In much the same way that family members resemble each other like in the above photos, if species are related, as evolution purports, one would expect to see similarities between related species. These similarities, especially if they are of traits that don’t seem to give an advantage to a species, allow us to infer a common ancestor that most likely shared this same trait.
Prediction: If species are related, they should share similarities. These similarities should become more frequent and intensify the more closely related species are.
Falsification: Species are completely unique, distinct from each other.
There are four main categories of similarities that we will look at:
- Anatomical similarities
- Genetic similarities
- Embryological development similarities
- Geographic distribution similarities
- Examples: homologous structures, fossils, embryological development, synapomorphy, etc
Most insects have 6 legs.Spider have 8 legs. Echinoderm pentaradial symmetry: Though this clade is diverse--sea star, sea urchin, sea cucumber, brittle star--they have something in common--they almost all have five sides as adults. Chimpanzees and humans have nearly identical skeletal structures; they are just stretched in different ways. Explore this here: http://www.pbs.org/wgbh/nova/evolution/compare-skeletons.html Embryological development similarities (much more on this under 'remnants'). Genetic similarities Why should it be that biological sugars are in the 'D' orientation? Why not a mix of 'L' and 'D'? Evolution! Our common ancestor had enzymes that operated on 'D' and therefore all its descendants (like us!) also uses the same type of sugar. Proteins used in Earth life are almost always 'L' conformation (the 'L' vs 'D' has to do with chirality and the direction that the constituents rotate according to their chemical priority designations when hydrogen is facing away from the observer). Amino acid or protein sequence similarities: This is a different way of representing genetic similarity (since genes obviously are the precursors to protein sequences). Many genetic mutations in genes do not get represented in protein sequences (since changes in the third codon site are usually synonymous mutations do not result in the translation of a different amino acid). These differences in genetic or protein sequences can easily be used to determine divergence. They are in essence molecular clocks that can be used to ascertain and predict how long ago species diverged. Genetic language similarities: Genes are transcribed from DNA into RNA and then are translated into amino acids which are put together to make protein. RNA used for translation comes in four "letters"-- A, C, G, U (adenine, cytosine, guanine, and uracil). These letters are put together into three letter words called codons. The spelling of these words is consistent across almost all life (though it does not have to be at all!). This exchangeability makes genetic modification by recombinant DNA (DNA from one organism put into another) possible Here are several charts representing these words: Biogeographic similarities: Species that are related to each other tend to live near each other. Why? Because their common ancestor was likely from a similar location. For example, why are so many marsupial species just on Australia (the columns labeled 'marsupial' are from Australia and its vicinity)? Why are placental species on other continents? Why is their distribution so separate even though their appearance is so similar? The simplest and best explanation is that they evolved separately and convergently from different lineages. The same environmental niches often help create the same evolutionary adaptations, like these ant eating mammals. This convergent evolution also results in similar species evolving separately (which we can verify with genetic testing) by analogous anole adaptations in the Caribbean and South America. Likewise, in the cichlids of two different lakes: Lake Tanganyika (left) and Lake Malawi (right). And in the saber toothed predators of Australia (Thylacosmilus--a marsupial) and North America (simlodon--a placental).