Who is black in America? Ethnic tensions flare between black Americans and black immigrants.

As soon as it was announced that filming would start for a Harriet Tubman biopic with British Nigerian actress Cynthia Erivo as the lead, a social media fury erupted.

An online appeal went up demanding that an African American woman be cast as Tubman, who, after escaping slavery, made more than a dozen trips to lead others to freedom on the Underground Railroad.

In the Change.org petition, which garnered 1,123 signatures by Oct. 17, organizer Tyler Holmes wrote: "We will boycott the film Harriet until you hire an actual black American actress to play the part."

Part of the anger directed at Erivo was that social media users unearthed an old tweet where Erivo had mocked a "ghetto American accent." Critics said she denigrated African Americans on one hand, but sought to portray an iconic African American hero on the other.

This came after a tangle in August when Nigerian-born blogger and author Luvvie Ajayi wrote that Tevin Campbell was too obscure a choice to sing at Aretha Franklin's funeral. "Under what rock did they pull that name from?" Ajayi quipped. The Twitter response was livid.

Such arguments, dubbed by some "the diaspora war," reveal more than preferences over movie roles and pop culture. The rancor provides a peek into a debate about identity in America, raising questions about how a changing black population — increasingly diverse with immigrants and refugees from Africa, the Caribbean, Britain, and elsewhere — sees itself and is seen by the majority.

Who is black in America? Can there be unity based on skin color alone? Who gets to speak for African Americans?

Although there is more nuance to the arguments, the sides often go like this: Black immigrants are respected more than black Americans, all the while benefiting from reparations meant to right evils of America's past. That's led to some black Americans redefining themselves as "American Descendants of Slavery" to spotlight their claim on America's promises. Meanwhile, immigrants discover they're newly identified as "black" in a white nation — an unnecessary distinction in Nigeria, Ghana, or Jamaica — and say that when pulled over by cops, no one cares whether they have a charming accent.

These identity issues are showing up at universities, during marches, and at theaters, and raise questions of whether these diverging groups can, or want to, build coalitions for political change.

We talked to a number of experts — immigrants and Americans — to help explain the origins of the tension and how the issue is playing out.

For decades, researchers have studied how universities are increasing the numbers of black students at majority-white colleges. But some of the current tensions between immigrants and African Americans can be traced to a theory that the nation's most selective universities have shifted away from racial-justice remedies — things like affirmative action that were put in place to right the wrongs of slavery and Jim Crow segregation — by using diversity as a goal instead.

A study published in the American Journal of Education in 2007 found that immigrants or children of immigrants, while making up 13 percent of the nation's black 18- and 19-year-olds — accounted for 41 percent of blacks admitted to Ivy League schools.

"If it's about getting black faces at Harvard, then you're doing fine," Mary C. Waters, the former chair of Harvard's sociology department, told the New York Times about a need for a philosophical discussion on affirmative action. "If it's about making up for 200 to 500 years of slavery in this country and its aftermath, then you're not doing well."

Compounding the tension is a fivefold increase in the black immigrant population in recent decades. There were 4.2 million black immigrants living in the United States in 2016, up from 816,000 in 1980, according to a Pew Research Center report. As more black immigrants experience success, they get what Fordham University professor Christina M. Greer calls "elevated minority status."

"Foreign-born blacks are often perceived by whites and even black Americans as different and 'special' — as harder-working and more productive citizens than their black American counterparts," Greer wrote in her book Black Ethnics: Race, Immigration, and the Pursuit of the American Dream.

It's a phenomenon that academics started noticing decades ago — that immigrants generally are "strivers" who work hard to better their lives.

It's not quite an apples-to-apples comparison, though.

Onoso Imoagene, a University of Pennsylvania sociologist born in Nigeria, who studies African immigrants and how they adapt to discrimination in America, said that although more than half of Nigerians in America are college-educated, just 7 percent of Nigerians living abroad have at least a bachelor's degree. So those who end up in the United States are the most educated — "a hyper-selected group," she said.

Even before immigrants come here, said Amy Yeboah, an assistant professor of African American studies at Howard University and the American-born daughter of Ghanian parents, they have an advantage American blacks often don't.

"If you are educated in Ghana, your level of education will be different from what you get in the Bronx," said Yeboah, who grew up in New York and earned her undergraduate and doctoral degrees from Temple University. "Students who apply from Ghana compared to those who are born here will do better, because they are prepared better."

Harvard professor Lani Guinier told the Washington Post that immigrants have an added benefit.

"In part, it has to do with coming from a country … where blacks were in the majority and did not experience the stigma that black children did in the United States," she said.

Immigrants are not oblivious to discrimination in their home countries, said Imoagene. It's just that those experiences haven't involved skin color.

"We have our own axes of stratification, when you think of ethnic lines [in Nigeria] — whether you are Yoruba or Igbo, or Christian or Muslim," she said. "Then you come here and find out you're [also] black, and have to learn the racial meanings attached to that status."

Some black Americans want to redefine themselves as an "American Descendant of Slavery," or ADOS, rather than African American.

Antonio Moore, a lawyer in California, and Yvette Carnell, a former journalist and congressional aide, appear to be leading the charge. The two make regular YouTube videos arguing that people whose ancestors were enslaved have a "justice claim" that black immigrants don't.

"We have been doing 'people of color' politics, but if you want to talk about what people who have been identified as African Americans need and what we are owed, then we have to change that definition." Carnell said.

On her videos, she has often criticized former President Barack Obama for saying this is a nation of immigrants. "We were not immigrants. We were property, we were chattel slaves. That's a difference."

Michelle Saahene's voice was heard around the world when she spoke in April at the Center City Starbucks where a manager called the police on two black American men because they asked to use the bathroom without placing an order.

"They didn't do anything," Saahene, of Philadelphia, said in the viral video of the incident.

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Raised in Central Pennsylvania, Saahene, now 32, said it was difficult for her to negotiate her racial identity as the daughter of immigrants growing up in Palmyra, a predominantly white town near Lebanon. Her teachers treated her well because she excelled in school. But at Pennsylvania State University and other places, she felt she got the cold shoulder from African American students.

Since those times, she has traveled back to Ghana. At Elmina Castle, where captured Africans were held before being taken on ships destined for the Americas, she wept.

"I … imagined what it was like to experience the torture, the rape and murder, and I looked out on the ocean and imagined being on a boat, sailing away, and I got sick to my stomach. When I got back to America, it was impossible for me to look at all African Americans and not see them as my possible brothers and sisters, neighbors and family and friends in Africa.

"To me, this feud between Africans and African Americans, it's terrible and it needs to stop."

Earlier this year, Rosita Johnson, a retired Philadelphia teacher, was honored by the South African government for her efforts starting in the '80s to support a school for children who fled to Tanzania after the Soweto protests of white rule.

At her Germantown home, the 86-year-old talked about the tensions between some Americans and Africans.  A fractured black population, she said, only helps those in power.

"It's a divide-and-conquer tactic," said Johnson, "because African Americans are Africans. These are our cousins. If you're African American, you're related to somebody over there. Unfortunately, because of slavery and colonization, all people of African descent have suffered from racism. I call it a mental illness."