Studies on racial diversity in society and race mixing

Make your own conclusions, I just post what's known. Some links might be dead. A lot of compiled info, decided to share.
1. Racial (ethnic) Diversity
It has been well documented in numerous studies, such as the famous Putnam study, that racial diversity has huge negative psychological effects for all members of the multiracial (multicultral) society.

Effects include; decreased trust, increased violence, increased mental illness, increased isolation, increased criminality.

STUDIES ON NEGATIVE EFFECTS OF DIVERSITY UPON SOCIETY:

More diverse neighborhoods have lower social cohesion. Source:  http://www.citylab.com/housing/2013/...munities/7614/

Diversity increases psychotic experiences. Source:  http://bjp.rcpsych.org/content/201/4/282.full

Diversity increases social adversity. Source:  http://bjp.rcpsych.org/content/201/4/282.full

A 10% increase in diversity doubles the chance of psychotic episodes. Source:  http://bjp.rcpsych.org/content/201/4/282.full

Diversity reduces voter registration, political efficacy, charity, and number of friendships. Source:  http://onlinelibrary.wiley.com/doi/1...176.x/abstract

Ethnic diversity reduces happiness and quality of life. Source:  http://onlinelibrary.wiley.com/doi/1...176.x/abstract

Diversity reduces trust, civic participation, and civic health. Source:  http://www.boston.com/news/globe/ide...ity/?page=full

Ethnic diversity harms health for Hispanics and Blacks. Source:  http://ajph.aphapublications.org/doi...PH.2012.300787

Diversity primarily hurts the dominant ethnic group. Source:  http://www.theindependentaustralian.com.au/node/57

Ethnic diversity reduces concern for the environment. Source:  http://link.springer.com/article/10....640-012-9619-6

Ethnic diversity within 80 meters of a person reduces social trust. Source:  http://curis.ku.dk/ws/files/13025117...ASR.pdf#page=2

Ethnic diversity directly reduces strong communities. Source:  https://www.msu.edu/~zpneal/publicat...versitysoc.pdf

Ethnically homogeneous neighborhoods are beneficial for health. Source:  https://www.mailman.columbia.edu/pub...nority-seniors

In America, more diverse cities have more segregation. Source:  http://fivethirtyeight.com/features/...st-segregated/

Homogeneous polities have less crime, less civil war, and more altruism. Source:  http://link.springer.com/article/10....464-013-9608-0

States with little diversity have more democracy, less corruption, and less inequality. Source:  http://www.theindependentaustralian.com.au/node/57

There is extensive evidence people prefer others who are genetically similar. Source:  http://psychology.uwo.ca/faculty/rus...n%202005-1.pdf

Borders, not multiculturalism, reduce intergroup violence. Source:  http://arxiv.org/abs/1110.1409

Diversity reduces charity and volunteering. Source:  http://onlinelibrary.wiley.com/doi/1...176.x/abstract

People who live in diverse communities rather than homogenous ones are poorer and less educated. Source:  http://onlinelibrary.wiley.com/doi/1...176.x/abstract

Black people trust their neighbors less than do White people. Source:  http://onlinelibrary.wiley.com/doi/1...176.x/abstract

Spanish speakers trust their neighbors less than do English speakers. Source:  http://onlinelibrary.wiley.com/doi/1...176.x/abstract

Asians trust their neighbors less than do White people. Source:  http://onlinelibrary.wiley.com/doi/1...176.x/abstract

Ethnically diverse workplaces have lower cohesion, lower satisfaction and higher turnover. Source:  http://jom.sagepub.com/content/23/3/...=1&ssource=mfc

Ethnic diversity reduces social trust. Source:  http://www.nber.org/papers/w5677

Ethnic diversity among members of the same race reduces infrastructure quality, charity, and loan repayment. Source:  http://onlinelibrary.wiley.com/doi/1...176.x/abstract

Diversity of any sort makes people more likely to defect in game theoretic scenarios. Source:  http://onlinelibrary.wiley.com/doi/1...176.x/abstract

