SUMMARY Adaptation, Natural selection and the evolution of species N4/5 Biology

Mutations

Most of the mutations that we think matter to evolution are "naturally-occurring."

When a cell divides, it makes a copy of its DNA — and sometimes the copy is not quite perfect.

The small difference from the original DNA sequence is a mutation.

Mutations are spontaneous and random and are the only source of new alleles!

Alleles are the forms of genes that organisms carry in their DNA.

Alleles are what determine inherited genetic characteristics.

Environmental factors like radiation (UV, x-rays) and some chemicals, can increase the rates of

mutation in DNA. These are called mutagenic agents.

Mutations can occur in all dividing cells. Cells divide throughout the body by mitosis.

Specialised cells also divide to produce gametes (sex cells), this type of division is called meiosis.

Cells undergoing meiosis may pass a mutation on to the next generation.

These mutations would be carried by eggs or sperm into the zygote.

Cells dividing by mitosis are not likely to pass the mutation to a new generation.

Mutations in mitosis are commonly the cause of cancers in fully developed organisms.

Mutations may be neutral or might give an advantage or disadvantage to the new organism.

It is mutations which produce new variations in organisms and they are the raw material

for evolution.

Natural selection

The variation that exists in a population allows environmental factors to have an influence on the population. This process is called Natural selection and it determines which organisms survive to reproduce.

Natural selection (survival of the fittest)

Organisms reproduce producing many offspring, variation exists among the offspring.

Individuals struggle to survive in their environment, only the “fittest” survive as they are

better adapted, these survivors reproduce passing on their successful genetic information (alleles).

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Darwin explained that those organisms best adapted for their environment would survive

and reproduce. Those not well adapted would not survive and eventually would disappear.

Darwin’s problem was he could not give a clear living example but used his observations

particularly of Finches in the Galapagos Islands.

Darwin’s finches support the idea that organisms

change with selection pressure.

In the finches case it was to evolve different beaks

shapes and sizes as mutations allowed them to make use of different food sources.

One original group of finches slowly spread to different islands and evolved to be best

adapted to the food on their island.

A modern example of Natural Selection in action is the Peppered Moth (Biston betularia)

Two forms of the moth exist pale and dark.

The distribution of the different forms is determined by the

environmental pressure of pollution.

Pollution levels alter the colour of the trees (soot and a change in lichen numbers).

This alters the camouflage effect on the different forms and this alters the numbers surviving.

In towns and cities where there is a lot of pollution the lichens on trees die and soot often covers

the trees making them dark. More dark coloured forms of the moth will be present.

In towns and cities where there is less pollution light coloured lichens will survive and less soot

will cover the trees. More of the light coloured form will be found.

In rural areas more of the light coloured moth would be found.

Bacterial resistance to antibiotics

is also an example of modern natural selection.

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Adaptations that have evolved through natural selection

Adaptations can either be structural, physiological or behavioural.

Skunk – produce a foul smelling liquid

to ward off predators.

Ostrich – runs fast to escape predators.

Squid – produces ink to confuse predators.

Limpets – covered in a hard shell to

protect from predators.

Hedgehogs - have spines to prevent them being eaten.

Butterflies – some have eye markings to give false impression of size.

Frogs – some are bright coloured to advertise poison.

Stick insects - look like sticks and are camouflaged.

Bison – form groups to protect their young.

Quail – create ring nesting to scatter in all

directions when surprised by a predator.

Baboons – have a group organisation to create

defensive ring around young.

Plants have adaptations to help them survive.

Thorns and spines help keep the plant from being eaten.

Stings give predators a reminder that the

plant is not very edible.

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Speciation

Speciation is the process that gives rise to new species.

It incorporates the process of natural selection. It takes a long time.

A species is a group of organisms that can interbreed and produce fertile offspring.

The process for speciation is as follows:

The population making up the species is divided. This might be geographically

(mountains, rivers, floods etc.), behaviourally or reproductively.

This isolates one group from another.

Different mutations occur spontaneously in the isolated populations.

Natural selection acts on the separate populations. Those organisms best suited to the

environment survive, reproduce and pass on their genes. Different selection pressures allow

different gene pools to develop. Each sub-population changes over time (long), it evolves.

Eventually the populations become genetically different. The differences will be great enough to

now be two distinct species. If put back together the two species will not be able to reproduce and

produce fertile offspring.

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