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The answer to that is that we can.
When you get a course of antibiotics from a doctor and they tell you to complete the entire course - this too is evidence of natural selection driving evolution.
Antibiotics contain toxins designed to kill microbes causing diseases. Too little dosage will not harm enough to cure the illness. Too much dosage and all the microbes are killed. An intermediate dose will kill most of the disease microbes but a few will survive. The survivors will be those that have some genetic property that gives them some resistance to the toxins. These survivors will then pass on the genes for antibiotic resistance to their descendents.
Ever since antibiotics were discovered, microbes have become immune to the vast majority of them. Microbes reproduce quickly and evolve quickly. Today we have MRSA and C Diff, both new strains of microbes that are immune to almost everything that humans can throw at them.
Travellers from developed temperate regions who visit the tropics have long taken anti-malarial drugs. In the past thirty years, the dosage has had to be increased and often supplemented with a different type of drug. This is to combat drug resistance that has developed in the malaria parasite.
Tuberculoses is a fatal and debilitating disease that was controlled by drugs for several decades. It is now making a comeback as drug resistance takes effect.
And diseases themselves evolve taking on new forms all the time. That is why humans are always catching influenza (the flu) and the common cold and never develop permanent immunity to them.
New diseases like Ebola and HIV have developed in recent years from mutations in previously harmless viruses and microbes.
All of these examples are evidence of evolution in action.
In 2009, the results of a 20 years experiment (the Lenski Experiment) were published.
This used microbes in a brilliant series of controlled laboratory experiments that showed many of the tenets of the Theory of Evolution.
The experiment used E coli, the common bacteria found in the human gut. Observations were conducted over several years covering 45,000 E coli generations.
The experiment set up twelve flasks of E Coli which were placed into a broth containing glucose (the only substance these microbes could use for food) plus a mixture of other nutrients that the E Coli could not metabolise.
At the end of each day, samples from each flask would be frozen to be used in later comparisons. E Coli can be frozen almost indefinitely and come back to life when thawed out.
Also each day, one tenth of the E Coli from each flask would then be migrated to another flask full of the same broth mixture.
The twelve flasks were kept completely isolated from each other so that the experiments ran in parallel.
This process meant that each day the E coli in the twelve flasks had an abundance of food which then got used up. The bacteria would initially multiply quickly. Once the food supply (the glucose) ran out the growth slowed down or stopped. This sequence of abundance and starvation provided the perfect conditions for natural selection to work on the microbes.
All twelve flasks evolved to better exploit the glucose.
The interesting point was that the E Coli in the twelve flasks evolved in different ways and at different rates. All the E coli increased in size but took a different number of generations to do so.
Two of the flasks followed the same evolutionary path over 20,000 generations. 59 genes had ended up changing in exactly the same way over those 20,000 generations. The DNA odds of this happening were enormous but it occurred. Supporters of Creation had often stated that these sorts of odds made it impossible for evolution to produce benefits to organisms. Of course, the truth is that it did not happen by blind chance. The organisms were under the same pressures and eventually arrived at the same solutions.
In a separate development, the E coli in one of the flasks suddenly increased by six-fold after 33,000 generations. What had happened was that this group of E coli had acquired the ability to metabolise citrate. Investigation showed that the adaptation was the result of two separate gene mutations. The first had occurred around generation 20,000 and the second around generation 33,000.
Either mutation on its own would not allow the E coli to metabolise citrate but the two together did. This violated another Creationist dogma called Irreducible Complexity. This says that some changes require more than a single mutation and the organism would not survive with just the single mutation. The odds against the two mutations happening at the same time were too great to make it likely.
The Lenski Experiment showed random mutations in genes followed by non-random natural selection. It showed that organisms could adapt to a new environment by different routes independently. It also showed that successive mutations, each non-useful by itself, could build up to produce evolutionary change. The experiment showed both gradual evolution and explosive evolution taking place.
Many such experiments showing "evolution in the laboratory" have since been run.
Supporters of Intelligent Design have been very fond of using the complexity of the mammalian eye as "proof" against evolution. They refer to the concept of "half an eye", saying that such an incomplete structure would not be useful. Since evolution works gradually in small steps the eye could never evolve as it needs to be complete or it is of no use.
This argument can be disproved simply by looking at living examples of eyes.
Flatworms have eyespots consisting of light sensitive pigments on the skin. These eyes cannot form an image but simply allow the creature to distinguish light and dark.
In limpets the eyespot is protected by a skin fold that forms a cup or dimple.
In the chambered nautilus the opening of the cup is narrow, providing a better image. This is a pinhole eye.
Ragworms have a transparent cover over the narrow opening of the eye to protect it.
Abalones contain fluid in the eye chamber. Some species have coagulated fluid at the top forming a simple lens.
In mammals, muscles formerly used for other things have been co-opted to move the eye.
The pinhole eye of nautilus, a mollusc.
This used a mathematical model.
The model began with a patch of light sensitive cells backed by a pigment layer, like a retina. It allowed tissue around the structure to deform randomly. These random changes were limited by the program to 1% of the size of the entire structure for each step. This part of the program mimics random genetic changes.
The model was then programmed to only accept changes that improved visual acuity. It rejected changes that degraded vision. This part of the program mimics natural selection.
The mathematical model yielded a complex eye, going through stages similar to those found in the real animals above.
The model eye folded inwards to form a cup. The cup became capped with a transparent surface. The interior of the cup gelled to form a lens. The lens eventually took on the best dimensions to produce a good, clear image.
The program achieved the transition from a patch of light sensitive cells to a complex eye in 1829 steps. Calculations based on that rate of evolutionary change indicate that evolution could produce a complex eye in just 400,000 years, a blink of geological time.
