It always amazes me how, sometimes, these ridiculously easy questions can be so ridiculously hard to answer. In this post in particular I want to propose three different example relevant in Biology: Life, Species and Genes. What amazes me even more than these answers is the fact that people don’t think enough about these questions! At first it feels easy to give a definition for the three words above. But from the moment you start digging deeper into the real meaning you just loose yourself and you realize that nobody still don’t know. And I mean it, the scientific community has still to come down to an agreement.
Let’s go from the tiniest to the biggest, let’s start with genes. As a molecular biologist the definition of a gene is one of my major concerns. We used to say that a gene is a stretch of the nucleic acid called DeoxyRibonucleic Acid, aka DNA. But how long of a stretch? Well, we still don’t know because if you can argue that every gene has a clear end, you can’t say the same thing for the beginning. Many other stretches of DNA play a role on the expression of that gene, so it may be that a gene is not even a continuous stretch. We used to think that a gene is part of DNA that codes for a protein. Well, now we can say almost surely that this is wrong, because if the old definition holds for some genes, it doesn’t hold for others. Many genes never express proteins even though they have huge roles in the cell. As you can see we struggle to definite it either structurally or functionally.
What about species? This should be easier right? It seems easy to describe the differences between two different species of fish for example. They may have several differences in the appearence, they may have different life cycles and different behaviours. That’s exactly the problem. For years we used phenotypic characteristics (external, fairly-easy-to-see properties of an organism) to determine species. And that’s the cause of problems we have now. It turns out that seemingly different species are actually belonging to the same one.
And many organisms described as beloging to the same species, are being discovered to be very different from the genomic point of view. The problem is that we can’t precisely pinpoint the border on where a species ends and a new species begins. We used to think that two organisms belong to the same species if they can copulate and give birth to fertile species. But it has been proven that this is not the case, because many crossbreeds can actually have children themselves. Also what about bacteria or plants that reproduce asexually? There has been some proposal for: ” A species is a group of individuals with high genetic similarity”. But still there’s no agreement on what define “enough similarity” to define species. All summed up the only thing we can do is define species appropriately slightly differently for each circumstance (there are ecological,genetic, reproductive, evolutionary and phylogenetic species). Some definition may be more appropriate in some cases than others, but we still lack a good, solid one.
You may start to think that we are getting pretty complicated, but we still need to talk about what is life. I’ll put it down quite simply. You may think that what distinguish living think from mere matter is the possibility to think, to move, to create social structures etc.. But that’s very superficial. What about plants? They’re alive, but still they neither think nor move. And what about all microorganisms? There can be some mono-cellular organism that still move and interact, are they alive? If you think about it, we, as human, are nothing more than a collection of cells. Everything in us, and everything in every organisms, is reducible to a collection of different sets of cells. So maybe the definition of life is within the cell, since mono-cellular bacteria can be alive as well. But what is a cell, if not a collection of chemicals inside a membrane? Each cell is just a container of chemical reactions occuring accordingly to the normal laws of physics. Each molecule, each enzyme and each substrate can’t have will. They just move kineticly in the medium and they pass through chemical reactions that lead to other products that will go into other reactions and so on.. An endless cycle of renewable reactions that can respond chemically to the environment. If you look careful you can see that every life-form has a thing in common: DNA (even if this is still arguable). DNA carry information which determine the production and the handling of every molecule. But what is DNA if not a collection of molecules and thus atoms? Well maybe an alive collection of atoms has no difference with a dead collection of atoms. Maybe it’s just the nature tendency to organize atoms into more complex structures that brings up apparent life. And maybe asking for a definition for life makes no sense because there’s no such a thing.
I hope you realize now that definiton of things can be challenging, sometimes even seemingly impossible. But just starting to scratch the surface of the true meaning of words and concepts can reveal profound insights. While you’ll thing about definitions of apparently common objects, let me tell you another story… But what is a story??