Problem Set 8 Key - BICD100 FA2011 Problem Set#8 KEY with...

BICD100 FA2011 Problem Set #8 KEY
with corrections on pg. 4 added 11/28 in red font
1. Name three observations discussed in class supporting the conclusion that new species arise via
evolution.
We are all genetically related. Highest degree of sequence similarity observed for species already
thought to be closely related based on other criteria such as similar morphology. For example, our
genes and proteins are 99% identical to those of chimpanzees.
The basic idea of evolution is that mutations occur at random, there is selection for the fittest
genotypes and/or selection against the least fit genotypes, and new species arise as a result of the
accumulation of genetic changes over time. Evidence discussed in class for both negative and
positive selection:
Non-synonymous substitutions accumulate more slowly than synonymous ones. Non-synonymous
substitutions are ones that change protein sequence – many will be deleterious to protein function,
reducing fitness and potentially selected against. The difference between accumulation rate of
synonymous vs. non-synonymous substitutions is evidence of negative selection.
Specific genes have been identified that are responsible for different aspects of the evolutionary
transition from teosinte to maize. For both tb1 and tga1 , allelic variation is greatly reduced in maize
compared to teosinte, as predicted for a gene that was subjected to strong positive selection during
evolution of maize from teosinte (due to “bottleneck effect”).
2. What is the evidence that the maize teosinte branched ( tb1 ) gene was the target of positive selection
during maize evolution from teosinte?
The promoter region of tb1 (which regulates its expression) shows only 2% as much allelic diversity
in maize as in teosinte (compared to 70% for a neutral gene)
3. What is the evidence that maize evolution from its wild ancestor, teosinte, involved changes in gene
regulation?
Increased expression of the tb1 gene during the transition from teosinte to maize caused
suppression of branch outgrowth.
4. What is the evidence that maize evolution from its wild ancestor, teosinte, involved changes in protein
function?
The tga1 gene is implicated in maize evolution as well (see lecture 24 pdf pg. 13). A single amino
acid change in the TGA1 protein brought about the softening of kernels during the transition from
teosinte to maize.
5. Many plants produce flowers consisting of four organ types arranged in concentric "whorls". In wild-
type Arabidopsis thaliana , whorl 1 organs are sepals (green leaf-like organs that protect the flower before
it has opened), whorl 2 organs are petals, whorl 3 organs are stamens (male reproductive structures that
make pollen), and whorl 4 organs are carpels (female reproductive structures containing eggs).