B2.8 Speciation
Summary
Changes in the environment of plants and animals may cause them to die out. The fossil record shows that new organisms arise, flourish, and after a time become extinct. The record also shows changes that lead to the formation of new species.
Origins of life
• The first organisms were very simple single-celled organisms, similar to bacteria.
• However, these rarely survive as fossils.
• Therefore, Scientists cannot be certain about how life began on Earth.
• It is only possible to develop theoretical models that show how life may have started.
Fossils
• We can learn from fossils how much or how little different organisms have changed as life developed on Earth.
• However, 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.
• 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.
Extinctions
• Various events can make it difficult for individuals to survive: o changes to the environment over geological time
o new predators may arise o new diseases may arise
o new, more successful, competitors may arise
o a single catastrophic event, eg massive volcanic eruptions or collisions with asteroids.
• If the organisms cannot evolve quick enough to become better adapted to the change they may become extinct.
• If they do evolve, they are likely to develop into new species.
Speciation
• A species is defined as a group of organisms that are capable of breeding together to produce fertile offspring.
• 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:
o The individuals with the most beneficial alleles are more likely to survive. o They are more likely to reproduce and pass on their genes.
o Therefore, the beneficial alleles become more common in future generations.
• Over a long period of time, the populations may become so different that successful interbreeding is no longer possible.
• They have no become separate species.
• Speciation has therefore occurred.
