Tonight we learnt about the birds and the bees; literally!  This set me thinking about how we as science teachers teach the topic.  We used to do a quick run through the actual birds and the bees part as it is something they seem to do a lot of at primary school.  Sometimes it feels like the only bits of science new pupils can do is fair testing and drawing (and labelling) a flower.  Actually, they seem to be able to grow cress as well.

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My own teaching of the subject does not start with the birds and the bees, but with the slugs and aphids.  The point is to get them thinking about the biological purpose of sex.  The first thing to get them thinking about is the fact that most organisms don’t reproduce sexually.  Instead they are quite happy to make copies of them selves.  All their daughters are clones.  This means there are more offspring and no energy wasted in looking for a mate.  A nice idea until disease comes along and all the population are suddenly hit at the same time.  No variation means massive loss of numbers.  The potato blight that caused the Irish famine is an example where the potatoes are often clones of each other.  The current banana crisis is another surprising example (type ‘banana crisis’ into google).

There is a solution; sex.  Sexual reproduction is a source of variation and change.  Many organisms make the best of both sexual and asexual reproduction.  This is when I introduce the humble aphid, or greenfly.  The aphid makes clones of itself for about seven generations or so before it gets a bit worried by the monotony and decides to look for a mate to ensure genetic diversity.  With the slug it gets even more interesting and disturbing.  The slug is hermaphroditic; having both male and female genitalia.  It can also make clones of itself just like the aphids.  However, when the slug decides to find a mate it does not want to invest time and energy into making and laying eggs.  So, the first thing two slugs will do when they meet to mate is to attempt to bite each other’s external genitalia off.  In the north of Britain it is usually too cold for slugs to even bother wasting the energy looking for each other and therefore it is a general rule that most slugs don’t have sex above Preston.

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