Even starting from an extremely mundane encounter list, it's a procedure that's guaranteed to lead to weirdness.
Evlyn uses the Labyrinth Lord "Forest/Wooded" Wilderness Encounter Table.
For fun, I thought I'd try it again, using the D&D 5e "Sylvan Forest Encounters" from the Dungeon Master Guide. (I've modified the list to remove the non-creature entries, and to separate entries where you would encounter 2 different creatures at once.)
Also for fun, I thought I'd see what would happen if two different islands separated off the same mainland.
Step 1: First we see what species survive on each island. Evlyn suggests that half the mainland die off or fail to migrate, and half survive on the island.
1 displacer beast
1 giant elk
1 elf druid
1 giant owl
1d4 blink dogs
Step 2: Next, we allow existing species to split off new species to fill the vacant ecological niches. Evlyn has a table to roll on to see which traits the new species "pick up" from convergent evolution into the niche, and any trait not "picked up" in this way should stay the same from the original species. 5e doesn't have Morale or Hoard Classes, but it does have official creature types and bolded descriptors used to organize the entry in the Monster Manual, so I'm going to use those instead.
Step 3: Third, we allow each of the surviving species to experience genetic drift, so that they chance from the mainland baseline.
1 displacer beast - dwarf variant (reduce HD/size, increase number)
1 dryad - giant variant (increase HD/size, reduce number)
1d4 satyrs - random characteristic variant (modify: descriptor)
2d4 pixies - dwarf variant (reduce HD/size, increase number)
1 owlbear - random characteristic variant (modify: appearance)
1 giant elk - random characteristic variant (modify: creature type)
1 elven druid - hyper variant (intensify: damage)
1 treant - hyper variant (intensify: saves)
1 unicorn - random characteristic variant (modify: saves)
2d4 hyenas - dwarf variant (reduce HD/size, increase number)
1 giant owl - giant variant (increase HD/size, reduce number)
1 dryad - giant variant (increase HD/size, reduce number)
1d4 satyrs - dwarf variant (reduce HD/size, increase number)
2d4 pixies - stunted variant (dilute: number)
2d4 sprites - dwarf variant (reduce HD/size, increase number)
1 owlbear - giant variant (increase HD/size, reduce number)
1d4 elks - dwarf variant (reduce HD/size, increase number)
1d4 blink dogs - hyper variant (intensify: HD/size)
Step 4: The final step is to put it all back together into an encounter list for each island.
A - THE ISLAND OF CATS, ELK, AND TREES
1d4 dwarf displacer beasts (as displacer beast, except: size medium, 10d8+20 hp)
1 gnoll-like unicorn (as unicorn, except: size medium, 5d8 hp, add Rampage ability, appearance "feral humanoid with one-horned horse head")
1 hyena-like treant (as treant, except: AC 11, attack bite +2 melee weapon (1d6 piercing damage), add Pack Tactics ability, appearance "huge fallen tree with knot-holes like spots, stalks on four limbs")
1 giant-owl-like treant (as treant, except: AC 12, save S+1 D+2 C+2 I-1 W+1 C+0, add Keen Hearing and Sight special ability)
1 giant dryad (as dryad, except: size large, 5d10+5 hp)
1d4 variant-descriptor satyrs (as satyrs except: remove Hedonistic Revelers descriptor, add descriptor Abstemious Perfectionists "satyrs spend long hours practicing and perfecting their music, forswearing any distractions or mind-altering substances, living only to prepare themselves for seasonal concerts which they carry off flawlessly")
1 centaur-like displacer beast (as displacer beast, except: alignment neutral good, creature type fey, speed 50 ft)
2d4 elf-scout-like giant elk (as giant elk, except: AC 13, size medium, 3d8+3 hp, add Scout descriptor, add Keen Hearing and Sight special ability, appearance "humanoid elk with antlers")
2d6 dwarf pixies (as pixies, except: 1d3-1 hp)
1 sprite-like giant elk (as giant elk, except: alignment neutral good, speed 10 ft / fly 40 ft, AC 15, size tiny, 1d4 hp)
1 variant-appearance owlbear (as owlbear, except: appearance "panther body, wooden face, lion's mane of leaves")
