Evidence for Evolution - 1. Lecture
Evidence for Evolution
Definition - the development from a simple to a complex formed organism (i.e. Procaryotes => Eucaryotes)
Lamarckism - theory of soft inheritance, inheriting traits that an organism acquired during its lifetime and passing it on to its offspring
1) Final genetic process - Speciation; phylogenetic tree
2) The main mechanism of evolution is natural selection
Evolution is a phylogenetic process
In order to demonstrate Evolution in the sense of Darwin, there are 3 Points that need to be shown:
- Species can go extinct
- Species can change over time
- Species trace back to a common ancestor
Extinction of Species
The main source for the fact that species can go extinct, comes from the fossil record.
Aside from that scientific paleontology & comporative anatomy had given enough convincing evidence.
Because extinction was an established fact, Darwinism brought a new view on why species go extinct.
Cuvier believed in external catastrophic events as the cause (some believe the biblical flood could be interpreted as a major extinction event)
Darwin’s view of a struggle for survival offers a different explanation: there is no external catastrophe needed, population growth and competition are reason enough.
Change of Species
That species change over time was already Lamarck’s point and he gave some good arguments for it.
Evidence for that comes from directly observable changes and, again, from the fossil record.
Sometimes the change is fast enough to be observed in human history or even a lifetime.
Human domestication is an obvious example that was discussed by both, Lamarck and Darwin. The phenotypic change that can be produced by breeding is staggering, a for example the variety of dogs show. Darwin collected pigeons and demonstrated that according to the taxonomic standards of his time they were sufficiently different to make “three good genera and about fifteen species”.
Evidence comes again from the fossil record. There are often striking similarities in the fossils found in a certain region with the living species in the same area (e.g. Australian fossil mammals are mostly marsupial, living and extinct sloths are only found in Central and South America, etc).
The more ancient a form is, the more it differs from the living forms. The pattern is so widespread that it had been known as the “law of succession” even before Darwin, who took this (this wonderful relationship in the same continent between the dead and the living) as important evidence for evolution.
The same holds true for differences among fossils: the strata where fossils are found are connected by a chain: each is more similar to the adjacent.
The fossil record also shows the origin of major new taxa.
In old strata only reptilian characters were found. In new ones, only mammalian characters were found. It was a gradual change of characters, where all kinds of intermediates are found.
! Not all change of the phenotype through time is automatically evolution - the change needs to be genetic !
Since the phenotype is also affected by the environment, phenotypic changes can also be affected by the environmental, phenotypic changes can also result from environmental changes (in good years plants grow to large size, in bad ones rather small - phenotypic/developmental plasticity).
In many cases it can though, be ruled out (major environmental shocks would have been documented).
The Common Ancestor
Darwin’s novel element for the theory of evolution was his claim that all species trace back to a common ancestor. The main evidence for this claim comes from striking similarities in the morphology and organs among living species.
Homologies refer to characters that are shared across species due to common decent. They appear at any level of organismal organisation - from molecular and genetic to morphologic and behavioral.
Analogies are similarities that are not due to common ancestry, but result from convergent evolution if similar environmental conditions favor similar adaptive solutions
Cause for these similarites can be either Functionalism or Formalism.
Functionalism - led by Cuvier with the claim that all species were optimally designed by their creator and therefore anatomy follows from the everyday problems they face in their lives
=> Form follows Function
Formalism - led by Geoffroy St-Hilaire, influence by Lamarck and with the belief that morphology of all species was based on common body plans. Although many changes are possible on top of these plans, every species still carries “reminders” of them
=> Explicit belief in homology instead of analogy in sense of body plans
Functionalism explains ‘perfect’ design - the perfect fit between an organism and its environment - weak spots in this theory are so called maladaptions and vestigial organs that don’t have a optimal use and therefore wouldn’t be expected under creation by an ominscient creator.
Maladaptions are often legcies of evolutionary history i.e.
- vertebrate eye: how could natural selection ever produce such a perfect structure - BUT it’s not perfect, it has a design flaw. The retina is upside down, light must pass through four diffrent kinds of neurons and capillaries before finding light sensitive cells. The optic nerve fibers are on the inside of the eye, the nerve bundle must dive through the retina => a blind spot without light sensitive cells is in the eye
- Whales have a pelvis but no hind limb => no function
- Boas and pythons have vestigial legs (thighs and hips)
- Cave organisms lose eyes to varying extents => some have lenses but no optic nerves/no lenses but optical nerves
- Humans: middle ear bones (reptile remnants), appendix (guinea pigs & horses use them for bacterial digestion), muscles to move ears and tail
For plants, Darwin experimented to demonstrate that the seeds of many species are still able to germinate even after being kept in salt water for a long time.
Embryology gives similar evidence against a special creation, under creation we would expect an organism to take a direct route from embryo to adult - instead the embryo passes various stages - showing homology between different species (which might be temporare)
The evolutionary answer is conservative, it makes use of what is there before. From the point of view of a natural process with limited resources, developing something new is more difficult than modifying what already exists.
Disproving Formalism - failed to disprove, provided more evidence against creator
Adaptation is difficult to explain and the concept of archetypes and universal body plans (while relatively flexible) was more mystical than scientific.
Darwin turned the archetype into the ancestor in his theory, this enabled him to give the concept of homology (which was formally defined through body plans) the scientific basis it needed.
He was also capable of explaining observations that needed experience, the most important evidence brought to by biogeography i.e.:
species in South America were more similar among each other than to Old World speices, even if they lived in environments that were much more similar than across the continents => strong example are the Galapagos Islands which were never part of a continent
- Oceanic islands have very few “kinds” of organisms - Galapagos has few species of birds, compared to the hundreds of species on continents
- Particular groups of organisms are present in uncommon numbers with very unusual distribution (unbalanced faunas)
- If organisms found on volcanic islands are compared to the ones that are missing, there is a very striking evidence: found are plants, insects and birds. Missing are amphibians, reptiles, freshwater fish and mammals (except bats)
Darwin did not claim that all living species would necessarily trace back to a single common ancestor. He suggested it could have been several independent ancestors. Today it is though, commonly assumed that all life indeed had a single origin.
The strongest claim for this is the universal genetic code. Why is the code universal?
The explanations that are possible are: chemical constraints or historic contingency.
Most people think that the original choice of the code (3 bases create 1 amino acid) was just an historic accident.
Current theoretical work suggests that code evolved early in order to reduce the number of harmful amino acid substitutions that occur by mutation. Once it had evolved however, it would be strongly maintained and any deviation from the code lethal.