Are there different species of human?
Walking down the street we all find it very easy to make judgements about the ethnicity of people we encounter. We use phenotypic characteristics to determine their lineage. From the exterior people all look different and we observe more similarities between people of the same race than those of different. There are many physical stereotypes associated with different races and these form the basis for our categorisation. But should we take this further and split different races of humans into different species? To determine this we must think beyond the physical and use our genetic material to make a judgement. We must consider the similarities of an individual population compared to that of any other and also the similarities of the human genome to other distinct species.
The human genome is 3.3x109 base pairs in length and is arrange on linear chromosomes. Each cell in the human body contains 23 pairs of human chromosomes of which 22 pair show homology to each other. The exceptions to this rule are the human sex chromosomes. The males of the species contain an X chromosome and a Y. the Y chromosome is significantly smaller than the X and contains very few genes most of which are associated with male development. The females however to contain homologous sex chromosomes, two X's.
These general principles apply to every human on the planet apart from those affected by chromosomal rearrangement. These can come from chromosome translocations where part of one chromosome moves to another leading to a new rearrangement of genes and non-homology. This usually results in a healthy phenotype but an inability to reproduce. Where there is gain or loss of a whole chromosome it usually is unviable. The exception to this is Down syndrome a non-disjunction during meiosis that leads to trisomy 21. These diseases are the minority and most human genetic variation comes in a much less extreme from.
Within a gene there are many single nucleotide changes called single nucleotide polymorphisms. Some of these result in a change in the protein they code for ability to perform its function. This gives rise to a different phenotype, and sequencing of the gene revealed the change responsible. A gene with two or more possible sequences is said to have alleles. Some alleles of genes are deleterious and give rise to disease. Whereas other are responsible for differences in appearance, such as eye colour.
Classification into species is the lowest form of classification and therefore shows the closest relationship possible. It is also defined by the ability for mating to place between individuals of the same species. sapiens is the name given to the human species usually heard preceded by Homo which referees to the genous.
Human genetic variation
A large amount of research has been conducted into investigating the level of genetic variation found between different races. When the human genome project sequenced the human genome they stated that all humans are genetically similar. It is also suggested that humans of one race can be genetically more similar to someone of a different race than a member of their own race. This was proposed by the American anthropology association in 1997, "data also show that any two individuals within a particular population are as different genetically as any two people selected from any two populations in the world"
It is also thought that the difference in allele frequencies seen in the entire human race can only partly be attributed to differences in populations. However studies looking at more than one loci in the human genome have found that this sort of research can accurately categories people into there various races just from the polymorphisms in several loci of their DNA. So the level of genetic diversity between people of different races is a hotly contested subject. An extensive study by Withersoon et al set about to answer the unanswered question of human genetic variation. They conducted their study using varying amounts of loci analysis. The found that for a small number of loci there is a high concordance of genetic similarity between people of different populations. When the number of loci examined reach into the hundreds still a large amount of similarity was seen between people of different population compared to that of the same population. However where thousands of loci where examined this argument no longer held up. They also found that despite the similarity of different races of people at around 100 loci it was still possible to accurately divide them into their ethnic groups. Other research that supports the findings of similarity is that of Bamshad et al 2004 who found that 38% of the time Europeans were classed as more similar to Asians than other Europeans. (withersoon)
There is only approximately 0.1% genetic variation between individuals and of this variation 85%-90% comes from within individuals of the same population. There is only a maximum of 15% of variation in the human genome found between populations. (jorde et al) This coincides with the research by Wetherspoon in that with a small number of loci examined it is possible to expect people from different populations to be more similar than those from their own. However once a more thorough powerful genetic screen is used the 15% expected variation between populations can be use to categorize.
Drug responses and disease susceptibility in different populations
Despite the debate about the relevance of the level of genetic variation between populations different populations have different drugs of choice for certain conditions. For example there is a 43% increase survival rate in black people if treated with both isosorbide dinitrate and hydralazine for heart failure. (Taylor)
Drug metabolising enzymes can contain mutations within them that lead to an impaired function. These mutations show different frequencies with different populations. For example a variant of the Cytochrome p450 monooxygenase gives rise to poor metabolism of debrisoquine oxidation. The frequency of this gene in the population of Europeans is between 5-10% compared to less than 1% in Japanese people. This defect is important in the metabolism of certain drugs and can lead to toxicity. Therefore clinicians should be more aware of this when treating Europeans. (Wilson)
One study comparing the effects of morphine on white and Chinese participants found that morphine had a lesser effect on respiration and blood pressure in Chinese people. However there is an increase in the likelihood of gastrointestinal complications such as vomiting in the Chinese. One clinical reason for not prescribing to much morphine is the effects on blood pressure and respiration. Therefore clinically it should be considered that Chinese people be given a higher dose of pain relief due to its less severe side effects. (zhou)
As well as varying responses to drugs seen in different populations there are also varying levels of diseased alleles. The allele responsible for sickle cell anaemia is seen at a much higher frequency in Africans than in Europeans. This is due to the advantageous nature of this allele in the prevention of malaria when seen in the heterozygous state. Malaria is a disease attributed to Africa and therefore there is no selective advantage to carrying the sickle cell allele in Europe. Africans living in Europe still have a higher chance of carrying the sickle cell allele and therefore screening antenatal screening in this population subgroup is much more important than any other.
