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Gene Flow

6/26/2016

If you tuned in last Sunday you now know what genetic drift is (http://anthropologicalconcepts.weebly.com/blog/genetic-drift) and that it is bad, that in mind, gene flow is the exact opposite and is very good. It is thanks to gene flow the human race is so diverse with different traits. In the following blog I will discuss how gene flow happens, give examples of it, and exactly why it is good. But first, what is gene flow?
The basic definition of gene flow is an exchange of genes between populations (image below). Another term that is sometimes used is migration, but this term also means “movement of people,” whilst gene flow is the exchange of genes. Last week I addressed the founder effect, in gene flow the separation of people from their home community is vital. Unlike the founder effect in which you have people within the same community mating, in gene flow they would mate with people from other communities. This is an important force of evolution, “the flow of alleles in and out of a population due to the migration of individuals or gametes.”

Seeing how this a blog entry on a physical anthropology focused site, you would think gene flow only happens in humans; it does not. Gene flow can be seen in all types of animals, even plants. Plants for example, the way they mate is by sending their pollen via the wind, insects, or birds “to pollinate other populations of the same species some distance away.” In the case of humans, it should be evident seeing how many types of humans we have. An over used, but very good, example of gene flow is of US soldiers who had children with Vietnamese women during the Vietnam war. This one act “altered the gene pool frequencies of the Vietnamese population.” The gene flow of many African Americans in the US is a wonderful example of how gene flow influences microevolutionary (“evolutionary change within a species or small group of organisms, especially over a short period.”) changes in modern human populations. African Americans mostly have West African descent, that said, there is also a good portion of their genes that have European American. “By measuring allele frequencies for specific genetic loci, we can estimate the amount of migration of European alleles into the African American gene pool.” This method of estimating is very interesting because through this estimation they found the data from western and northern states (“including New York, Detroit, and Oakland”) it shown the percentage of foreign genes (non-African) as 20 to 25 percent. But in the southern states it was a much lower percentage, 4 to 11 percent (which is not really surprising with the history of discrimination towards African-American in the United States).
     In the case of mutation in gene flow is good (as opposed to mutation in genetic drift). This is an important factor for the diversity in populations. Light coloured eyes for example. Most of Europe has light coloured eyes, and seeing how we all originated in Africa, it is easy to see how these light eyes are a mutation. It all started with one person with light eyes, someone to say to themselves “they will be my mate.” Even if their children, or their children's children did not have light eyes, that gene is in them; this is how most of Europe has ended up with light eyes...it all started with one person. These light eyes are yet another example of gene flow. How else could the majority of Europe have them if it was not for gene flow?
     For humans gene flow is one of the things we have to thank for the survival of our species. What I mean by this is with gene flow our bodies have furthered its adaptability. Recently the human race has gotten very touchy about social rules of who they can and cannot mate with, but, in our millions of years of evolution the past thousand or so years is a spit in the bucket, thus why I use “recently.” That said, it is nice to see how we are over coming these social “norms.” Seeing a light skinned man with a dark skinned woman, or vice versa is becoming more acceptable in modern days (light skin is another example of gene flow and adaptation; see this blog entry for further information about that: http://anthropologicalconcepts.weebly.com/blog/-evolution). It is because of this wonderful force that when new genes are brought into a population, we can gain diversity. “Of course, migration patterns are a manifestation of human cultural behavior, and this emphasizes once again the essential biocultural nature of human evolution.”
     Please feel free to comment on what you thought of the blog, or other physical anthropological subjects you would like me to cover.

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Genetic Drift

6/19/2016

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Genetic drift is an evolutionary change in which it “occurs because the population is small.” How small is “small?” Did this happen and can it happen again? Is genetic drift good? But really, was IS genetic drift? In the following blog I will address these questions, and hopefully answer all questions you have about this topic.
     So, what is genetic drift. I know I gave a definition in the first paragraph, but it was a very vague definition that would only make sense to those who already know what genetic drift is. It is a random factor in evolution which means it has ties in mutation. When an allele, which I mentioned in an earlier blog (http://anthropologicalconcepts.weebly.com/blog/you-have-your-mothers-eyes) is rare in a population, a small population (a few hundred), it may not be able to pass on to the off spring because if the allele is rare, that means it is recessive. This may seem like a small thing, but this means the “genetic variability in this population has been reduced.” And in the unlikeliness of that allele being passed on, that trait dies in that population.
     That said, it answers the “is genetic drift good” question; no it is not. There is a particular type of genetic drift that can lead to another more drastic type. The particular type is called “founder effect,” in which “allele frequencies are altered in small populations that re taken from, or are remnants of, larger populations.” This type did occur, and still does occur in modern human (and nonhuman) populations. Founder effect can happen when a small party breaks off a bigger one and establishes themselves elsewhere. When they choose to procreate they will most likely choose one of their party. So over time the genes of the original founders will be the only one's in their expanded group. “In such a case, an allele that was rare in the founders' parent population but, just by chance, was carried by even one of the founders can eventually become common that group's descendants.”
  Now, as I mentioned in the previous paragraph, there is a more drastic type of founder effect; this type is known by the term “genetic bottleneck.” The reason I call this type “drastic,” is because the effects can be extremely damaging to a species. While a small population (“with considerable genetic variability”) has a chance to get some of the same variability, an original population with the same considerable genetic variability, when a small group leave to colonize another area is where the bottleneck occurs (image below). When the population size is restored, there will be less of the genetic variability as the original population. Cheetahs for example, genetically they are an “extremely uniform species.” Biologists believe in the past there was a huge decline in numbers, and for reasons that are left up to skepticism, the male cheetah produces a “high percentage of defective sperm as compared to other cat species.” So this decreased reproductive potential, and other factors (hunting by humans for example), but the cheetah in very dangerous waters. On the subject of species, we humans are also very uniform genetically as well. Reason being a good amount of our evolution (the last 4 – 5 million years), we most likely lived in small groups, and in doing so the “drift would have had a significant impact.”

