X-Men #1 (1963)
Same as that of the predominant society in which they inhabit.
Various and unpredicatable.
While records of Mutants on Earth appear as early as 5,000 years ago (with the emergence of Apocalypse in Egypt) the Mutant population did not grow to significant numbers until the early 20th century. Mutant births, however, began to increase throughout the 20th century (presumably due to environmental factors related to industrialization). In 1920 one of the most prominent mutants was born: Namor the Sub-Mariner, who is considered by many to be the first modern-day Mutant. The Second World War saw the emergence of Mutants as soldiers as several of the species including Toro, the Sub-Mariner, and Wolverine fought in Allied or Axis armies. The World War II era also saw the beginnings of programs which used mutants as genetic soldiers, such as the Weapon Plus program. Mutant births steeped upwards after the end of the war (following the atomic blasts in Hiroshima and Nagasaki).
As Mutant populations grew, human governments and populations reacted harshly to them, and the Mutants began to form widely divergent ideas about how best to live within the larger human society. Mutants broke off in to two rough camps. Professor Charles Xavier articulated and epitomized the ideal that humans and mutants can live together in harmony. Seeking to help his fellow mutants and strengthen the relationship between humans and mutants, he created the Xavier Institute For Higher Learning, a school and safe haven for mutants. He also created a team of mutants called the X-Men who would help protect and recruit mutants while at the same time protecting the people who feared them. Another mutant, a Holocaust survivor named Eric Lensherr, foresaw a war between humans and mutants. Styling himself Magneto, he prepared for that coming war by creating the Brotherhood of Evil Mutants. The ideals of Professor X and Magneto clashed (as did various incarnations of their respective teams) as each tried to prove their points to one another and to the world.
As the years passed many mutants joined both Magneto and Xavier's side, and the mutant population began truly to boom. Neither Xavier's nor Magneto's predictions came to pass: the world never entirely accepted mutants, but neither did outright war between mainstream humans and mutants break out. With some difficulty, Magneto was able to establish a Mutant homeland on the island-nation of Genosha, off the coast of Africa. With a species-population numbering in the millions and powers beyond that of their genetic peers, war or no war, it appeared that over time, mutants would eventually supplant humans as the dominant species on Earth.
This dominance came to an abrupt end when two shocks occurred in quick succession. First, the parasitic twin of Xavier, Cassandra Nova caused the extermination of over 16 million mutants worldwide and the utter annihilation of Genosha. Soon after that, Magneto's children Scarlet Witch and Quicksilver conspired to alter reality to ensure Mutant ascendancy (and protect the Scarlet Witch's life). When the Scarlet Witch reverted reality back to it's normal course, (uttering the words "No More Mutants") many former mutants had been de-powered and were now normal human beings. The mutant population is now estimated to be in the hundreds, but a full worldwide census has not been taken.There have been no Mutant births since M-Day.