Because evolution is largely treelike in large, multicellular animals (interesting cases of lateral gene transfer notwithstanding) the easiest way to display the history of life is in the form of a tree. Trees are great at packing a lot of information into a small space; they can represent the entire set of relationships between a group of organisms very compactly.

image credit: wikipedia

However, they can also be difficult to read and so a reader’s interpretation of them can be easily affected by the way in which they’re drawn. Take a look at this evolutionary tree, showing the evolutionary relationships between several different animals.
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One of the nice things about having a reliable knowledge of the relationships between groups of organisms is that it lets us figure out when different traits have evolved. This can be of great practical importance – for instance, if we see that a particular mutation has evolved independently in several species that are all parasites, then it suggests that the mutation might be important for parasitism. It also lets us make more general statements about the history of life on earth – for example, we can say with great certainty that complex eyes have evolved independently multiple times.

Another trait that has evolved multiple times is powered flight. We know that there are four groups of organisms that can fly – birds, bats, insects, and (extinct) pterosaurs. And we know that each of these groups evolved flight independently because, in each case, the most closely-related group cannot fly. For example, bats are most closely related to carnivores like dogs and cats than they are to any other flying animal. (The situation with birds is slightly more complex because their closest relatives are all extinct, but the principal is the same). We can be pretty confident that powered flight evolved independently four times.
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Biology is by far the “fuzziest” of the sciences, dealing as it does with complicated and messy living organisms rather than platonic forces and molecules.  As such it has a distinct lack of the kind of elegant laws that crop up in the harder sciences. Pithy equations like E=mc² (which relates energy, mass and a fundamental physical constant), or the mathematical relationship e^pi*i=-1 (which relates the three most important constants in mathematics) are in short supply in biology.

There is one biological law, however, that shares the elegance of these equations. It comes from the field of theoretical population genetics, and concerns a scenario where a species is split into two populations. Imagine a species of lizard that is found only on a single island. The island has a mountain range running from north to south down its centre. The mountains make it difficult for the lizards to cross from one side of the island to the other, effectively splitting them into two populations.
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Readers with a passing interest in biology might be familiar with the name Drosophila melanogaster, a type of fruit fly that has been studied intensely for about a hundred years. Drosophila has been the subject of such interest not because there is anything unusual about it, or because it’s important to human affairs, but for straightforward practical reasons – fruit flies don’t take up much room, they breed quickly, and they’re easy to take care of.

The things we learn by studying Drosophila can hopefully be applied to animals that are of more practical importance, such as disease vectors, livestock, and, of course, humans. The last item on that list usually jumps out at us and makes us wonder: just how much we can learn about humans by studying flies? Surely humans and flies are completely different animals!

Well, as with many things in biology, it all depends on how closely you look. Let’s start with the obvious picture that springs to mind – human on the left, fly on the right.
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The variability hypothesis is an idea, first suggested in the early nineteenth century, that there is more variation in men than women. The idea suggests that if we measure a given trait – for example, height – in both men and women we will find that regardless of whether their is a difference in average height, the distribution of heights in men will be more extreme than the difference in women. In other words, there are more very short and very tall men than there are very short and very tall women.

This idea has had a controversial history when applied to humans, for a number of reasons. It’s been proposed to apply not only to easily measurable traits like height and weight, but also more nebulous ones like intelligence and criminal behaviour and has been used to justify some outrageously sexist viewpoints. Unfortunately, there has been much less debate about increased male variability in non-human animals. However, thinking about the possible mechanisms of variability reveals an interesting quirk of natural selection that suggests how it might come about.
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One of the assumptions that we take for granted when trying to figure out the relationships between living things is that all life on earth is related. Some organisms might be more closely or distantly related than others, but if you go back far enough, all living things share a common ancestor.  This seems like quite a big claim to make, especially when considering pairs of organisms that seem to have absolutely nothing in common.

To understand why we are so confident about the relatedness of living things, consider the following thought experiment. Imagine that we take a hundred volunteers who have never seen a boat, and ask them to build one. We’ll put them on widely-spaced islands, so that they can’t communicate with one another, give them plenty of tools and materials, and come back in a year to see how they get on. What do you think will happen?
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