Whale Evolution Shows Constraints
While I am in the grip of a soul crushing sinus and ear infection, Dr. Christine Janis, professor of Biology at Brown University, has provided me with another guest post.
With regards a creationist’s complaint that “evolution can do anything” when it comes to whale evolution (someone who thought that changing feet into flippers meant a radical change in body plan that couldn’t possible happen). Actually, evolution is extraordinarily constrained by history — if it were not then a dog could give birth to a cat, and octopus to a whale, and we’d see no pattern of common descent, and thus would never know that evolution had happened. Here’s how whales are constrained by evolution. (Note: this is far from comprehensive in terms of characters — I left out entire systems like circulation — and I could have done quite a few more subcategories. However, I think I got most of the basics — if I left out something glaringly obvious, please let me know!)
Whales belong to the phylum Chordata. Like all chordates they have (at some point in their life cycle) the following features:
- pharyngeal pouches (that perforate as gill slits in some adult chordates);
- a notochord for support down the back (forms the intervertebral discs in us and whales as adults);
- a tail that extends out beyond the end of the gut (as do we, of course, as embryos);
- segmental muscles in the trunk with a complex infolding pattern (as seen in the “6 pack” stomachs of atheletes);
- a nerve cord that runs down the back (rather than the front), above the notochord, and is hollow in the center;
- a thyroid gland (which is the “endostyle” in non-vertebrate chordates, and larval lampreys –metamorphosing tp the thyroid in the adult).
All chordates, including whales, have these features: no chordate ever completely loses these features, and they will always be chordates; any evolution has to involve tinkering of this basic chordate body plan.
Whales belong to the subphylum Vertebrata: here are some modifications of the chordate body plan seen in all vertebrates:
- they have some sort of segmental cartilaginous supports around the notochord (i.e., “vertebrae”, reduced in the living jawless fishes);
- they have a distinct kidney and liver (these functions are carried out in other chordates, but not by distinctive, separate organs);
- they have a three-part brain, with portions corresponding (from front to back) the three basic sense organs: nose, eyes (with the “backwards” retina retained in us), and inner ear (originally for balance, containing a posterior and anterior semi-circular canal on each side);
- they have a new kind of tissue in development, neural crest, that makes up much of the front end, including distinctive cartilaginous bars that support the pharyngeal pouches (thanks to Metaspriggina we now know that the primitive condition of these gill bars was to be jointed, with the gill tissue external to the bars);
- the pharyngeal pouches now also contain muscle tissue that power the pumping of water over the gills in adult fish (or that move our jaws, and work our larynx);
- these muscles are innervated from the brain in a characteristic fashion —– from front to back from cranial nerves five (first pouch), seven (second pouch), nine (third pouch) and ten (4th to 8th pouches);
- the eyes are moved by a set of six muscles, all innervated in a characteristic fashion — most by cranial nerve 3, but one by cranial nerve 4 and one by cranial nerve 6; a doubling of the original set of Hox genes (now two strands).
All vertebrates, including whales have these features (most whales have lost the sense of smell and the olfactory portion of the forebrain, but they still have the pseudogenes for olfaction): no vertebrate ever completely loses these features, and they will always be vertebrates; any evolution has to involve tinkering of this basic chordate/vertebrate body plan.
Whales belong to the grouping Gnathostomata, or jawed vertebrates: here are some additional features to the basic chordate/vertebrate plan:
- They have jaws, formed from the cartilaginous bar in the first (mandibular) pharyngeal pouch, that contain teeth (made from enamel and dentine) (although baleen whales lack teeth as adults, they form tooth buds as embryos);
- they have bone tissue, formed from cells called osteocytes;
- they have a two pairs of limbs, in the front and back of the trunk, with limb girdles embedded in the body;
- they have a cerebellum in the hind brain, and a horizontal semi-circular canal on each side in the inner ear (in addition to the other two on each side);
- their nerves are covered with a lipid insulation, myelin;
- they have a distinct pancreas, that secretes both hormones (insulin) and digestive enzymes;
- they have distinct reproductive ducts draining the gonads (rather than the gametes being dumped into the body cavity as in other chordates);
- they have a further doubling of the hox genes (now 4 strands).
All gnathostomes, including whales, have these features: no gnathostome ever completely loses these features, and they will always be gnathostomes; any evolution has to involve tinkering of this basic chordate/vertebrate/gnathostome body plan.
Whales belong to the grouping Osteichthyes. In addition to the previous features they also have:
- bone tissue in the internal skeleton (originally just cartilaginous, always preformed in cartilage in development);
- a lung or similar type of air bladder (in addition to gills in bony fishes) (secondarily lost in a few forms, such as plethodontid salamanders);
- dermal bones (formed in the skin, not preformed in cartilage) around the originally cartilaginous jaws, with embedded teeth, in a characteristic pattern also seen in us—– in the upper jaw the premaxilla and maxilla, in the lower jaw the dentary.
All osteichthyans, including whales, have these features: no osteichthyan ever completely loses these features, and they will always be osteichthyans; any evolution has to involve tinkering of this basic chordate/vertebrate/gnathostome/osteichthyan body plan.
