Evolutionary Anthropology: Microevolution & Macroevolution: Lecture Four

Microevolution Cont'd

• Term test on June 16th, 5-7pm.

• Readings for Week #5:

  • Fuentes (2022)

  • Harrison (2013)

Evolution of Skin Tone

• Skin is a type of epithelium, serving as a protective layer.

• It's the largest organ in the integumentary system, including skin, hair, glands, and fingernails.

Skin Functions:

• Protection of tissues.

• Thermal regulation.

• Excretion.

• Sensation.

• Storage.

• Vitamin D synthesis.

• Protection from UV radiation.

Skin Layers:

• Hypodermis.

• Dermis.

• Epidermis.

Melanin:

• Produced by melanocytes.

• Located between the dermis and epidermis.

• Pigment granules are concentrated in melanosomes.

Human Skin Tone Phenotype:

• Melanin production varies based on environmental factors like tanning.

• Dispersion from basal cells.

• Thickness of skin, blood vessels, and carotene pigment play minor roles.

Human Skin Tone Genotype:

• Polygenic, involving >60 genes.

• MC1R gene is highly polymorphic; found in Neanderthals.

Natural Selection and Skin Tone:

• Gloger’s Rule: Greater pigmentation at lower latitudes.

• Clinal distribution.

UV Radiation:

• Disadvantages: Sunburn, disruption of skin barrier, cancer, chemical and vitamin breakdown.

• Melanin acts as a protective barrier.

• Advantages: Necessary for Vitamin D production when stimulated by UV rays.

Vitamin D:

• Function: Skeletal growth, calcium transportation, kidney function.

• Required exposure to UV light.

• Insufficiency leads to Rickets and Osteomalacia.

Adaptation to UV environment:

• Darker skin serves a protective function from intense UV radiation.

• Lighter skin allows for higher Vitamin D production in less UV radiation.

• Trade-off between Vitamin D production and sun protection.

Folate:

• Acquired from diet and important for physiological functions.

• Disadvantage of UV radiation: degrades folate.

Vitamin D and Folate Importance:

• Both are important in reproductive outcomes.

• Folate is needed for pregnancy, neural development, DNA synthesis and repair, and sperm.

• Vitamin D is needed for pregnancy, sex hormones, semen, and gonads.

• Need balance between sufficient Vitamin D and protection of folate.

Evolution of Human Skin Tone:

• Initial dark skin pigmentation evolved in humans, followed by lighter skin with pelts in hominins with fur.

• When pelts were lost, more melanin production was needed for protection.

• Light skin mutation is also found in Neanderthals (M1CR).

• Migration to higher latitudes with less sunlight decreased melanin production.

• SLC24A5 depigmentation gene in Europe around 6000 years ago.

Crawford et al. (2017):

• Genotype-phenotype mapping shows almost all skin tones exist in Africa.

• Many pigmentation gene variants are found in Africa.

• Some light skin tone variants in Europe have origins in Africa.

Cheddar Man:

• Prehistoric remains in the UK.

• aDNA genetic sequencing used for estimations of skin and eye color.

Vitamin D and Skin Tone - Gozdzik et al. (2010):

• Seasonal variation of Vitamin D in Toronto, Canada.

• Circulating Vitamin D is lower in East and South Asian groups.

• Implications for health in cold, heavily clothed environments.

Biology of Races and Racism

Racial Categories:

• How racial categories were created.

• How they have been used/abused.

• Influence they have today.

Historical Classifications:

• Linnaeus:

  • Europaeus albus: ‘governed by laws’

  • Americanus rubescens: ‘governed by customs’

  • Asiaticus fuscus: ‘governed by opinion’

  • Africanus niger: ‘governed by caprice’

• Cuvier:

  • Caucasoid

  • Mongoloid

  • Negroid

Characteristics:

• Usually geographical but often included ‘cultural’ components like religion, language and dress.

• Problems: Socially constructed, highly variable, do not account for variation, focus on only a few features.

Problems with racial categories:

• Phenotypes/visual appearance (skin tone, hair and eye color, body shape, facial characteristics).

  • Problems: Which features are important? What about non-visual variants? Polygenic traits. Continuous, not discrete, variation. Different genes experiencing different selection pressures.

• Variation exists in response to different selective pressures.

  • E.g., Skin tone: Pattern of variation doesn’t fit within certain racial categories; it's continuous, clinal.

• Conventionally based on place of origin.

  • BUT more genetic variation exists in Africa than everywhere else combined; all other populations are subsets of the variation.