There are also differences observed in the prevalence of alleles of the Rhesus blood system, in different populations. In Europe the two alleles making up this system are virtually equal within the population. Whereas the Asian population has virtually only one allele, making up more than 95% of the total. Again Africans have the same allele ay a higher frequency than Europe. This again has clinical significance as blood groups are one of the main determinants of transplant success and therefore there is an increased chance of someone finding an organ donor within their own population if they contain the most common blood group type.
Relationship between Humans and other species.
We are constantly being told that the closest evolutionary relatives to humans are apes. These currently are classed in the same family as humans, the hominidea. However they are not even considered within the same genus as humans. Humans are within the genous Homo whereas apes are found in the genous Pan. The member of apes with the closest genetic relationship to humans in the chimpanzee. More specifically the subset of chimpanzees called the Homo (Pan) paniscus or the bonobo chimpanzee are believed to be our actual closest relatives in evolutionary terms. They are believed to have diverged form us about 5-6 million years ago and before that at approximately 6-7 million years ago we both diverged from gorillas. At important coding sequences we show 99.4% homology with the chimpanzee genome and 98.4% at non-coding sequences. These non-coding regions are under less evolutionary pressure and this therefore accounts for the increase in divergence. However as you see can our genomes are extremely closely related to that of something in the same family as us. Therefore does this mean we should speciate humans due to the small amounts of variation seen?
Out of Africa
The origin of human evolution is a much-debated topic with two theories as the front-runners. The first theory called the multi regional theory of human evolution suggests that the human ancestors homo erectus moved out of Africa more than a million years ago and that evolution into modern humans took place independently. Obviously there was virtually no contact between these separately developing continents.
The second theory called the out of Africa theory suggests that it was not the Homo erectus that left Africa but the Homo sapiens. This brings the date forward for the segregation of humans considerably. The first study highlighting this theory suggested that the departure from Africa was about 100,000 years ago. However the methodology of this study was brought into question and the actual date is believed to be around 156,000 years ago.
With either of these theories there is a prolonged period of separation and inbreeding on a large scale that could easily lead to speciation. The geographical separation during evolution would lead to the variation in allele frequencies seen in populations. However as we are aware the variation seen between populations is small and suggests a greater likely hood for the out of Africa theory. Whichever way evolution occurred it is apparent that despite phenotypic changes due to environmental factor genetic variation is still small. (Nei)
Social implication of speciation
We all like to believe that the lessons of the past are well learnt. When we move forward from human evolution and look at human social development we see cases of extreme racism. When humans segregated in Africa there was a prolonged period of separation before the human species became mobile again. As soon as this was achieved the first wave of racism was seen. The slave trade and inequality to black people because of there skin characterises a long period in history.
Another display of racism almost leading to genocide at the start of the last century show how close and relevant racism is today. Therefore the ethics of trying to subdivide the human race are staggering. The social implications that would follow speciation of the human race are unpredictable. It is unknown whether one race would claim superiority and the effects this would have on multicultural populations. Some people are already wrongly discriminated against without science adding fuel to the fire. The question from a social point of view should not be can we find genetic evidences that suggests sub speciation within the human race but should we be looking at all?
From the genetic analysis studies that have been conducted it is apparent that variation between populations only accounts for a small proportion of all human variation. There can be larger amounts of variation seen between people of the same population than of two people from different populations. However in spite of this by conducting large-scale SNP analysis of a person's genome it is possible to determine their ethnicity. These results are contradictory and perhaps do not form a basis for action.
Alleles are known to be at different frequencies within different populations. From a clinical point of view this should be considered when making a diagnosis of certain diseases. If two people are presenting with the symptoms of a disease, then it is much more likely that the person from the high-risk population will have the disease. Therefore in a medical setting people from different populations are already being treated differently due to the variations seen in their genomes. This is a small step on the way to personalised patient treatments determined by genetic analysis. And it could be argued that sub-speciation of the human race would enhance medical care.
When looking at the amount of genetic variation seen in between human populations within the human race, it is determined to be very small. However when you consider that our closest ancestor genome is only changed from ours by 1% and yet they are not considered to be within the same genus as us it makes you wonder if the small amount of human variation found is significant enough to warrant sub- speciation.
The ethical concerns previously highlighted are an important consideration when conducting genetic analysis in this field of science. The results must be interpreted correctly and used wisely.
Due to all of these points I conclude that the human race is one species and should remain that way. In a clinical setting it is appropriate and important to use general population variations in genetics when treating patient. However sub speciation of the human race is not only a dangerous decision but one also lacking in any genetic basis. There is far too much debate and contradictor results to implement any form of adequate divide, not to mention the difficulties in classifying those people of mixed decent. However I think that the argument of our closet ancestor having such a small difference in variation and yet in terms of taxonomy being so far apart is one that stands up. Therefore I support the idea that instead of sub categorising the human species the chimpanzees should be moved into the Homo genus.