Evolution Follows the Path of Least Resistance

Over at Jerry Coyne’s blog, biologist Greg Mayer wrote:

One of the most important lessons of comparative anatomy is that evolution usually proceeds by the modification of pre-existing structures (or, stated more precisely, the modification of the pre-existing developmental programs that produce those structures). Certain changes are easier to evolve because the developmental system can be modified to produce them—evolution follows the developmental path of least resistance. In terms of the skeleton of vertebrates, this means that most evolutionary changes are reduction, fusion, loss, lengthening, shortening, thickening,  and narrowing of bones. Evolution uses what’s already there, and rarely do wholly new structures arise.  (from “Tinkering with elephants’ feet”)

All of this is true, yet we can proceed beyond this conventional thinking and ask a couple of questions:

WHY does evolution follow the developmental path of least resistance?

WHAT are the implications of evolution following the developmental path of least resistance?

Notice a pattern?

Only have the time these days to share the abstracts of studies whose data support the hypothesis of FLE.  Check this one out:

SNARE protein-driven secretion of neurotransmitters from synaptic vesicles is at the center of neuronal communication. In the absence of the cytosolic protein Munc18-1, synaptic secretion comes to a halt.

Front-loading with teneurins

So why did I bring up the teneurins? Let’s consider the abstract of a paper that was published a few weeks ago [1]:

Teneurins are type II transmembrane proteins expressed during pattern formation and neurogenesis with an intracellular domain that can be transported to the nucleus and an extracellular domain that can be shed into the extracellular milieu.

In other words, we have a protein that connect the nucleus to the environment outside the cell.

In Drosophila melanogaster, Caenorhabditis elegans and mouse the knockdown or knockout of teneurin expression can lead to abnormal patterning, defasciculation and abnormal pathfinding of neurites, and the disruption of basement membranes.

In other words, this is protein that plays an important role in the formation of brains and nerves. The fact that is carries out the same basic functions in worms, insects, and animals strongly suggests its role in nervous system is quite ancient and may have coincided with the emergence of the nervous system itself. So when did the tenurins arise?

Teneurins

According to Wikipedia,

Teneurins are transmembrane proteins. The name refers to “ten-a” (from “tenascin-like protein, accessory”) and “neurons”, the primary site of teneurin expression.

Teneurins are highly conserved between Drosophila, C. elegans and vertebrates. In each species they are expressed by a subset of neurons as well as at sites of pattern formation and morphogenesis. In Drosophila, a teneurin known as ten-m or Odz is a pair-rule gene, and its expression is required for normal development. The knockdown of teneurin (ten-1) expression in C. elegans with RNAi leads to abnormal neuronal pathfinding and abnormal development of the gonads.

And according to this article,

Teneurins are a unique family of transmembrane proteins conserved from C. elegans and D. melanogaster to mammals. In vertebrates there are four paralogs (teneurin-1 to -4), all of which are expressed prominently in the developing central nervous system.

So why mention these proteins?

 

Evolutionary Thought Experiment

Let me provide you with a little evolutionary thought experiment.  Stephen J. Gould once noted that “evolution is a bush, not a ladder.”  Quite true.  Consider the following representation:

 

Notice that the evolution of mammals does not entail a straight shot from some ancestral chordate to mammal.  On the contrary, the evolution of mammals also involves the evolution of sharks, bony fish, frogs, snakes, and birds along the way.  What’s more, the bush shows a nesting pattern, where the tetrapods, for example, nest together to the exclusion of all other chordates.  This is because the tetrapods derive from an ancestral tetrapod state that was not shared by the other chordates.  In fact, the following arrangement makes this more clear.

First questions about LECA

We have seen that science has discovered the last eukaryotic common ancestor was essentially as complex as a modern day eukaryotic cell (see here and here and here).  Furthermore, I have argued that this complex cell plan that has defined eukarya since the time of LECA has worked to facilitate the eventual emergence of metaozoan-type complexity.

So perhaps it is time to begin contemplating the origin of this complexity.

Electric Faces

From here:

For the first time, Tufts University biologists have reported that bioelectrical signals are necessary for normal head and facial formation in an organism and have captured that process in a time-lapse video that reveals never-before-seen patterns of visible bioelectrical signals outlining where eyes, nose, mouth, and other features will appear in an embryonic tadpole.

Genetic Switch for Limbs and Digits Found in Primitive Fish

From here:

Genetic instructions for developing limbs and digits were present in primitive fish millions of years before their descendants first crawled on to land, researchers have discovered.

Genetic switches control the timing and location of gene activity. When a particular switch taken from fish DNA is placed into mouse embryos, the segment can activate genes in the developing limb region of embryos, University of Chicago researchers report in Proceedings of the National Academy of Sciences. The successful swap suggests that the recipe for limb development is conserved in species separated by 400 million years of evolution.

“The genetic switches that drive the expression of genes in the digits of mice are not only present in fish, but the fish sequence can actually activate the expression in mice,” said Igor Schneider, PhD, postdoctoral researcher in the Department of Organismal Biology and Anatomy at the University of Chicago and lead author on the paper.

Google Mystery

What does it mean?

Scientific discovery, not tautology

I previously showed that the scientific discovery of a complex LECA was not a tautology.  DrREC replied:

Mike Gene, do you recognize the difference between FIRST and LAST?

L as in LUCA or LECA is LAST-The Last Eukaryotic Common Ancestor, the most RECENT (not oldest) organism from which all organisms/all Eukaryotic organisms (respectively) living on Earth descend.

Not the First! Do you understand LECA isn’t the first Eukaryote?

This is a very strange line of questioning given that nowhere did I argue that the last eukaryotic common ancestor was the first Eukaryote.  Nor do I think so.  Thus, DrREC’s first objection amounts to shadow boxing with the straw man he invented.

But it gets worse.