Diana's Post

I am Diana Mindreci, an Olathe North Student. I am in Mr. Thompson's naturalists class. I will be talking about genetic drift, mutation and selection and how they work.

Genetic Drift, Mutation and Selection

The three major forces of evolution are genetic drift, mutation and selection. These forces cause changes in genotypes and phenotypes over time, and also determine the amount and kind of variation seen in a population.


Genetic Drift

Genetic drift is the random changes in the frequency of alleles (One member of a pair or series of genes that occupy a specific position on a specific chromosome.) in a gene pool, usually of small population. Random genetic drift is a stochastic process (by definition). One aspect of genetic drift is the random nature of transmitting alleles from one generation to the next given that only a fraction of all possible zygotes become mature adults.

In the population of five worms below, each worm gives rise to exactly one worm in the next generation. There are five alleles (skin colors) at generation 0 and the same five alleles at generation 4.



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The model above starts with a diverse population (5 worms, 5 alleles).



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With no diversity in generation 0 and no forces of evolution acting on the population, the model above begins and ends with all worms in the population having the same allele.


In the above examples, the populations of worms are not evolving. Neither the genotypes nor phenotypes are changiing. For the changes to occur, there must be mutation, selection or random genetic drift.



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When genetic drift is introduced into the model above, these are the results.
In generation 2, the pink worm produces 1 offspring, the 3 green worms produced none, and the dark blue worm produced 4.



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In a population model with genetic drift, alleles will eventually become "fixed". When an allele is fixed, all members of the population have that allele. In the model above,the dark blue allele fixed after 4 generations.


Mutations

A mutation is a permanent change in the DNA sequence of a gene.

Example:

A butterfly may produce offspring with a new mutation caused by ultraviolet light from the sun. In most cases, this mutation is not good, since obviously there was no 'purpose' for such change at the molecular level. However, sometimes a mutation may change the butterfly's color, making it harder for predators to see it; this is an advantage and the chances of this butterfly surviving and producing its own offspring are a little better, and over time the number of butterflies with this mutation may form a large percentage of the species.


A mutation event is how the allele sequence changes.
Two things must happen:

1)A change in the molecular structure of DNA
2)Failure of editing enzymes to correct the change; copying into new DNA

Mutant strain: A population of descendents of the individual in which the original mutation event occurred. The “mutation” is now inherited by the regular reassortment and recombination mechansisms, as are other alleles.

Rate of mutation: How often a given map position mutates. In practice, this is hard to measure.

Frequency of mutation: What percent of alleles contain a sequence defined to be “mutant,” in a given population at a given point in time. This is easy to measure.

Mutation events are rare. How to detect them?

1)Observation of large numbers of progeny.
2)Positive selection. For traits which confer survival advantage: Subject them
to the selective environment. Example: plate bacteria on agar containing an
antibiotic.
3)Negative selection. For traits which prevent survival, under a given
condition. Example: Replica plate bacteria colonies on agar lacking a
nutrient which the “wild type” strain can make with its own enzymes.

Note that different types of mutations have different effects on the gene product and on the resulting phenotype as shown in the table below.



Click on picture for better view.


Natural Selection

Natural selecion is the process in nature by which, according to Darwin's theory of evolution, only the organisms best adapted to their environment tend to survive and transmit their genetic characteristics in increasing numbers to succeeding generations while those less adapted tend to be eliminated.


Darwin's grand idea of evolution by natural selection is relatively simple but often misunderstood. To find out how it works, imagine a population of beetles:

There is variation in traits.
For example, some beetles are green and some are brown.


There is differential reproduction.
Since the environment can't support unlimited population growth, not all individuals get to reproduce to their full potential. In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do.

There is heredity.
The surviving brown beetles have brown baby beetles because this trait has a genetic basis.

End result:

The more advantageous trait, brown coloration, which allows the beetle to have more offspring, becomes more common in the population. If this process continues, eventually, all individuals in the population will be brown.