Homogeneous military units have less desertion than diverse units. Source:  http://www.nber.org/papers/w8627

Diversity correlates with low GDP. Source:  https://www.washingtonpost.com/news/...rse-countries/

Ethnic homogeneity correlates with strong democracy. Source:  https://www.washingtonpost.com/news/...rse-countries/

Genetic diversity causes societal conflict. Source:  https://www.nber.org/papers/w21079

Ethnic diversity causally decreases social cohesion. Source:  http://esr.oxfordjournals.org/conten...sr.jcv081.full

STUDIES ON NATURAL HUMAN TRIBALISM:

Ethnocentrism is rational, biological, and genetic in origin. Source:  http://www.pnas.org/content/108/4/1262.abstract

Babies demonstrate ethnocentrism before exposure to non-Whites. Source:  http://onlinelibrary.wiley.com/doi/1...2.01138.x/full
Ethnocentrism is universal and likely evolved in origin. Source:  http://www-personal.umich.edu/~axe/r...Hamm_Ethno.pdf

Races are extended families. Ethnocentrism is genetically rational. Source:  http://www.amazon.com/The-Ethnic-Phe.../dp/0275927091

Ethnocentrism is biological in origin and a superior evolutionary strategy to altruism. Source:  http://jasss.soc.surrey.ac.uk/16/3/7.html

Humans are more altruistic to individuals who they are more closely related to. Source:  http://www.ncbi.nlm.nih.gov/pubmed/17456276

People subconsciously prefer those who are genetically similar to them for biologically rational reasons. Source:  http://www.psychology.uwo.ca/faculty...ity%201989.pdf

Kinship between members of an ethnic group is greater than expected. Source:  http://citeseerx.ist.psu.edu/viewdoc...0.1.1.372.1009

ALSO -
https://www.nber.org/papers/w21079
https://www.pnas.org/content/108/4/1262.abstract
Ethnocentrism is universal and likely evolved in origin -  http://www-personal.umich.edu/~axe/r...Hamm_Ethno.pdf
Ethnocentrism is biological in origin and a superior evolutionary strategy to altruism -  http://jasss.soc.surrey.ac.uk/16/3/7.html.bak
Humans are more altruistic to individuals who they are more closely related to -  https://www.ncbi.nlm.nih.gov/pubmed/17456276
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3461446/
https://journals.plos.org/plosone/ar...l.pone.0095660
The kinship between random co-ethnics can exceed that between grandparent and grandchild, kinship between members of an ethnic group is greater than expected -  http://citeseerx.ist.psu.edu/viewdoc...0.1.1.372.1009
https://www.nber.org/papers/w8627  ;

also -
"Study paints bleak picture of ethnic diversity
John Lloyd in London OCTOBER 9, 2006
A bleak picture of the corrosive effects of ethnic diversity has been revealed in research by Harvard University’s Robert Putnam, one of the world’s most influential political scientists.

His research shows that the more diverse a community is, the less likely its inhabitants are to trust anyone – from their next-door neighbour to the mayor.

This is a contentious finding in the current climate of concern about the benefits of immigration. Professor Putnam told the Financial Times he had delayed publishing his research until he could develop proposals to compensate for the negative effects of diversity, saying it “would have been irresponsible to publish without that”.

The core message of the research was that, “in the presence of diversity, we hunker down”, he said. “We act like turtles. The effect of diversity is worse than had been imagined. And it’s not just that we don’t trust people who are not like us. In diverse communities, we don’t trust people who do look like us.”

Prof Putnam found trust was lowest in Los Angeles, “the most diverse human habitation in human history”, but his findings also held for rural South Dakota, where “diversity means inviting Swedes to a Norwegians’ picnic”.

When the data were adjusted for class, income and other factors, they showed that the more people of different races lived in the same community, the greater the loss of trust. “They don’t trust the local mayor, they don’t trust the local paper, they don’t trust other people and they don’t trust institutions,” said Prof Putnam. “The only thing there’s more of is protest marches and TV watching.”

