Irreducible Complexity? Not Really
In 1996, Michael Behe, a biochemist from Lehigh University published his contention that many structures found in nature were what he termed "irreducibly complex", and could not be the product of natural processes.
His arguments were reminiscent of those made by William Paley (1743 - 1805), who used the analogy of finding a pocketwatch dropped in the heath.
"In crossing a heath, suppose I pitched my foot against a stone, and were asked how the stone came to be there;
I might possibly answer, that, for anything I knew to the contrary, it had lain there forever:
nor would it perhaps be very easy to show the absurdity of this answer.
But suppose I had found a watch upon the ground, and it should be inquired how the watch happened to be in that place;
I should hardly think of the answer I had before given, that for anything I knew, the watch might have always been there.
(...)
There must have existed, at some time, and at some place or other, an artificer or artificers, who formed [the watch]
for the purpose which we find it actually to answer; who comprehended its construction, and designed its use. (...)
Every indication of contrivance, every manifestation of design, which existed in the watch, exists in the works of nature;
with the difference, on the side of nature, of being greater or more, and that in a degree which exceeds all computation.
-- William Paley, Natural Theology (1802)
Behe's arguments were similar: that the complexity of living structures could mean only that they had an Intelligent Designer.
Evolutionary biologists have met this contention with many examples of how neutral evolution and natural selection
could quite reasonably have "invented" such complex structures.
Reconstructing the Evolution of a Complex System: Snake Venom
Venom-like proteins first appeared more than 200 million years ago in the common ancestor that gave rise to snakes and their closest saurian (lizard) cousins.
These genes were expressed in mucus-producing glands of the earliest venomous lizards.
Not necessarily fatal. Just enough to slow down the prey, or cause excessive bleeding (anticoagulant venom) and confer a selective advantage on the lizards with early venom.
For example, it's now known that the gene coding for a venom called crotamine (found in rattlesnakes, genus Crotalus) is very closely related to the genes coding for defensins, small proteins found widely in the animal kingdom (and even in plants) that provide anti-bacterial protection. In vertebrates, defensins are found in immune system cells where they help the cells kill bacteria.
These precursor defensin genes appear to have been readily mutable.
There are hundreds of different kinds in the many different species that have them.
By the time the earliest ancestor of snakes appeared (about 60 million years ago), it already had multiple genes coding for venom proteins.
Snake venoms are (molecularly) more similar in related species than in distant species: They have been inherited from common ancestors.
But they do show specialization, even within a lineage, that marks the effects of natural selection:
Green mambas and black mambas have chemically similar venoms. But green mambas hunt in trees, and black mambas hunt on the ground.
Natural selection is no miracle:
- Green mamba venom is more effective against birds.
- Black mamba venom is more effective against rodents.
Fun with venom:
What is the difference between poison and venom? (If you didn't come to class, you'll have to look it up.)
Venoms have evolved from proteins that still function (for different purposes) in other body organs in modern snakes.
Duplication, Mutation and Recruitment:
- gene for a particular protein is accidentally duplicated
- one copy continues to function normally, leaving the new copy "free" to mutate without undue harm
- the gene mutates and is expressed in a different part of the body (salivary or venom glands)
- Different types of venom have different action on the prey's systems.
Example:
- The ancestor of the Inland Taipan produced a protein in the cardiac muscles that caused slight relaxation of the snake's heart.
- Gene duplication, recruitment, activation in a new tissue (mouth glands), and further mutations resulted in a venom that causes precipitous drop in blood pressure, and quick unconsciousness in prey. (why might this be an advantage for the snake?)
From Simple Beginnings: The Vertebrate Eye
Charles Darwin himself was confounded when he tried to understand how a structure as complex as the vertebrate eye could possibly have evolved by means of natural selection.
Today, however, we have the luxury of a vast trove of molecular and morphological data that make clear how such a progression could have been not only possible, but likely.
