How did Tolkien model diachronic changes in his Elvish languages?

Question

Wikipedia explains that Tolkien modelled changes to his Elvish languages, something we'd call diachronic conlanging these days:

Tolkien conceived a family tree of Elvish languages, all descending from a common ancestor called Primitive Quendian. He worked extensively on how the languages diverged from Primitive Quendian over time, in phonology and grammar, in imitation of the development of real language families.

What is a summary of the major changes he modelled? For example, did he describe something akin to the Great Vowel Shift?

Answer 1

OK, here are some details on classical Quenya and classical Sindarin (based on Helmut W. Pesch, Das große Elbisch-Buch, Bastei-Lübbe 2009)

The phoneme inventory of Primitive Quendian was

p   t   k
pʰ  tʰ  kʰ
b   d   g
m   n   ŋ
    r,l
        ɣ
w     j
a, e, i, o, u, ai, oi, ui, au, eu, iu

An early addition to this repertoire were prenasalised consonants mb, nd, ŋg that developped differently in Quenya and Sindarin.

Sound shifts for Quenya

pʰ   tʰ   kʰ  -> f   θ   x  (later: θ -> s)
b-   d-   g-  -> v-  l-  Ø
-b-  -d-  -g- -> -mb- -nd- -ŋg-
mb-  nd-  ŋg- -> m-  n-  ŋ-   (later: ŋ- -> n-)

The loss of almost all b's, d's, and g's makes Quenya phonological similar to the Finnish language.

Quenya has some old compounds that preserve the pre-shifted state of a root, e.g., for dome > lome "night, darkness" there is an old compund tindome and a newer regular one Aldalome. This adds to the diachronical depth of the language.

Sound shifts for Sindarin

p    t    k   -> b   d   g
pʰ   tʰ   kʰ  -> v   ð   ɣ (later: ɣ -> Ø)
mb-  nd-  ŋg- -> b-  d-  g- 
kʷ  -> p
w-  -> gw-

Sindarin also has a lot of lenitations (i.e., in certain contexts the initial consonsonant of a word is changed to some "softer" consonant) inspired by Welsh. The lenitations are different for original b d g and b d g that come from mb nd ŋg. Sindarin also has acquired an umlaut y.

EDIT 3: Evolution of initial sp-, st-, and sk-

Both in Sindarin and quenya, consonant clusters with an initial s were simplified in two steps: First, the stop became aspirated and the s was dropped

 sp st sk -> pʰ tʰ kʰ

and than the aspirated stops became voiceless fricatives

 pʰ tʰ kʰ -> f θ h

In Quenya, also the so-created θ's became s later, such that the net development of Quenya is

  st -> s

EDIT 1: Some more information gleened from the dictionary

Quenya Words beginning with h- derive from Primitive Quendian roots beginning in kʰ-. The initial consonant cluster stʰ was simplified via θ to s.

Some words beginning with ç- like hyarmen "south; left side" come from roots beginning in kʰj.

Sindarin has simplified some consonant clusters like kl- kr- via intermediate voiceless l and r but in classical Sindarin those l's and r's are the usual voiced ones (despite being retained in orthography as lh rh).

EDIT 2: The grammatical changes aren't explained in great detail. It seems that Tolkien applied some handwavium here when he states that Quenya was consciously changed for greater clarity. On the other hand, Sindarin has evolved naturally (in Tolkiens internal history) in the woodlands of Middle Earth where everything is interwoven with everything.

Answer 2

I don't know the exact changes Tolkien enacted, but here are a few in no particular order. You can tell they are quite inspired by real world sound changes in Celtic languages, which I'll mark in brackets. A good summary for quenya can be found here.

• Aspirates /pʰ tʰ kʰ/ become fricatives /ɸ θ x/
• Lots of syncope, especially the second vowel in trisyllabic words
• In Quenya /kʷ/ is retained (Irish), while in Sindarin it becomes /p/ (Welsh)
• Vowels are reduced or lost finally, especially in Sindarin (Welsh)
• Sindarin features i-affection (Welsh)
• Various forms of lenition (Irish, Welsh)
• Plenty of assimilation in clusters
• Quenya features some palatalization with y /j/
• Syllabic consonants become non-syllabic
• Some sounds such as /ɣ/ and /ŋ/ are lost or merge with other sounds
• In Quenya usually /w/ > /v/, while in Sindarin initial /w/ > /gw/ (Welsh)

Hope this answers your question somewhat.

Answer 3

What is a summary of the major changes he modelled? For example, did he describe something akin to the Great Vowel Shift?

For Sindarin, there are some big changes in vowels comparable to the Great Vowel Shift.

For example, under certain circumstances and in different periods:
á > au
au > o

That's one of the reasons why Sindarin looks quite different from Quenya sometimes. e.g.
Q. - S.
náre - naur
anar - anor

If you want a more detailed "summary", or rather "study", David Salo spent some forty pages to analyze Sindarin's historical sound changes in his book: a Gateway to Sindarin. And here's an essay by Helge K. Fauskanger discussing the Evolution of Quenya.

Or if you want the description from Tolkien himself, you can get Parma Eldalamberon #19, where you can find big phonetic tables by Tolkien.

The biggest Elvish dictionaries made by Tolkien himself are organized by roots - Words in different Elven languages are listed under the proto root from which they are derived. Here's an example from the Etymologies.

KHER rule, govern, possess. Q[uenya]. heru master, heri lady ... O[ld ]N[oldorin] khéro master ... N[oldorin]. hîr. N. herth house-hold ...