Post by Admin on Jun 23, 2020 19:18:01 GMT
5. Are Human Races Defined By Adaptive Traits?
Races, when they exist, occupy a subset of the geographical range of their species. Sometimes environmental factors vary over the geographical range of the species, and some of these environmental factors induce natural selection that results in local adaptation. Hence, when races exist, they sometimes display local adaptations to the environment associated with their geographical sub-range that are not adaptive in the remainder of the species’ geographical range. In such cases, these geographic subpopulations also represent an ecotype. As stated earlier, the ecotype concept can be and has been applied to many different types of populations and even individuals within a local area, but at least in some circumstances in some species an ecotype and subspecies or race can refer to the same populations. This reasoning leads to the idea that local adaptations can sometimes be biological markers of racial status in humans; that is, human races are ecotypes (Pigliucci & Kaplan, 2003). However, human ecotypes do not correspond to races under either subspecies definition. Even the advocates of the ecotype race concept acknowledge that the same adaptation can arise independently in different parts of the species’ range (Pigliucci & Kaplan, 2003). Hence, ecotypes do not in general correspond to evolutionary lineages, and specifically human ecotypes cannot correspond to evolutionary lineages in humans since the hypothesis of multiple evolutionary lineages within the human species is rejected, as noted earlier. Moreover, variation in environmental factors can induce natural selection that results in local adaptations even in species that are not genetically subdivided at all (Templeton, 2006); that is, the ecotypes are only genetically differentiated at the gene loci under selection and show little to no genetic differentiation over the remainder of the genome. In these cases, the geographic distributions of the local adaptations reflect the geography of environmental factors and not boundaries of overall genetic differentiation. Hence, the ecotype concept in general does not correspond to populations demarcated by sharp boundaries of genetic differentiation that exceed some threshold. This is also certainly the case in humans because there are no human populations with sharp genetic boundaries that exceed the thresholds used in the non-human literature.
One solution to these problems of using ecotypes as races in humans is to simply abandon the subspecies concept of race entirely and define human races solely through ecotype status (Pigliucci & Kaplan, 2003). This solution does not solve the main problem that the subspecies concept of race addressed: avoiding cultural biases in a definition of human race. All species, humans included, adapt to many environmental factors, not just one. Frequently, different adaptive traits display discordant geographical distributions because the underlying environmental factors have discordant geographical distributions. As a result, one will get different ecotypes for different adaptations. This is not a problem for how the ecotype concept is used in the general evolutionary literature, but it raises a critical problem for implementing the ecotype concept as a definition of race in humans. Depending upon which adaptive trait is chosen, one will get very different “races”. So which adaptive traits should be used and which should be ignored? Evolutionary biology provides no objective way of addressing the question of choice of adaptive traits, so the ecotype concept of race in humans is yet another subjective, culturally sensitive concept of “race.”
Skin color is historically the locally adaptive trait most commonly considered by European cultures as a “racial trait” in humans. Skin color is an adaptation to the amount of ultraviolet (uv) radiation in the environment: dark skins are adaptive in high uv environments in order to protect from radiation damage that can kill and burn cells and damage DNA if not protected by melanin, and light skins are adaptive in low uv environments in order to make sufficient vitamin D, which requires uv (Hochberg & Templeton, 2010; Jablonski & Chaplin, 2010). The geographical distribution of skin color follows the environmental factor of uv intensity. Skin color differences do not reflect overall genetic divergence. For example, the native peoples with the darkest skins live in tropical Africa and Melanesia. The dark skins of Africans and Melanesians are adaptive to the high uv found in these areas. Because Africans and Melanesians live on opposite sides of the world, they are more highly genetically differentiated than many other human populations (Figure 2) despite their similar skin colors. Europeans, who are geographically intermediate between Africa and Melanesia, are likewise intermediate at the molecular genetic level between Africans and Melanesians, even though Europeans have light skins that are adapted to the low uv environment of Europe. Skin color differences in humans are not a reliable indicator of overall genetic differentiation or evolutionary history. Moreover, skin color varies continuously among humans in a clinal fashion rather than categorical ecotypes (Relethford, 2009). Hence, there is a compelling biological reason to exclude skin color as the racially-defining adaptive trait under the ecotype concept of race.
Another adaptive trait in humans is resistance to malaria, which is widespread in African populations. However, malaria is also common in some areas outside of Africa, and malarial resistance is found in many European and Asian populations as well. Indeed, one of the alleles underlying malarial resistance, the sickle-cell allele, has its highest frequency in certain populations on the Arabian Peninsula and in India despite frequently being regarded as a disease of “blacks”. Each adaptive trait in humans has its own geographical distribution that reflects the distribution of the underlying environmental factor for which it is adaptive. The discordance in the distributions of adaptive traits in humans means that different adaptive traits will define different ecotypes/”races”. No guidance, other than cultural preference, is given for choosing which adaptive trait should define the ecotypes that are regarded as races, and which ecotypes should not be regarded as races. Hence, equating ecotypes to races, even if limited just to humans, does not yield an objective, culture-free definition of race.
