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