All vertebrates have light-sensitive photoreceptor cells (rods and/or cones) comprising the outermost layer of the retina:
When light strikes this molecule, it dissociates into its two components (opsin and retinal), triggering a nervous system stimuls that is eventually perceived by the brain as a spot of light (corresponding to that photoreceptor in the visual field).
The opsin portion of the molecule is variable, and controlled by the genes in the particular species in which it occurs.
- rods
- contain rhodopsin
- confer high sensitivity to light (night vision)
- provide a low-resolution (grainy) image
- cones
- contain somewhat different pigments
- confer lower sensitivity to light (day vision)
- provide a high-resolution (sharp) image
- confer color vision, if more than one type of cone is present
- Light-sensing opsins belong to a family of proteins (G-protein coupled receptors, or GPCRs, a.k.a., "serpentine" proteins because they snake in and out of plasma membranes) found in all eukaryotes, and are involved in sensing external stimuli (such as chemical signals).
- In the earliest ancestor of animals, a GPCR apparently mutated and became able to sense a different type of stimulus: LIGHT.
- This early protein eventually developed into an opsin
- opsins further differentiated in animals into different types of opsins sensing light in different tissues
- They may only later have been recruited to sense light in actual light-sensing organs (eyes)
Eyes themselves may have evolved from simple patches of light-sensing cells on the skin to the complex structures we see today:
Perhaps the most amazing feature of the vertebrate eye is the light-focusing lens, composed of proteins called crystallins
- transparent
- stable (cataracts result from crystallins clumping with age)
- evolved from recruited ancestral genes that coded for heat-shock proteins
- a heat-stressed cell can be protected from death by being stabilized by small heat-shock proteins
- the lens itself has heat-shock protein properties
When it comes to evolution, there seems to be no end to the amazing variety and...grandeur.
Early Development
As a zygote develops and undergoes its orderly cleavages, the DNA in each new cell is repackaged and modified:
- some genes turn on (are expressed)
- some genes turn off (are not expressed)
- some genes code for proteins that turn other genes on or off
- these proteins are called transcription factors because they affect the cell's ability to transcribe DNA into RNA (transcription)
- such transcriptional control can be either positive or negative
Recall the Central Dogma:
DNA is transcribed into RNA, and RNA is translated into protein
The above is synonymous with gene expression.
(We now know that there are exceptions to this general rule. For example, some genes are transcribed only into functional RNA, such as transfer RNA (tRNA) or ribosomal RNA (rRNA), and never translated into protein.)
As the genetic instructions guiding a vertebrate embryo's development change in each new cell, the cells themselves follow those instructions, and are modified...
- some change form and function to become specific types of cells
- later, specialized cells clump together to form organs
- many cells migrate from one location to another, and contribute to tissues and organs far form where the cell first appeared
- still other cells die via programmed death (apoptosis) to facilitate proper formation of structures
All of this is governed by instructions on the DNA. Each cell has the same genome, but different genes are active and inactive in each type of cell.
Hox Genes and Genetic Toolkits
As we already have seen earlier in the semester, every animal embryo has a set of genes that determines its body axes, morphology, segmentation, limbs, and other features. This has been called the Genetic Toolkit.
Among these are the Hox Genes that determine the identity of the body segments in animals as diverse as fruit flies and mammals. How did these toolkits expand to make such different organisms? Duplication and Recruitment, as we have seen before. The Hox genes in fruit flies and mammals are homologous: inherited from a common ancestor.
In the two major lineages of animals, protostomes and deuterostomes, whom we have met before, major organ systems (circulatory, digestive, and nervous) are reversed in body position:
- Protostomes (flatworms, earthworms, mollusks, insects, crustaceans)
- nervous system is ventral
- intestine is dorsal
- circulatory system is dorsal
- Deuterostomes (starfish, vertebrates)
- nervous system is dorsal
- intestine is ventral
- circulatory system is ventral
Yet the genes that determine where these systems will develop are, once again, HOMOLOGOUS.
