BIOB51 Lecture 4 Notes
How does similarity of characteristics provide evidence for Darwinian evolution? For descent
Similar location, similar embryonic origin but different function.
If organisms were perfectly adapted and placed perfectly in the environment that they’re in, why
do they show these similarities?
Descended from ancestors that didn’t necessarily live in the dark, they’ve just been able to
exploit whatever resources are in the cave and become adapted to cave living.
Vestigial structures: how evolution creates this, not created by selection acting against a
particular structure. If selection was acting to reduce or remove a particular structure, you might
not see any evidence of it at all. Also shows us that as species change, have adapted more to
their particular environments by using the energy from one function for a more advantageous
one making it thrive in its environment (possibly) and these adaptations get passed on. Because
it wasn’t directly selected against, it’s still there and because it’s not selected for, it’s become
You can see different structures becoming more important, more adaptive, and more common
and these other structures that are less advantageous becoming reduced. Energy shifted from
developing structures that aren’t necessarily aiding the individual to survive and reproduce
going towards different structures that are actually adaptations.
Whales have a non-functional pelvis that’s really tiny.
If we look back at the fossil record of extinct species that we think are ancestors to whales, we
see this pattern of a reduction.
Fetuses of different animals look similar because lots of animals have really similar embryonic
precursors to the same structures that act in different ways/forms. Slide 72:
These are all vertebrates, which binds them together in a phylogenetic perspective. All
vertebrates shared a common ancestor so they all share a certain amount of similarity in their
development. They all have a similar spinal development. Most vertebrates have limbs. They all
share similar limbic development. They’re all amnions. They all have an amnion sac that they
The similarity in development. They don’t develop like fish eggs. They’re not laid out in the open
but within an egg. Within that egg is an amniotic sac and that’s what the organism develops off
We see in all vertebrates, embryos develop in slight tails even in humans. We see a lot of
similarities even in things that disappear as embryos develop. Pharyngeal pouches are very
important at the very beginning of the embryo and they kind of create the space for different
hemispheres of the brain to form for our bilateral symmetry across our body and across the
bodies of all vertebrates. So even these things that end up disappearing as we develop are
really similar across vertebrates.
Alternative codes are possible. Maybe they can work better and make fewer errors. Since
humans have a similar genetic code, those viruses can not only attack birds but they can attack
us too so they [viruses] can use the same impact methodology.
This is the idea that mistakes give you insight into common origin. Textbook example: Two
students in a class. The male student is cheating off of the female student. If they both get the
answer right, we don’t really know if there was cheating going on. If they both get the same
answers wrong, that’s way more evidence for cheating than if they both get the same answers
A genetic flaw that exists within human chromosome 17. Duplication mutation part is
problematic because it results in unequal crossing over. So when the chromosomes line up and
cross over certain genetic material, what happens is…
Because what orients the lineups right is like really similar patterns on each chromosome but if
you have similar patterns that are really close together, you can get an improper lineup. This
results in unequal crossing over. So you’ll have some chromosome arms with way more
information and some arms with way less information. Then there’s a misalignment in meiosis
and you can get chromosomes that lack info and chromosomes that have too much info going to gametes. These specific lacks and specific gains and info (too much/too little) result in
phenotype: peripheral nerve disease. Problematic for humans. Without our modern day
treatments, individuals with peripheral nerve disease will probably have died without
The same duplication, the same problem can be found in our closest relatives. So they’re found
in both chimpanzees and bonobos. This is evidence that we have this common mistake. What
makes the most sense is that our common ancestor acquired this mistake and it’s been passed
down to us.
Similar Molecular homology with similar species origins.
Even though the structures may change form, when you look at it, they’ve got similar embryonic
precursors, similar locations in the body and potentially similar formal structures even though
there may be variation and maybe variation in function.
Your adaptations are based on what your ancestors had to work with. They’re not based on
something totally new and different that you just acquired.
We can check through historical collections of these shells to check for differences over time. So
if we look through the historical record, people who have collected these shells around the
1870s, we can get a measure of distribution of traits of these shells (spire height and shell
Do we have any evidence that natural selection has favoured adaptations in the shells to avoid
Experiment: Tethered two shells to a common line in a bunch of different places. The
experimenter tethered those (shells) in an area that had yet to be invaded by the crabs (no
crabs). Then she got further into the area where the crabs invaded so there was some crab
presence but not a lot and tethered another set of shells. Finally, she tethered the last set of
shells in an area where there were abundant crabs. Difference in the predator would provide
different strength of selection. She used shells that were on the edges of the distribution to test
these things so she thought maybe having a thicker shell might be adaptive, might reduce
predation from crabs so if the shell’s too thick and the crabs can’t crack it, they leave you alone. So she took the thinnest and thickest shell (respectively) she could find to see if there was
natural selection on shell thickness.
No crab area: Thinner shells and thicker shells do exactly the same and at 16 days, 100% of the
snails were alive. Shell thickness didn’t matter. High survival in both groups.
Her results (cont.):
Different survivorship in the