British Home Office research has pointed in the same direction and Prof Putnam, now working with social scientists at Manchester University, said other European countries would be likely to have similar trends.

His 2000 book, Bowling Alone, on the increasing atomisation of contemporary society, made him an academic celebrity. Though some scholars questioned how well its findings applied outside the US, policymakers were impressed and he was invited to speak at Camp David, Downing Street and Buckingham Palace." ;

https://www.ncbi.nlm.nih.gov/pubmed/22753852
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3519326/
https://link.springer.com/article/10...640-012-9619-6

http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf
"Target articles that examine how genes bias the development of complex human social behavior in one direction over alternatives evoke particularly strong reactions; for some readers they seem to threaten not only the way we conceptualize human nature but also what we think is right and what we think is wrong. Studies of altruism and political attitudes in twins estimate that about 50% of the variance is associated with direct genetic inheritance, virtually 0% with the twin's common family environment, and the remainder with each twin's specific environment. Studies of human marriages and friendships show that people choose each other on the basis of similarity, assorting on the most genetically influenced of a set of homogenous attributes. These data imply a genetic canalization of social influences so that, within the constraints allowed by the total spectrum of cultural alternatives, people create environments maximally compatible with their genotypes.Too many studies of twins and adoptees have been conducted for the genetic contribution to individual differences in aggressiveness, intelligence, law-abidingness, sexuality, and – yes, probably – preference for similarity in social partners, to be legitimately ignored. The notion of the "species typical individual" still too often prevails. Surely it is now time for genetic variance, the first postulate of Darwinian theory, to be more formally incorporated into theorizing about human behavior in both the evolutionary and the social sciences. By implication, it would then more readily be seen to follow that variance in human behavior, including such complex social endeavors as mate choice, selective friendship, and
in-group patriotism, has a genetic basis."

https://onlinelibrary.wiley.com/doi/...9.2005.00216.x
https://papers.ssrn.com/sol3/papers....ract_id=319762

2. Race mixing

https://youtu.be/-gTRVKsBiSU?t=41
Also
Mixedrace children suffer from more health problems

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1448064/

A massively well-funded study of over 100,000 schoolchildren found that “Adolescents who identify themselves as mixed race are at higher health and behavior risk than those of 1 race.” Indeed, even when controlling for education, socioeconomic status, and other factors, there is an across-the board higher rate of health risks amongst mixed race adolescents than mono-racial adolescents.

One study found that White-Asian mixes had a 2x higher rate of being “diagnosed with a psychological disorder, such as anxiety, depression or substance abuse.”
http://www.eurekalert.org/pub_releas...-baa081108.php

A study on Black-White mixes in agreement found that ”When it comes to engaging in risky/anti-social adolescent behavior, however, mixed race adolescents are stark outliers compared to both blacks and whites.” This holds true despite being raised in similar environments to mono-racial children.
http://www.msu.edu/~renn/RHE-_mixed_race.pdf

Often race-deniers and cultural Marxists will bring up Haldane’s rule, arguing that since races can mix and create fertile offspring, the genetic distance is not too great. Haldane’s rule is “when in the offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterogametic [XY] sex.”
http://www.nature.com/scitable/topic...metic-sex-1144

Indeed, although Black-White mixes are not sterile and males are not absent, males (the heterogametic sex) are more rare than females.
http://www.jstor.org/pss/1537084

The argument regarding Haldane’s rule is also meaningless because different species in the animal kingdom can breed and still produce fertile offspring. The wolf (Canis lupus) and the dog (Canis lupus familiaris), the coyote (Canis latrans), and the common jackal (Canis aureus) are separate species yet can all interbreed and produce fertile offspring.
http://www.jstor.org/discover/10.230...47699085472247

Two species of orangutan (Pongo abellii from Sumatra and Pongo pygmaeus from Borneo) can interbreed despite having different chromosomal numbers.
http://www.nytimes.com/1995/02/28/sc...ted=all&src=pm