1 elk-like treant (as treant, except: AC 10, appearance "huge fallen tree with crown of antler-like branches, bounds on four limbs")
1 variant-creature-type giant elk (as giant elk, except: creature type plant)
1 blink-dog-like displacer beast (as displacer beast, except: alignment lawful good, attack bite +3 melee weapon (1d6+1 piercing damage), creature type fey)
1d4 faerie-dragon-like treants (as treants, except: alignment chaotic good, add Innate Spellcasting ability, appearance "huge fallen tree with pair of leafy wing-like branches and root tail, hops and flits about")
1 hyper-damage elven druid (as elven druid, except: all attacks increase dice by two sizes, quarterstaff deals 1d10+2, produce flame deals 1d12+2, shillelagh deals 1d12+2, thunderwave deals 2d12+4)
1 hyper-saving treant (as treant, except: save S+12 D-2 C+10 I+2 W+6 C+2)
1 variant-saving unicorn (as unicorn, except: save S+0 D+3 C+3 I+4 W+2 C+2)
B - FAERIE AND GIANT ISLAND
1 displacer-beast-like dryad (as dryad, except: alignment lawful evil, speed 40ft, AC 13, attack tentacle multiattack, two +6 melee weapons (each 1d4 bludgeoning, 1d8+4 bludgeoning with shillelagh), save S+4 D+2 C+3 I-2 W+1 C-1, appearance "woman with dark green skin, black leaves instead of hair, two legs, four arms, two leafy vine tentacles growing from her back, cruel laugh, glowing emerald eyes")
1d4 gnoll-like satyrs (as satyrs, except: speed 30 ft, attack bite +4 melee weapon (2d4+1 bludgeoning damage), attack spear +4 melee or ranged weapon (1d6+3 piercing damage), attack longbow +3 ranged weapon (1d6+3 piercing damage), add Rampage special ability)
2d6 dwarf hyenas (as hyenas, except: size small, 1d6 hp)
1 giant giant owl (as giant owl, except: size huge, 3d12+6 hp)
1 giant dryad (as dryad, except: size large 5d10+5 hp)
1d6 dwarf satyrs (as satyrs, except: size small, 7d6-7 hp)
1 centaur-like giant owl (as giant owl, except: Speed 50 ft, Attack: Talons +3 melee weapon attack (2d6+4 bludgeoning damage), creature type monstrosity)
2d4 elven-scout-like pixies (as pixies, except: attack multiattack, two shortswords +4 melee (each 1d6+2 piercing), two longbows +4 ranged (each 1d8+2 piercing), save S+0 D+2 C+1 I+0 W+1 C+0, creature type humanoid (elf), add Keen Hearing and Sight special ability)
1 stunted-number pixie (as pixie)
2d6 dwarf sprites (as sprites, except: 1d3-1 hp)
1 giant owlbear (as owlbear, except: size huge, 7d12+28 hp)
1d6 dwarf elk (as elk, except: size medium, 2d8 hp)
1 giant-elk-like sprite (as sprite, except: save S+4 D+3 C+2 I-2 W+2 C+0)
1d4 hyper-sized blink dogs (as blink dogs, except: size huge, 4d12+12 hp)
1d4 faerie-dragon-like hyenas (as hyenas, except: speed 10 ft / fly 60 ft, size tiny, 4d4+4 hp, attack deals 1 piercing damage, appearance "cat-sized hyenas with rainbow-hued fur and butterfly wings, they wear sharp-toothed grins and their tails twitch with merriment")
2d4 elf-druid-like sprites (as sprites, except: speed 30 ft, AC 11, size medium, 5d8+5 hp, attack deals 1d6 damage, add Spellcasting special ability)
1 treant-like giant owl (as giant owl, except: speed 30 ft, AC 16, size huge, 12d12+60 hp, appearance "a huge owl with feathers like green leaves, its face like a mask carved from wood")
1d4 unicorn-like blink dogs (as blink dogs, except: AC 12, attack multiattack, horn +7 melee (1d8 +4 magical piercing), paws +7 melee (2d6+4 magical bludgeoning), save S+4 D+2 C+2 I+0 W+3 C+3, creature type fey, add Divine Guardians descriptor, appearance "white furred dogs that twinkle like starlight as they blink in and out of existence, a single spiral horn grows from each of their foreheads")
Final Thoughts: There's always something meditative about solo procedural generation, but trying to do this for a 20-item list (twice!) is maybe pushing the boundaries of what's feasible as preparation. This would probably work best with a shorter initial encounter list. More cosmetic and fewer mechanical changes might actually affect the player experience more.
It might also be interesting to utilize something like this method for an island-hopping game where the players will get to see multiple alternate ecosystems - especially if they can see them without having to fight all of them.