The Pleistocene Period

The Pleistocene period was a time when ecological changes emerged. During this period glacier formations began to arise and have been accredited with playing a role in the divergence of different species. For different species of avian there has been controversy between their origins and speciation events. Studies have been conducted for this controversy to try and find which theory to accept for the songbirds. Even though a majority of studies have accepted Late Pleistocene events role in the speciation of songbirds, few studies go against this theory and hypothesize a much farther speciation of songbirds in reference with today.
In the Klicka and Zink paper, they rejected the widely accepted Late Pleistocene Origins Model when it came to
the origin of the songbirds (1666). They sequenced DNA for 35 pairs of sister taxa found within the species of songbirds. Using molecular clock methods to determine estimated divergence times between closely related pairs, they found the divergence times went against the LPO model. Based on the LPO model, Klicka and Zink, predicted the graph would be “left-skewed and leptokurtic”, however when they plotted the graph they found a majority of the species that fell within the Pliocene period, with an average of 5.1% sequence divergence (1666). An average of 5.1% correlated with a divergence time of 2.4 MA, meaning divergence of these species happened before the Pleistocene. They also sequenced 13 pairs of species not estimated to have speciated in the Pleistocene with the 35 original pairs and found a divergence time much similar to the average time previously found. Within their data of sister taxa, they only were able to find one species pair that coincided with the time of the Late Pleistocene origin, which was the Timberline Sparrow and Brewer’s Sparrow (Klinka and Zink). According to Klinka and Zink, their analysis of mtDNA pointed to a Late Pliocene origin, meaning the glacial cycles played no role in the speciation of these species, because the species had already speciated before the event took place. Instead perhaps it was bottlenecking of populations that contributed to this speciation.
On the contrary, Avise and Walker accepted the theory that Pleistocene biogeographic factors did have a major role in the speciation of songbirds (457).
In their study, they increased the sample size, sequenced mtDNA, categorized them phylogeographically, and calculated a net sequence divergence (Avise and Walker). To correct the divergence within one species, Avise and Walker took samples from different populations within a species and compared their divergence. Out of the phylogeographic group in category one, they found a “strong geographic orientation” and that “a total of 28 pairs of intraspecific phylogroups show evidence of separation dates that fall within the Pleistocene” (Avise and Walker). There was also a variety of different data types (RFLP, cty b, etc.) from different sources taken into account. A big difference between Avise and Walker from the other study was that a net sequence was taken to correct within species divergence and the bias found in gene trees. Getting rid of the outliers in the data and using the same molecular clock calculations as Klicka and Zink, Avise and Walker obtained a graph that had a vast majority of the songbird species within the Pleistocene period. Correlation between the sequence divergence and the time in this data set accepted the theory that Pleistocene events did have a major effect on the speciation of the
songbirds.
The molecular clock has given researchers a dependable way to estimate divergence times within species.
The lower the sequence divergence between species, the more recently they have split. Pleistocene origin in songbirds is a theory I have come to accept because the small number of DNA sequence difference showed a more recent speciation time. The Pleistocene is a more recent event in reference with today than the Pliocene. There is much divergence just within a species that to use one individual from a species would not be sufficient to represent the species as a whole. Many samples, such as populations within a species should have their divergence calculated for more reliable data. Correcting within species divergence can give more reliable data to accept. Environmental events can attribute to the variance between species. For an event as major as the glacier cycle, this could have played a role in the geographic isolation between different species of songbirds. This barrier limits the amount of cross breeding between species if there are physical barriers. This makes individuals within the same geographical region to have access to the same resources which can play a role in the development of an individual and ultimately the divergence of a species from
another.