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B2.8 Speciation

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.