Encyclopedia of Evolution.pdf - Online Reading Center

Methylation can have lifelong effects on an organism. In
mice, when a mother neglects her offspring, the gene for
the glucocortoid receptor is inactivated, resulting in lifelong
anxiety in the offspring. Interestingly, it is not just the
genes that can be inherited from one generation to another,
but also the status of their activation. A gene can be passed
on into later generations in an inactivated form. Examples
include coat color in mice and flower form in toadflax.
Genetics is the study of the inheritance of genes; epigenetics
is the study of the inheritance of gene activation patterns.
Sometimes, small RNA molecules (such as the interfering
RNA molecules, RNAi) can chemically interfere with the
transcription of a gene.
Once expressed, genes interact in complex ways. A cascade
of gene expression occurs during embryonic development,
in which homeotic genes activate other genes, which
in turn activate still other genes (see developmental evolution).
Even after development of the organism, one gene
can affect another, a phenomenon called epistasis. Because of
epistasis, two mutations that are individually deleterious can
work together to produce a beneficial change in the organism;
and two mutations that are individually beneficial can work
together to produce a harmful effect on the organism. Because
of development and epistasis, genes do not usually have individually
recognizable effects on an organism. Therefore natural
selection does not generally act upon a single gene.
Amount of DNA in Cells
Only a small part of the DNA of eukaryotic cells consists
of genes. About 90 percent of the DNA of many eukaryotic
species does not encode genetic information. Some of
this noncoding DNA plays an essential role in controlling
the expression of the genes. For example, some noncoding
DNA may play essential roles in coiling and uncoiling the
DNA during cell division, and other noncoding DNA produces
RNAi (see above). A great deal of the DNA appears
to have no present function in the cells and has been called
“junk DNA.” Because scientists cannot be sure that it is useless,
they prefer to call it noncoding DNA. What, if anything,
DNA Comparison among Species
DNA (raw material of evolution)
most of the noncoding DNA does is one of the major unanswered
questions of modern genetics.
Most eukaryotic cells have two chromosomes of each
type. That is, most eukaryotic organisms consist mostly
of diploid cells. In the cells of some species, extra copies of
chromosomes can be present. Triploid cells have three chromosomes
of each type, tetraploid cells have four, and pentaploid
cells have five. Collectively, cells with more than two
chromosomes of each type are called polyploid cells. In many
plants, polyploid cells can function normally. Polyploidy
allows hybrid plants to reproduce (see hybridization).
Most animals cannot tolerate polyploidy. This is the reason
that some plants have a huge amount of DNA in the nuclei of
their cells.
Largely as a result of noncoding DNA and polyploidy,
cells of different species differ enormously in the amount of
DNA that they contain, and there is no relationship between
the complexity of an organism and the amount of DNA that
its cells contain. When only the genes are taken into account,
a general pattern emerges in which more complex organisms
have more genes (see table).
The number of genes is not a complete explanation for
the complexity of an organism. Humans have about 35,000
genes, which may be the most of any organism. Mice have
almost as many genes as humans do. Even the nematode
worm Caenorhabditis elegans, which has only 959 cells in its
entire body, has 20,000 genes. The complexity of an organism,
therefore, cannot be explained simply by the number of
its genes. The emergence of complexity in humans is largely
due to the pathways of interaction among the genes.
Where New Genes Come From
Sometimes, a gene can be copied more than once, and the
copy is inserted into another location among the chromosomes.
This process is called gene duplication. The cell now
has two copies of the gene instead of one. The two copies are
now free to evolve in different directions and encode two different
proteins rather than just one. One example of this is
the globin gene, which encodes a protein that binds oxygen
gas. This gene is found widely among animal species. This
Million base % of DNA
Million
base pairs
Organism Genus pair total that is coding of coding DNA
Bacterium Escherichia 4 100 4
Yeast Saccharomyces 9 70 6
Nematode Caenorhabditis 90 25 22
Fruit fly Drosophila 180 33 60
Cress plant Arabidopsis 200 31 60
Human Homo 3,500 9–27 320–950
Newt Triturus 19,000 1.5–4.5 290–860
Lily Fritillaria 130,000 0.02 30
Lungfish Protopterus 140,000 0.4–1.2 560–1680