CH. 3 (Vestiges, Embryos, and Bad Design) Flashcards
Vestiges, Embryos, and Bad Design
Palimpsest
recycled manuscript, “palimpsestos” meaning scraped again in greek. Organisms are palimpsests of history.
Vestigial Trait
a feature of a species that was an adaptation in its ancestor but no longer is useful or no longer performs the function for which it evolved.
Examples of vestigial traits
flight - metabolically expensive, large wings are burden. flightless birds evolved on islands where there aren’t predators or comp. for food on ground
eyes - in cave dwelling creatures vision isnt useful and also metabolically expensive. They lost sight either because mutations favoured their loss or mutations that reduced vision were not selected against and thus accumulated (blind mole rats have vestige of eye that sees minimal light and helps them regulate activity)
whales - have tiny pelvic bones and hand limbs embedded in tissue
Humans - have an appendix, coccyx, arrector pili, wiggly ears
Atavism
come from re-expression of genes that were functional in ancestors but silenced by natural selection when they were no longer needed.
- Must recapitulate ancestral trait in an exact way
- occur occasionally rather than in every individual
- dormant genes reawakened but the DNA is degraded after millions of years of non use
Examples of atavisms
- 1 in 500 whales born with rear leg that protrudes
- coccygeal projection (human tail) we start with tail but it disappears 7 weeks into development
Experiment done with hens and mice tissue?
developing tissue from jaw of mouse was grafted onto chicken embryo, the chicken developed tooth like structures thus proving that molecules from the mouse awoke the dormant developmental program in chickens (important protein was missing).
Dead gene
silenced gene (vestigial gene no longer intact or expressed). Our genomes are graveyards!
pseudogene
gene that doesn’t function either because control regions for gene have been inactivated or mutations have changed gene such that a usable protein can no longer be made (of 30 000 human genes, 2000 are pseudogenes)
GLO
gene used to produce enzyme called C-gulano-y-lactone oxidase (makes vit. C from glucose). Pseudogene in humans, guinea pigs, and fruit bats (deactivation different in guinea pigs than in humans but had same effect). Mutation that silenced this gene was present in ancestor of all primates and was passed along.
endogenous retroviruses
make copies of their genome and insert them into the DNA of species they infect. If the virus infects cells that make sperm/eggs that can be passes on to future generations (human genome contains thousands of such viruses rendered harmless by mutations) .
Mice OR?
Mice have 1000 olfactory receptor genes. Each produces a different protein that recognizes a different airborne molecule, combos of signals allows them to recognize a lot of smells. Humans have and express fewer OR genes than mice (we have 800, express 400).
What pattern does our embryonic development show?
fish -> amphibian -> reptiles -> mammals
ontogeny recapitulates phylogeny
Branchial arches
arches that lie on each side of the embryo by future head—contain tissues that develop into nerves, blood vessels, muscles, bones, cartilage
Why is our development so convoluted (3 of our branchial arches disappear, 2 of our kidneys, etc)
We follow the evolutionary path of our ancestors, so it’s easier to tack on to that developmental program than to cut and edit it (we also often require biochemical cues from features that appear earlier).
examples of developmental add ons/rearrangement later on…
Languo - fine hair that covers 6 month human fetus and shed 1 month before birth
Dolphins and whales have embryonic hindlimb buds that are reabsorbed
Flatfish - born as normal fish but as they develop (out of utero) their skull changes shape and one of their eyes migrates
Women give birth through pelvises obviously very painful for such huge headed animals (if we gave birth through abdomen that would be fine but we didn’t evolve from creature who did)
Also, ectopic pregnancies. We used to shed eggs directly out of ovary to outside of body (fish reptiles), humans have a uterus and the fallopian tube is an imperfect connection (egg has to jump)
