Animals in the sea develop into new species in a different way than on land

The study also provided further support for the idea that differentiation, which does not depend on the physical barriers between the sexes, is more common than previously thought. The necessary isolation may also be achieved through the differences in breeding times or the depth at which it is done.

How do new species of animals evolve?

There are many answers to this simple question, but most scientists believe that the most dominant mechanism through which new species develop is, quite simply, the distance between different populations. As soon as part of the population of a certain species moves to another area, those items can no longer mate with the previous population, and can change at a higher rate. A good example of this idea is the Pharisees who came to the Galapagos Islands and lived in isolation on each island. Since they could not fly between the islands and mate with the other Pharisee populations, the Pharisees underwent a separate evolution on each island that adapted them to the environmental conditions on that island.

This theory is well suited to the land conditions, where there are mountains, valleys and many obstacles that allow different populations to develop in isolation from each other. It is common knowledge that the theory also works in the sea and oceans, but a month ago the research of Dr. Philip Sexton and Dr. Richard Norris, both from the Scripps Institution of Oceanography in San Diego, was published, suggesting that species do not evolve in the same way in the sea and on land.

Just as on land there are obstacles that prevent different populations from meeting each other, similar barriers exist in the oceans. Seawater differs from place to place in its temperature and salt level, and these differences act as barriers to the movement of plankton - tiny organisms that are unable to actively swim against the current. The existence of these buffers has until now been accepted as a working assumption among the scientists in the field, from which they concluded that the differentiation according to the distance between the populations is also valid and robust for the plankton in the oceans. The new study, published in the journal Geology, presents a completely different picture.

Sexton and Norris examined the fossils of a microscopic plankton species called Truncorotalia truncatulinoides buried in layers of sediment beneath the sea floor. The researchers surveyed different sediment layers from different regions of the world that contain fossils of this species, and were able to follow its spread from its ancient home to its current distribution in the oceans.
Previous research work done on this species showed that it first appeared 2.8 million years ago, in the southwest Pacific Ocean, and it took 800,000 years for it to spread to the rest of the oceans. The popular belief was that the fact that the organism was restricted to the southwest Pacific indicated the existence of some sort of buffer (probably caused by the pattern of ocean currents) that stopped its ability to spread beyond a certain limit, which was consistent with the theory that differentiation is related to the distance between populations.

Saxkton and Norris found that T. truncatulinoides appeared for a short time in the Atlantic Ocean 2.5 million years ago, then disappeared again. This appearance and disappearance surprisingly correspond to an important change in the earth's climate at that time. A more careful examination of the sediments showed that the second appearance of the species in the Atlantic Ocean, 2 million years ago, was also in 'beats'. The length of each pulse was 19,000 years, corresponding to the fluctuations that occurred in the orbit of the earth around the sun, which are themselves linked to the ice ages.

The holes suggest that it is climate that influences the availability of the underwater habitats that have limited the spread of T. truncatulinoides, rather than the presence of physical barriers in the oceans. From this new point of view, the plankton is allowed to disperse freely throughout the ocean, but the local environmental conditions determine whether the species is able to survive and thrive in the different areas. As an analogy for the idea, you can bring the coconuts, which sometimes wash up on the coast of Great Britain. The cold temperatures prevent the nuts from sprouting, but if the climate were to suddenly become subtropical, then coconut trees would begin to grow on Britain's shores.

This new idea, according to which there are very few, if any, barriers blocking the spread of plankton in the world's oceans, is consistent with genetic research showing that the rate of gene flow in the oceans is noticeably high. Moreover, the distribution of several ocean dwellers such as the tuna and the large molluscs (such as the squids) shows that although those species have preferred habitats, a small part of them can be found regularly in other areas as well.

Sexton and Norris' findings provide further support for the idea that differentiation, independent of the physical barriers between the sexes, is more common than previously thought. In this way of differentiation, the necessary isolation may also be achieved through the differences in the times of reproduction or the depth at which it is done. Sexton and Norris claim that this is the main mechanism for differentiation in the oceans, but it is clear that further research is needed to get a clearer picture.

For information on the website of the National Oceanographic Center in Southampton

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