6 How Do Humans Differ?
We can look around any place we are at with other humans and see variation, both biological variation and cultural variation. This chapter will focus on why there is biological variation in human populations. This necessitates a discussion of not only biological explanations, but cultural and environmental explanations as well. We will explore why there are different allele frequencies in populations and how that is related to adaptive significance.
Human Variation
Genetic Variation
When thinking about human variation, it is difficult not to think about Salt ‘n’ Pepa’s 1990 hit, “Let’s Talk about Sex” (Blacks’ Magic album; released by Next Plateau records). The act of sexual reproduction is critical in the production of human variation from a genetic level to a cultural level, e.g., rules for mate choice.
“All humans share mostly identical genetic material—100% of the same genes and 99.9% of their variations…almost all our species’ genetic variations are found within populations, not between them” (Welsch et.al. 2017, 265). Data from the Human Genome Project, which began in 1990, and subsequent research projects indicate that “between 83 and 97% of genetic variation is found within human populations and between 3 and 17% is found between populations” (Welsch et.al. 2017, 266). Why is this?
Recall in chapter 3, we discussed meiosis, the process of cell division in sex cells. Eggs and sperm each carry half of our parents’ chromosomes. When an egg is fertilized by a sperm a zygote is formed with a full complement of chromosomes. The mix of the chromosomes is random (we cannot know which our parents’ chromosome we will get) which is why offspring look similar to their parents but may look more like one parent than another or not. Plus, genes may exchange genetic material in a process called recombination, creating a new genetic mix in offspring. Other forces of evolution also contribution to variation. Mutations can cause new genetic variants to arise. Genetic drift, which occurs in small populations isolated through either geography or cultural behavior, will make populations different from one another but cause less variation within a population. Gene flow, the exchange of genetic material between populations, causes a decrease in variation and is a primary explanation of why contemporary humans are so alike genetically. We can use genetics to track Pleistocene AMH migration patterns out of Africa and through the rest of the world. As glaciers melted, populations in Australia, New Guinea, and the Americas became isolated for a short time, allowing genetic drift to impact their genetic makeup. As better modes of travel were invented, gene flow would once again impact populations the world over.
One reason humans are so alike genetically is because there have been a few genetic bottlenecks, a form of genetic drift. Bottlenecks are caused by events that wipe out a part of the gene pool, leaving less genetic variation. Computer