The common chimpanzee (Pan troglodytes) and the bonobo (Pan paniscus) and many species of birds, such as the pintail (Anas acuta) and the mallard (Anas platyrhynchos), can interbreed as well.
http://www.sciencedaily.com/releases...0420104723.htm
http://birdaz.com/blog/2011/02/24/ma...the-wonderful/

The gibbon and the siamang can also interbreed to produce a hybrid
http://www.ncbi.nlm.nih.gov/pubmed/451603
http://www.ncbi.nlm.nih.gov/pubmed/1060807

Some species that aren’t even in the same genus can interbreed.
http://aob.oxfordjournals.org/content/74/2/111.full.pdf

Black Americans are a hybrid race of around 22% White ancestry
http://genomebiology.com/content/10/12/r141

This 22% ancestry is the cause of multiple negative health effects due to genetic incompatibility. Indeed, consistent with Haldane’s rule, unmixed Blacks from Africa and White Americans do not have the same rate of birth problems that hybrid American Blacks have: “In 2005, the mortality rate for black infants was 4.4 times higher than that of white infants… African women who come to the United States and have babies experience the same low rate of infant deaths as white American mothers.”
http://t1nyurl.com/6tr9e6t  (docs.google)

“Indiscriminate interbreeding between distinct forms, whether ‘species’ or markedly different races, is not generally beneficial. The defect may show in a change in the sex-ratio of the offspring, probably caused by the early abortion of members of one sex, generally the male in the case of mammals.”
http://www.amazon.com/Race-John-R-Baker/dp/0936396040

LTA4H, or “leukotriene A4 hydrolase” is found on chromosome 17. An allele of this gene increases the risk of heart attack (the #1 cause of death in America) in Blacks by more than 250%, but only 16% in Whites. The 30% of Whites with this allele have counteracting genes, while the 6% of Black Americans who obtained it through race mixing do not.
http://www.ncbi.nlm.nih.gov/pubmed/16282974

The average rate of success for mixed race couples is around half that of same race couples, 0.127 compared to 0.213.
http://www.jstor.org/stable/4145377

There is evidence that the more similar the two people are, the happier their marriage tends to be.
https://www.sciencedirect.com/scienc...9188699190057I

A study in Iceland showed that 3rd cousin marriages are the most fertile and successful. Suggesting that mating within one's ethnic/racial group would be more beneficial than outside of one's ethnic/racial group.
http://www.sciencemag.org/content/319/5864/813.abstract

One study showed that people tend to find their own face when morphed into the opposite sex most attractive, even when he/she doesn't know it's his own face, strongly suggesting that people typically prefer those who look like themselves, in other words their own racial/ethnic group.
http://www.psyc.nott.ac.uk/research/...onvoak1999.pdf

Hybrid vigor (heterosis) does not seem to apply to humans as we are already very heterozygous, at 0.776 [ http://mbe.oxfordjournals.org/conten.../707.full.pdf ] Compared to dogs for example at 0.401. [ http://www.eebweb.arizona.edu/Course...1996-Wolf.pdf ]

Although some argue that race mixing "averages out" traits and since average faces are considered more attractive [ http://cs.brown.edu/courses/cs143/2009/lecture13.pdf ] then mixed race offspring are more attractive.

However this is fallacious as it assumes we are designed for perfect symmetry, when in fact the magnitude of asymmetry is the correct measurement and this random component fulctuates, also called "fluctuating asymmetry" - so more honestly a lower level of asymmetry correlates with increased attractiveness, not symmetry.
http://beheco.oxfordjournals.org/con...5/864.abstract

But does increased heterozygosity cause lower asymmetry? A meta analysis of 118 datasets in 14 studies showed a very weak inverse correlation between the two, showing that increased heterozygosity has no beneficial effect on symmetry or asymmetry
http://www.nature.com/hdy/journal/v8.../6885550a.html

One study on the craniofacial morphology in White-Amerindian mixed race individuals found that over half of the 52 shape variables deviated from the mathematical average completely shattering the above argument.
http://majorityrights.com/images/upl...ish.indian.pdf

