Natural SelectionNatural Selection

• Is the process by which organisms that are better adapted to their environment survive and breed while those less well adapted fail to do so.

• The better adapted organisms are more likely to pass their characteristics to succeeding generations.

A wee story to set the scene..

Mice are not genetically identical. They are

produced by sexual reproduction which

ensures that they possess different

combinations of genes from their parents.

Important characteristics such as coat colour

and thickness, speed and reactions, food

finding ability, resistance to disease and

aggressiveness will vary from mouse to mouse.

Only those mice with the best combinations of genes for their habitat will survive. This means that their gene combination will be passed on to their offspring. Mice withless useful combinations of genes die and so these genes are not passed on.

This is the story of natural selection and if it operates over millions of years then it is thought that it can give rise to new species.

Natural Selection in the Natural Selection in the Peppered MothPeppered Moth

The peppered moth is a nocturnal insect which rests during the day on tree trunks. Insect-eating birds prey on the resting moths by picking them from the tree trunks.

In the early 19th century, the moths were well adapted to survive as they had light–coloured speckled wings which camouflaged them against the lichen covered tree trunks.

This meant that the birds could not see them and it gave the moths some protection.

There is another form of the peppered moth which is melanic – it is nearly all black coloured.

(If these moths were on a lichen coveredtree such as the one in the picture, do youthink they would survive the beady eyes of an insect-eating bird?)

Melanic moths show up well against the lichen-covered trees and birds can easily hunt them. so melanic moths rarely survive and pass on their genes.

Environmental change and Natural Selection in the Peppered Moth

During the late 19th century, Britain saw the Industrial Revolution.

Large towns and cities were formed where factories and industries burned up large volumes of coal to power them.

Burning the coal caused lots of sooty smoke and sulphur dioxide.

As you know, lichens are very sensitive to pollution and so will not grow in polluted areas.

The trees became less lichen-covered and more coated with black soot.

Which moths would be more camouflaged now? (The melanic form or the light speckled form?)

Which form would have the greatest survival and increase in number and pass on their genes to future generations?

Which form would die out and not be able to pass on their genes?

Lichen-covered tree trunk in

unpolluted area

Soot-covered tree trunk in

very polluted area

Today, due to the Clean Air Act, the pale form of moth is becoming more numerous again in what were once polluted areas. Perhaps the situation we had before the Industrial Revolution will return one day?

Stick In and complete the

‘Peppered Moth Story’

Selective BreedingSelective Breeding

• So far we have looked at natural selection and how this has led to the great biodiversity found today.

• We are now going to look at Selective Breeding where humans deliberately choose which individuals to breed. This can happen in both plants and animals.

Selective Breeding in Domesticated Animals

• Humans deliberately breed desirable characteristics so that the offspring will also show these characteristics.

• Over time and several generations later, an improvement may result.

• However, it takes quite a long time and the results are not always guaranteed.

Example: cows and milk yieldFarming is a huge industry in Britain today.

It would not be cost effective to our farmers if only some of their cows produced large volumes of milk.

So, they will only breed cattle with desirable characteristics (large milk yield). The aim is that the female offspring should be able to produce large quantities of milk.

Draw a line graph of the milk yields in cattle from Selective Breeding

YEAR Average yield (litres per cow per year)

1905 1900

1920 2300

1935 2700

1950 3400

1980 4700

Average yield

Year

Title: Milk Yields in Cattle

More Examples of Selectively Bred More Examples of Selectively Bred AnimalsAnimals

Selectively Bred Animals Characteristics Selected For

Cattle Milk and Beef production

Sheep Milk and Wool quality

Pigs Growth rate and litter size

Poultry Growth rate and egg production

Horses Strength, size and speed

Selective breeding in Selective breeding in plantsplants

• In 1895 a group of American Biologists began a breeding experiment using a variety of maize (sweet corn) whose seed grains varied slightly in oil and protein content.

• Firstly they selected only those plants that produced seed grains with the highest oil content – strain O.

• Then they selected only those plants that produced seed grains with the highest protein content – strain P.

• They repeated this over 50 generations. Strain O showed improved oil content and strain P showed improved protein content.

• Stick in Figure 10.2

Limitations of Selective Breeding

• Relies on sexual reproduction

- fertilisation not always guaranteed and there may be difficulties getting individuals to breed.

- can take months or even years to get just one generation so can be very slow.

• Inbreeding is a common method used in selective breeding.

- can lead to an increase in recessive genes.

- since many of these normally hidden recessive genes give inferior qualities, inbred lines may show disadvantages.

Limitations of Selective Breeding….

Limitations of Selective Breeding

• Individuals may not have the genes for the desired characteristic.

- If they are not present, they cannot be created e.g. if a gene for disease resistance does not exist in a species then the characteristic cannot be developed in this way.