Great question. There are a few answers. One is that conditions on continents can be very different. As species evolve in different environments, they are under different pressures.

For example, a species that survives and reproduces well in a warm, wet environment may die in a cool, dry environment. So two species may even belong to the same family and share an ancestor that was different from both of them, but be very different from each other.

The opposite of this is when very distantly related species end up looking similar because they evolved in the same sort of environment. For example, in our desert conditions in North America, we expect to see various species of cacti, with no leaves, sharp needles, and a waxy-waterproof surface. In African deserts, they have euphorbia, which have no leaves, sharp needles, and a waxy-waterproof surface. This site:
Na href="http://davesgarden.com/guides/articles/view/2149/"> pictures has great pictures of both. These two different families of cacti are not the descendants of ancestors with no leaves, sharp needles, and a waxy-waterproof surface. They evolved these traits independently from each other. This is an example of convergent evolution.

Can you think of other examples of closely-related species that have adapted to very different environments or distantly-related species that look alike because they have adapted to similar environments?

Good question! It has to do with what species were where when they evolved, and is thus a historical accident, but it also has to do with different conditions in different parts of the world, and with the ability of species to disperse across parts of the world that they cannot live (trees, for example, do not grow in oceans normally).

There is a lot of research ongoing in this area as we try to find the answer. It depends on the species, of course - each species has its own, unique history, but there are patterns, and those patterns are still being studied.

Thanks for the great question.

The tectonic plates that make up the crust of the earth move around over the course of millions of years. As they do, the continents and oceans on top of the tectonic plates move as well. Eventually populations of the same species get split up as the plates move and continents split apart, with oceans or mountains forming in between them. (Note this is over millions of years and countless generations!)

Now, when the same species is separated by a new mountain range or ocean, evolution by natural selection may begin to act on the two different populations in different ways. This is because there might be important differences between the two locations, including the types of food available or what predators there might be. Different selective pressures in these different locations therefore favor the evolution of different traits in different environments, and new species may even arise from this process (this is called allopatric speciation).

The same process can happen when some members of a species migrate to a new environment and don’t go back, for instance from Asia into the Americans over an ice bridge in the Arctic. Species that made it across were acted upon by natural selection in their new environment and subsequently evolved.

So different continents have different types of living organisms as natural selection has acted on individuals of species isolated from those members on other continents. We should expect, and we find, species that are highly genetically related across the different continents. However, millions of years of natural selection have led to the evolution of different species across the world.