What does it mean, in practice?

100%

100%100%

Members of our community are only slightly less different from us than members of distant populations

85%

85%100%

Variances among continents are small, but not zero. Do genotypes naturally cluster in continental or subcontinental groups?

Assigning a genotype to its continent by discriminant analysis

training dataset

query genotype

?

Genes, as well as morphology, suggest inconsistent clusterings of genotypes

Africa

Asia, Europe, Australia, Americas

Americas

Africa, Asia, Americas,Oceania

Asia Europe

Africa, Asia,EuropeOceania

Y chromosome: Romualdi et al. 2002

Alu insertions: Romualdi et al. 2002

X chromosome: Wilson et al. 2001

Europe,Ethiopia

S. Africa N. Guinea

Asia

Genes, as well as morphology, suggest inconsistent clusterings of genotypes

377 STR loci: Rosenberg et al. 2005

Melanesia Eurasia N Africa N America

Maya

S. Africa

377 STR loci: Barbujani and Belle 2006

E Africa

C Africa

Piapoco

Suruì

Karitiana

Kalash

W. Eurasia

E. Asia

Africa

Americas

Oceania

8

6

2

45

9

10

1

7

3

Genomic boundaries inferred from diversity at 377 STR loci(Barbujani and Belle 2006)

Less than 8% of the alleles are continent-specific, and more than half of these are African

Rosenberg et al. (2002)

Avg. numbers of haplotype blocks in 51 genome regions (1.5 million base pairs): limited continental

differentiation

Africa is special: Genetic diversity in all continents is often a subset

of African genetic variation

Tishkoff et al. (1998)

Approximate inferred colonization dates

Human genetic diversity largely reflects patterns of migration

A pointillist view of human evolution and variation: 1

 © 1999 Kenneth K Kidd, Yale University

A pointillist view of human evolution and variation: 2

© 1999 Kenneth K Kidd, Yale University

A pointillist view of human evolution and variation: 3

© 1999 Kenneth K Kidd, Yale University

Genetic evidence on modern humans’ origins

• Extensive allele sharing across continents• Extensive haplotype sharing across continents• Largest proportion of human genetic diversity within

populations• No obvious continental clusters of populations• Genetic diversity out of Africa a subset of African

diversity• Broad-scale clines

We are a young

and mobile species

• Il nostro genoma è molto piccolo• Il nostro genoma è molto grande• I nostri genomi sono molto simili• I nostri genomi sono molto differenti

La diversità genomica umana

Mind the numbers

Humans and chimps share >98% of their genomes

Among the 2% differences, 1.9% are fixed differences within species

The remaining fraction, 0.1%, contains all human genomic variation

85% of that 0.1% represents differences among members of the same population

The differences among the main racial or continental groups represent 10% of 0.1% of the total, that is, 0.01%

But 0.01% of 3 billion DNA sites means 300 000 variable sites

DNA-based forensic identification

Many DNA regions are characterised by a Variable Number of Tandem Repeats (VNTR): …CTAGACCCGAGAGAGAGAATTCCATGC… [5] …CTAGACCCGAGAGAGAATTCCATGC… [4] …CTAGACCCGAGAGAGAGAGAGAATTCCATGC… [7]

Each of us carries a potentially unique combination of alleles at VNTR loci

Isolation of VNTRs

DNA fingerprintingAlec Jeffreys et al.: Hypervariable minisatellite

regions in human DNA, Nature, 314:67-73, 1985.

DNA fingerprinting: an application

DNA fingerprinting: an application

DNA fingerprinting: a paternity case

A case of sexual violence

But if races do not exist, how come forensic scientists are so good at finding them?

a. UK forensic classification (before 2005):European, Afro-Caribbean, Indian Subcontinent, South East Asian, Middle Eastern

b. UK forensic classification (after 2005)White-British, White-Irish, White-other, Asian-Indian, Asian-Pakistani, Asian-Bangladeshi, Asian-other, Black-Caribbean, Black-African, Black-other, Chinese, + 4 razze miste e Other

c. USA forensic classification Caucasian, African-American, East Asian, Hispanic, Native American

d. Bribri classificationBribri, ña

Any list can be Ok for certain purposes, no list gives an all-purpose biological classification of humankind

Think about Hispanics

Consigli e sconsigli