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The Fact of Evolution: Observable Evidence
Biologists who study the distribution and diversity of living things over time and space are engaging in the study of Lines of Evidence: Biogeography . biogeography
As the ancient, single land mass known as
Pangaea split apart and gave rise to the continents we know today, living things rode those pieces of land as they drifted apart.
The results of the long, slow allopatry-generating event known as
continental drift can be seen in the fossil record.
The continents are dancing
- area where the edge of one plate is being forced
underneath the edge of another one. Deep trenches form in these
subduction zone - area where volcanic activity causes the
ocean floor to split and spread apart. Oceanic ridges form in these
seafloor spreading zone
The evolution of related lineages can be traced by comparing the descendants' current geographic distributions with their genetic makeup.
There are thousands of scientifically documented cases of Lines of Evidence: Microevolution Observed . Here are just a few. microevolution
Staphylococcus aureus ( S. aureus) is ubiquitous and usually harmless
Opportunistic infections of S. aureus can usually be treated with a penicillins, a β-lactam antibiotic.
These antibiotics interfere with the activity of an enzyme used by the bacteria to construct their cell walls during cell division
In the early 1960s, strains of S. aureus began to show up that were not killed by methicillin (a β-lactam relative of penicillin)
The β-lactam antibiotics are so called because they all contain a β-lactam ring (the square portion of the molecule's structure in the diagram).
The new S. aureus were found to be making a new enzyme, , that disrupts the β lactam ring, rendering the antibiotic inactive.
β lactamase These new bacteria were dubbed
Methicillin-resistant Staph aureus (MRSA) These can be some of the nasty flesh eating" bacteria that are becoming more and more common as overuse of antibiotics (especially in hospital settings) selects for resistance not only in S. aureus, but many other bacterial species.
β lactamases (there are many!) are believed to be derived from bacterial enzymes that originally functioned in cell wall construction.
Mutations of the genes encoding these original enzymes changed those enzymes into the various β lactamases.
Bacterial genes encoding enzymes commonly mutate.
As antibiotics evolve by human manipulation, adaptive bacterial mutations are selected.
So goes the Evolutionary Arms Race.
2. Antibiotic-resistant Mycobacterium tuberculosis
Mycobacterium tuberculosis causes tuberculosis (TB) in
humans and other primates.
In the middle of the 20th century, new antibiotics nearly
Late 1980's: new resurgence of TB in developing nations
Some strains were resistant to antibiotics to which they had been
How did this happen? "Felt need?" "Natural Selection?"
The answer came from an AIDS patient...
HIV+ individual diagnosed with TB
treated with two antibiotics: rifampin and isoniazid, which are usually effective against M.
Infection cleared; chest radiographs were normal
Two months later, the patient was readmitted with TB symptoms.
He was treated with same antibiotics, but died 10 days later.
Culture and sensitivity test on lung discharge revealed M.
tuberculosis resistant to rifampin.
Was this an example of natural selection? Let's put Darwin to the ( post hoc) test.
1. Did variation exist in the bacterial population?
YES - both sensitive and resistant strains were present in the patient)
2. Was the variation heritable?
YES- Rifampin's mode of action is to bind specifically to RNA polymerase, interfering with transcription of all genes.
M. tuberculosis had a point mutation in the RNA polymerase gene ( rpoB): It changed leucine (TCG) --> serine (TTG).
This was enough to render the mutant RNA polymerase unrecognizable to rifampin.
The mutation was heritable; all progeny of the mutant, resistant strain
had the same point mutation.
3. Did differential reproduction among variable bacteria occur?
YES - M. tuberculosis with the point mutation left more offspring than those
that lacked the mutation. 4. Did a non-random subset of the original population remain after
YES - The populations before and after rifampin administration were significantly
different, with resistant bacteria replacing rifampin-sensitive individuals.
3. North American House Sparrow body mass and wing surface area
A tale of The American Acclimatization Society and Shakespeare...
Bergmann's Rule: Animals in higher latitudes/colder climates tend to have larger bodies (with a lower surface area to volume ratio) than those in lower latitudes/warmer climates. (Why should this be so?)
This can be true not only across species, but within a species with a wide geographic range.
What would you predict with respect to body size (northern hemisphere) in northern and southern populations?
Now consider color. Darker colors (pigments) absorbs light/heat. Lighter colors reflect light/heat.
What would you predict with respect to pigmentation (northern hemisphere) in northern and southern populations?
This was studied in House Sparrows by Johnston and Selander, who published their work in the early 1970s.
All House Sparrows in the U.S. are descended from a few pairs imported from central England.
Yet the North American populations at higher latitudes are darker and larger than those in southern populations.
5. Citrate-feeding E. coli
In Michigan State University's
run by Dr. Richard Lenski, observations of evolution in action are ongoing. Escherichia coli Evolution Lab
One of 12 culture flasks became cloudy, indicating bacterial overgrowth
A lab worker suspected contamination, but upon examining the populations, found only descendants of original E. coli culture
Someone in the flask had mutated, and somehow become much better adapted to the flask environment.
Analysis revealed that the new, wildly growing strain was--unlike wild type E. coli--able to feed on citrate, which was part of the culture medium.
In fact, this new strain didn't even need glucose, which had been the main food in the culture medium.
Lenski's lab is now determining which genes mutated, and are observing new mutations that make the bacteria even better at getting their energy from citrate.
Natural selection is happening, live, in their flasks.
6. Evolution as Compromise: Corolla shape in flowering plants.
The whorl of petals surrounding the reproductive parts of a flower (the male stamens and the female stigma(s) are collectively termed the
Selection by pollinators has resulted in the shapes and colors of flowers.
Flower morphology may be shaped by multiple factors, and may reflect a compromise reached by the plant to both New Theory:
In 2001, Candace Galen, et al. discovered such a balance in the attract the right pollinator
thwart "cheaters" who steal flower rewards without pollinating .
Alpine Skypilot Polemonium viscosum Skypilots are pollinated primarily by bumblebees, which prefer a wide, tubular corolla for easy access to nectar at the base of the flower.
In some ecosystems where the Skypilot lives, certain ants enter the flowers to rob nectar, cutting off the flowers' reproductive parts to gain access.
Galen and her colleagues showed that artificially constricting wide-based flowers helped to exclude ants, while still allowing bumblebees in.
They noted that in areas of high ant-predation risk ( krummholz), Skypilot corollas were more tubular.
In areas of low ant-predation risk ( tundra), Skypilot corollas were wider.
Their conclusion: In areas of high predation risk, plants have been selected that have been able to strike a balance between a corolla wide enough for a bumblebee, but narrow enough to deter ants.
In areas of low predation risk, no such pressure exists, and mutant forms with narrow corollas are not as attractive to bumblebees (which have to squeeze a bit to get in).
Different genes. Microevolution
You can read about more examples HERE. (Which might help you when it comes to extra credit questions on the exam. Just maybe.)
Over time, microevolutionary changes can, under certain circumstances, result in Lines of Evidence: Macroevolution Observed reproductive isolation (<-- required link), which defines a . biological species
includes the generation of new species from previously existing species. Macroevolution
Some who do not wish to accept the observable, verifiable process of evolution are sometimes heard to say
"Well, yes, we can see microevolution happening. But no one has ever observed speciation."
All the lines of evidence we have examined so far (fossils, homology, biogeography) provide strong evidence for past macroevolutionary events.
There are examples of speciation (<-- required link) having happened in the observable present.