Question

For hundreds of years, most people of European descent brought up on Martha’s Vineyard (Massachusetts) knew sign language. This lasted until the 1950’s, after which sign language use declined. Explain the factors underlying this historical trend in the context of key principles and processes related to population genetics and microevolution.

Answer 1

It is thought that a history of genetic relatedness among some parents, combined with intermarriage on the island, contributed to the spread of the genetic defect that caused vineyard deafness. Individuals with vineyard deafness used a highly developed sign language, probably based on a language brought from Kent.

It was caused by a recessively inherited genetic mutation that was traced to Kent county, England. Emigrants from that region, some of whom came specifically from an area called The Weald, settled on Martha's Vineyard in the late 17th century

One in every 155 people on Martha's Vineyard born deaf. ... Their deafness was believed to be the result of a recessive gene traced to Kent County in England, where some Martha's Vineyard settlers had originated in the late 17th century. Some came from an area in Kent called the Weald.

Certainly, having a large number of people with hearing loss motivated the people of Martha's Vineyard to improve communication opportunities for those who are deaf. Some censuses taken of the 19th century Vineyard population reveal the extent of deafness. In 1817, two families had deaf members, with a total of seven deaf. Just a few years later, by 1827, there were 11 deaf. The 1850 Chilmark census identified 17 deaf out of 141 households, in the Hammett, Lambert, Luce, Mayhew, Tilton, and West families.

In 1855, it was 17 plus four in nearby Tisbury. The 1880 Chilmark census had 19 deaf in 159 households. New deaf families in the 1880 census included the Nobles and the Smiths. To put this into perspective, compared to the mainland U.S. where the frequency of deafness was 1 in almost 6,000, on the Vineyard it was as high as 1 in 155 (1 in 25 in Chilmark, and 1 in 4 in the Chilmark village of Squibnocket

Gradual Decline in Deaf Population

Intermarriages persisted and the deaf population of Chilmark and the rest of the Vineyard continued to propagate. It would have kept growing if not for the growth of deaf education on the mainland. As deaf Vineyard children attended schools off-island, they tended to settle off-island and marry mainland mates.Gradually, the deaf population on the Vineyard declined. The last deaf Vineyard native passed away in the 1950s.