Divergent evolution in animals

Creation, definition, and usage

The term divergent evolution is believed to have been first used by J. T. Gulick. Divergent evolution is commonly defined as what occurs when two groups of the same species evolve different traits within those groups in order to accommodate for differing environmental and social pressures. Various examples of such pressures can include predation, food supplies, and competition for mates. For example, the tympanal ears of certain nocturnal insects are believed to be a result of needing the ultrasonic hearing that tympanal ears provide in order to hear predators in the dark.^[1][2] Non-nocturnal insects - that do not need to fear nocturnal predators - are often found to lack these tympanal ears.

Causes

Animals undergo divergent evolution for a number of reasons. Predators or their absence, changes in the environment, and the time at which certain animals are most active are chief among them.

Predators

A lack of predators – predatory birds and mammals - for cliff-side nest residing kittiwake caused that particular group of kittiwake to lose their ancestral mobbing behavior that had been exhibited up until that point for protecting young.^[3] The mobbing behavior normally displayed by the kittiwake is lost when the kittiwake take residence in this area with little threat from predators towards their young. The mobbing behavior was originally developed to protect ground-level nests containing young from various predators such as reptiles, mammals and other birds.^[4]

Environment

The cliff-side nesting area itself was similarly responsible for the kittiwakes losing their mobbing mentality – predatory mammals small enough to fit on the cliff edges along with the kittiwakes and their offspring would not be able to make the climb up while predatory birds would not be able to maneuver near the cliff face while also being afflicted by the weather conditions of the area.^[5]

Distinctions

Divergent evolution is always coupled with convergent evolution, as they are both similar and different in various facets such as whether something evolves, what evolves, and why it evolves. It is instructive to compare divergent evolution with both convergent and parallel evolution.

Divergent versus convergent evolution

Convergent evolution is defined as a similar trait evolution that occurs in two otherwise different species of animal as a result of those two species living in similar environments with similar environmental pressures like predators and food supply. It differs from divergent evolution in that the species involved are different while the traits they obtain do not differ from each other. An example of convergent evolution is the development of horns in various species for sparring over mates, resources, and territory ^[6]

Divergent versus parallel evolution

Parallel evolution is the development of a similar trait in species descending from the same ancestor. It is similar to divergent evolution in that the species descend from the same ancestor, but it differs in that the trait is the same while in divergent evolution the trait is not. An example of parallel evolution are certain arboreal frog species, 'flying' frogs, in both Old World families and New World families having developed the ability of gliding flight. They have "enlarged hands and feet, full webbing between all fingers and toes, lateral skin flaps on the arms and legs, and reduced weight per snout-vent length".^[7]

Darwin’s Finches

One of the most famous examples of divergent evolution is the case of Darwin's Finches. During Darwin’s travels to the Galapagos Islands he discovered several different species of finch that shared a common ancestor. They lived on varying diets and had beaks that differed in shape and size reflecting their diet. The change in beak shape and size was believed to be a result of the lengths the birds had to go to in order to support their change in diet. Some Galapagos finches have beaks that are larger and more powerful to crack nuts with. A different type allows the bird to use cactus spines to spear insects in the bark of trees.

Divergent evolution in dogs

Another good example of divergent evolution is the domesticated dog and the wolf. Dogs and wolves both diverged from a common ancestor called the Grey Wolf. Dogs and wolves diverged in many places all over the world, each their own separate occasion, rather than just one single isolated event.^[8] Then through artificial selection, dogs diverged from the Grey Wolves into the domesticated dog that is known today.^[9] To further support divergent evolution of dogs and wolves, genomic research was conducted to compare mitochondrial DNA to indicate the presence of shared ancestry. Taking 162 wolves from various parts of the world as well as 140 dogs of 60 different breeds, it is found that dogs and wolves have shared ancestry by how similar their DNA sequences are.^[10] On a macro scale, to support shared ancestry, comparison of the physical characteristics of each animal can be conducted. Dogs and wolves have similar body shape, skull size, and limb formation, further supporting identical genetic makeup and thus shared ancestry.^[11] An example of this would be how physically and behaviorally similar malamutes and huskies are to wolves. Huskies and malamutes have very similar body size and skull shape. Huskies and wolves share similar coat patterns as well as tolerance to cold. In the hypothetical situations, mutations and breeding events were simulated to show the progression of the wolf behavior over ten generations. The results concluded that even though the last generation of the wolves were more docile and less aggressive, the temperament of the wolves fluctuated greatly from one generation to the next.^[12] Although it can be concluded that evolution may not be happening based on the amount of generations tested, the continuation of generations will become fixated and later support the hypothesis that wolves became more docile to a point where human interaction was capable of existing. By concluding that dogs and wolves are extremely similar physically, behaviorally, and genetically, it is evident that millions of years ago, dogs diverged from the common ancestor called the Grey Wolf and that also dogs and wolves are subject to divergent evolution.

References

1. Yack, J.E.; J.H. Fullard (April 2000). "Flapping Ears". Current Biology. 10 (7): R257. doi:10.1016/s0960-9822(00)00412-7.
2. Yack, J.E.; J.W. Dawson (2008). "Insect Ears". 3: 35–53.
3. Cullen, Esther (April 2008). "Adaptations in the kittiwake to cliff-nesting". Ibis. 99 (2): 275–302. doi:10.1111/j.1474-919x.1957.tb01950.x.
4. Alcock, John (2013). Animal Behavior: An Evolutionary Approach, Tenth Edition. pp. 101–109.
5. Cullen, Esther (April 2008). "Adaptations in the kittiwake to cliff-nesting". Ibis. 99 (2): 275–302. doi:10.1111/j.1474-919x.1957.tb01950.x.
6. Alcock, John (2013). Animal Behavior: An Evolutionary Approach, Tenth Edition. p. 182.
7. Emerson, S.B.; M.A.R. Koehl (1990). "The interaction of behavioral and morphological change in the evolution of a novel locomotor type: 'Flying' frogs.". Evolution. 44 (8): 1931–1946. doi:10.2307/2409604.
8. Vila, C., JE Maldonado, and RK Wayne. 1999. Phylogenetic Relationships, Evolution, and Genetic Diversity of the Domestic Dog. J Hered 90:71-77
9. Driscoll C.A, D.W. Macdonald, and S.J. O'Brien. 2009. From Wild Animals to Domestic Pets, an Evolutionary View of Domestication. PNAS Vol. 106. Sup. 1.
10. Vila C., P. Savolainen, J.E. Maldonado, I.R. Amorim, J.E. Rice, R.L. Honeycutt, K.A. Crandall, J. Lundeberg, and R.K. Wayne. 1997. Multiple and Ancient Origins of the Domestic Dog. Science 13 Vol. 276, no. 5319: 1687-1689
11. Honeycutt, R.L. 2010. Unraveling the Mysteries of Dog Evolution. BMC Biology 8:20
12. Romanchik, J. 2011. From the Wild Wolf to Man’s Best Friend:An Analysis of a Hypothetical Wolf Population and the Change in Temperament, Possibly Leading to Their Domestication. Old Dominion University http://d2oqb2vjj999su.cloudfront.net/users/000/082/618/962/attachments/Scientific%20Paper-%20Wolves%20to%20Dogs.pdf