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.

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:

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:

    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:

    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...

    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:

    Yet the genes that determine where these systems will develop are, once again, HOMOLOGOUS.