The neurotransmitter oxytocin "makes people more co-operative, benevolent, loyal, generous and trusting of others. It is involved in the parent-child bond - new mothers and fathers have raised levels of oxytocin. Production also increases when people hug and when they have sex and, recent research suggests, when they receive psychological warmth." However, oxytocin has been alleged to "foster racism."
http://www.guardian.co.uk/technology...hormone-racism

The study found that "intergroup bias that fuels prejudice, xenophobia, and intergroup violence… Which maybe modulated by brain oxytocin." This suggests that the instinctual desire to pursue the interests of one's own ethnic group to further ensure the existence of one's people is linked to racial bias, coinciding with the observation that diversity is a weakness, not a strength.
http://www.pnas.org/content/early/2011/01/06/1015316108

One study found that during the menstrual cycle when women have a higher likeihood of conception, racial bias is increased, especially when the likeihood of sexual intercourse was increased; "increased conception risk was positively associated with several measures of race bias. This association was particularly strong when perceived vulnerability to sexual coercion was high."

>Although egalitarians suggest that only a racist would consider race when it comes to reproduction and mate choice, this study observed a general, innate tendency to prefer one's own race especially when most likely to be impregnated.
http://www.cdnresearch.net/pubs/private/racebias.pdf

The more people of different backgrounds who produce offspring = the more types that are harder to match.
Multiracial patients have uncommon profiles and since there can be many possible racial and ethnic combinations in multiracial societies, finding a match can still be extremely difficult.
http://m.phys.org/_news162659550.html

Race Is A Social Construct?
The Immune System: How Important Is Race?
>"If Nick Glasgow were white, he would have a nearly 90 percent chance of finding a matching bone marrow donor who could cure his leukemia."

The reason that mixed-heritage patients are so hard to match can be found in the immune system. Different races have developed certain proteins, or markers, that are part of the body's natural defenses. These markers help the immune system determine which cells are foreign and should be rejected.

Are cells racist?
A match between two people who share more genetically in common significantly reduces the risk of the donor and recipient cells attacking each other. (READ: cells from people of two different races will attack and fight each other as though they are a virus)

The more people of different backgrounds who produce offspring = the more types that are harder to match.
Multiracial patients have uncommon profiles and since there can be many possible racial and ethnic combinations in multiracial societies, finding a match can still be extremely difficult.
http://m.phys.org/_news162659550.html

As an example, a White mother of mixed-race child would have more genetically in common with a random White person on the street than with her own child.
If such a dramatic and fundamental alienation from your own parents if not horrific enough, With a mixed-race child diagnosed with leukaemia, every member of the child's own family becomes an incompatible donor for a bone-marrow transplant, and finding a compatible donor is unlikely:

>One obstacle to finding a matching donor was her mixed ethnic background. Her father is black, with West Indian and Panamanian roots. Her mother is white, with Russian-Jewish roots. In addition, only 5 percent of registered donors are black.
http://www.nytimes.com/1996/04/18/sp...hter-dies.html

Bone Marrow Transplants: When Race Is an Issue
>''Compared to organ transplants, bone marrow donations need to be even more genetically similar to their recipients. Since all the immune system's cells come from bone marrow, a transplant essentially introduces a new immune system to a person. Without genetic similarity between the donor and the patient, the new white blood cells will attack the host body.
http://www.time.com/time/health/arti...993074,00.html

Race matters when a patient needs a stem cell or marrow transplant
>If you become ill with a blood cancer or other disease that requires a stem cell transplant, here's an uncomfortable fact: Your race matters. Diversity is a strength in much of life, [citation needed!] but it's a curse when finding a stem cell donor match.
http://articles.washingtonpost.com/2...hell-ancestors
---------
The Race for Ancestral Genetics in Clinical Trials
>A seasoned cancer researcher would never set up a study in which all the ill patients were, say, Canadian, and all the healthy controls were Japanese. And yet cancer researchers risk making a similar mistake if they overlook genetic information that fleshes out what many of us like to think of as race or ethnicity, some experts say.

