sábado, agosto 23, 2008

pajaritos con auto reconocimiento

Pensado antes como exclusivo de los primates... pero visto ahora también (y únicamente) en este pajarito.


sábado, agosto 16, 2008

Conduta e Evolução dos Cetáceos

Exaptação e conduta - seguidos por câmbios epigeneticos desengatilhados pelo novo nicho e deriva estrutal de estruturas livres para se transformarem - são dois agentes poderosos no processo evolutivo. Certa vez li um caso semelhante discutido por Vargas e Rubilar em relação a especilização das aves para o ovo. Encontro agora um outro exemplo não menos atrativo - a evolução inicial da linhagem dos cetáceos:

Nature 450, 1190-1194 (20 December 2007)

Whales originated from aquatic artiodactyls in the Eocene epoch of India

J. G. M. Thewissen1, Lisa Noelle Cooper1,2, Mark T. Clementz3, Sunil Bajpai4 & B. N. Tiwari

"The modern artiodactyl morphologically most similar to Indohyus is probably the African mousedeer Hyemoschus aquaticus. Hyemoschus lives near streams and feeds on land, but flees into the water when danger occurs40. Indohyus had more pronounced aquatic specializations than Hyemoschus does, and it probably spent a considerably greater amount of time in the water either for protection or when feeding. As indicated by the evidence from stable isotopes, Indohyus spent most of its time in the water and either came onshore to feed on vegetation (as the modern Hippopotamus does) or foraged on invertebrates or aquatic vegetation in the same way that the modern muskrat Ondatra does.

Raoellids are the sister group to cetaceans, and this implies that aquatic habitats originated before the Order Cetacea. The great evolutionary change that occurred at the origin of cetaceans is thus not the adoption of an aquatic lifestyle. Here we propose that dietary change was the event that defined cetacean origins; this is consistent with the cranial and dental synapomorphies identified. Molars of Indohyus are markedly different from those of pakicetids, and it is widely assumed that pakicetids ate aquatic prey18, 23.

Our working hypothesis for the origin of whales is that raoellid ancestors, although herbivores or omnivores on land, took to fresh water in times of danger. Aquatic habits were increased in Indohyus (as suggested by osteosclerosis and oxygen isotopes), although it did not necessarily have an aquatic diet (as suggested by carbon isotopes). Cetaceans originated from an Indohyus-like ancestor and switched to a diet of aquatic prey. Significant changes in the morphology of the teeth, the oral skeleton and the sense organs made cetaceans different from their ancestors and unique among mammals."

Poderia ser diferente? Os pobres cetaceos, a partir de uma nova conduta, se tornaram aprisionados en el mar, como Glaucus: "According to Ovid, Glaucus began life as a mortal fisherman living in the Boeotian city of Anthedon. He discovered by accident a magical herb which could bring the fish he caught back to life, and decided to try eating it. The herb made him immortal, but also caused him to grow fins instead of arms and a fish's tail instead of legs forcing him to dwell forever in the sea".

By the way, hoje morreu o grande compositor brasileiro Dorival Caymmi, que, junto a Jorge Amado, escreveu a linda canção "Doce é morrer no mar".

domingo, agosto 10, 2008

Advertencias de Darwin sobre selección natural

The Origin of Species by Means of Natural Selection Or, The Preservation of Favoured Races in the Struggle for Life By Charles Darwin: "If green woodpeckers alone had existed and we did not know that there were many black and pied kinds I dare say that we should have thought that the green colour was a beautiful adaptation to conceal this tree frequenting bird from its enemies and consequently that it was a character of importance and had been acquired through natural selection. As it is the colour is probably in chief part due to sexual selection. A trailing palm in the Malay Archipelago climbs the loftiest trees by the aid of exquisitely constructed hooks clustered around the ends of the branches and this contrivance no doubt is of the highest service to the plant but as we see nearly similar hooks on many trees which are not climbers and which as there is reason to believe from the distribution of the thorn bearing species in Africa and South America serve as a defence against browsing quadrupeds so the spikes on the palm may at first have been developed for this object and subsequently have been improved and taken advantage of by the plant as it underwent further modification and became a climber. The naked skinhead of a vulture is generally considered as a direct adaptation for wallowing in putridity and so it may be or it may possibly be due to the direct action of putrid matter. But we should be very cautious in drawing any such inference when we see that the skin on the head of the clean feeding male Turkey is likewise naked. The sutures in the skulls of young mammals have been advanced as a beautiful adaptation for aiding parturition and no doubt they facilitate or may be indispensable for this act but as sutures occur in the skulls of young birds and reptiles which have only to escape from a broken egg we may infer that this structure has arisen from the laws of growth and has been taken advantage of in the parturition of the higher animals""

lunes, agosto 04, 2008

Fisiologia (Cardíaca) Desenvolvimental II

O artigo citado por Gustavo em um post anterior mostrava um elegante experimento de Waddingon. Waddinton sacou o coração de embriões de galinha em um estágio em que estes eram capazes de sobreviver e se desenvolver sem a o coração. Uma das consequências morfogenéticas da ausência do coração foi a perda do dobramento cefálico característico de aminiotas. O artigo de Waddington foi citado, surpreendentemente, apenas dez vezes. Uma das citações - "Correlation between the embryonic head flexures and cardiac development (1993)"- de Manner et al., fez o experimento ao revés. Impediu o dobramento da cabeça colocando um pêlo dentro do tubo neural (Figuras 1 e 2). O resultado: mal formação cardíaca.

The aim of the present study is to examine whether the formation of the cranial and cervical flexures is involved in the process of cardiac looping, and whether looping anomalies are causally involved in the development of cardiac malformations. For this purpose, the formation of the cranial and cervical flexures was experimentally suppressed in chick embryos by introducing a straight human hair into the neural tube. In the experimental embryos, the absence of the cervical flexure, alone or in combination with a reduced cranial flexure, was always associated with anomalies in the looping of the tubular heart.

Os autores, intrigados, discutem:

These results are in accord with the hypothesis that certain positional and morphological changes of the embryonic heart loop are caused by the formation of the head flexures (His 1881 ; Patten 1922). However, we must concede that there are experimental findings published by Waddington (1937) and Flynn et al. (1991) which seemingly support a totally different hypothesis, namely that the curvature of the cervical region is caused by the positional changes of the heart loop. Our results do not give direct information as to which of the two correlated processes - embryonic flexures and cardiac looping - is the cause and which is the effect.

A dúvida dos autores surge desde um ponto de vista mecanicista dos seres vivos. No entanto, o coração contribui para a dobramento da cabeça e o dobramento da cabeça contribui para a formação do coração. Organismos possuem uma organização circular que os diferencia de máquinas. Como diria um velho filósofo alemão: "Organismos são seres em que as partes são meio e fim". Ou: