Extinction ans Speciation

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How Fossils are formed (3)


Teeth, shells, bones etc. dont decay easily

They are eventually replaced by minerals as they decay

They form a rock-like structure, exactly the same shape as the original hard part

The surrounding sediment turns to rock, but the fossil satys distinc

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The remains of organisms from many years ago, which are found in rocks.

Provide the evidence that organisms lived millions of years ago

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How Fossils are formed (1)


An organism is buried in a soft material like clay

The clay hardens around it and it decays, leaving a cast of itself

E.g. animal burrows, plant roots

Footprints cna be pressed into materials when soft, leaving an impression when it hardens

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How Fossils are formed (2)


Amber and tar pits

No oxygen or moisture; microorganisms can't survive


Too cold for decay microbes to work

Peat Bogs

Too acidic for decay microbes

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What is a species?

A group of similar organisms that can reproduce to give fertile offspring.

What is speciation?

The development of a new species.

When does speciation occur?

When 2 populations of the same species become so different they can no longer reproduce to give fertile offspring.

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Reasons for Extinction

1) Environment changes too quickly (e.g. destruction of habitat)

2) New predator

3) New disease

4) They can't compete with other species for food

5) Catastrophic event

6) A new species evolves (speciation)

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Theories of how life began

There are various hypotheses, but no one really knows

1) The first life forms came into existance in a primordial soup

2) Simple organisms arrived here on comets and more complex organisms evolved from these

These can't be supported - lack of valid and reliable evidence

Because some scientists believe many early organisms were soft-bodied, and soft tissue tends to decay away completely

So the fossil record is incomplete.

Fossisl formed millions of years ago may have been destryoed by geological activity

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How does Speciation occur? (2)

6) So the alleles that control the beneficial charateristics are more likely to be passed on to the next generation

7) After some time the majority of the popultion will show this charateristic

8) Eventually individuals from the different populations will have changed so muh that they won't be able to breed with each other to produce fertile offspring

9) The 2 groups will have become seperate species 

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How does Speciation occur?

1) Isolation; population of the same species are seperated

This can happen due to physical barrier

E.g. floods, earthquakes or mountain ranges can cause barriers that geographically isolate individuals from the main popualtion

2) Conditions on either side of the barrier will be slightly different     e.g. climate

3) Because the conditions are different on either side, different charateristics will become more common in each population, due to natural selection

4) Each population will show variation because htey have a wide range of alleles

5) Individuals with charateristics make them better adapted to their environment

This gives them a better chance of survival , so they are more likely to reproduce

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