing is a good way to organize information.

Summarize

Briefly summarize paragraphs or sections of a text. Summarizing is a good way to keep track of essential information while condensing lengthier passages.

Summaries will...

  • state what the paragraph is about

  • describe what the author is doing

  • account for key terms and/or ideas. 

2.  The Writing in the Margin strategy that I have students use for Background Essay Red Queen Hypothesis is the Clarify strategy.  The text discusses the complex idea of the benefit of sexual reproduction therefore pausing to clarify ideas will increase your understanding of the ideas in the text. 

Clarify

Clarify complex ideas presented in the text. Readers clarify ideas through a process of analysis, synthesis and evaluation. Pausing to clarify ideas will increase your understanding of the ideas in the text.

In order to clarify information you might...

  • define key terms.

  • reread sections of the text.

  • analyze or connect ideas in the text.

    paraphrase or summarize ideas. 

Evaluate

10 minutes

The Exit Slip requires students to develop an evidence-based argument to answer "Why Sex?" (W.7.1 - Write arguments to support claims with clear reasons and relevant evidence./SP7 - Engaging in Argument from Evidence)