B2.8.1 Old and new species (part 1)
Evidence for early forms of life comes from fossils.
Fossils are the ‘remains’ of organisms from many years ago, and are found in rocks. Fossils may be formed in various ways:
- from the hard parts of animals that do not decay easily
- from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent
- when parts of the organism are replaced by other materials as they decay
- as preserved traces of organisms, eg footprints, burrows and rootlet traces.
Many early forms of life were soft-bodied, which means that they have left few traces behind. What traces there were have been mainly destroyed by geological activity.
We can learn from fossils how much or how little organisms have changed as life developed on Earth.
B2.8.1 Old and new species (part 2)
Extinction may be caused by:
- changes to the environment over geological time
- new predators
- new diseases
- new, more successful, competitors
- a single catastrophic event, eg massive volcanic eruptions or collisions with asteroids
- through the cyclical nature of speciation.
New species arise as a result of:
- isolation – two populations of a species become separated, eg geographically
- genetic variation – each population has a wide range of alleles that control their characteristics
- natural selection – in each population, the alleles that control the characteristics which help the organism to survive are selected
- speciation – the populations become so different that successful interbreeding is no longer possible.