Several mechanisms are known for generating new species, but studying these processes in the ocean is much more difficult than on land because such processes are frequently not apparent. The work presented here aims to better understand how the movement and changing shape of continents and oceans affect living organisms over time. The Gulf of California is only about 6 million years old and is separated from the Pacific Ocean by the Baja California peninsula. This work examines how fishes originally colonized the Gulf, whether in the past a seaway once connected presently separated populations, and whether currently separated fish populations are becoming different species today. First, their DNA is studied to reveal the relationships between these populations and reveal their past migratory events. Second, the rocks on the peninsula are examined because they reflect the region?s past environment and would indicate whether or not some of the land was under water in the past.
Formation of new species is a principle contributor to biodiversity across the world, and Baja California has a greater number and diversity of species than expected from its size and location alone. The size, location, and climatic characteristics of estuarine habitat are always changing, but on too long a timescale for scientists to observe. This molecular work will help understand what processes have generated and are maintaining southern and Baja California?s diverse estuarine life. Disseminating information from this work to high school and undergraduate students (through mentorships) and to residents (through educational YouTube videos) promotes conservation awareness. Working with authorities like the NFWS and NERRS helps focus conservation efforts not only on regions that are diverse today, but that are likely to preserve and generate new species in the future.
Environmental change over time imparts a first-order influence on how species are distributed and how populations within species are isolated, reconnected and evolve through time. The geographic patterns within genetic markers can reveal this history. Though understanding the sequence of events that produced these genetic patterns is important to understand and a common goal, identifying such influences are often difficult. In this context we chose to examine fishes in estuaries along the Pacific coast. These estuaries are isolated from one another, limiting migration (and therefore gene exchange) between them, providing an ideal system for the genetic study of population evolution. We used a two-step approach. First we developed a model that predicted estuarine habitat area through time since the last ice age, then we examined genetic data from fishes living in this region today to infer the history of their gene exchange. Estuaries in which these fishes live can only form near sea level where the shore is not steep, as evidenced by modern estuaries. To understand the location and size of estaurine habitat, we used the well-established sea-level history since the last ice age 20,000 years ago when sea-level was about 130 meters lower than it is today due to glaciers on land. As sea-level rose it flooded a landscape which is now underwater. By understanding the slope of this underwater landscape using detailed satellite imagery and the sea level history, we were able to determine where and when estuary habitat formed along the Pacific coast on a millennial timescale from 20,000 years ago to preset. We found that during the last ice age, estuarine habitat was reduced to three sites along the coast: one in the mid-California coast north of Point Conception, a second in the Viscaíno region in the middle of the Baja California (Mexico) coast and a third in Magdalena Bay, along the southern coast of Baja California. The more extensive modern distribution of estuary habitat formed between 15 – 10 thousand years ago as glaciers melted and sea levels rose onto flatter shelf areas. Our genetic analyses reveal that once these habitats formed, migration between previously isolated habitats became possible and genetic markers associated with the isolated populations during glacial times rapidly mixed across this newly formed habitat. Once sea level stopped increasing about 7,000 years ago, wave power and increased storm activity converted these habitats to smaller, more closed lagoon settings, which support different types of species. Estuaries provide natural water filtration, recycle nutrients, mitigate storm damage, and act as nurseries for the larvae of commercially and recreationally important coastal and marine fish species. Therefore, estuaries are a target for restoration and conservation in California. Our results reveal the dynamic nature of estuaries over millennial and sub-millennial timescales. They indicate that restoring an estuary to its natural state does not imply maintaining it in a static state, like dredging the mouth to keep it open to the sea, but rather letting the system change over time. Native species, like the fishes we studied here, seem to be adapted to tremendous variation in habitat availability over time as demonstrated in our models. Therefore, stakeholders might benefit from a shift in the restoration mindset from preserving a specific type of habitat, to preserving natural processes that may provide different types of habitat over time based on freshwater input from rivers and oceanic processes along the coast.
