Encyclopedia of Evolution.pdf - Online Reading Center

meiosis
Zoology. He became director in 1961. He retired as director
in 1970 and from the museum in 1975. This gave him the
opportunity to become a prolific writer, particularly on the
historical development of biological science.
Throughout his career, Mayr emphasized the allopatric
mode of speciation that results from geographical isolation,
which he considered the predominant mode of speciation in
birds and mammals. However, he recognized that many other
types of speciation are possible, especially in microorganisms.
Rather than feel threatened by the emergence of sympatric
speciation, speciation by hybridization and polyploidy, horizontal
gene transfer, and symbiogenesis, he embraced
them: At age 100, he wrote, “There are whole new worlds to
be discovered with, perhaps, new modes of speciation…” He
died February 3, 2005.
Further Reading
Margulis, Lynn. “Ernst Mayr, Biologist extraordinaire.” American
Scientist 93 (2005): 200–201.
Mayr, Ernst. “80 years of watching the evolution scenery.” Science
305 (2004): 46–47.
———. The Growth of Biological Thought: Diversity, Evolution, and
Inheritance. Cambridge, Mass.: Harvard University Press, 1982.
———. One Long Argument: Charles Darwin and the Genesis of
Modern Evolutionary Thought. Cambridge, Mass.: Harvard University
Press, 1991.
———. This Is Biology: The Science of the Living World. Cambridge,
Mass.: Harvard University Press, 1997.
———. What Evolution Is. New York: Basic Books, 2001.
———. What Makes Biology Unique?: Considerations on the Autonomy
of a Scientific Discipline. New York: Cambridge University
Press, 2004.
meiosis Eukaryotic cells (see eukaryotes, evolution of)
have nuclei that contain chromosomes. Chromosomes are the
structures that contain DNA (see DNA [raw material of
evolution]). When a cell reproduces itself, the chromosomes
of its nucleus replicate, then the rest of the cell divides. There
are two types of cell and nuclear division. In mitosis, two
cells result, which have the same number of chromosomes
as the original cell. Mitosis occurs in most of the tissues and
organs of organisms as they grow and as they replace old
cells. Meiosis is a type of nuclear and cell division that occurs
during sexual reproduction. During meiosis, the nuclei reduce
their chromosome number by half when they divide. Meiosis
produces the sex cells: the spores of fungi and plants, and the
eggs and sperm of animals. Eggs and sperm are collectively
called gametes. When an egg and a sperm fuse together in
the process of fertilization, they form a zygote. Fertilization
restores the original chromosome number. In every generation
in which sexual reproduction occurs, meiosis alternates
with fertilization.
Chromosomes of almost all eukaryotic cells occur in
pairs. Each organism inherits one member of each pair from
its female parent, the other from its male parent. What is it
about the chromosomes that make them “a pair”? They function
as a pair because each chromosome carries genes that
code for the same traits as the other chromosome in the pair.
Consider a chromosome in a plant cell that carries the gene
for red flowers. The chromosome with which it is paired also
has the gene for flower color, at the same location on the
chromosome—but not necessarily the same form of the gene.
The other chromosome may have the same form of the gene,
for red flowers; or it may have another form of the gene, coding
for white flowers. Different forms of the same gene are
called alleles. The two chromosomes of this pair are homologous
to one another (they are homologs). Chromosomes
in pairs are homologous to one another just as people are
homologous to one another. People have eyes, noses, mouths,
chins, arms, etc., at the same relative locations on their bodies,
but the forms of these organs can be very different. Chromosomes
that form pairs with one another are homologous,
usually not identical, just as people are homologous, usually
not identical.
Because the chromosomes of most eukaryotic cells occur
in pairs, the cells are called diploid (di- denotes two), sometimes
abbreviated 2N. Spores and gametes have chromosomes
that are unpaired; they are called haploid, sometimes abbreviated
1N or simply N. Diploid cells of humans normally have
46 chromosomes, in 23 pairs; human haploid cells have 23
chromosomes, with no pairs. Meiosis produces haploid cells
from diploid cells; fertilization unites haploid cells back into
diploid cells.
Meiosis consists of two divisions (referred to as Meiosis
I and Meiosis II). In the first division, one cell becomes
two; in the second, two cells may become four. The stages
of the cell cycle during each division have the same names in
meiosis and in mitosis (interphase, prophase, metaphase, anaphase,
telophase).
Interphase. During the interphase that precedes meiosis,
each chromosome replicates and forms two chromatids.
However, chromosome replication is not quite completed.
Each chromosome contains short stretches of DNA that have
not yet been duplicated.
First division. The first division of meiosis separates
homologs from one another:
• The homologs recognize and line up next to each other as
the first phase, prophase I, begins. This happens because
the short unduplicated segments of DNA in one chromosome
recognize corresponding segments of its homolog.
There are special proteins whose only function is to promote
the recognition between homologs so that they can
come together during meiosis. Because this stage of meiosis
requires pairs of chromosomes, cells with unpaired sets of
chromosomes cannot undergo meiosis.
• Once the homologs have paired with one another, chromatids
of the two homologs may cross one another at one or
more locations, break, then rejoin. This process is called
crossing over. At this time special proteins that function
only during meiosis clip the DNA of each homolog at the
same location and fuse the DNA strands back together,
having switched the strands of the two homologs. As a
result of crossing over, some alleles that had been on one
homolog now occur on the other homolog. That is, some