Geology Reference
In-Depth Information
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Figure 18.6
Meiosis and Mitosis
Nucleus
Nucleus
A cell prepares
to divide.
Chromosomes
become visible
2 pairs of
chromosomes
Cell
division
occurs
along an
equator
Sperm
Nuclear membrane
disappears and
chromosomes
duplicate
Nuclear
membrane
disappears,
chromosomes
duplicate and
line up along
an equator
Egg
Four sperm
are the result of
each cell
dividing
down the
middle
Fertilized egg
(b)
Cell divides and
the nuclear
membrane reappears
(c)
(a)
During meiosis, sex cells form that contain
one member of each chromosome pair. The
formation of sperm is shown here: eggs form
the same way, but only one of the four fi nal
eggs is functional.
The full number of
chromosomes is restored when a
sperm fertilizes an egg.
Mitosis results in the complete
duplication of a cell. In this example, a cell
with four chromosomes (two pairs) produces
two cells, each with four chromosomes.
Mitosis takes place in all cells except sex
cells. Once an egg has been fertilized, the
developing embryo grows by mitosis.
a
c
b
effect on the type of protein synthesized. In other words, the
same protein is synthesized before and after the mutation,
and thus the mutation is neutral.
What causes mutations? Some are induced by
mutagens
,
agents that bring about higher mutation rates. Exposure to
some chemicals, ultraviolet radiation, X-rays, and extreme
temperature changes might cause mutations. Some muta-
tions are spontaneous, taking place in the absence of any
known mutagen.
offspring. Lions and tigers can also interbreed in captivity,
although they do not interbreed in nature and their off-
spring are sterile, so they too are separate species. Domes-
tic horses that have gone wild can interbreed with zebras to
yield a
zebroid
, which is sterile; thus, horses and zebras are
separate species. It should be obvious from these examples
that reproductive barriers are not complete in some species,
indicating varying degrees of change from a common ances-
tral species.
The process of speciation involves a change in the genetic
makeup of a population, which also may bring about changes
in form and structure. According to the concept of
allopatric
speciation
, species arise when a small part of a population be-
comes isolated from its parent population (
Speciation
, the phenomenon of a new species arising from an
ancestral species, is well documented, but the rate and ways
in which it takes place vary. First, though, let us be clear on
what we mean by
species
, a biological term for a population
of similar individuals that in nature interbreed and produce
fertile offspring. Thus, a species is reproductively isolated
from other species. This defi nition does not apply to organ-
isms such as bacteria that reproduce asexually, but it is nev-
ertheless useful for our discussion of plants, animals, fungi,
and single-cell organisms called protistans.
Goats and sheep are distinguished by physical char-
acteristics, and they do not interbreed in nature, thus they
are separate species. Yet in captivity they can produce fertile
Figure 18.7). Iso-
lation may result from a marine transgression that effectively
separates a once-interbreeding species, or a few individuals
may somehow get to a remote area and no longer exchange
genes with the parent population (Figure 18.1). Given these
conditions and the fact that different selective pressures are
likely, they may eventually give rise to a reproductively iso-
lated species.
Although widespread agreement exists on allopatric spe-
ciation, scientists disagree on how rapidly a new species may
evolve. According to Darwin and reaffirmed by the mod-
ern synthesis, the gradual accumulation of minor changes
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