In nature eyes have developed independently in many creatures: vertebrates (fish, amphibians, reptiles, mammals), molluscs (squid, octopus), arthropods (crustaceans, insects, spiders). Evolution is capable of forming even the most complex structures.
These experiments using microbes and mathematical models are all well and good. What about higher creatures. Can they be observed in the process of evolution?
The answer is yes.
Off the coast of Croatia there is a small island called Pod Kopiste. This island is home to a common Mediterranean lizard called Podcarcis sicula. The lizard is an insect eater which aggressively defends its territory.
In 1971, five pairs of the lizards were taken to a nearby island, Pod Mrcaru, where they were unknown.
Over 30 years later in 2008 a group of biologists visited Pod Mrcaru to check on the introduced lizards.
First they checked the lizards' DNA to confirm they were the same species as the lizards on Pod Kopiste. They then compared the two populations to see if there were any changes between the two.
What they found surprised the biologists, not because change had occurred but because the changes were so extensive and had happened over a relatively short period.
Firstly the transplanted lizards had larger and wider heads. The jaws were larger. This translated to a stronger bite. The reason for this was that the transplanted lizards were eating a more vegetarian diet. Plant material requires more force to bite and chew compared to the former diet of insects. In addition, the hind legs were smaller in the new population.
Secondly, the Pod Mrcaru lizards were a larger population and did not defend their territories, unlike their stay-at-home cousins.
Finally, the newly herbivorous lizards had developed a caecal valve in the gut. This is essentially a small appendix used by plant eating creatures to digest cellulose. This structure did not exist in the original Pod Kopiste lizards and was not common in this type of lizard.
After just thirty years of separation, the lizards had begun to show changes in appearance and features. They had changed their behaviour and they had begun to change internally.
The lizards were showing the first signs of speciation, the appearance of a new species. Exactly as predicted by the Theory of Evolution.
200 years ago the peppered moth in England was lightly coloured. This effectively camouflaged them against the light coloured trees and lichens on which they rested. Then came widespread pollution during the Industrial Revolution. This caused many of the lichens to die out and the trees themselves became blackened by soot. The light-coloured moths died off as they were easier to spot by predators.
At the same time the dark-coloured moths flourished because of their ability to hide on the darkened trees.
As environmental legislation cleaned up indutrial pollution, the light coloured peppered moths made a comeback.
The peppered moth is a very easy to understand example of evolution brought about by natural selection.
Light coloured Peppered Moth
Dark coloured Peppered Moth
Radiotrophic fungi are fungi that have recently evolved to use the pigment melanin to convert gamma radiation into chemical energy for growth. They were first discovered in 2007 as black molds growing inside and around the Chernobyl Nuclear Power Plant in Ukraine.
In North America there is an insect called the Hawthorn Fly which feeds on hawthorns and related fruit. 200 years ago apples were introduced to the continent from Europe. One population of these flies began to feed on apples. As a result of their new feeding habits, they began to mature earlier in order to match the apple growth cycle.
Shortfin Molly (Poecilia mexicana) is a small fish that lives in the Sulfur Caves of Mexico. There are two populations. One lives in the dark interior and other in the lit surface regions. Experiments show that if the two populations are moved into each others' habitats, they are more at risk from a preditor. Studies have shown that the two populations are becoming genetically diverse.
Finally, let us look at the recently discovered phenomenon of Ring Species, which demonstrate evolution in action.
The European Herring Gull lives in Northern Europe including the British Isles, Ireland and Iceland. It has grey wings, a white head and a yellow beak with a red spot.
In North America there lives a similar bird called the American Herring Gull. The European and American Herring Gulls are close enough to be able to interbreed.
The American Herring Gull can also interbreed with the East Siberian Herring Gull which lives, as its name suggests, in the Eastern part of Siberia. The latter gull can interbreed with Birula's Gull found in Central and North Siberia. This gull is also capable of interbreeding with the Heuglin Gull, resident in Western Siberia.
The Heuglin Gull itself can breed with the nearby Siberian Lesser Black Backed Gull which is found in many parts of Siberia.
This last gull can interbreed with the Lesser Black Backed Gull.
The Lesser Black Backed Gull is found in North West Europe, including the British Isles. The Lesser Black Backed Gull is smaller than the European Herring Gull and has black wings and yellow legs.
The European Herring Gull and Lesser Black Backed Gull, although overlapping in the British Isles and often nesting in the same areas, do not interbreed and are, in fact, two different species.
A whole series of closely related birds form a ring around the North Pole. No gull can cross this area so migrations and spreading can only occur in a ring around the polar regions. These birds live in the lands surrounding the North Pole. Neighbouring populations are similar enough to be able to interbreed. However the populations at the ends of the ring are so different that they are different species.
Ring Species demonstrate a pair of distinct species plus their intermediate forms. It shows that species can be formed by a series of gradual steps. It is stunning evidence for Evolution.
The Greenish Warbler (Phylloscopus trochiloides) forms a ring species, around the Himalaya. The ring is found around the inhospitable Tibetan Plateau and the plumbeitarsus and the viridanus appeared to no longer be able to mutually procreate.
In 2014 the first example of a ring species in plants was described. The Euphorbia tithymaloides is a group within the Spurge family, that has reproduced and evolved in a ring through Central America and the Caribbean, meeting in the Virgin Islands where they appear as distinct species in appearance and reproduction.
A potentially serious problem arising from the ability of microbes to adapt and evolve quickly.
Details of a pivital experiment in evolution.
Evolution of the Eye
The eye is a complex organ that can be produced by small evolutionary steps.
Lizards Undergo Rapid Evolution
Lizards undergo rapid evolution after introduction to a new island home.