Tyler's Post

My name is Tyler. I am in Mr. Thompson’s' fifth hour Student Naturalist class. In the little free time I do get the chance to enjoy, I mostly listen to music and hang out with my buddies. I also like to read.
I am going to be covering adaptations, genetic drift, and speciation of different animals such as Darwin’s finches, other animals, and even humans. I am not seeking to prove that the theory of speciation is fact, regardless of my own beliefs, but simply to inform the reader.
Adaptations happen in nature when an animal has a special ability that makes them better at competition, which is where animals compete for certain resources that are not easily ascertainable. An adaptation makes competition much easier for that animal, like the Western Hognose Snakes' fangs, which are located a little farther back in their mouths, and are used to literally pop some amphibians, such as some frogs and toads. Another example of this is the Gray Tree Frogs' adaptation to change colors, and to blend almost seamlessly into its background. It can blend so well that to most predators it is nearly invisible. Let’s say that an animal is hunting the Gray Tree Frog, and another frog. The predator is much more likely to pursue the other frog if it can't see the Gray Tree Frog, thus giving it an advantage. Not all advantages must be physical; however instincts or behaviors that only certain animals do are also adaptations. The Western Hognose, hen threatened, will bluff like it is very tough, and try to scare the predator off. If this fails it will turn over on its back and expose a red spot on its belly, and play dead. For a human example, the skin tones of different people around the world differ by the climate they live in. In Africa, the skin tones of the people are much darker than of those in Denmark. Now, this is not because the animals/people decided to adapt, or that their body decided to adapt. The reason that these adaptations occur is because of mutation. Now, the organisms with successful mutations breed with the other organisms of the species, and the species in that area becomes different, for the better, even though by chance. This leads to Genetic Drift, a term to describe how in different areas of the world, there are different organisms, and why in different areas of the world there are similar organisms with different adaptations. Darwin's Finches are a great example of this. Darwin examined at the finches on an island and discovered that they all came from a common ancestor. The important part was that on the island, there wasn't just one kind of finch, there were several. The finches that mutated and began to have longer beaks got better at eating grubs, and thus went to a different area to hunt grubs. They reproduced in that area, leading into a totally different kind of bird in this area that in the starting point. This happened several times to the finches, making several kinds. Another example would be the marsupials in Australia. They are similar to lots of the animals in the rest of the world, but somehow obtained pouches and look a little different. Their adaptions made them different, even though they probably had a common ancestor with another animal in another part of the world. Another Human example would be that in a place like Norway, the adaptation of dark skin would not help a certain person survive. Neither would a taller, skinnier profile. The people in Norway probably adapted to be stouter, where the people in Africa were more successful with their taller, skinnier profile. Their skin being darker also helped them to better suit their environment. Since it is more successful to be stout in Norway, and better to be tall and dark skinned in Africa, the two people's look very different today, even though it has been proven that all humans originated from Africa, from a common ancestor. Genetic drift, and specific adaptations that helped them survive in their environment led to this. The term for when one species becomes two or more species' is speciation. When Darwin’s' finches started as one species and ended up as several, it was speciation.

This is not one of the easiest things to prove in the Animal Kingdom, on account of the fact that it is not very observable. Things like speciation, like its broader heading Evolution takes a very long time to occur, and thus is difficult to prove. Genetic Drift it very closely associable to speciation. When Genetic Drift is where one creature becomes different because of separation of the species, speciation is the extreme of this. Speciation is when one species changes enough to where the common ancestor of a species is completely different than something that mutated from it. IF humans came from monkeys, than this would be an example of speciation, because they would be a completely different species. In some ways, speciation is the extreme of genetic drift, because genetic drift can cause speciation. A very well known example is the way that all dogs came from a common ancestor- the wolf. The wolf is a completely different species than the dog, or the many species' of dogs. This example can, in many ways, can be related to genetic drift as well, which comes from mutations and adaptations. In a way, you could say that mutations cause adaptations, which in turn can cause genetic drift. The opposite could also be said, however, that being separated can cause adaptations of the species to change in different areas, causing genetic drift. Either way, you could say that the cause of speciation can be genetic drift, so they make a chain, in which they all are related. Using the Human example again: A. A human adapts so that it is not so good in hotter climates, and is better in colder regions. Thus, it migrates to a colder area, being more successful there, and continues to change from the original human. Now, the two are completely different. B. A group of humans get separated from Africa and somehow end up in a colder climate. Some of them mutate and are better in this climate, and thus survive longer and reproduce more. In time, after further mutations, that part of the species is completely different than how it began back in Africa. This chain is not necessicarily what causes speciation or evolution, just one possible way that it can happen. This way of thinking of Evolution is much more reasonable than the way that many think of it. It is certain that if more people actually understood this process, they would be more accepting of it.
I'm not trying to prove anything, just to state a perspective on the issue. If something is learned through this, then I've accomplished my goal.


http://en.wikipedia.org/wiki/Genetic_drift

http://en.wikipedia.org/wiki/Speciation

http://www.talkorigins.org/faqs/evolution-definition.html

http://www.talkorigins.org/faqs/faq-speciation.html

http://www.wooster.edu/biology/mloveless/Drift.html