Near-shore island lizard fauna shaped by a combination of human-mediated and natural dispersal

Aim: Island biotas provide opportunities to study colonization and
adaptation to novel environments. Islands, especially near-shore islands,
may have a long record of human habitation such that some lineages result
from human-assisted introductions. Here, we combine phylogenetic analyses
with fossil data and historical specimen records to reconstruct
colonization histories, characterize among-island divergence, and assess
the role of humans in shaping the evolutionary history of lizards
inhabiting a near-shore island archipelago. Location: Channel Islands and
adjacent mainland of California, USA. Taxa: Western fence lizard (
Sceloporus occidentalis), southern alligator lizard (Elgaria
multicarinata), common side-blotched lizard (Uta stansburiana). Methods:
We sequenced mitochondrial DNA (ND1, cyt-b) from each of three lizard
species, covering their entire island distributions plus the adjacent
mainland. For each, we estimated diversity within and among each island,
obtained maximum likelihood bootstrapped phylogenies, constructed
haplotype networks, and tested for population expansion. We used museum
specimen records and microfossil evidence to infer colonization scenarios.
Results: Sceloporus occidentalis is characterized by a single
island-colonization event, and exhibits the deepest divergences from
mainland relatives and the highest among-island divergence. Elgaria
multicarinata and Uta stansburiana each have at least three distinct
colonization events, with fossil and historical data indicating that some
of these occurred after humans arrived to the islands. Main Conclusions:
The evolution of Channel Island lineages for two lizard taxa has been
mediated by ancient and contemporary anthropogenic activity, while the
evolution of the third is shaped by natural dispersal and vicariance
caused by sea-level rise. Genetic divergence corroborates the treatment of
S. occidentalis as an endemic island species, S. becki. The unique
histories of these three taxa are synthesized with other Channel Island
lineages highlighting that taxa inhabiting islands with long histories of
human activity should be carefully studied to assess the role of people in
facilitating colonization and subsequent gene flow.