Whales belong to the grouping Tetrapoda. In addition to the previous features they also have:
- fins now forming limbs, with digits, with the homologous bones in each set: i.e., humerus/femur, radius + ulna/tibia + fibula, carpals/tarsals, metacarpals/metatarsals, phalanges (finger and toe bones) (even though extant whales have lost the hind limbs, they retain hind limb buds in development);
- a urinary bladder (secondarily lost in some birds and reptiles);
- salivary glands and lachrymal glands;
- a muscular tongue innervated by cranial nerve 12 (unlike the muscular tongue of lampreys, which is innervated by cranial nerve 5);
- a stapes that transmits airborne sound, originally the hyomandibular bone (part of the cartilaginous suppor of the second pharyngeal pouch) that supported the upper jaw and the operculum in bony fishes (but the enclosed middle ear and eardrum evolved separately in different tetrapod groups);
- a transversus abdominus muscle for exhaling on land.
All tetrapods, including whales, have these features: no tetrapod ever completely loses these features, and they will always be tetrapods; any evolution has to involve tinkering of this basic chordate/vertebrate/gnathostome/osteichthyan/tetrapod body plan.
Note here that tetrapods have all lost the internal gills: the embryonic gill buds now form the parathyroid gland, used for calcium regulation. Because the embryonic gills have been repurposed, tetrapods never regain them.
Whales belong to the grouping Amniota. In addition to the previous features they also have:
- an embryo with three extraembryonic membranes in addition to the yolk sac — chorion, amnion, and allantois;
- lungs that are linked to the mouth via a trachea (a tube with cartilaginous rings);
- ribs that are mobile and used to inhale air into the lungs;
- an ankle joint with a distinct astragalus and calcaneum (seen in the earliest whales such as Pakicetus);
- a compact (metaneprhic) kidneys drained by a new tube, the ureter.
All amniotes, including whales, have these features: no amniote ever completely loses these features, and they will always be amniotes; any evolution has to involve tinkering of this basic chordate/vertebrate/gnathostome/osteichthyan/tetrapod/amniote body plan.
Whales belong to the grouping Mammalia. In addition to the previous features they also have:
- mammary glands in the females producing milk to nourish the young;
- hair made of alpha keratin, and formed in a different way from the beta keratin scales of reptiles/feathers of birds (although whales lack hair, they have the pseudogenes for the production of hair);
- a diaphragm, to help with respiration;
- a backbone that now primarily bends up and down (as in a whale swimming), rather than side to side (as in a fish swimming);
- facial muscles, including ones around the lips that allow for suckling — these muscles are innervated by cranial nerve 7, in contrast to the near by muscles of mastication that are innervated by cranial nerve 5;
- a secondary palate separating the nose and mouth cavities, plus a means of sealing off the back of the pharynx with the tongue pressed to the secondary palate (again essential for suckling);
- teeth that differentiated into incisors, canines, molars and premolars, and are only replaced once (at maximum) during the lifetime — the molars are now more durable with double roots and prismatic enamel;
- a lower jaw composed of a single bone, the dentary, and which also forms the jaw articulation with the squamosal bone on the skull;
- a three-boned middle ear, with the bones in addition to the stapes, the malleus and incus, being homologous with bones forming the original gnathostome jaw joint (as they still do in embryonic mammals);
- short fingers and toes, with never more than two segments in the thumb or big toe, and never more than three in the other digits;
- eyes where two of the original four opsin genes are now pseudogenes, giving mammals at best dichromatic color vision (anthropoid primates get back tricolor vision by a gene duplication, whales loose one more opsin to have monochromatic vision, no mammal regains the original vertebrate tetrachromatic color vision except for perhaps some female humans).
All mammals, including whales, have these features: no mammal ever completely loses these features, and they will always be mammals; any evolution has to involve tinkering of this basic chordate/vertebrate/gnathostome/osteichthyan/tetrapod/amniote/mammalian body plan.
Whales belong to the grouping Theria. In addition to the previous features they also have:
- an external ear (pinna) (secondarily lost in whales, and many other aquatic mammals);
- live birth (viviparity);
- separation of the alimentary (gut) and urogenital ducts so that they now form separate openings (as opposed to a single opening of other vertebrates, the cloaca);
- as a consequence of this, male therians now use the penis for urination as well as sperm transmission;
- the coracoid bone of the shoulder girdle has essentially been lost, allowing the scapula (shoulder blade) to be mobile and to function as an additional limb segment (for bounding in terrestrial forms).
All therians, including whales, have these features: no therian ever completely loses these features, and they will always be therians; any evolution has to involve tinkering of this basic chordate/vertebrate/gnathostome/osteichthyan/tetrapod/amniote/mammalian/therian body plan.
Whales belong to the grouping Placentalia. In addition to the previous features they also have:
- young that are retained in the uterus past the duration of a single estrus cycle;
- a corpus callosum linking the two sides of the brain;
- a sperm duct that loops over the ureter before entering the urethra;
- a maximum of six upper and six lower incisors.
All placentals, including whales, have these features: no placental ever completely loses these features, and they will always be placentals; any evolution has to involve tinkering of this basic chordate/vertebrate/gnathostome/osteichthyan/tetrapod/amniote/mammalian/therian/placental body plan.
So, as you can see, whales are indeed quite constrained as to how much they can modify their body plan in evolution. Still, a bit of tinkering with the basic terrestrial placental plan did the job quite nicely.
Note that with the exception of this last sentence, and a few asides, there is absolutely nothing about evolution in this post: all of these features (and their persistence through the phylogenetic hierarchy) can be observed in the diversity of chordates living today.