>Fortunately, awareness of how ancestral genetics might contribute to risk of disease and drug response in people has risen over the last several years. Studies that look directly at the problem are on the rapid rise, and this increased interest has biotechnology companies lowering the cost of tests that determine genetic ancestry, thanks to a little competition. However, experts have yet to decide on how to genetically define ancestry, suggesting examining anywhere from a handful to hundreds of gene variants.

>Not including information on the race or ethnicity of study volunteers could skew disease risks as stronger or weaker than they really are.

>"Alzheimer's disease is the poster child for this problem," says pharmacogeneticist Esteban Burchard, M.D., of the University of California in San Francisco. A variant of the gene ApoE4 is a strong genetic risk factor for early-onset Alzheimer's disease, and the characteristic most likely to raise or lower that risk is race.

>"It occurs in about 20% of the African American population, and it means nothing. It occurs in about 6% of the Japanese, and it makes their risk six times higher [than that for white people]," Burchard said. "Something about being Japanese unleashes the wrath of the gene, and something about being African American attenuates it."
http://jnci.oxfordjournals.org/content/98/18/1270.full

BiDil
>The FDA have approved a heart failure drug called Bidil, but only for blacks, as blacks do not benefit from conventional heart failure drugs.
http://online.wsj.com/article/SB1160...tml?mg=com-wsj

Sickle-Cell Disease
Sickle cell anaemia is a condition found almost exclusively in blacks, about 1 in 4 west Africans carry the gene for it, almost no whites do. Any whites who carry the gene most likely have a black ancestor.

As a result of population growth in African-Caribbean regions of overseas France and immigration from North and sub-Saharan Africa to mainland France, sickle cell disease has become a major health problem in France. SCD has become the most common genetic disease in this country.
https://en.wikipedia.org/wiki/Sickle_cell_anemia

Invasive species: Genes Are Destroyed With Admixture/Racial Mixing

Extinction by hybridization:
>Nonindigenous species can bring about a form of extinction of native flora and fauna by hybridization and introgression either through purposeful introduction by humans or through habitat modification, bringing previously isolated species into contact. These phenomena can be especially problematic for rare species coming into contact with more abundant ones. Increased use of molecular techniques focuses attention on the extent of this underappreciated problem that is not always apparent from morphological observations alone. Some degree of gene flow is a normal, evolutionarily constructive process, and all constellations of genes and genotypes cannot be preserved. However, hybridization with or without introgression may, nevertheless, threaten a rare species' existence.
http://www.annualreviews.org/doi/abs...colsys.27.1.83

The Island Model with Stochastic Migration:
>It is supposed that the migration rate and the immigrant gene frequency are independent of each other in each generation, and each of them is independently and identically distributed in every generation. The treatment is confined to a single diallelic locus without mutation. If the diploid population is infinite, selection is absent and the immigrant gene frequency is fixed, then the gene frequency on the island converges to the immigrant frequency, and the logarithm of the absolute value of its deviation from it is asymptotically normally distributed.
The above implies that if you have two genes in some frequency in the immigrant population, under one way migration that frequency becomes the dominant frequency on the island.

>We investigated various cases of the island model with stochastic migration. If the population is infinite, the immigrants have a fixed gene frequency and the alleles are neutral, the gene frequency on the island converges to that of the immigrants.
What this means is that the genes initially on the model island, in effect, disappeared.
http://www.ncbi.nlm.nih.gov/pmc/arti...28/pdf/163.pdf

Evolution with Stochastic Fitness and Stochastic Migration
>Migration between local populations plays an important role in evolution - influencing local adaptation, speciation, extinction, and the maintenance of genetic variation.

>A number of studies have shown that when per capita population growth rate is treated as a random variable, rather than a fixed parameter, then directional evolution is influenced not only by the expected fitness of each phenotype, but also by the variance and other moments [23], [28]-[33]. This follows from the fact that, in a closed population, change in mean phenotype is inversely proportional to mean population fitness
http://www.plosone.org/article/info%...l.pone.0007130

Figure 1. The consequences of a negative correlation between fitness and immigration rate:
http://www.plosone.org/article/fetch...entation=PNG_M

Figure 2. The influence of the variance in immigration rates, on the expected change in mean phenotype:
http://www.plosone.org/article/fetch...entation=PNG_M

Essentially, when the variance in the immigration rate is small it results in a negative phenotypic change on the mean phenotype of a population, as the migrant phenotypes predominate.

