Chapter 3 - Remnants: Vestiges, Embryos & Bad Design

Question 1

Molecular palimpsets

Answer 1

• In species’ genomes is inscribed much of their evolutionary history, including the wrecks of genes that once were useful.
• What’s more, in their development from embryos, many species go through contortions of form that are bizarre: organs and other features appear, and then change dramatically or even disappear completely before birth.
• And species aren’t all that well designed, either: many of them show imperfections that are signs not of celestial engineering, but of evolution.

Question 2

Vestigial Trait

Answer 2

• A feature of a species that was an adaptation in its ancestors, but that has either lost its usefulness completely or, as in the ostrich, has been coopted for new uses.
  (Ostrich wings)
• Evolutionary theory doesn’t say that vestigial characters have no function.
• A trait can be vestigial and functional at the same time.
• It is vestigial not because it’s functionless, but because it no longer performs the function for which it evolved.
• Vestigiality is diagnosed not by its usefulness but because it no longer has the function for which it originally evolved.
• Goos bumps (arrector pili) are a vestigial trait.

Question 3

Ancestral features

Answer 3

a

Question 4

Atavism

Answer 4

• These sporadically expressed remnants of ancestral features are called atavisms.
• They differ from vestigial traits because they occur only occasionally rather than in every individual.

(sometimes whales are born with legs - atavism and also an evidence for evolution)

(human tail - coccygeal projection)

Question 5

Dead Genes

Answer 5

• Atavisms and vestigial traits show us that when a trait is no longer used, or becomes reduced, the genes that make it don’t instantly disappear from the genome.
• Evolution stops their action by inactivating them, not by snipping them out of the DNA.
• The normal function of a gene is to make a protein—a protein whose sequence of amino acids is determined by the sequence of nucleotide bases that make up the DNA.
• A gene that doesn’t function is called a pseudogene.
• Out of about 30,000 genes, for example, humans carry more than 2,000 pseudogenes.
• A dead gene in one species that is active in its relatives is evidence for evolution.
• We also harbor dead genes that came from other species, namely viruses.
• Some, called “endogenous retroviruses,” can make copies of their genome and insert them into the DNA of species they infect. (HIV is a retrovirus.)
  If the viruses infect the cells that make sperm and eggs, they can be passed on to future generations.
• This points to common ancestry.

Question 6

Palimpsets in Embryos

Answer 6

• All vertebrates begin development in the same way, looking rather like an embryonic fish.
• Why do different vertebrates, which wind up looking very different from each other, all begin development looking like a fish embryo?
• Why doesn’t natural selection eliminate the “fish embryo” stage of human development, since a combination of a tail, fish-like gill arches, and a fish-like circulatory system doesn’t seem necessary for a human embryo.
• The probable answer: involves recognizing that as one species evolves into another, the descendant inherits the developmental program of its ancestor: that is, all the genes that form ancestral structures.

Question 7

Ernst Haeckel

Answer 7

• German evolutionist.
• Formulated a “biogenetic law” in 1866
• “Ontogeny recapitulates phylogeny.”
• This means that the development of an organism simply replays its evolutionary history.

Question 8

Problems with Biogenetic Law

Answer 8

• Some species, like plants, have dispensed with nearly all traces of their ancestry during development.
• Haeckel’s law wasn’t strictly true.

Question 9

Embryology in the Origin & Lanugo

Answer 9

• Darwin considered embryology his strongest evidence for evolution.
• Today he’d probably give pride of place to the fossil record.
• One of my favorite cases of embryological evidence for evolution is the furry human fetus.
• We are famously known as “naked apes” because, unlike other primates, we don’t have a thick coat of hair.
• But in fact for one brief period we do—as embryos.
• (6th month) We become completely covered with a fine, downy coat of hair called lanugo.
• Lanugo can be explained only as a remnant of our primate ancestry: fetal monkeys also develop a coat of hair at about the same stage of development. Their hair, however, doesn’t fall out.