Races, when they exist, occupy a subset of the geographical range of their species. Sometimes environmental factors vary over the geographical range of the species, and some of these environmental factors induce natural selection that results in local adaptation. Hence, when races exist, they sometimes display local adaptations to the environment associated with their geographical sub-range that are not adaptive in the remainder of the species’ geographical range. In such cases, these geographic subpopulations also represent an ecotype. As stated earlier, the ecotype concept can be and has been applied to many different types of populations and even individuals within a local area, but at least in some circumstances in some species an ecotype and subspecies or race can refer to the same populations. This reasoning leads to the idea that local adaptations can sometimes be biological markers of racial status in humans; that is, human races are ecotypes (Pigliucci & Kaplan, 2003). However, human ecotypes do not correspond to races under either subspecies definition. Even the advocates of the ecotype race concept acknowledge that the same adaptation can arise independently in different parts of the species’ range (Pigliucci & Kaplan, 2003). Hence, ecotypes do not in general correspond to evolutionary lineages, and specifically human ecotypes cannot correspond to evolutionary lineages in humans since the hypothesis of multiple evolutionary lineages within the human species is rejected, as noted earlier. Moreover, variation in environmental factors can induce natural selection that results in local adaptations even in species that are not genetically subdivided at all (Templeton, 2006); that is, the ecotypes are only genetically differentiated at the gene loci under selection and show little to no genetic differentiation over the remainder of the genome. In these cases, the geographic distributions of the local adaptations reflect the geography of environmental factors and not boundaries of overall genetic differentiation. Hence, the ecotype concept in general does not correspond to populations demarcated by sharp boundaries of genetic differentiation that exceed some threshold. This is also certainly the case in humans because there are no human populations with sharp genetic boundaries that exceed the thresholds used in the non-human literature.
One solution to these problems of using ecotypes as races in humans is to simply abandon the subspecies concept of race entirely and define human races solely through ecotype status (Pigliucci & Kaplan, 2003). This solution does not solve the main problem that the subspecies concept of race addressed: avoiding cultural biases in a definition of human race. All species, humans included, adapt to many environmental factors, not just one. Frequently, different adaptive traits display discordant geographical distributions because the underlying environmental factors have discordant geographical distributions. As a result, one will get different ecotypes for different adaptations. This is not a problem for how the ecotype concept is used in the general evolutionary literature, but it raises a critical problem for implementing the ecotype concept as a definition of race in humans. Depending upon which adaptive trait is chosen, one will get very different “races”. So which adaptive traits should be used and which should be ignored? Evolutionary biology provides no objective way of addressing the question of choice of adaptive traits, so the ecotype concept of race in humans is yet another subjective, culturally sensitive concept of “race.”
Skin color is historically the locally adaptive trait most commonly considered by European cultures as a “racial trait” in humans. Skin color is an adaptation to the amount of ultraviolet (uv) radiation in the environment: dark skins are adaptive in high uv environments in order to protect from radiation damage that can kill and burn cells and damage DNA if not protected by melanin, and light skins are adaptive in low uv environments in order to make sufficient vitamin D, which requires uv (Hochberg & Templeton, 2010; Jablonski & Chaplin, 2010). The geographical distribution of skin color follows the environmental factor of uv intensity. Skin color differences do not reflect overall genetic divergence. For example, the native peoples with the darkest skins live in tropical Africa and Melanesia. The dark skins of Africans and Melanesians are adaptive to the high uv found in these areas. Because Africans and Melanesians live on opposite sides of the world, they are more highly genetically differentiated than many other human populations (Figure 2) despite their similar skin colors. Europeans, who are geographically intermediate between Africa and Melanesia, are likewise intermediate at the molecular genetic level between Africans and Melanesians, even though Europeans have light skins that are adapted to the low uv environment of Europe. Skin color differences in humans are not a reliable indicator of overall genetic differentiation or evolutionary history. Moreover, skin color varies continuously among humans in a clinal fashion rather than categorical ecotypes (Relethford, 2009). Hence, there is a compelling biological reason to exclude skin color as the racially-defining adaptive trait under the ecotype concept of race.
Another adaptive trait in humans is resistance to malaria, which is widespread in African populations. However, malaria is also common in some areas outside of Africa, and malarial resistance is found in many European and Asian populations as well. Indeed, one of the alleles underlying malarial resistance, the sickle-cell allele, has its highest frequency in certain populations on the Arabian Peninsula and in India despite frequently being regarded as a disease of “blacks”. Each adaptive trait in humans has its own geographical distribution that reflects the distribution of the underlying environmental factor for which it is adaptive. The discordance in the distributions of adaptive traits in humans means that different adaptive traits will define different ecotypes/”races”. No guidance, other than cultural preference, is given for choosing which adaptive trait should define the ecotypes that are regarded as races, and which ecotypes should not be regarded as races. Hence, equating ecotypes to races, even if limited just to humans, does not yield an objective, culture-free definition of race.