Friday, March 7, 2008

When your mouth is your anus is your vagina: reproduction in phylum Porifera

Orifices are sophisticated inventions. Having a hole for every function is the mark of a recently-evolved animal. More ancient animals like birds and reptiles have a cloaca, which is an opening used for both defecation and reproduction. 

Even having a digestive tract with two ends is a complicated arrangement. Animals like jellyfish, corals and sponges, who were the earliest evolved animals, have a large body cavity with just one opening leading to the outside environment. They eat, shit and have sex using the one hole. 

Sponges reproduce sexually by shooting their sperm into the water. (Copulation is a land animal invention, developed to cope with the lack of a hospitable, moist environment and free transport provided by the ocean. In light of this, a uterus can be thought of as a mini, internal, ocean substitute.) Each sponge hopes (metaphorically, sponges have no brain) that its drifting sperm will be 'eaten' by another sponge. If one's vagina doubles as one's mouth, there is a danger that the sperm will not be used to make baby sponges, and instead be digested as a nutritious food source. Happily, sponge cells are capable of telling the difference between food and the sperm of other sponges. 

Sunday, March 2, 2008

Why Anna Should Probably Not Sleep With Her Brother OR Consanguinous Matings and Darwinian Fitness

I've always assumed jokes about inbred, two headed, Tasmanian congenital idiots were exaggerations. Apparently not. The statistics on inbreeding are quite frightening. 

Amongst live births resulting from incest, 6/18 children die in infancy or childhood, and 3/18 have intellectual disabilities. 

Here are some statistics on the offspring of first cousins, compared to the children of unrelated parents: 

Trait Unrelated 1st cousins

stillbirth and neonatal death* 4% 11%
major congenital malformation 1% 1.7%
genetic abnormality 7.9% 18.8%
mental deficiency 1.3% 5.5%
recessive muscular dystrophy 0.0034% 0.051%
*1941 figures

So perhaps it's wise to think twice before taking a plunge into the shallow end of the gene pool. Your sibling may be intelligent and attractive, but chances are your resulting daughter-niece or son-nephew will not be.

Whales: Neo-cons of the Sea OR Cetacean Phylogeny and Convergent Evolution

Every now and then life throws up an evolutionary backslider: a creature who, having successfully evolved into a new way of life, turns around and evolves right back into something that resembles its primitive ancestor. 

The ancestors of tree kangaroos (inhabitants of PNG and far North Queensland) evolved from mammals who came down from the trees and adapted to life on the ground. Then, at some point, they began to re-adapt themselves to life in the trees, taking their big, ungainly, land-adapted feet with them. 

Whales have taken things a step further. The ancestors of whales were land dwelling mammals with limbs and toenails, or possibly hoofs. Life had emerged from the ocean 400 million years ago, and was ticking along nicely for more than 300 million years before these ancestral whales decided it would be a bright idea to crawl back into the primal soup. At this point, they grew their limbs back into flippers and went back to live in the ocean like a bunch of fat, bloated fish. 

How, whales, is this a positive lifestyle choice? You are now WARM BLOODED creatures who live in a FREEZING OCEAN. You're the size of a TRUCK and you eat things that are the size of a FULL STOP. You breathe AIR and live UNDER WATER. And you wonder why Japanese people want to kill you!