Natural Selection

Learning intentions

I know that sexual reproduction results in variation in a population

I can explain natural selection as survival of the fittest

I know that if variation exists in a population the population can adapt if the environmental conditions change

I can describe some examples of natural selection

Success criteria

I can state that sexual reproduction results in variation in a population

I can carry out an experiment that allows measurement of success of individuals in a population of brine shrimps

I can state that if variation exists in a population the population can adapt if the environmental conditions change

I have completed notes on some examples of natural selection

Natural Selection If mice arrived in a habitat that had no other

mice and ideal conditions existed eventually the habitat would become over run.

So under ideal conditions the number of mice increase rapidly because there is; Plenty food to eat so none die of starvation No predators to eat the mice Plenty of shelter so do not need to compete for

shelter (will not die from exposure)

There are a number of ways in which mice can be killed off and prevented from breeding;

Mice with poor camouflage or slow reaction will be eaten

Some that are not as good at competing for food will die of starvation

Some weaker mice will die of disease

Mice not as good at competing for shelter or that have thinner coats will die of exposure or cold

However, this does not happen as only a few mice survive to produce offspring.

This means that only the mice with the best characteristics will survive to breed.

This for example could be mice with the allele for thick fur or speed

So only mice with useful combinations of genes survive to pass on the genes. Mice with less useful combinations of genes die and so these genes are not passed on!

Your turn………

See if you can survive the process of natural selection?

Natural Selection over time……..

It is this process that allows species to adapt to environments, survive and breed (weeding out)

This process is called natural selection and it has operated for millions of years to give rise to new species

Evolution is the appearance of new species as a result of natural selection

A definition

The environment ‘selects’ the best individual members of a species to survive and pass on their genes. Over a long period of time this leads to evolutionary changes and results in specialised organisms able to exploit their environment effectively.

Factors involved

Reproduction Variation Selection pressure of environment ‘Survival of the fittest’

1858 Charles Darwin + Alfred Wallace ‘evolution is the result of natural selection’ 1859 ‘The origin of species’

Some thoughts about Natural selection

Fact 1.

All species have great potential fertility. Their populations would increase exponentially if all their offspring reproduced successfully.

Fact 2

Apart from annual fluctuations populations are normally stable

Fact 3

Natural resources are limited. In a stable environment they remain relatively constant

Idea 1.

If more individuals are produced than can be supported by resources but the population remains stable – there must be a struggle for existence among individuals

Survival of a very small part of each generation

Fact 4 No two individuals are the same, every population displays enormous variation

Fact 5Much of the variation is inherited

Idea 2.

Survival is not random but depends on inherited characteristics of individuals ‘survival of the fittest’. This unequal survival constitutes the process of natural selection

Idea 3

Over the generations the process of natural selection will lead to a continuing gradual changes of populations and the production of new species = evolution

Darwin's theory – summary

Overproduction of offspring Consistency of numbers Struggle for existence (environmental

resources determine survival) Variation among offspring Survival of the fittest by natural

selection Like produces like Formation of new species

Darwins Finches

From 1 common mainland ancestor populations of finches on the Galapagos islands developed into 13 different species by the process of natural selection leading to evolution.

The diet of the food they ate fruit, seeds, insects, fruit and the habitat they lived in ground, tree cactus resulted in very different types of finches with varying beak shapes and sizes

Peppered Moth

This nocturnal insect rests by day on exposed surfaces such as tree trunks.

Insect eating birds prey on the resting moths.

In the early 19th century moths were well adapted to survive as they had light coloured speckled wings to camouflage them against the lichen covered tree surfaces.

There is also a genetic variety of this moth which is melanic (nearly all black coloured).

These moths showed up easily so the birds ate them.

This meant that the melanic moths very rarely survived to pass on their genes

Starting in the 1840s significant changes took place – the Industrial Revolution

Burning loads of coal produced sulphur dioxide (lethal to plants) and sooty smoke.

This meant in industrial areas the speckled moths were easy to see and the melanic moths were more camouflaged.

So in industrial areas there were more melanic moths and outside these

areas there were more speckled moths

This is an example of how changes in theEnvironment leads to changes in the adaptation Of a species (can still interbreed so not yet a Different species

Antibiotic resistance

Antibiotics are more effective against bacteria in their early years.

In bacteria variation in the population sometimes leads to resistance

Antibiotics will not work if a bacteria becomes resistant to it

The gene for resistance can be passed from one bacterium to another

Staphylococcus aureus

This strain of bacterium is responsible for 1 in 5 hospital infections.

Previously it was controlled by antibiotics. However one stain of it ‘super Staph’ is

resistant to all but 1 antibiotic. It is only a matter of time before a strain

appears that is resistant to all know antibiotics.

This is why its vital to find new antibiotic sources (fungi)