(Part II)2005


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Janine DNA and Evolution Part II

Changes in the DNA sequence of nitrogen bases result in variations.  Variations in DNA occur naturally during the production of gametes and fertilization.  Sometimes variations are lethal.  Sometimes variations in DNA cause a disadvantage.  Other times variations can be an advantage to the individual and allow it to better survive in its particular environment.  When the variation is lethal of course the individual is not going to be able to survive and pass on its variation to future offspring. When the variation proves to be a disadvantage, it will probably be out-competed by individuals without this variation and so will not survive well enough to pass on its variation to future offspring.  When the variation proves to be an advantage, it will probably be better able to compete and so will be more able to survive to pass on the variation to its offspring.  Overtime, this advantageous variation in DNA may be seem more frequently in a population therefore becoming the norm.  If you were to compare the population from before the variation became the norm to a population after it became the norm, you would notice a change in the population - or evolution.  


Speciation is thought to have occurred mainly through geographical isolation.  At one time all the land on Earth was connected into a super continent called Pangaea.  Due to continental drift or separation of tectonic plates, the continent was broken up and now exists as seven distinct continents.  Organisms on the separate continents became isolated from each other and were no longer able to reproduce together.  As the environments on each continent varied, the favorable traits that were naturally selected differed for each group of organisms on each continent.  Over a very long period of time, the variation seen in the different groups of organisms on different continents account for the reason that they can no longer reproduce and so are no longer considered to be of the same species. 

Speciation is also thought to occur on a much smaller scale.  If a group of organisms become isolated from the general population, they too can undergo changes in their population possibly producing a new species.  For example, suppose a group of deer travel higher on a mountaintop than most of the herd.  Suppose also that a large snowstorm blocked the path for the deer to return to the herd.  Over the winter, the isolated deer with longer hair are able to survive and the deer with shorter hair die from the cold.  In the spring, the long hair deer mate passing on the gene that codes for longer hair.  Most of the fawns contain the gene for longer hair.  If isolation continues, the population will continue to produce more and more fawn with long hair as any fawns with short hair will always die out every winter leaving only long haired deer alive to reproduce.  As more winters go on, the longer the hair the better the chances are for survival.  Eventually, it may reach the point that the deer with long hair are at a disadvantage in lower elevations because it gets too warm for them.  Therefore, the deer chose to remain in the higher elevations and will never rejoin their original population. 

The example with the deer looks at one trait.  However, survival involves many traits.  Even with the deer, since the deer now live in higher elevations the ability to climb on rocks would be an advantageous trait that would also be seen in more and more offspring in the population.  Since water if frozen in the winter in higher elevations, the ability to obtain or conserve water in those conditions would also be an advantageous trait that would be passed on to future offspring.  Enough of theses changes in a population over time could easily result in enough variation so that the animals would not be able to reproduce with the original population even if they did come in contact again. 

Evidence for Evolution

There are several pieces of evidence for evolution.  The following in no way is a complete list of evidence for evolution.    This is a basic primer on evolution and only covers the basics of evidence on evolution.  Since the embryological comparison can be so easily disputed because of the drawings of the scientist who proposed this piece of evidence, it is not included in this list.

Evidence for Evolution - Fossil Record
The first type of evidence for evolution can be seen in the fossil record.  The fossil record does not account for every species that exists nor does it account for every change a species went through over time.  The creation of a fossil is a somewhat rare occurrence.  However, there are some fossils that show adaptations have occurred over time.  The best example of this is with the ancestors of whales.  A fossil was found of a whale like creature that is now thought to have been the ancestors of whales.  It is believed this creature walked on four legs near or in shallow waters.  The creature hunted its food in the weeds of the water.  As competition increased limiting the amount of food available, creatures with the ability to search for food in deeper water had a survival advantage.  Overtime, the traits enabling the creatures to search for food in deeper water would be seen more frequently in the offspring.  Leg bones shaped more like fins would have been an advantage over leg bones enabling them to walk.  Therefore, over time, the shape of legs on whales became shaped like
fins.  Today, many types of whales still have leg bones that aren't even attached to the spine.  Since the leg bones are still present but have no use today they are called vestigial structures.  Please refer to the diagrams below to see drawings of whale adaptations over time.

The drawing above is an artist's rendition of what the land ancestors of whale looked like. 



The top two drawings to the right are artist's Renditions Of fossils found that are believed to be the ancestors of whales.


