In The Chrysalids, what are the events that cause David to have any character changes within the novel?

Certainly Chapter 1 when David befriends Sophie and
catches a glimpse of her feet causes the child David to find a discrepancy between what
he has been taught and the truth as he experiences it. Consider how he thinks about it
at the end of Chapter 1:

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Clearly there must be a mistake somewhere.
Surely having one very small toe extra - well, two very small toes, becuase I supposed
there would be one to match on the other foot - surely that couldn't be enough to make
her "hateful in the sight of God...?"

The ways of the world
were very puzzling.

David
begins to question and secretly doubt the rules and culture of his world. Of course, his
befriending of Sophie and the discovery of her genetic "deviation" foreshadows his own
discovery.

Chapter 4, with Uncle Axel's stern insistence
that David and the others promise never to talk about their "gift" to others represents
another milestone in David's development:

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There had been no acknowledged, co-operative
policy among us. It was simply as individuals that we had all taken the same
self-protective secretive course. But now, out of Uncle Axel's anxious insistence on my
promise, the feeling of a threat was strengthened. It was still shapeless to us, but it
was more real.

As David goes
on to comment, this decision they make together was what began their collective
consciousness as a group - "it made us into a group." From this point on, the group
thinks and acts as a group, working together (except of course when one of them chooses
to marry outside of the group).

The discovery of Sophie and
her fleeing and capture, combined with the suicide of David's Aunt Harriet and
presumably the death of her baby in Chapter 7 equally serves to reveal to David what is
at stake if their "gift" becomes known. As he comments, "And I went on being very
frightened." David even prays to be "normal".

When Anne
chooses to marry outside of the group and in Chapter 10 is found hung after the death of
her husband, it is clear that this fear they have of discovery is heightened. It was
just by chance that Rachel managed to obtain Anne's note of confession and thus saved
the group from discovery. The group live in a dangerous world where at any moment they
could be found out with horrifying consequences, as displayed by Sophie and Aunt
Harriet. These really are the key events that form David Storm and prepare us for the
group's discovery of Petra, which of course equally destroys the group and gives some of
them hope for the future.