• Most genetic variation is found WITHIN groups (93-95%); less genetic variation is found BETWEEN groups (3-5%).

Genetic Variability:

• Greatest genetic variability within Africa, decreasing with distance from Africa.

• Out-of-Africa model and serial founder effects.

Summary:

• Racial categories are inadequate when observing continuous variation across populations experiencing evolutionary processes.

• No set of features that can assign people to different groups.

• Humans exhibit real variations but avoid overinterpreting superficial traits, consider biological and cultural interactions.

Commercial ancestry tests:

• Problematic promotion and interpretations.

• Perception of DNA overriding all else.

• How it works: Degree of similarity to contemporary populations.

  • How common are Single Nucleotide Polymorphisms (SNPs) in a given population?

  • Estimates are based on a small subset of the genome.

  • Reference populations: biased sampling, self-reported race/ethnicity.

  • E.g., twin example: Different results from different companies.

Does Race Matter?

• Races are not biological units.

• Racial categories are socially constructed but have real consequences.

Health and Social Implications:

• Categories used in social and medical contexts lead to marginalization and justification of practices towards certain groups.

• E.g. Health disparities among black women.

  • Risk of pregnancy-related death 3-4x higher among black women than white women in US (CDC).

  • Black infants twice as likely to die than white infants in US (CDC).

  • Trends remain among healthy, wealthy, highly educated black women.

• Racial bias in pain assessment and treatment recommendations, and false beliefs about biological differences between blacks and whites as per Hoffman et al. (2016).

  • Misconceptions and unconscious biases.

  • Black Americans systematically undertreated for pain relative to white Americans.

  • Aims to redress in medical training.

• Racialized groups experience different health events differently due to socioeconomic factors (City of Toronto Open Covid Data).

• Misinterpretation of genetic research for political purposes.

• Appreciation of human variation, much of which is clinal.

• ‘Populations,’ ‘Population affinity’ rather than ‘race.’

  • A local interbreeding group with reduced gene flow between themselves and other groups of humans’ (Shook et al., 2019).

Summary:

• Race is not a valid concept when describing human biological variation.

• Concepts of race are socially constructed and highly variable.

• Continuous variation cannot easily be categorized.

• Geographic patterning does not match ideas of discrete racial groups.

Macroevolution

• Evolution is characterized on two scales: Microevolution and Macroevolution.

Microevolution vs. Macroevolution

• “Macroevolution” refers to evolution at levels higher than the population.

• Coined by T. Dobzhansky.

• Large-scale history of life on Earth.

• The accumulation of changes resulting from microevolutionary processes over time can lead to macroevolutionary changes.

Speciation

• Speciation connects micro-to macroevolution.

1. What is a species?

2. How do new species arise?

The Biological Species Concept (BSC):

• “Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups” - Ernst Mayr.

  • For different populations to belong to the same species: (1) be capable of interbreeding, (2) naturally, (3) produce fertile offspring.

Limitations of the BSC:

• The biological species concept cannot be applied to:

  • Asexual organisms.

  • Organisms about which little is known regarding their ability to interbreed.

  • Fossils.

  • Intermediate states between two species.

  • Hybridization zones.

Other species concepts

• Species are difficult to classify because of variability within populations:

  • Reticulate evolution.

  • Incomplete lineage sorting.

  • Hybridization.

Phylogenetic species concept

• A character-based definition.

• Groups of organisms that share a unique, shared evolutionary history (descent) and can be distinguished by a unique combination of character states

• Used, in some form, by most paleontologists

Patterns of Speciation

• Anagenesis: Over time, a species can change from one form to a different form.

• Cladogenesis: One or more new species branch off an ancestral species.

Adaptive Radiation

• Formation of many new species following the availability of new environments or the development of new adaptations.

• Many adaptive radiations are evident in the fossil record, including the primate fossil record.

Speciation Factors

For speciation to occur, two factors need to be in place:

1. Reduction or elimination of gene flow between populations: typically through geographic isolation (allopatric speciation).

2. Genetic divergence: other evolutionary forces (mutation, genetic drift, natural selection) act to increase the genetic differences between populations.

The end result is reproductive isolation; populations become new species.

Allopatric Speciation

Allopatric speciation is a type of speciation (the formation of new species) that occurs when a population is geographically separated into two or more isolated groups. Over time, these groups evolve independently, and due to differences in natural selection, mutations, and genetic drift, they may become so genetically different that they can no longer interbreed, even if brought back together.