As I said before, genetic drift is not a good thing, mostly because the retention of a disease or condition in a population. There are some that can happen to anyone (cystic fibrosis, a variety of Tay-Sachs, sickle – cell anemia, but the last one is actually an advantage, I will explain why later), but most common within small populations. For example there is a recessive but fatal condition called Amish microcephaly, in which a mutation causes abnormally small brains and heads of fetuses. Unlike my previous statement some can happen to “anyone,” this specific condition is only found within the Old Order Amish community of Lancaster County, Pennsylvania, United States. Even though this is recessive it occur in approximately 1 in 500 births. Now I mentioned that sickle – cell anemia is an advantage, reason being those with this benign condition are more likely to be a survivor of malaria (“an intermittent and remittent fever caused by a protozoan parasite that invades the red blood cells.”) And the reason why those with sickle – cell, which provides 60% protection against overall mortality, is because, it is suggested, the hemoglobin (the protein responsible for transporting oxygen in the blood) gets in the way of the parasite and in doing so the infected results with a low percentage of infected red blood cells.
After this blog, I hope I have opened your minds to go beyond your neighbourhood or ethnic group to put a stop to the damage genetic drift can cause (and has caused thanks to our evolutionary ancestors). Please feel free to comment on what you thought of the blog, or other physical anthropological subjects you would like me to cover.

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The Evolution of Mammals

6/12/2016

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I have discussed and addressed the evolution of Homo sapiens sprinkled in several previous blogs, but have not how the entire mammalian population came to be. How did a planet that was ruled by an enormous beings who were a cross between reptiles and mammals become a planet where mammals are the most common? In the following blog I will discuss how and when the mammal evolved.
First thing is first, who, or what was the first mammal? It may seem unbelievable but the common ancestor of all living mammals today looked like a rodent (image below). “The scientists then worked with an artist to illustrate this ancestor. In addition to a furry tail, the researchers suggest the four-legged creature likely ate insects, weighed from 6 grams (about the weight of some shrews) up to 245 grams — less than half a pound — and was more adapted for general scampering than built for more specialized forms of movement, such as swinging from trees.” This small mammalian ancestor appeared about 200,000 to 400,000 years after the extinction of the dinosaurs, which when compared to how long it took humans to evolve (5 to 8 million years), it was as if they were just waiting for the extreme threat to go away. But how did what makes a mammal a mammal come along?

If you were to ask a child who was just learning about mammals they would tell you that mammals have hair. We do not have a grasp why all mammals have hair, but there is a theory which, after what is theorized what happened to the dinosaurs makes sense. The reason mammals have hair was to regulate temperature, or in the case of the mammalian ancestor, to keep warm. Another big sign an animal is a mammal is if they give live birth. Again, the drop in temperature after the incident that caused the extinction of the dinosaurs played a big part in this one as well. Mammals evolved from a reptile named synapsids. Let us say the mammalian ancestor laid eggs, she would risk dying from staying still whilst sitting on her eggs. With the live birth, the female mammal is free to move gather food, protect herself, along with a myriad of other tasks. Live birth is also a huge advantage to the take over of mammals because of the fragility of eggs as opposed to incubating the offspring in the womb.
The type of mammal that gives live birth (placental), is the type of mammal that is populating the Earth. We are currently in the “Age of Mammals,” which, as I stated earlier, started 200,000 to 400,000 (roughly 65 million years ago) after the “Age of Dinosaurs” came to an end. The synapsids were around millions of years before they evolved into mammals. “These reptiles arose during the Pennsylvanian Period (310 to 275 million years ago). A branch of the synapsids called the therapsids appeared by the middle of the Permian Period (275 to 225 million years ago).” In hindsight, evolving over a span of 100+ million years into a mammal, the evolution of humans is not so bad. That said, throughout these millions of years, the therapsids had to lose their scales thus growing hair (or fur), produce milk producing glands, and above all adapt to the changing times by having a uterus.
All that said, it really is not that hard to believe the mammalian ancestor was so small. If you were a new species, a completely new branch of animal, being small was the best to ensure your survival. Primates for example, our, chimpanzee's, gorilla's, lemur's, every primates out there, we are all closely related to tupaiids (tree shrews...image below). Even though our mammalian ancestor evolved from a reptile, modern mammals cannot in anyway be linked to reptiles, or to any other group. “Since it can be shown that each of the 32 orders of mammals are separate and distinct groups set apart from one another and from all other creatures by unbridged gaps, it seems evident that collectively as mammals they are set apart as well.”

Earlier I did state that to be a mammal live birth is a factor, but in the case of monotremes this does not apply. What I mean by this is that monotremes are mammals who lay eggs. The species that fall into monotremes are not many, in fact, there are only two; the platypus and the echidna. I mentioned placental earlier, aside from it meaning to give live birth, it also means “presence of a placenta, which facilitates exchange of nutrients and wastes between the blood of the mother and that of the fetus.” This, of course, does not apply to monotrems, but it also does not apply to marsupials, even though they, in a way, to give live birth. But no matter our differences, we are all mammals, and this is our age.
Please feel free to comment on what you thought of the blog, or other physical anthropological subjects you would like me to cover.

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Gene Flow

Genetic Drift

The Evolution of Mammals

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