genes and morphology of humans provide excellent examples of geographic variation (as a result of extraordinary efforts to measure and to determine the genotypes of humans throughout the world)

fossils of humans provide perhaps the best documented record of evolution in vertebrates (as a result of extraordinary efforts to find human fossils)

so are current human populations one species?

is human geographic variation different from that in other vertebrates?

in the following paragraphs, MYA = million years ago, KYA = thousand (kilo) years ago

for example ... the Cambrian Period with the first fossils in most modern phyla began about 550 MYA ... dinosaurs became extinct at the end of the Tertiary Era about 65 MYA ... the Pleistocene Epoch or Ice Ages with at least four episodes of major continental glaciation began about 2.5 MYA ... 0 A.D. was about 2 KYA

chimpanzees and humans are more closely related to each other than either one is to other apes -- hence humans and chimpanzees must share a common ancestor not shared by other apes -- similarity of DNA sequences has made this conclusion clear

notice that this recent finding indicates that humans cannot be placed in a separate family of primates (unless we want to place every lineage of apes -- gibbons, orang-utans, gorillas, chimps, humans -- in its own phylogenetic family!) -- the family Hominidae as previously recognized is not phylogenetic!

two living species of chimpanzee are restricted to Africa (both endangered) -- over 98% of human DNA has the same base-pair sequence as chimp DNA -- similarity is greater than that between most closely related species of vertebrates -- most recent common ancestor lived only 5-10 MYA

during the evolution of humans, several phylogenetic lines became extinct -- only one line persisted to the present

Australopithecus ape-humans appeared more than 3 MYA in Africa -- apparently two or three distinct species -- fully erect, tool-using Homo habilis appeared about 2 MYA in Africa and coexisted with Australopithecus for about 1 MY before latter became extinct

Homo erectus with larger brains (but still not as big as modern humans') appeared more than 1 MYA in Africa and quickly spread throughout Africa, Europe, Asia

Homo sapiens with modern skulls (and thus modern brains) appeared during last interglacial period about 100 KYA -- they too first appeared in Africa and spread rapidly throughout Africa, Europe, Asia -- everywhere they replaced erectus which became extinct -- in Europe and western Asia a similar group of humans (Neanderthals) coexisted with modern humans until near the end of the last glaciation then also became extinct

appearance of Homo sapiens at locations all across Europe and Asia at about the same time has raised the possibility that modern humans might have had three (or more) separate origins -- but rapid spread from Africa is more likely

fossil and molecular evidence increasingly suggests that each of the three stages of human evolution (Australopithecus, Homo erectus, and Homo sapiens) had a single origin in Africa -- human evolution came "out of Africa" ... three times in a row!

modern humans colonized nearly the entire world in 100 ky (100 thousand years)

modern humans from Africa occupied Eurasia about 100-50 KYA, Australia about 30 KYA, North and South America about 12 KYA, the western Pacific islands about 5 KYA, Madagascar about 1.5 KYA (across the Indian Ocean from southeast Asia), Hawaii and New Zealand about 1 KYA (across the ocean from the southwest Pacific)

the only places not occupied by humans before 1500 were Antarctica and a few remote islands in the South Atlantic and Indian Oceans and in the eastern Pacific (notably the Galapagos Islands)

human populations today (or before 1500) show as much geographic variation as do other widespread vertebrate species

well-studied examples of geographic variation in humans include ...

• skin color (result of melanin produced by melanocytes in the skin, influenced by many genes)

• hair texture (determined by cysteine cross-linking in keratin)

• hair and eye color (several genes)

• details of body build and skull shape

• height

• genes that provide protection against specific diseases (for instance, sickle-cell hemoglobin provides some resistence to malaria -- many other alleles have been suggested to provide resistence to various diseases)

• genes that determine blood antigens (many genes, including ABO and Rh)

• mitochondrial and nuclear DNA sequences (not known for as many human populations as the above examples -- but new information is accumulating rapidly)

human populations vary in continuous clines like those in many other vertebrates

where human populations have lived in contact with each other for long periods of time, morphology and allele frequencies vary geographically in continuous clines

notice gradual variation in skin color between Africa and Europe -- also gradual variation in gene frequencies across Europe and Asia -- gradual clines indicate gene flow (absence of reproductive isolation) between adjacent populations (just as in other vertebrates)

notice also that the same features that vary between human populations also vary within human populations -- often differences among individuals within a population are greater than differences among averages of different populations (often stated "variation is greater within populations than between populations") -- this pattern almost always applies to populations in contact with each other for long periods of time

there is no one way to define distinct categories of human populations (subspecies or races) -- recall that classifications of subspecies of other vertebrates are similarly arbitrary -- better procedure is to refer to geographic location or recent origin of populations

our impression of separate races in humans probably results from experience with people whose ancestors originated in widely separated parts of the world -- western Africa, western Europe, eastern Asia, North America

we can conclude that ...

clinal variation in human populations indicates no effective reproductive isolation between contiguous populations -- humans are one species by the biological species concept

clinal variation also means that there are no distinct differences between contiguous (parapatric) populations -- humans are one species by the phylogenetic species concept

human populations are not reproductively isolated but they also do not mate randomly -- they are instead intermediate between these extremes

again there are parallels with other vertebrates . . .

• most humans choose mates close to home -- natal dispersal is short -- as in many other species -- only in the past few centuries has modern transportation increased natal dispersal

• humans tend to choose mates that resemble themselves in various measures of appearance and personality (mates would thus also resemble siblings and parents but whether resemblance to parents, siblings, or self is the most important influence is not known) -- Snow Geese and some other birds are known to choose mates that resemble parents or siblings

• cultural differences also restrict matings -- for example, language differences or rules about acceptable marriage partners -- these cultural norms tend to creat barriers to gene flow between populations -- a possible analog in other vertebrates is bird song -- many birds learn their songs by listening to older individuals -- as a result populations of the same species often have different dialects -- an example of culture in nonhuman vertebrates -- but it is not clear whether these dialects in birds influence mating (nor is much known about this possibility in humans!)

• some reproduction does not follow social rules -- for instance, even when monogamous marriage is the rule, genetic studies show that actual reproduction is not so restricted -- other vertebrates like Snow Geese also form life-long monogamous bonds and nevertheless copulate with other individuals (about 10% of matings in Snow Geese) -- so some offsrping are not the putative father's!

clinal geographic variation in human populations suggests that cultural differences have not prevented gene flow among contiguous human populations over long periods of time -- cultural restrictions have probably decreased gene flow in some places at some times but have not prevented gene flow in the long run

biological studies of humans raise questions for everybody

scientific (including biological) results are used (and sometimes misused) in medical, social, political, and international programs

scientific (including biological) results can agree with, contradict, or have equivocal relationships with religious or political convictions

how should information from scientific studies affect our lives?

what separates credible results from dubious claims?   or use from misuse?

do studies of human populations differ in these ways from other scientific studies?

how do you personally determine what the questions are and what the evidence indicates?

does the possibility of misuse mean that scientific study of human populations should stop?