Evolution of Planktonic Larval Forms from Benthic Bilateral Animals - Essay Example

Evolution of Planktonic Larval Forms from Benthic Bilateral Animals The evolution of marine invertebrates hasraised a hot debate among scholars and biologists. Assertively however, marine invertebrates have biphasic life cycle. The biphasic cycle consists of planktotrophic larvae that feed on plankton. The larvae are pelagic and live on the water column. These larvae have cilia that make them motile. The larvae have a mouth and gut essential for feeding. In addition, the larvae have a neural system. On the other hand, the benthic adults have special appendages for locomotion, brain, eyes, skeleton, reproductive organs, respiratory system, nephridia, circulatory system, and axes (Raff 1475). Some scientists argue that the pelagic larvae existed first and these evolved to form the benthic adults. On the hand, other scientists argue that the benthic adults existed first and slowly evolved into the pelagic larvae. Both are but hypotheses that have their limitations. Scientists from both sides have supported the theories and provided possible explanations of how evolution took place. The hypothesis suggesting that the benthic metazoans were the original and then the pelagic larvae evolved later seems more logical. According to this theory, the first bilaterians consisted of triploblastic animals that exhibited bilateral symmetry. Phylogeneticists have discovered that the most basal bilaterians are acoels (Raff 1474). This serves as evidence that direct development probably occurred because they contain posterior, interior, and middle Hox genes. These genes are responsible for direct development that took place. Other bilaterians displaying more complexity than acoels could transcribe genes responsible for development of eyes, mesoderm, nephridia and heart. The presence of such machinery proves that development of bilateral body features occurred in steps. This makes it possible for one to visualize how the process of larval evolution occurred. Subsequent steps in the evolution produced a different generation of phyla of these organisms. The planktotrophic larvae developed indirectly producing the intercalation of larval origins. The intercalation of larval origins occurred during the Cambrian period that brought along radical changes. The Cambrian period resulted to formation of diverse phyla of these organisms. The diversity is a reality on observation of the modern phyla. Evolution resulted to basal clades that define the precursors of modern phyla. Evolution of planktonic larvae was a secondary pathway. The larvae-first theory asserts that larvae existed in the Precambrian era (Nielson 208). However, studies defy this claim. This theory is self-supporting because an understanding of the key features exhibited by larvae gives the theory logic. Since the structure and adaptations of the larvae are minimal compared to adults, it is possible that co-option of developmental genes in the adult resulted to secondary development of larvae. Studies on some systems of the larvae and adults support this possibility. For example, in sea urchins, one of the modern phyla, scientists have that there is a similarity in gene expression in the gut of larvae and adult bodies (Raff 1476). This implies that the larval gut evolved from the adult gut system. Supporters of the larvae-first theory argue that the complex systems in the adult resulted as an advanced form of the simple systems in larvae. However, there is lack of phylogenetic evidence to support this claim. In addition, an analysis of the larvae indicates that they have originated from distinct origins. This forms additional evidence that the adults of the bilaterians were the primitive organisms. From the precursors of modern phyla, secondary evolution gave rise a diverse range of larvae. This fact also exposes a weakness of the larvae-first hypothesis. It is quite difficult for the supporters of that theory to explain why the larvae indicate different origins. Geneticists have been studying the co-option of genes in evolution and such studies are sources of more evidence supporting the adult-first theory (Raff 1476). There is a correlation between functional genes in the gut of larvae and adults. This supports the fact that secondary evolution happened and that larvae resulted from the precursors of the modern phyla. This implies convergence of genes that lacks ion the larvae-first hypothesis. Given that, the structure of larvae is simpler, convergence of genes from adults to larvae is a possibility. In addition, fossil analysis providers a new set of evidence concerning the evolution of planktonic larvae. The study of fossils in order to establish what happened in history is a source of biological evidence. Fossil records reveal that bilaterians existed, 100 million years before the larvae. This ascertains the fact that bilaterians emerged first and larvae later. Emergence of larvae was probably a result of selection. Plankton was on the increase in the sea and the bialterians were facing predation. For these organisms to survive, they evolved into planktonic larvae to camouflage from predators. Camouflaging into larvae gave them an opportunity to utilize sea plankton that was apparently on the increase. Fossil study also indicates that evolution of larvae took place in the Cambrian region. Further analysis of embryos from fossil records provides proof that at some point in evolution, indirect development resulted (Raff 1477). The study