HeredityAccumulation of Variation During Reproduction
Every new generation receives two things from the one before it: a common basic body design, and small changes to that design. Now take one more step. When this new generation reproduces in its turn, its offspring inherit the differences their parents already had — and pick up some new differences of their own. Generation after generation, the variations accumulate.
The simplest place to watch this is asexual reproduction. One bacterium divides into two, each of those divides again, and so on. Copying DNA is very accurate but not perfect, so now and then a small copying error gives a daughter cell a new feature — and every one of its descendants inherits it.
Grow the family tree. Drag the generations and watch features being passed down and new ones appearing.
Generations:
A heat wave is on its way, and a thick, dark cell wall helps a bacterium withstand heat. Once all eight bacteria of the third generation are on screen, tap the ones you expect to survive. Then bring on the heat.
Did every variation have the same chance of surviving? Obviously not. Depending on the nature of the variation, different individuals have different kinds of advantages: the thick-walled bacteria survived the heat wave, while in a cold spell some other variation might have won. Variations can be inherited, and some of them help their owners survive. This selection of variants by the environment is the basis of evolution, which you will study in higher classes.
Look at the bottom row: eight bacteria, and no two of them exactly alike. Some of their differences are unique, appearing for the first time in that generation. Others were inherited from their parents, who were already different from each other. With asexual reproduction the differences are tiny, because they come only from small inaccuracies in DNA copying. With sexual reproduction, where every offspring combines the DNA of two parents, far greater diversity is created — you will see how when we come to the rules of inheritance.
Which Variation Came First?
Every variation in the bottom row started in one individual, at one moment, and was then copied into all of that individual's descendants. That gives us a way to tell old variations from new ones.
Trace the variations through the tree and complete all three targets.
A variation that appeared in generation 1 had two more rounds of division to spread through: it is in
Exam tip
This reasoning is behind a favourite 1-mark question: in an asexually reproducing species, a trait found in 60% of the population is likely to have arisen earlier than a trait found in 10% of it. It has had more generations in which to be copied.
Speed Round: Who Survives?
The surroundings change, and two varieties of the same organism face the change together. Decide which of them is more likely to survive — or whether the difference between them does not matter at all.
Get five right to move on.