Evolution Q&A: How does it cross the species boundary?

Great Tree of Life Image by Leonard Eisenberg

I often run into folks who have questions or misunderstandings about Evolution, so I’ve decided to write a series of articles to address these recurring ideas. Hopefully, this will be helpful! Today’s commonly asked question:

I accept that animals can adapt and change, but how can they cross the species boundary? Dogs only give birth to dogs. Wolves only give birth to wolves. How does a fish become a lizard become a dog and eventually become a human?

Species Boundaries

In short, they don’t exist. No such thing.

Many people assume there are boundaries between species because…

a) the way we’ve chosen to name them implies there are separations between them and

b) when you look at two animals of different species there obvious distinctions between them.

First, let’s talking about the naming.

Our entire method of categorizing animals was created before the idea of Evolution existed. Categories are designed to break things up into separate groups… which is where the problem begins. We currently categorize using Kingdom-Phylum-Class-Order-Family-Genus-Species. Even if you aren’t familiar with KPCOFGS, you’ve heard parts of it your entire life. When people refer to the “Animal Kingdom” that’s the “Kingdom” they are referring to! Humans are called “Homo Sapiens”. “Homo” is our Genus, “Sapien” is our species. “Homo Erectus”, a precursor to humans, is in the same Genus with us (“Homo”), but is a different Species (“Erectus”).

The full description of humans is:

Kingdom Phylum Class Order Family Genus Species
Animalia Chordata Mammalia Primates Hominidae Homo Sapien

Why does the naming cause a problem though? Let’s jump into our imaginations for a second, and imagine that you have to decide on an organized naming scheme for the following colors (for our example, we’ll assume these are the only colors that exist currently). Let’s keep it reasonably simple for this analogy, don’t come at me with your color wheel!


Starting on the right, it looks like we have red, orange, yellow, light green, two medium greens that are reasonably similar, cyan, a soft blue, a dark blue, a navy blue perhaps, then a purple, maybe a magenta and a hot pink? We could argue about the specifics, but they all generally fall into those buckets. It’s not that difficult to name them because they are all reasonably distinct from each other because of those white gaps in between them. Those white gaps are what people might consider “species boundaries.”

However, those gaps only exist because some of the species/colors went extinct in the past. They don’t exist now, but they did exist sometime in the past. What would that strip of color blocks look like if we extended it back in time a bit. Think of the bottom of the image as “now” and as we go upward in the image, we are going backward in time.

Now we can see that there used to be a lot more colors, but not all of them made it to the bottom (current day), so when we were looking at the gaps between the colors (the “boundaries”) we mistakenly thought of them as entirely separate. We previously thought that green and yellow were entirely different things. But, look at this blown up segment and look across it, pixel by pixel, and tell me where exactly does yellow become green?


There is a tiny difference from one pixel to the next, but it is such a small change that for most purposes the two pixels next to each other are the same. Now imagine if the colors were animals throughout time. One “yellow-1” animal could mate with another “yellow-2” animal because they are incredibly similar even if they aren’t identical. Right? Humans are all very similar, but we aren’t genetically identical and we can still produce offspring. Would the “yellow-1” and “yellow-2” animals produce yellow offspring? You bet! Would that yellow baby be exactly the same yellow as the parents? Nope! Maybe he’s one tiny little pixel closer to green than his parents were. And then he has offspring which are still yellow, but maybe one tiny little pixel closer to green that he is… over and over. If you watched every single generation, you wouldn’t be able to tell when they stopped being yellow and started being green. If we were able to preserve all of the animals from every generation and then stack them all next to each other, we could put together a perfectly smooth gradient from yellow to green. However, over the course of time, the previous generations got old and died and maybe no new animals of exactly their color were born in the next generation. Their exact color of yellow-green dies out and no new yellow-greens of exactly their shade are born. All of the new children are either yellow-yellow-green or yellow-green-green. Then the gap in the colors starts to form.

In an effort to relate this to a more tangible example, perhaps the medium green on the left represents gorillas, the light green in the middle represents chimpanzees, and the yellow on the right represents humans. The jagged bits at the top of the white gap represent each of the “transitional” species that went extinct. So, in that white gap is where you would have seen Australopithecus Afarensis, Australopithecus Africanus, Homo Neanderthalensis, Homo Erectus, etc. They went extinct so their “color” didn’t make it all the way to the bottom. If all of those species had survived, you’d see a nice smooth gradient frmo yellow to green.

If you weren’t able to see all the previous generations and you only saw what was alive in the current day, you’d see green animals (gorillas) and yellow animals (humans) and think they were completely different. This is why the naming method causes us problems and interferes with people’s ability to understand how Evolution can cross the “species boundary.” The reason is: The species boundary doesn’t exist, but we name them as if it does/did.

When you can see the entire history with every single animal/color, you realize there are no boundaries between them, just tiny, tiny changes from generation to generation.


Keep in mind the above image is really just to demonstrate the idea of the gradual change from one to the other. The color bands don’t look exactly like they should if it was really showing the progression of Evolution. 🙂

This can be a complicated topic and I’m hoping this analogy will make it more clear. If this helped or if this just made it more confusing, please let me know in the comments!