Week 12 · Lesson 2 Natural Selection and Artificial Selection: populations and their growth, carrying capacity, environmental and genetic factors, natural selection, artificial selection and its techniques · 3 lessons
2
Natural selection: moths and rabbits
Science Prep 3 Term 1, Unit 3: pp. 75–83
GoalExplain natural selection with a simple model: variation, survival, passing on traits and change over many generations.
New words
variation · differences between individuals of the same speciesnatural selection · individuals whose traits suit their environment survive and reproduce more, so those traits become more commonadaptation · a trait that helps an organism survive in its environment
Explanation
Individuals of a species are not all the same: there is variation. Some variations are inherited. When the environment changes, some variations help their owners survive and reproduce more, and they pass these traits to their offspring. After many generations those traits are more common. This is natural selection, and it works on the whole population, not on one individual, who does not change by wishing. A simple model: moths come in light and dark. On light tree bark, birds find the dark moths more easily, so more light moths survive. If the trees become darker, the opposite happens. Note: it is not “the strongest survives”; the better fit to the environment survives. Another model: a beetle population is 80 % green and 20 % brown. On brown soil birds see green beetles more easily. In a model of 500 beetles, 400 are green and 100 are brown. If only 1 in 4 green beetles survives but 3 in 4 brown ones, the survivors are 100 green and 75 brown, so brown is now about 43 % of the survivors. Numbers in models are invented to show the idea.
Worked examples
Check the beetle model: 400 green × 1/4 = 100 survive; 100 brown × 3/4 = 75 survive. Total survivors = 175. Brown share = 75 ÷ 175 = about 43 %. At the start it was 20 %. Selection changed the proportions.
A dry region becomes sandy. In a population of lizards, some have pale skin and some dark skin. The pale lizards blend with sand and are caught less by birds. Over many generations the population has more pale lizards. No lizard changed colour on purpose; the proportions changed.
Can you say “the strongest survives”? No. A small, well-hidden insect can survive when a strong, easily seen one does not. What matters is the fit to the environment.
Class activity
Paper moth model, with a grown-up. Cut 20 small light squares and 20 small dark squares of paper. Scatter them on a sheet of light paper (the bark). Close your eyes, then pick 20 squares in 10 seconds. Count the light and dark squares you picked and those left. Repeat on a dark sheet. Which colour is picked less on each sheet? Write what the model shows and what it cannot show.
Practice
1
Name two things needed for natural selection.
Inherited variation, and a difference in survival or reproduction (any two sensible).
2
Does natural selection act on one individual or on a population?
On the population over generations.
3
In the beetle model, how many survive in all?
100 + 75 = 175.
4
What do we say instead of “the strongest survives”?
The best fit to the environment survives and reproduces more.
