13 HUMAN EVOLUTION
Learning Objectives
- Explore genetics of human skin coloration
- Describe factors that impact genetic variability of skin coloration
- Identify genetic complexities in the variation of skin coloration
Exercises
- Can animals develop a suntan or sunburn? Discuss with your group and try to determine specific examples of this from your experience.
- What about fish? Can you find any reliable evidence from an internet search on this topic?
- What are some factors that contribute to variation in human individuals ability to tan?
Population Genetics
A population’s individuals often display different phenotypes, or express different alleles of a particular gene, which scientists refer to as polymorphisms. We call populations with two or more variations of particular characteristics polymorphic. A number of factors, including the population’s genetic structure and the environment (Figure 19.3) influence population variation, the distribution of phenotypes among individuals. Understanding phenotypic variation sources in a population is important for determining how a population will evolve in response to different evolutionary pressures.
Figure 19.3 The distribution of phenotypes in this litter of kittens illustrates population variation. (credit: Pieter Lanser)
Heritability is the fraction of phenotype variation that we can attribute to genetic differences, or genetic variability, among individuals in a population. The greater the heritability of a population’s phenotypic variation, the more susceptible it is to the evolutionary forces that act on heritable variation.
The diversity of alleles and genotypes within a population is known as genetic variability. When scientists are involved in the breeding of a species, such as with animals in zoos and nature preserves, they try to increase a population’s genetic variability to preserve as much of the phenotypic diversity as possible. Human skin is an example of genetic variance in populations due to melanin. Th pigment is found in a wide variety of living organisms including plants and fungus. It is the pigment that gives skin and hair its color. Within skin cells called melanocytes, melanin has two main forms eumelanin and pheomelanin. Eumelanin gives rise to a range of brown skin tones, as well as black, brown, and blond hair and pheomelanin, which gives rise to reddish browns of freckles and red hair.
Genetic variability
Natural selection and some of the other evolutionary forces can only act on heritable traits, namely an organism’s genetic code. Because alleles are passed from parent to offspring, those that confer beneficial traits or behaviors may be selected, while deleterious alleles may not. The type and amount of melanin in your skin
The theory of natural selection stems from the observation that some individuals in a population are more likely to survive longer and have more offspring than others. This increases the chances they will pass on more of their genes to the next generation. Over the course of generations, humans living near equatorial Africa adapted to have a higher melanin production threshold and more eumelanin, giving skin a darker tone. This built-in sun shield provided benefits like melanoma protection, making them more likely to survive and pass genetic traits to future generations.
Gene Flow
Another important evolutionary force is gene flow: the flow of alleles in and out of a population due to the migration of individuals or gametes (Figure 19.6). While some populations are fairly stable, others experience more flux. Humans migrated northward moving far from the equator away from higher UV direct sunlight. This revealed a new selective pressure. UV light can damage skin, but it brings the benefit of helping us produce vitamin D to strengthen bones enhance absorption of vital calcium, iron, magnesium, phosphate, and zinc. Lack of vitamin D in humans can lead to rickets with symptoms of serious fatigue and weakened bones. There were individual humans in the migratory population that happened to produce less melanin.
Figure 19.6 Gene flow can occur when an individual travels from one geographic location to another.
Mutation
Mutations are changes to an organism’s DNA and are an important driver of diversity in populations. Species evolve because of mutations accumulating over time. The appearance of new mutations is the most common way to introduce novel genotypic and phenotypic variability. Some mutations are unfavorable or harmful and are quickly eliminated from the population by natural selection. Others are beneficial and will spread through the population. Whether or not a mutation is beneficial or harmful is determined by whether it helps an organism survive to sexual maturity and reproduce. Some mutations do not do anything and can linger, unaffected by natural selection, in the genome. Some can have a dramatic effect on a gene and the resulting phenotype.
Nonrandom Mating
If individuals nonrandomly mate with their peers, the result can be a changing population. There are many reasons nonrandom mating occurs. One reason is simple mate choice. For example, female peahens may prefer peacocks with bigger, brighter tails. Natural selection picks traits that lead to more mating selections for an individual. One common form of mate choice, called assortative mating, is an individual’s preference to mate with partners who are phenotypically similar to themselves.
Another cause of nonrandom mating is physical location. This is especially true in large populations spread over vast geographic distances where not all individuals will have equal access to one another. Some might be miles apart through woods or over rough terrain, while others might live immediately nearby.
If there is gene flow between the populations, the individuals will likely show gradual differences in phenotype along the cline. Restricted gene flow, alternatively can lead to abrupt differences, even speciation.
Human skin color pigmentation exists due to genetic, environmental and cultural variables that scientists are still being illuminated by research. The videos in this page offer some contrasting views. Checked out this citation for a peer-reviewed journal article on this topic from 2021.
Jablonski NG. The evolution of human skin pigmentation involved the interactions of genetic, environmental, and cultural variables. Pigment Cell Melanoma Res. 2021 Jul;34(4):707-729. doi: 10.1111/pcmr.12976. Epub 2021 May 4. PMID: 33825328; PMCID: PMC8359960.
Activity
HHMI Biointeractive worksheet The Biology of Skin Color linked here.
Assessment
- Describe how human skin coloration can provide an advantage for survival and reproduction to people groups settled far from the equator for generations.
- Describe how human skin coloration can provide an advantage for survival and reproduction to people groups settling at high altitudes for generations.
- Explain how phenotypes for human skin coloration have changed based on the principles of genetic variability during modern times of globalization.
