gradual replacement-bones, shells, teeth etc don't decay easy, get buried, replaced by minerals forming rock-like substances shaped like original part, surrounding sediment turns to rock
casts- organism is buried in soft material, material hardens, organisms decay, leaving a cast
impressions- things like footprints pressed into soft materials, materials harden leaving an impression of the print
preservation- in places where no decay happens due to unsuitable conditions for decay microbes so they cant survive e.g. amber and tar pits-no oxygen, glaciers-too cold and peat bogs-too acidic
fossil record not complete because:
early organisms were soft-bodies, soft tissue decays completely
fossils destroyed by geological activity e.g movement of tectonic plates crushed fossils
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Extinction
term species means organisms that can breed together to give fertile offspring
species that don't exist anymore are extinct
reasons:
environmental changes
new predator
new disease
another species is more successful
catastrophic event
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Speciation
speciation=development of a new species
occurs when populations of same species become so different they cant breed successfully anymore
isolation-population of species separated e.g by geographical barrier, conditions either side are different so different characteristics will become more common in each population due to natural selection:
variation because of wide range of alleles
most suited characteristics to the environment=best chance of survival=more likely to breed so get passed on to the next generation
eventually the separated populations become so different they cant breed so they are different species
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