Evolution and inheritance, Year 9

Heredity (inheritance)

提示It is the reason features run in families.

为什么Parents pass genes to their offspring in the egg and the sperm, which is why children resemble their parents and why features run in families. Features gained during life, such as a scar or a suntan, are not passed on, because they do not change the genes.

half

提示The two parents contribute equally.

为什么An egg and a sperm each carry one set of genes, half the number found in a normal body cell. At fertilisation the two sets join, so the baby has a full set, with an equal share from each parent.

Every egg and every sperm carries a different mix of the parents' genes

提示Mum and Dad hand on only half of what they hold, and not the same half twice.

为什么Each parent passes on only half of their genes, and which half is different every time an egg or sperm is made. Each child is therefore a new combination. Only identical twins, who grow from a single fertilised egg, share all their genes.

survive; reproduce; offspring

提示Staying alive comes first, then having young, then what those young inherit.

为什么These steps, repeated over many generations, make a useful feature more common in a population. Individuals without it are more likely to die before breeding, so their features become rarer. Nothing is chosen on purpose: the environment decides who lives long enough to breed.

No — the thick-furred ones survived and bred more, so the feature spread

提示Ask which rabbits were still alive each spring to have young.

为什么Natural selection works on differences that are already there. The thin-furred rabbits were more likely to die before breeding, so each generation contained a larger share of thick-furred ones. The population changed; no single rabbit did.

food

提示A thick, strong beak cracks hard seeds; a thin one picks out insects.

为什么The finches had descended from one kind that reached the islands long before. On each island, the birds whose beaks happened to suit the local seeds, fruit or insects fed best and left more offspring. This variation was part of the evidence Darwin used when he worked out natural selection.

Species change gradually over many generations, by natural selection

提示He pictured living things as branches growing from one shared tree.

为什么Darwin argued that living species have descended from earlier ones, and that the fit between an organism and its surroundings is produced by natural selection acting on variation over a very long time. Many people found the idea shocking because it challenged what they had been taught.

Dodo

提示Its name survives in the saying "as dead as a …".

为什么The dodo had no natural predators and nested on the ground. It was first recorded by Dutch sailors in 1598, and the last widely accepted sighting was in 1662. It is the best-known example of a species driven to extinction by human activity.

Introduced animals such as pigs, which ate its eggs

提示The dodo nested on the ground, and the settlers brought livestock and pets.

为什么Pigs, dogs, cats, rats and monkeys brought to Mauritius raided dodo nests and competed with the birds for food, while forest was cleared for farming. A change in the environment that a species is not adapted to cope with is the usual cause of extinction.

No individuals are left to reproduce

提示New animals only ever come from parents.

为什么Every organism is built from the genes of its parents. Once the last member of a species dies, its particular set of genes is gone and there is nothing to breed from. That is why extinction is final, and why endangered species are protected while some still survive.

The plants were genetically alike, so none could resist it

提示If one plant had no defence, ask what was true of its neighbours.

为什么With little genetic variety, a disease that can attack one plant can attack them all. A crop or a wild population with plenty of variation is far more likely to contain some individuals that survive. Loss of variety makes living systems fragile.

B

提示Count the kinds of animal, not the individuals.

为什么Biodiversity is about variety. Both ponds hold the same number of animals, but the second contains ten times as many species, each with its own part to play in the food web.

Species lost before they are studied may take useful substances with them

提示Think about the plants nobody has tested yet.

为什么Wild species are a source of medicines, foods and materials, many of them still undiscovered. Each extinction removes possibilities for good. This is one of the practical reasons, alongside keeping ecosystems stable, for maintaining biodiversity.