Imagine being born as the third child of a devoted and attentive family but then celebrating your 50th birthday as one of hundreds of siblings claiming the attention of increasingly distracted and financially desperate parents.

This is roughly the position of BBC2, which reaches the half-century mark on Saturday . When the network arrived, on 20 April 1964, a television station was a special and treasurable thing. The BBC's single visual service was 28 years old and its ITV rival only nine. And although the new baby had a difficult delivery – a power cut on the first night wiped out everything except a short news bulletin – its early upbringing was privileged.

From 1967, it was given the gift of colour, then a technological novelty. Snooker, which became and remains a fixture in the BBC2 schedules, was chosen for showing because it is the sport in which different shades are most significant. That was one of the decisions made by David Attenborough, then controller of the network, whose own broadcasting personality – reflecting intelligence and culture – continued to define BBC2's brand long after he left to talk to animals.

Civilisation, the Kenneth Clark chronicle of western art that is regarded as a benchmark of public service TV, was also broadly the word with which the channel became associated. Viewers knew BBC2 as a place to find high-end documentary (The Great War, Molly Dineen's The Ark, Adam Curtis's The Century of the Self); arty chat (from Late Night Line-Up to The Late Show); and classy drama in a line that stretches from Boys from the Blackstuff and Edge of Darkness in the 1980s to the recent Parade's End and Line of Duty.

The fact that Line of Duty is ranked among the best TV fiction for years suggests there is no crisis with the channel. There is, though, a problem that has been created by two rival broadcasters with a numerical value double its own. First, Channel 4, a commercial network with a public service remit, challenged the BBC's second child as the place where edgier material – and younger audiences – went. And, subsequently, when the BBC itself counted up to four, a damaging perception developed that BBC4 had become a safe haven for the sort of cultural stuff that BBC2 no longer wanted to do, and the former home of Civilisation packed itself with cookery and gardening series. Though exaggerated, this slur has been a burden for recent controllers, along with an increasingly doddery demographic among the audience.

Reviewers and viewers have struggled to compute the value of BBC2. Breakout shows – Have I Got News for You, The Apprentice and The Great British Bake Off – have routinely been lifted from the channel to its older sister because their ratings were so high. And yet BBC1 franchises that deliver secondary ratings – such as Question Time and Imagine – have not been sent in the opposite direction because of a perception that some minority topics, such as arts and politics, should be represented on a mass audience channel.

Evidence of this identity crisis is that, although the BBC director general, Tony Hall, recently announced a new version of Civilisation, the remake may find itself under pressure to go out on BBC1 to lend cultural prestige to that network.

The task for Kim Shillinglaw, a distinguished factual programme maker who last Friday was announced as the 13th controller of BBC2, is to avoid the channel becoming caught in a trap in which its lighter pieces migrate to BBC1 while the heavier ones go out on BBC4. Promisingly, she has a connection with Attenborough, having worked on one of his later series. But he ran the channel when it was one of three, while she has to make it stand out among hundreds.

Fifty years after it began by literally losing power, BBC2 has metaphorically done so.