John Endler was an evolutionary biologist who studied evolution under controlled conditions. Endler’s most famous studies involved guppy fish, and how their coloration would change based on various selective pressures in their environment.
Evolution is a process that passes on beneficial genes to the next generation to increase the success of a species. Evolution has produced the amazing biodiversity found on Earth today after billions of years. Evolution takes extremely long periods of time to develop new species, however scientists also study ‘micro-evolution’ which is changes in the frequency of genes in a population from generation to generation. This micro-evolution is what Endler set out to observe.
The Guppies that Endler studied, Poecilia reticulate, had spots on their bodies. Endler set out to determine if predators had an influence on the coloration of the guppies after as few as 15 generations. Endler set up environments with substrates of varying color and size at the bottom of pools. The substrates that he used were either fine-grained gravel, or course grained gravel.
He had a stock population of fish which he used to supply each experiments initial population with. He had initial populations placed into environments of both substrate types and with and without predators. Endler and his group hypothesized that environments with predators were going to cause the populations of guppies to begin to develop traits to allow them to blend into their surroundings more easily.
Environment 1 – Fine-grained with predator
Environment 2 – Fine-grained without predator
Environment 3 – Course-grained with predator
Environment 4 – Course-grained without predator
The fish began each experiment with an average relative spot size of 0.80. The spot size is relative to body size in order to have a way to compare spots on fish of differing sizes. The data in the table below shows the changes in spot size over the course of 15 generations in each environment:
|Relative Spot Size
|Relative Spot Size
|Environment 1 - Fine-grained with predator||0.80||0.62|
|Environment 2 - Fine-grained without predator||0.80||0.94|
|Environment 3 - Course-grained with predator||0.80||1.03|
|Environment 4 - Course-grained without predator||0.80||0.69|
Changes were easily seen after 15 generations. To the scientists surprise, when no predator was present, they spot size changed as well. Since no predator was present to influence the changes, they hypothesized that the changes may be attributed to mating; so another experiment was set up using the fish at the end of generation 15. Their results are below.
|Relative Change in Spot Size after 15 Generations||Coloration Comments in General||Number of Successful Matings in 24 hr Period|
|Environment 1 -
Fine-grained with predator
|Environment 2 -
Fine-grained without predator
|Environment 3 -
Course-grained with predator
|Environment 4 -
Course-grained without predator
What might change in the results in the substrates if the environments varied in color rather than gravel size?