How is sex determined?

A fun video introducing the variety of sex determination systems:



I like this as an introduction, which sets some good rules of thumb, but there are some excellent exceptions to these rules that we can get into. For example, in mammals (even humans), the part of the Y chromosome that is most responsible for turning on the male-determining pathway doesn't always function as it should, resulting in individuals with an X and a Y chromosome who have female physical characteristics (for more see Ed Yong's article about Chen et al.'s 2013 paper).

Convergent evolution: tenrecs and hedgehogs

The hedgehog and tenrec diverged from one another over 100 million years ago. That is a lot of time. To put that in perspective the lineages leading to human and mouse also diverged roughly 100 million years ago (maybe closer to 90ish). And yet, the tenrec and hedgehog have independently evolved very similar features, likely because of similar environmental pressures. This independent evolution of features is called convergent evolution, and it is just fantastic to observe.

Tenrecs are found in Madagascar and Africa:

Tenrecs at Berkeley's Museum of Vertebrate Zoology, Own work

Hedgehogs are found in Europe, Asia, and Africa.

Hedgehog, by Nino Barbieri, via Wikimedia Commons

Based on their physical features, hedgehogs and tenrecs were once thought to be closely related species. But, Murphy et al. (2007) showed that their genomes are very different from each other, suggesting the two species have been separated for more than 100 million years.

Extra tidbit: 

Recent efforts have begun to domesticate hedgehogs, and the result of some of those efforts is the long-eared hedgehog:

It looks like a bat-eared hedgehog to me. 

Have a great day.