Fossils
Fossils show ancestral forms of species that exist today, as well as those that have gone extinct hundreds, thousands and millions of years ago. The further down you find a fossil, the older it is. Comparing fossils helps to construct an idea of how particular species evolved.
Sediment layers can provide information to help understand the fossil more, such as
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Molecular Biology
Nucleic DNA (regular DNA)
Scientists can extract DNA from preserved remains of organisms and compare the DNA sequence of particular genes. The more similar the DNA sequence, the more likely the organisms are closely related (they would have shared a common ancestor quite recently). If the nucleotide sequences contain a large amount of genetic differences (due to mutations) then they are not closely related at all (they would have shared a common ancestor a very long time ago).
One way of determining the similarity of nucleotide sequences is to undergo hybridization using two single-stranded molecules of DNA, one from each species. The harder the hybrid DNA is to separate, the more base-pair matches (thus more hydrogen bonds formed) showing a large number of complementary base pairs, indicating similarity. If the strands are easy to separate, there are many mispairings which indicates a large number of differences, indicating a lack of similarity. |
Mitochondrial DNA
Mitochondria have their own DNA and this gets passed down from mother to offspring. Males can inherit their mother's mtDNA but cannot pass it to their offspring as offspring inherit the mtDNA from the egg (from their mother).
As mitochondria are located outside of the nucleus, they are not subject to meiosis. This means they are unable to undergo any recombination (crossing over) prior to being passed on to offspring. The result of this is that mtDNA is identical between mother and offspring. The only way mtDNA can be different is if mutation occurs. When two groups diverge, mutations occur at a relatively consistent rate, therefore the number of mutations present indicate the time since divergence.
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Y-chromosome
The Y chromosome is passed down the paternal line, from father to son. As it is a little chromosome and part of a special homologous pair, the effect of crossing over on the genetic material within the chromosome is very limited. 95% of the chromosome (everything but the tips) remains unchanged from generation to generation, unless a mutation occurs. The Y chromosome, like mtDNA, can be used to track back many generations and identify where divergent evolution has occurred.
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Comparative Anatomy
By comparing the structures of different organisms you can find similarities. Looking at the structure on the left side of the picture, each have a pentadactyl limb (bascially a 5-fingered hand) and if you think about it, it makes no sense as to why a whale would have 5 fingers in its flipper? Such oddities suggest that whales share a common ancestor with humans, birds and lizards, and that over time function of the 'hand' changed. Such comparisons indicate divergent evolution. |
By comparing the structures of different organisms you can also find differences. Looking at the picture on the right, whales, turtles fish and penguins have very different structures, yet they all act as a flipper. These different structures show different ancestors yet the similarity in function shows those organisms all live in an aquatic environment (they all experience similar selection pressures). Such comparisons indicate convergent evolution. |
Biogeography
How come there are no polar bears in Antarctica even though its icy and cold? Why are there no penguins in the Arctic? Some people believe animals live in similar places all over the world. However, the changes in geography over time supports the theory of evolution and dispersal and divergence of species.
The underlying concept of biogeography supporting evolution is that isolated environments seemed to give rise to new species that weren't found anywhere else, even if conditions were similar elsewhere. The rise of particular species is not based on environmental conditions but the distribution of ancestral species due to the geography of the Earth. Darwin observed the animals of the Galapagos Islands and noticed that they were very similar to the animals on the South American mainland but very dissimilar to animals on other islands that had similar environments. From this, he concluded that the animals on the Galapagos had migrated from South America and after a long period of time became new species as the populations adapted to their new environment. |