690 Chapter 20 NEL
Inferring Relationships from DNA Sequences
At one time, scientists could compare and classify species based only on their mor-
phology and behaviour. For example, Charles Darwin found evidence for the theory of
evolution by comparing anatomical features ofdifferent species (see Chapter 6). Today,
biologists can compare the genetic makeup of different species for evidence of rela-
tionships among them.
Phylogenyis the proposed evolutionary history of a group of organisms, or of a
species. Overall, species that are closely related will share very similar DNA sequences,
while those that are more distantly related will have more genetic differences. For example,
you might expect that the sequence of DNA in a house cat’s genome would have more
similarities to that of a lion than to a sparrow. As we have seen, the DNA of any organism
can mutate. Natural selection acts on beneficial and harmful mutations in a popula-
tion, changing the relative proportions of these mutations that are passed on from gen-
eration to generation. The genomes of two species with a recent common ancestor would
have had less time and opportunity for mutations to accumulate and be selected, and so
we can predict that they would show fewer differences.
Mutations do not occur only in genomic DNA. Nuclear DNA is often quite a large
genome, so for some research it is more efficient for scientists to examine the changes in
the smaller genomes of mitochondria or chloroplasts. In particular, mitochondrial DNA
(mtDNA) can be used to trace inheritance through the maternal line in mammals, as the
egg is the only source of the mitochondria that are passed on to new offspring.
Mitochondrial DNA has also provided some fascinating clues about the evolutionary
history of modern humans. Two theories are proposed to explain the current distribu-
tion of humans around the world. One proposes that modern humans,Homo sapiens,
evolved simultaneously in different regions of the world from an earlier species,Homo
erectus. This theory is called the multiregional model and proposes that the different
ethnic groups observed worldwide today would have begun their evolution to Homo
sapiensbetween one and two million years ago. According to this model, the groups
interbred to some degree, and so didn't form into different species. The second theory,
called the monogenesis model, proposes that Homospecies moved out of Africa twice:
first as Homo erectus,and second as Homo sapiensbetween 100 000 and 200 000 years
ago, and that modern ethnic groups are all descendants of the second migration.
Mitochondrial DNA analyses for a variety of individuals, representing the ethnic
groups found around the world, seem to support the monogenesis model. The greatest
variety of mtDNA mutations exist in African ethnic groups, which is consistent with
the theory that mutations accumulate over time and that the population that has existed
the longest will demonstrate the largest accumulation of mutations. Additionally, the
mtDNA from ethnic groups on continents other than Africa were traced back to Africa
rather than to each other.The most common oncogene, ras,is found in 50 % of colon
cancers and 30 % of lung cancers. Present in normal cells, ras
makes a protein that acts as an “on” switch for cell division.
Rasensures that cells divide to replace damaged or dead
cells. After a sufficient number of cells have been produced,
the rasgene should be turned off. But the cancer-causing
oncogene produces a protein that blocks the “off” switch.
With the switch left on, cell division goes on continuously.Case Study Questions- Why do many scientists believe that certain viruses cause
cancer?
2.How does sunlight cause cancer?
3.List three environmental carcinogens and suggest a
possible source for each.
4.Distinguish between oncogenes and proto-oncogenes.
5.Explain how oncogenes are activated.
6.What is the ras gene?
phylogenyproposed evolutionary
history of a species or group of
organisms
Learning Tip
Lab Exercise 5. A in Chapter 5
shows an example of how
differences in genomic DNA
sequences provide evidence for
the relationships among
various species.DID YOU KNOW??
The Romanovs
Mitochondrial DNA was used to
identify the suspected remains of
the imperial Romanov family in
Russia, who were murdered by the
Bolsheviks in 1918. To do so,
mitochondrial DNA from Prince
Philip of England, a close relative of
the former Tsarina Alexandra
through his maternal side, was
compared to mitochondrial DNA
recovered from the remains,
resulting in positive identification
and the resolution of an 80-year-old
mystery.