Variety Is The Spice Of Life…. Literally…

 “Variety’s the very spice of life…” was a quotation from the English poet William Cowper (1731–1800) in The Task (1785). For biologists whose specialty is “Conservation Biology” it means that maintaining genetic diversity in a species is in the best interests of the species’ long-term survival. Genetic diversity in turn means maintaining many different “alleles” or versions of genes (previous blog) in a species’ population and in the populations of that species throughout the species’ geographic range. The theory is that a species evolves or is adapted to subtle differences in its environment throughout its geographic range and that these adaptions are made possible by equally subtle changes in its genome (previous blog). That is, the genetic modifications fine tune the species to different conditions dictated by differences between the habitats throughout its geographic distribution.

Biodiversity vs Genetic Bottleneck.

Widescale habitat loss (e.g., deforestation of the Amazon basin) leading to fewer total number of different species is referred to as loss of biodiversity. Loss of a single species’ populations is termed “genetic bottlenecking” or a “genetic bottleneck” when the losses constitutes a large proportion of the species.

The two most common ways that a species experiences loss of genetic diversity due to humans are: 1) over-exploitation and the resultant reduction in the species abundance, and 2) habitat fragmentation which results in isolated populations which prevents genetic exchange between adjacent populations. Natural means of loss of genetic are founder populations and isolation by means of geologic processes.

Two Extreme Examples Of A Genetic Bottleneck By Over-Exploitation: The Northern Elephant Seal and The American Bison

The Northern Elephant Seal was hunted beginning in the 1700’s for their oil-rich blubber, which was used to make oil for lamps and lubrication. It was thought extinct in the late 1800’s but about 100 elephant seals remained on Guadalupe Island off the coast of Baja California in 1892. This small residual population is the source of all Northern Elephant Seals (see map below). All the allele diversity which was present in the historic populations up the North American coast were lost. Those alleles are gone forever. Today, the northern elephant seal population has rebounded to approximately the size it was before hunting. It’s estimated that there are about 150,000 individuals all derived from the 100 which survived on Guadalupe Island. The Northern Elephant Seal is an example of a highly bottlenecked species. 

The American Bison – The American Bison. Bison once ranged from Florida to Maine and west to the Rocky Mountains of North America. Several sub-species existed including a slightly smaller woodland bison here in the Eastern U.S. The Bison was extirpated from the east and nearly extirpated from the Great Plains and western U.S. No exact population counts were made but most experts agree that there probably were 30 million bison before European settlement and westward expansion.  By 1900 the bison was nearly extinct. Thirty million Bison to only 325 in 1884 to 500,000 today. There may be 500,000 Bison but genetically speaking, the effective size is much smaller. 

There are formulas which Conservation Biologists use to calculate the “effective population size” based upon the “bottleneck” (e.g., the mid to late 1800’s for the American Bison. The roughly 5,000 bison roaming the Yellowstone National Park today are descendants of just two dozen individuals that found a haven in Yellowstone’s rugged interior. 

In 2015, a purebred herd of 350 individuals was identified on public lands in the Henry Mountains of southern Utah via genetic testing of mitochondrial and nuclear DNA. Efforts are being made to diversify the genomes of the Bison’s fragmented populations. The American Bison is genetically bottlenecked. We will never get the Eastern Woodland Bison of the Southeast U.S. back.  The European Bison (the Wisent) is also bottlenecked genetically.  Historically, the European bison’s range encompassed almost all of Europe, including southern England, and Russia. 

African Mega Fauna – A genetic bottleneck also occurs when a species’ populations become fragmented. This occurs because of the physical distances between populations.  Alleles can’t be shared between fragmented populations because individuals at the edges of populations can no longer breed with adjacent populations. The African Lion is a good example of a highly fragmented species. It once roamed over most of sub-Saharan African but now is highly fragmented. Of course other large mammals (e.g., African Elephant) in Africa have experienced the same population fragmentation.

Founder Populations – When a small subset of a species migrates to an area which it previously did not occur in and establishes itself there the migrants take a small subset of the allele diversity present in the source population. This can create a genetic bottleneck and can drive evolutionary change. It is believed that birds that populated the remote Galapagos Islands were probably “founders” from mainland South America and that they subsequently took unique evolutionary paths. Population bottlenecks are believed to lead to rapid changes in gene/allele frequencies through genetic drift, to facilitate rapid emergence of new phenotypes, and to enhance reproductive isolation. For such effects to occur, founding populations must be very small.

The Devil’s Hole Pupfish – In the desert southwest near Death Valley (California/Nevada) a unique example of fish evolution by means of isolation exists. During a wetter period in the geologic history of North America related to glaciation large inland seas existed in what is now Death Valley. One sea was “Lake Manly” and fossils show a single species of small fish in the genus Cyprinodon.  Over 10’s of thousands of years the area became arid and the inland seas turned into dry “flats”, small streams and isolated pools fed by groundwater. The most extreme example is Devil’s Hole where there is now one unique species, the Devil’s Hole Pupfish (Cyprinodon diabolis) . The pool it lives in (Devils Hole) is about 72 ft long by 11.5 ft wide. The fish feed on a small shelf with algae growing on it because the shelfs exposure to sunlight for part of each day.  The entire population of this species is about 200 and fluctuates. For decades depletion of groundwater from the aquifer has threatened  this species with extinction.

Addendum For Genetics Students: Computing Effective Population Size, Ne.

Calculating Effective Population Size Ne:  

Effective Population Size = Number of Generations / Sum of ( I /N_i ) 

where Ni = Population Size for ith generation.

Number of Generations in Data / SUM (1/Ni) 

where Ni is population size in ith generation

Using this formula for the following data. Suppose a population of animals went from 1000 to just 10 and then recovered to 2000 in span of five generations (e.g., 1000, 10, 100,1000, 2000).

Ne = 5 / [ 1/1000 + 1/10 + 1/100 + 1/1000 + 1/2000 ]

Ne = 44

Work Problem:

The Norway Beaver was extirpated from most of its range just as the American Beaver was.

The remnant population of beaver in Norway, for example, grew from a precariously low 100 or so individuals in 1880 to 7,000 by the 1930s; not only providing ample numbers to spread across the country, but also for reintroductions into neighboring Sweden and nearby Finland.

Presently an estimated 70,000.

1880, 100 beaver
1930, 7000 beaver
1960, 30000 beaver
2000, 70000 beaver

Effective Population Size =

ANSWER: N=4/(1/100 + 1/7000 + 1/30000 + 1/70000) = 392