supports the fact that indirect development came after direct development implying that natural selection took place. Other studies reveal that the larvae body plans and the adult body plans of annelids and moluscs differ morphologically to a great deal. The difference brings out a new explanation of the evolution process of the larvae body plans. This fact has made scientists realize that the evolution of these larvae took place at a slow pace (Raff 147). It also supports the fact that some larvae resulted from interrelated taxa. Convergence of genes likely involved interrelated lineages that brought about the morphological differences exhibited in the larvae. This receives support from phylogernetic studies. The adult-first hypothesis presents an explanation why there is a continued evolution among the larvae. Studies reveal that most of the larvae have been undergoing further adaptation and changes. Some of the larvae have evolved longer arms. An analysis of the deuterostomes reveals that novel features must have evolved first and that more changes occurred later. Continued evolution explains why some of plankton-feeding larvae have evolved into non-feeding larvae. The larvae-first theory cannot explain why evolution persists in larvae. The adult-first hypothesis makes it possible for scientists to establish the possible reasons explaining the continued evolution. From the continued evolution, scientists conclude that radical changes take place as ecological conditions change. New adaptations allow the larvae to survive in the changing conditions (Raff 1478). The adult-first evolution hypothesis is consistent in describing how the larvae resulted and relies on evidence from reliable sources. The ancestral acoelomorph may have resulted from radical changes. Being the ancestors of modern phyla and planktonic larvae, scientists positively assert that the simple acoels had genes that on expression would lead to the formation of novel characteristics. As ecology changed, evolution selected for the expression of genes that enabled easier survival.co-option of genes and convergence of interrelated lineages was nature’s way of ensuring that the organisms that survived had features to exploit the increasing plankton. Nielson explains the larvae-first theory and evolution of metazoans and gives a chronological elaboration of how the benthic adult forms resulted (246). Although these are strong points supporting this theory, it becomes impossible to describe how dedifferentiation occurred to allow for the choanoflagellates dividing. The hypothesis lacks adequate explanations on how the internal cells have the capacity to divide. In addition, the larvae-first theory cannot explain the emergence of larvae from different origins. Studies suggest that the morphology of larvae indicates that they have resulted from varying origins. The future gives promises that molecular genetics will resolve the controversy resulting from the two hypotheses (DeSalle and Schierwater 1043). Before then, the only coinciding assertions of the two hypotheses are that the ancestors were able to feed and that indirect development occurred. Each side has been providing evidence from closer analysis of phylogeny (Dunn et al. 745). Other researchers have suggested that sponges formed the ancestors of all organisms. A clear analysis of the relationship between the different taxa of the metazoans and a defined analysis of the larvae and adult plans will give more insights on the exact order of evolution. The adult-first theory presents a lot of consistency in evidence and explanations. It proves more valid because it elaborates the evolution process from pre-Cambrian period through the Cambrian to the current continued evolution in larvae. The possibility of co-option of genes and convergence of interrelated species are logical explanations of the modern phyla of metazoans and the morphology of the planktonic larvae. The hypothesis asserts with evidence that adult body plans of metazoans resulted first and later came the larvae as ecological conditions changed. Due to the strong evidence and logic it presents is offers more validity. Work Cited DeSalle, Rob and Schierwater, Bernd. An even “newer” animal phylogeny. BioEssays 30: 11-12: 1043-1047. Web 14, Mar. 2012, from http://desalle.amnh.org/pdf/DeSalle.2008.BioEssays.pdf Dunn, Casey. et al. Broad phylogenomic sampling improves resolution of the animal tree of life. Nature 452, (2008): 745-750. Web 14, Mar. 2012, from http://www.nature.com/nature/journal/v452/n7188/suppinfo/nature06614.html Nielson, Claus. How Did Indirect Development With Planktotrophic Larvae Evolve? Biol. Bull. 216 (2009): 203–215. Web 14, Mar. 2012, from http://www.biolbull.org/content/216/3/203.full.pdf Nielson, Claus. Six major steps in animal evolution: are we derived sponge larvae? EVOLUTION & DEVELOPMENT 10.2 (2008): 241–257. Web 14, Mar. 2012, from http://www.google.com/url?sa=t&rct=j&q=Six+major+steps+in+animal+evolution:+are+we+derived+sponge+larvae%3F+pdf&source=web&cd=1&ved=0CCAQFjAA&url=http%3A%2F%2F163.178.108.3%2Fprofesores%2FCampos%2520Manuel%2FEmbriologia%2520Animal%2520articulos%2Fpasos%2520para%2520en%2520la%2520evolucion%2520para%2520embriologia%2520animal.pdf&ei=Wb1gT6mHKo2HrAeAxrmFBg&usg=AFQjCNG--shoysSC6KJqBNQLTTKrscllLQ&cad=rja Raff, Rudolf. Origins of the other metazoan body plans: the evolution of larval forms. Phil. Trans. R. Soc. B 363 (2008): 1473–1479. Web 14, Mar. 2012, from http://rstb.royalsocietypublishing.org/content/363/1496/1473.full.pdf Read More