~~~~~~~~~~~~~~~~~~~~~~~~
DNA is what makes a species what it is. The Genome (a complete set of DNA)
is completely responsible for the biological identity of an organism.
True, the environment can influence development, but the environment can
only shift variables one way or the other based on what is already made
possible by the Genome.DNA is made of long strands of Nucleotides, of which there are 4: Adenine, Guanine, Thymine, and Cytosine (A, G, T, and C). These four Nucleotides are the four Letters in the alphabet of life. The Letters form Words, all of which are 3 Letters long. Three Letter triplets of Nucleotides code for 20 Amino Acids. The combinations of letters results in 20 Words.
The 20 Words are combined to form a multitude of Sentences, ranging from a 20 to 33,000 Words in length: Amino Acids are the Words that are strung together to form the Sentences, called Proteins. Proteins perform most of the functions of living cells.
The combination of all the Sentences makes up a Book. A Genome is a Book. The Book is made of DNA.
In Books, the Sentences dictate the story. In the Genome, Proteins dictate the function.
~~~~~~~~~~~~~~~~~~~~~~~~
A book is defined by its letters. If you change one letter in a book, the book isn't exactly what it used to be. Change 10 letters, and it is a bit different. Change 100 letters--even more. Change 50% of the sentences, and 50% of the identity of the book has changed.
Each of us is a unique book, filled with a unique set of letters. The entire book is the dictionary entry under the word "You". The dictionary of life contains entries for every organism that ever lived--each perfectly defined by its unique genome.
Here is one tiny bit of the dictionary entry for one organism, the "human" whose genome was sequenced in The Human Genome Project:
https://vimeo.com/11711801
As we look into the evolutionary past, however, we run into "species" that share 99% of our letters, and others that share 98%. Still others share 97%, and 94%. We keep moving backward in evolutionary time and we see forks in the road--several groups with 84% of the same letters, but none of them with the same differences, and EACH individual in EACH group never sharing exactly the same letters as its parents or offspring.
Every book is unique.
Biological life is not black and white. Biological life is shades of gray.
Define for me, then, a biological human.
Yes, you could assign groups of individuals in specific places that have the ability to reproduce with one another a species name--but this is merely a crutch to learn things about our world at the present time--not a suitable definition for actual species. If we grant that definition, then all species are equivalent. We simply shift the time range from the present back 30 years, and then 30 more years, and then 30 more all the way back to the origin of species, and the whole while we have groups of individuals in specific places that have the ability to reproduce with one another--but at the end we have modern "humans", and at the beginning we have a single celled organism or protobiont.
If we grant that definition, all books are "Moby Dick".
It is far better to define each book as it is--by its unique sequence of letters. Yes, genres and sub-genres can be argued over, but such things are subjective matters of opinion. There is no invisible wall that truly separates one genre from another--no magical percentage that can objectively differentiate between groups of letters.
Is what makes humans "human" a subjective matter of opinion?
Biological life comes in shades of gray.
Based on biology, all of us are individuals--we must look to the philosophers if we want to know anything more.
Read Parts 1-6 for my Philosophical
Perspective on Human Life
"Yes, you could assign groups of individuals in specific places that have the ability to reproduce with one another a species name--but this is merely a crutch to learn things about our world at the present time--not a suitable definition for actual species."
ReplyDeleteI think you're missing something fundamental in your counter-argument - modern biologists, who decide what "species" means, obviously include "specific time" along with "specific place," because common descent, a fundamental part of their reasoning, makes ignoring the temporal component obviously broken, as you point out at length. I think it isn't often discussed because only creatures that are alive at the same time have any chance of reproducing together, and thus there is little risk of confusion.
We (somewhat arbitrarily) define 'human' as 'Homo sapiens sapiens,' and if you go back through the tree of common descent, it won't take you long to find an organism that a modern human could not breed with, thus being a different species ('not human').
Since degree of genetic difference defines whether two creatures can reproduce, if you take common descent and genetic mutation over time as a fundamental part of your reasoning, it is immediately clear that _distance in time_ is roughly equivalent to _difference in genetics_ along the same line of descent, and thus to reproducibility, and thus to speciation.
Pointing out that the principle of common descent makes species a continuum rather than a set of discrete steps is basically restating the fundamental hypothesis of evolution.
If your point is that nothing fundamentally separates humans from other organisms biologically, well, see previous :)