Tusk-less elephants are being born, just as they have been for a very long time. It’s likely a recessive trait that pops up in populations from time to time. The difference with now is that because poachers kill elephants for their tusks, there is a selective advantage for tusk-less elephants, and their relative rarity as a percentage of the total population is decreasing.
It works like this — let’s say 1% of the elephant population shows the tusk-less trait. The total elephant population is 100,000, and this has been rather steady for a long time. 1% of 100,000 is 1,000. So if there was no selective advantage to being tusk-less, that number would remain relatively constant.
OK, now we introduce poachers, who kill elephants for their tusks. They won’t bother killing tusk-less elephants. After a year of poaching, the current total population of elephants drops to 50,000. Since none of the tusk-less elephants were killed, their numbers are still 1,000 — let’s pretend that no elephants were born in this year just to keep it simple. So their population remains the same in absolute number, but relative to the whole population, their frequency has increased from 1% to 2%. Now factor in reproduction — the chances of the tusk-less elephants encountering one another and breeding together has increased. Repeat this for years and the relative frequency of tusk-less elephants in the population increases.