The lowest drawing to the right is an artist's rendition of a fossil of a right whale believed to have died in the past century.  Note the leg bones that are not attached to the spinal cord. 

Evidence for Evolution - Comparative Anatomy

 Another piece of evidence for evolution found when examining the anatomy of various organisms.  Occasionally anatomical structures are similar but vary according the function performed by that structure.  These are called homologous structures.  Homologous structures have similar structures, suggesting similar ancestors, but perform different functions.  An example of this is seen in the forelimbs of humans, birds, porpoises, and elephants.  All of the forelimbs mentioned contain the same amount of bones but the bones are shaped differently according to the function the forelimb performs.  In other cases, the function of anatomical structures may be the same but the structure is very different.  These are called analogous structures.  Analogous structures have similar function but different structure suggesting different ancestors.  An example of analogous structures can be seen in the wings of birds and insects.  Both types of wings are used to fly.  However, bones give structure to the wings of birds and chitin gives structure to the wings of insects.  Please refer to the diagrams below for visual examples of homologous and analogous structures.

Evidence of Evolution - Vestigial Structures

As mentioned with the leg bones of whales, vestigial structures that have no function today but are thought to have served a purpose to ancestors.  A vestigial structure found in humans is the coccyx or tailbone.  There is no purpose today for the coccyx but it is believed that our ancestors at one time had a tail. Please refer to the pictures below for a visual representation.

Evidence for Evolution - Comparative Biochemistry

The DNA in every single organism consists of the same four nitrogen bases: adenine, thymine, cytosine, and guanine.  Also, these four nitrogen bases code for the same 20 amino acids that are arranged in various ways to create the proteins needed by all organisms on earth.  If we compare DNA between different species, we find that a lot of the DNA is exactly the same suggesting a common ancestor.  The more similar the DNA, the closer related the species are to each other. 

Mechanisms of Evolution

When the gene pool of a population remains stable or does not change over many generations it is said to be in genetic equilibrium.  If there are no changes, then evolution is not occurring.  A horseshoe crab today is the same as a horseshoe crab from over five hundred years ago or more.  The horseshoe crab has not evolved in over five hundred years or more.  It can be reasoned that the horseshoe crab had no reason to evolve.  Its basic structure was sound enough that any variation did not necessarily provide any great advantage so that it out-competed horseshoe crabs without the variation. 

Evolution only occurs when there is a change in the genetic equilibrium.  These changes can come about due to a mutation that proves to be an advantage like in the case of sickle cell anemia.  Individuals with only one mutated allele had an advantage over individuals with two mutated alleles and those with no mutated alleles.  Therefore, populations from regions where malaria is common also common tend to carry the allele for sickle cell anemia.           

Evolution can also occur in small, isolated populations where what is called genetic drift occurs.  In small, isolated populations the gene pool of the original ancestors represent only a small fraction of the possible alleles for any given trait.  As a result, it is more common for recessive alleles to be paired with other recessive alleles causing the expression of rare forms of a trait.  Examples of this can be seen in the Amish people of Lancaster Pennsylvania.  Due to religious isolation, they usually mate among their own population.  The ancestors of the people evidently carried recessive alleles for extra fingers and toes.  Because of the small gene pool, the frequency of extra fingers and toes is quite high in the Amish people with 1 in every 14 children being born with extra fingers and toes as opposed to 1
in 1000 in the larger population of the United States. 

Another mechanism resulting in evolution is migration.  The movement of organisms into a population can introduce new alleles into the population bringing about a change in the population over time.  The movement of organisms out of a population can remove alleles from a population.  If deer born with longer hair tend to move to higher elevations and do not mate with the deer with short fur, then that gene for logn hair will be removed from the population over time. 

Effects of Natural Selection on Evolution

Nature can select traits that alter the direction of evolution in a population.  For example, suppose a species of spiders of average size had a survival advantage.  Large spiders were easily seen by birds and were therefore eaten and prevented from reproducing.  Smaller spiders had a harder time of obtaining food and so died of starvation before they could reproduce.  In this case, natural selection would have a stabilizing influence on the population. 

Natural selection can also select traits to one extreme or the other.  For example, suppose a species of woodpecker feed on a particular type of insect that lived in a tree.  Woodpeckers with longer beaks would have an advantage to obtain the insects and thus survive better enabling them to pass on this trait to offspring.  In this case, natural selection would have a directional influence on the population. 

