snake venom is an example of
a complex adaptation
complex adaptations are
phenotypic traits requiring multiple, specific mutations to yield a functional advantage
coexpressed traits that experience selection for a
common, often novel, function
DNA can code for
gene, proteins, and RNA molecules
gene control regions
an upstream section of DNA that includes the promoter region as well as other regulatory sequences that influence the transcription of DNA
a promoter is a
region of DNA upstream of a gene where revelant proteins (such as the RNA polymerase and transcription factors) bind to initiate transcription of that gene
gene control regions can also
bind repressors or transcription factors to regulate the expression of nearby genes
a repressor
is a protein that binds to a sequence of DNA or RNA and inhibits the expression of one of more genes
a transcription factor
is a protein that binds to specific DNA sequences and acts like a light switch by turning all the sequences on or off simultaneously
regulatory networks are often
involved in complex adaptations
regulatory network is a
system of interacting genes, transcription factors, promoters, RNA, an other molecules. It functions like a biological circuit, responding to signals with output that controls the activation of genes, during development, the cell cycle, and the activation of metabolic pathways.
hox genes
a set of transcription factor genes show unusual property, provide a glimpse of one way where gene expression is translated into the many different forms that metazoans (animals) exhibit.
hox genes, in contrast, are genes that
specify segment identify and they are all clustered together in one (usually) tidy spot. Within that cluster, there is even further evidence of order. The genome seems to have various randomly scattered genes, with no order present in the arrangement on a chromosome, the order is only shown in expression thru the process of development.
drosophila has
8 hox genes in a row, and the genes' order within that row shows their order of expression in the fly body. (3' end of DNA strand [denoted lab: labial] is shown in the head, right of the DNA strand [abd-B: abdominal-B] is shown at the end of the fly's abdomen)
hierarchical gene organization controls
development of animal embryos
some expressed genes shut down other genes, for example Distallelss (DII), involved
in the production of antennae and legs, is repressed by homothorax (hth) and so is not expressed in the abdomen of the fly
hox genes are expressed
during development (development genes)
knocking out individual Hox genes in Drosophila causes
homeotic transformations (one body part develops into another)
Antennapedia mutant
lega develop on the fly's head instead of antennae
Hox genes enable the
development of morphologically distinct regions in a segmented animal (activation from the 3' end s the start of a segment to develop into part of the head)
mutations to genes at the top of the hierarchy can have
drastic effects
gene duplication can produce
novel functions
promiscuous proteins
capable of carrying out two functions; likely to take on new functions if duplicated
paralog
homologous gene that arises by gene duplication
gene recruitment
co-option of a particular gene or network for a totally different function as a result of a mutation; reorganization of a preexisting regulatory network can be a major evolutionary event
New adaptations in microbes: E.coli long term evolution experiment
12 initially identical populations of asexual Escherichia coli bacteria, evolved the ability to feed on citrate in the growing medium in the absence of oxygen
snake venoms evolved through
duplication and co-option
crotamine genes in snake venome are
closely related to beta-defensins the bacteria-fighting molecules found in many vertebrates
regulatory mutation led to
production of defensin gene in the mouth
venom genes have been recruited from
genes expressed in many organs in snakes
venom evolved before
snaked evolved
directional terminology
anterior: front
posterior: rear
dorsal: back
ventral: belly
proximal: close to center of mass
distal: away from the center of mass
hox genes are part of a conserved
"genetic toolkit" among animals (common ancestor passed this gene to flies and mammoths[diverse animals])
dorsal-ventral patterning is conserved; flies and mice use
homologous genes for dorsal-ventral patterning, though they are expressed as "mirror images"
the phylogeny of animals informs as to how
the genetic toolkit was deployed (dorsal-ventral patterning and single nerve cord in annelids, anthropods, and vertebrates)
mouse legs start as a bulge known as a limb bud
mice build proximal structures then more distal structures
fly legs develop as a series of concentric structures
the "bulls-eye" is the most distal part of the leg
ortholog
is one of two or more homologous genes separated by a speciation event
paralogs
homologous genes produced by gene duplication, that are both possessed by the same species
fly and mouse leg genes reflect
derivation from genes in a common ancestor
in flies, Engrailed (En) gene helps
define the posterior portion of the limb bud in mice. its ortholog in mice has an identical role in the leg disk
in flies, engrailed proteins
turn on the expression of the signaling gene Hedgehog (Hh)
combination of signals (high Hh and absence of En) causes
cells to begin expressing two additional signaling genes Decaptenaplegic (Dpp) and Wingless (Wg), these interact to produce (DII) and (EGFR) affecting the development of limb
blocking Shh gene expression
stops limb patterning pathway
fins appear in the
foosil record 400 mys
in fish mesodermal tissues produce
a cluster of skeletal bones near the base of the fin
AER (apical ectodermal ridge) (Hoxa11 and Hoxd13 overlap)
stimulated distal growth of the limb bones in the mesodermal tissue (when Hox genes ceases, fin rays begin to grow from extodermal ridge tissue)
in tetrapods, skeletal bones derived from
the mesoderm become much longer, forming the long bones of the limb (later the smaller bones develop)
in tetrapod limbs, Hox genes become
active a second time later in development and their domains of expression no longer overlap (evolution of tetrapods limbs involves changes to the timing and location)
Hoxd13 in development of a fish fin (create an appendage with a striking resemblance to a tetrapod limb)
this is a pleiotropic effect, simultaneously shrinking the outer area of the fin where fin rays develop and expanding the region where bone grows
expression differences in a single gene give
rise to limb elongation
increased expression of Bmp2 stimulates
extra growth in the finger bones of bat forelimbs
complex eyes have evolved in
several different lineages
each kind eye contains ___ for directing incoming light and ___ opsins for capturing it. But particular molecule for each eye are different.
crystallins, opsins
crystallin
water-soluble structural protein found in the lens and the cornea of the eye for transparency
opsins
proteins that bind to light-reactive chemicals to underline vision, phototaxis, circadian rhythms, and other light meditates responses of organisms
opsins evolved in a
common ancestor around 1 bya
crystallins evolved though
gene recruitment
gene recruitment (or co-option)
the placement of a new gene under a foreign regulatory system, can lead to increase in genomic complexity, is recognized as major driving force in evolution
laws of physics
atmospheric O2 concentrations constrain insect size
antagonistic pleitropy
single gene affects expression of many traits mutations may have positive effects for one trait but negative effects for another trait
antagonistic pleiotropy: number of cervical vertebrae in all mammals
7 in their necks
note the routing of the recurrent laryngeal nerve in the giraffe neck,
carries the constraint of looping around gill arch blood vessels
convergent evolution
independent evolution leading to similar traits in two different lineages (similar selection pressures)
parallel evolution
independent evolution of similar traits in multiple lineages, all starting from a similar ancestral condition
cavefish species exhibit
parallel evolution in depigmentation
deep homology
growth and development of traits in different lineages result from underlying mechanism inherited from a common ancestor
deep homology may help to
explain cases of parallel evolution