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Q J Med 2003; 96: 545-546
© 2003 Association of Physicians

Biologic

The molecular clock

Colin Berry

The first 10% of the full text of this article appears below.

Sea squirts (Ciona sp.), flies, ascidians and human beings all have vitamin D receptor and retinoid X receptor genes. However, thyroid hormone receptor, retinoic acid receptor, and peroxisome proliferator-activated receptor genes are found in sea squirts and Man, but not in flies, implying that these genes are an invention of the chordates. Similarly, sea squirts do not have oestrogen receptor genes, genes for androgen, glucocorticoid or mineralcorticoid receptors: these appear to be associated with the appearance of the vertebrates. Such steps may be of evolutionary significance, and their identification is part of a redefinition of taxonomy, much as new disease classifications (in oncology, say) depend on new genetic information.

A measure of the genetic distance between species (now often known with great accuracy) can be converted to a time estimate for their separation from a common ancestor by using a . . . [Full Text of this Article]


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