Module One 11.7 Why have some species of plants and animals died out? How do new species of plants a
Life is thought to have originated about 3 Billion years ago. During that time the range of species that have lived on Earth has changed. For examples dinosaurs are now extinct. It is believed we evolved from apes. The evidence we have for these changes lies with in the history of fossils.
FossilsFossils are the remains of plants and animals that lived millions of years ago.
There are three main ways by which fossils can be formed:
1. From the hard parts of animals (e.g. bones, teeth, shells), i.e. the bits that take a long time to decay when buried. They are eventually replaced by minerals, forming a rock like substance.
2. From the softer parts of animals which haven't decayed fully because one of the conditions of decay is absent (warmth, moisture, oxygen). The soft material is gradually replaced by minerals as it slowly decays. This is very rare and is sometimes known as PETRIFICATION (turning to stone!).
3. In places where NO decay happens and the whole organism is preserved. E.g. :
a) Amber- no oxygen or moisture is present so decay cannot occur. The yellow stone is made out of a tree resin that can engulf insects and fossilise them.
b) Glaciers- a hairy mammoth has been discovered preserved as it was too cold for decay.
c) Water-logged bogs- a 10,000 year old man was found in a peat bog a few years ago. Here the conditions were too acidic for bacteria to live and decay.
Fossils as evidence for evolution
The fossils in rock layers tell us:
1. Information about the size and shape of plants and animals
2. Information about how long ago they existed through the type of rock and the depth of the fossil within a rock layer.
Fossils are usually dated by GEOLOGISTS who already know the age of the rock. Fossils show how species have changed and developed over millions of years.
" The Grand Canyon is 1 mile deep and the rocks at the bottom are around 1 billion years old "
Missing Links- most plants and animals decay away completely. Very few turn into fossils thus the fossil record may well be incomplete.
Fossils
How Fossils are made
Extinction
Different species become EXTINCT (died out completely) if the environment they need changes, if they are hunted by predators, wiped out by disease or through new competitors.
Think of the number of endangered or extinct species there are today and why this has happened!
E.g. The Dodo
Red squirrels
Pandas
Elm Trees
Mutations
A mutation is when a new gene is formed due to a change in an existing gene.
For example a useful mutation is the peppered moth
Here the gene that determines the colour of the moth is normally peppered. When industrialization began and buildings were covered in black smoke a new variety was formed from a mutation. So the gene for colour mutated to a new "black" colour. This meant that the moth could continue to survive in cities.
Mutations occur naturally over time and allow species to change and adapt to their environment when the new gene is passed onto the offspring.
The chance of mutations occurring can be increased dramatically by exposure to ionising radiation. This includes:
- UV light
- Radioactive substances (a,b and g rays)
- X-rays
- Certain chemicals such as those in cigarettes
Evolution and Natural Selection
Charles Darwin was a scientist in the nineteenth century who gave a lot of time and thought to the changes that had occurred in species over millions of years and he came up with the theory of evolution. This theory suggest that changes have occurred in species over a long period of time.
Evolution occurs via NATURAL SELECTION.
This is summarised as:
Variation- Each species has many individuals. These individuals show a wide range of variation because of differences in their genes (as a result of sexual reproduction)
Over Production- Most organisms produce more young than will survive to adulthood
Struggle for existence- Hunting, predation, disease and competition means that a large number of each species die.
Survival of the fittest- Individuals with characteristics best suited to the environment are more likely to survive and breed successfully
Useful genes passed on- The genes that are advantageous are passed on to offspring and gradually the species changes as it adapt to changes in it environment.
Ideas about evolution
Darwin's theories were only gradually accepted because:
- There was a lack of fossilised evidence.
- Darwin's theories went against religious beliefs (god created all living things and species did not change).
- In the eighteenth and nineteenth centuries more and more was found out about fossils and it became clear that new species had evolved and others had become extinct .
- More and more was found out about and genetics (Mendel's work)
mutations and how they could produce new alleles so the theory became more widely accepted.
Another French scientist called Chevalier de Lamarck suggested an alternative to natural selection. He believed that an organ would change when it was necessary. E.g. giraffes developed long necks because they needed to reach up to the trees for food. He also thought that characteristics that and animal developed during its life, such as stronger muscles, would be passed onto its offspring.
In summary:
|
Scientist |
Theory |
Arguments for or against |
|
DARWIN |
Natural Selection: competition, survival of the fittest, genes passed on. |
Genetic evidence. Mutations produce new alleles and therefore new characteristics which can be passed onto offspring |
|
LAMARCK |
Organs will change when necessary |
No evidence to show if an animal develops strong muscles during its life they are passed on to offspring. |
Variation
Variation, Inheritance and Evolution
The Story of Mendel - some Interactive Exercises
Evolution
pbs - Evolution - links to different aspects of evolution
Natural Selection
Natural Selection for Antibiotic Resistance
Natural Selection - the Black Peppered Moth - an interactive exercise