Natural selection can also have a disruptive influence on the traits in a population.  Consider a population of marine organisms called limpets.  The shell-color of limpets varies from white to tan to dark brown.  Adult limpets live attached to rocks.  The limpets with white colored shells are easily camouflaged on light colored rocks and so are not preyed upon by birds.  The limpets of dark colored shells are easily camouflaged on dark colored rocks and so are not preyed upon by birds.  However, tan colored limpets are easily seen on light and dark colored rocks so they are easily preyed upon and prevented from reproducing and passing on their trait.  In this population the variations that will generally be seen in the population will be to the extremes of white colored shells or dark colored shells. 

Patterns of Evolution

There are three main patterns of evolution.  The first pattern is called divergent evolution.  This pattern occurs when there is a common ancestor that evolves into many new species due to the different environments that offspring can be found in.  The best example of this is with the finches on the Galapagos Islands.  It is thought that all the finches arose from a common ancestor that lived in South America.  As the ancestral finches migrated onto the Galapagos Islands, different environments favored different adaptations.  As a result the offspring are different than the common ancestor.  Please refer the simple diagram showing the differences in beak shapes according the to the food eaten by the finch on different islands.

Beak Shapes on the Finches of the Galapagos Islands

Another pattern of evolution is called convergent evolution. This occurs when organisms from distantly related organisms develop similar adaptations because they inhabit the same environment.  An example of this can be seen when one compares a shark with a dolphin.  A shark is a fish that breathes through its gills.  A dolphin is a mammal that breathes through its lungs.  The body shape of sharks and dolphins, even they are from very different ancestors do not look all that much different because they both live in the ocean.  Adaptations that help the shark swim better also help the dolphin swim better.  The ability to swim better has helped each animal survive better.  Therefore over time, sharks have dolphins have evolved to look very similar. 

A final pattern of evolution is called CO-evolution.  This occurs when two species find an advantage in developing a close relationship to each other.  An obvious example is between insects and flowering plants.  Insects use the nectar in the flowering plants for food.  Flowering plants reproduce by having their pollen picked up on the legs of insects and carried to other flowers when they search for more nectar.  The flowers that appeal the most to the insects available are more commonly selected by the insects and thus are more able to reproduce.  The insects that are most able to obtain the nectar of the available plants are better able to survive and thus pass on their traits to future offspring. 

Timeframe of Evolution

There are two different hypotheses on the time frame for evolution.  One hypothesis is that evolution occurs gradually over a long period of time as evidenced in the fossil record of horses.  The other philosophy is that evolution occurs in rapid bursts with long periods of time without any changes as evidenced in the fossil record of elephants.  The later hypothesis tends to correlate to the mass extinction and emergence of new species that have known to occur during different periods of earth's history.  For example, it is generally accepted that when a meteorite crashed down to earth it resulted in a series of geographical disruptions such as the eruptions of many volcanoes.  The ash that was released in the air was so great that it prevented plants from being able to photosynthesize.  When the plants died, the dinosaurs that required a large amount of plant material also died.  When the vegetarian dinosaurs died, the carnivorous dinosaurs died because they also weren't able to eat enough meat to stay alive.  Smaller animals were able to survive and reproduce because they didn't require as much meat or vegetation to stay alive.  As the earth's climate began to stabilize, the mammals began to find their own niches and develop traits that proved advantageous in competitions for survival.  Hence we see the great variation in species today with very little signs of evolution still occurring.                 

Theory of Evolution

Recall that a theory is an explanation of natural phenomenon based on evidence.  Since the time that Darwin proposed the Theory of Natural Selection, there has been great advancement in scientific knowledge as well as in scientific technology.  If any evidence had been found that disproved the Theory of Natural Selection, the theory would have been rejected.  Instead, scientific advancement has only led to more evidence to help support the Theory of Natural Selection.  The Theory of Evolution is of course related to the Theory of Natural Selection.  Many would just say they are the same theory.  In my view they are different.  The Theory of Natural Selection refers to how adaptations are inherited whereas the Theory of Evolution refers to how enough adaptations over time can lead to the creation of a new species.     

The basic premise of evolution has been explained in this paper and has been supported with evidence.  The Theory of Evolution is still a work in progress.  There are many evolutionary scientists still researching evolution.  If sufficient evidence is ever found to disprove evolution, the theory will be rejected.  To date, advancements in science only help support the Theory of Evolution




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