In 2003, scientists announced the discovery of the fossil remains of some extremely short early humans on the Indonesian island of Flores.

Question

In 2003, scientists announced the discovery of the fossil remains of some extremely short early humans on the Indonesian island of Flores.

The new species had been named Homo floresiensis (Nickname: the Hobbit). One hypothesis is that H. floresiensis evolved from Homo erectus, another early human species. It is possible that early H. erectus populations constructed boats and rafts, so modern scientists speculate that they were blown far off course by strong winds during a storm and colonized the small Island of Flores were they speciated into H. floresiensis.

Data shows, that H. erectus averaged almost 6 feet in height, but the remains show that adults of H. floresiensis were only about 3 feet tall.

Please propose two hypothetical natural selection factors/pressures that could have been exerted on H. erectus on the island of Flores so that succeeding generations began to shrink in size.  The Island of Flores is hot and humid, and only 31 square miles small.

Explain how these two natural selection pressures could have caused populations of H. floresiensis to get smaller over time. 

You must propose two environmental factors and their possible effect for full credit.

Use the following structure to answer your question:

1A. Name the first possible natural selection factor/pressure _______________________

1B. State how this natural selection factor/pressure could have led to the observed change in height

 

2A. Name the second possible natural selection factor/pressure__________________________

2B. State how this natural selection factor/pressure could have led to the observed change in height

Answer 1

One can imagine a number of possible new environmental pressures that could have selected for smaller stature as Homo erectus populations migrated to the island of Flores. The phenomenon of Island (sometimes called Insular) dwarfism can be observed in many island species that are closely related to larger mainland counterparts. While the exact mechanism for this pattern of speciation is not certain, there are several often-proposed hypothesis that could be used to explain the smaller stature of Homo floresiensis. I will discuss them in the format provided in the above question:

1A (first possible environmental pressure): A small, geographically isolated island environment would likely not have been able to support the same number of large predators as Homo erectus populations experienced in their prior mainland environment.

1B (explanation of first environmental pressure): A relatively high predation pressure on the mainland environment could conceivably select for Homo erectus individuals that were larger in stature. These larger individuals would have been able to defend themselves more effectively against predation, or perhaps were simply less prone to become targets of predation. However when Homo erectus arrived on Flores, they may have encountered significantly fewer predators, and therefore those individuals previously selected for because of their comparatively large stature would no longer have necessarily experienced a greater likelihood for producing offspring (i.e. avoiding being eaten long enough to mate). This could open the door for genetic drift towards smaller stature (in the eventual Homo floresiensis population), or other environmental factors may have rewarded smaller stature, which brings me to our second pressure...

2A (second possible environmental pressure): In a new island environment, food resources may have been scarce compared to the previous mainland environment.

2B (explanation of second environmental pressure): Whether there were simply less of the same food resources that Homo erectus was adapted to exploiting on the mainland, or Homo erectus was less adapted to exploiting the new food resources of the island environment, we can say the new transplant population may have had less calories per capita available to them when they moved locations. As such, individuals with big, energetically demanding bodies may have suddenly been put at a reproductive disadvantage as they were no longer able to find enough calories to support themselves. In this environment of food scarcity, Individuals that just happen to have been smaller may have enjoyed the competitive advantage of being able to provide ample calories for their smaller, less-energetically-demanding bodies. Those small individuals with full bellies would then be more likely to reach a reproductively active age (by avoiding starving), and have access to the additional calories necessary to engage in the energetically taxing task of reproduction. In this scenario, the population would then eventually trend towards smaller individuals as genes for smallness proliferate in the newly forming Homo floresiensis popluation.