Convergent evolution

Lesser hedgehog tenrec © Peter J Stephenson

You might think you have seen one of these in your garden. But that is not likely – unless you happen to live in Madagascar. The lesser hedgehog tenrec (Echinops telfairi) certainly bears a superficial resemblance to a European hedgehog (Erinaceus sp.). Until quite recently even zoologists put these two species in the same order. This changed with the advent of molecular phylogenetics, a science that uses sophisticated statistical tools to compare the genes of different species and construct trees to show how they might have evolved. It turned out that tenrecs are more closely related to elephants than to hedgehogs. Indeed a new term was needed to describe this eclectic group of mammals. The term chosen was Afrotheria.

So hedgehogs and some tenrecs acquired their spiny pelage quite independently. This is a prime example of convergent evolution. I shall be having quite a lot to say about convergent evolution.

How does this concept relate to the evolving placenta? A simple example must suffice. Most pregnancy tests look for the presence of a placental hormone, human chorionic gonadotrophin (hCG). The hormone is found only in anthropoid primates (monkeys and great apes) and is coded by a gene that arose by duplicating and modifying the gene for the beta-subunit of luteinising hormone (LH), which is made by the pituitary. A similar hormone arose by convergent evolution in horses: equine chorionic gonadotrophin. It is secreted by fetally-derived trophoblast cells that invade the uterine wall and form structures called endometrial cups.

What both these hormones do is mimic the action of LH on the ovary, prolong the life of the corpus luteum, and thereby maintain the pregnancy. So if only humans, monkeys and horses have chorionic gonadotrophins, what do other mammals do? That is for a later post, but rodents have placental lactogens and ruminants use interferon tau.