Evolutionary Trends and Why

Trends in evolution are not like the trends we speak of in every day life, where the trend pops up and disappears just as swiftly as it appeared. When regarding evolutionary trends, these are considered change to a lineage or multiple lineages over a period time, these changes improve or remove characteristics of a species for specific reason regarding environment.

I like to think of amphibians when considering evolutionary trends. Amphibians are some of the most diverse creatures on planet earth and continue to provide excellent examples of evolution. Amphibians have evolved to live in many diverse areas but one trend all amphibians have kept is moist skin. This trend is a valuable factor of their evolution, whether it is a tree frog or a salamander all amphibians require water for respiration, but why is this important? 

The development of amphibians begins in water; like fish they have gills when hatched but throughout development the gills disappear and lung fuction is a main source of gas exchange. However, amphibians never retained a 4 chamber heart for effective gas exchange nor did they develop an effective diaphragm; both of these factors have made gas exchange (breathing) a high energy loss task in amphibians. In return, evolution or there lack of has allowed amphibians the ability to exchange gas by not only lung function but also oxygen exchange through their skin! But this type of gas exchange can only happen if their skin remains wet and so a trend among amphibians begun and has yet to end.