Figure 3. The relation between variance in immigration, selection, and in an island-continent model:
http://www.plosone.org/article/fetch...entation=PNG_M
>bottom-right in picture.

>The curves show the change in mean phenotypic value assuming a mean phenotypic value as decided by a population's selection.
>The variance in migration is shown in descending value on the curves.

Though the mean rate of migration is the same on all the curves, it is clear that the the island can still get close to fixation of the phenotype favoured there, so long as the variance in immigration is high.

So for this figure, assuming that migration is an indefinitely persisting condition, migrants will prevent the local island favored phenotype from remaining intact, as they are essentially working against selection.
_____________________________________

The models/simulations in these studies are based on understanding evolutionary genetics, on how migration influenced evolution, which makes them excellent sources for metaphors on the effects of modern immigration rates in human populations.

Genes Are Destroyed With Admixture/Racial Mixing
Nonindigenous species can bring about a form of extinction of native flora and fauna by hybridization and introgression either through purposeful introduction by humans or through habitat modification, bringing previously isolated species into contact. These phenomena can be especially problematic for rare species coming into contact with more abundant ones. Increased use of molecular techniques focuses attention on the extent of this underappreciated problem that is not always apparent from morphological observations alone. Some degree of gene flow is a normal, evolutionarily constructive process, and all constellations of genes and genotypes cannot be preserved. However, hybridization with or without introgression may, nevertheless, threaten a rare species’ existence.
http://www.annualreviews.org/doi/abs...colsys.27.1.83

Allopatric speciation
Allopatric speciation or geographic speciation is speciation that occurs when biological populations of the same species become vicariant — isolated from each other to an extent that prevents or interferes with genetic interchange. This can be the result of population dispersal leading to emigration, or by geographical changes such as mountain formation, island formation, or large scale human activities (for example agricultural and civil engineering developments). The vicariant populations then undergo genotypic or phenotypic divergence as: (a) they become subjected to different selective pressures, (b) they independently undergo genetic drift, and (c) different mutations arise in the populations’ gene pools.[1]

The separate populations over time may evolve distinctly different characteristics. If the geographical barriers are later removed, members of the two populations may be unable to successfully mate with each other, at which point, the genetically isolated groups have emerged as different species. Allopatric isolation is a key factor in speciation and a common process by which new species arise.[2] Adaptive radiation, as observed by Charles Darwin in Galapagos finches, is a consequence of allopatric speciation among island populations.

https://en.wikipedia.org/wiki/Allopatric_speciation
https://en.wikipedia.org/wiki/Speciation

Reproductive isolation - Sexual isolation by behavior or conduct
Even where there are minimal morphological differences between species, differences in behavior can be enough to prevent mating. For example, Drosophila melanogaster and D. simulans which are considered twin species due to their morphological similarity, do not mate even if they are kept together in a laboratory.[3][12]Drosophila ananassae and D. pallidosa are twin species from Melanesia. In the wild they rarely produce hybrids, although in the laboratory it is possible to produce fertile offspring. Studies of their sexual behavior show that the males court the females of both species but the females show a marked preference for mating with males of their own species.

https://en.wikipedia.org/wiki/Reproductive_isolation

>Picture: drosophila fruit flies are popular choices for these types of experiments.

http://evolution.berkeley.edu/evosit...eciation.shtml

Just imagine what a few extra years of separation could have done for certain human populations…

Well...

>"Human races are evolving away from each other," Harpending says. "Genes are evolving fast in Europe, Asia and Africa, but almost all of these are unique to their continent of origin. We are getting less alike, not merging into a single, mixed humanity." He says that is happening because there has not been much flow of genes between the regions."

http://unews.utah.edu/old/p/120607-1.html