SEX! Its evolutionary origin.

Glenn R. Morton (grmorton@waymark.net)
Sat, 10 Oct 1998 10:04:10 -0500

Hi Bob,

Anything with a title containing the word sex will be read. :-)

At 07:08 AM 10/10/98 EDT, RDehaan237@aol.com wrote to George Murphy:

>You understate the discrepancies and problems. Take the origin of life.
>Natural selection accounts for how cells and organisms adapt and survive, but
>says nothing about the _origin of life_. This is not a mere discrepancy or
>problem. This is a fundamental matter, and laboratory research on it has
>practically ceased because of the intractibility of the problem. Take the
>origin of sexual reproduction. The entire section in the 25 September issue
>of _Science_ was devoted to research on the evolution of sex. Yet all the
>articles deal only with _maintenance_ of sexual reproduction., but not a word
>about the _origin_ of sex.

It is frustrating to those of us who are evolutionists to constantly find
claims like yours that there is no evidence of how sexual reproduction
originated. It is out there but anti-evolutionists seem never to read
these things. I would suggest that you take a look at what can be inferred
from single-celled and primitive multicellular animals concerning the
origin of sex.

Starting from single celled animals, each of which has the capability to
reproduce there is no sex in the sense that we think of the term. Selective
pressure has been observed to convert single-cellular forms into
multicellular forms. A case was observed in which a single celled form
changed to multicellularity. Boxhorn, a student of Boraas,writes:

"5.9.1 Coloniality in Chlorella vulgaris Boraas (1983) reported the
induction of multicellularity in a strain of Chlorella pyrenoidosa (since
reclassified as C. vulgaris) by predation. He was growing the unicellular
green alga in the first stage of a two stage continuous culture system as
for food for a flagellate predator, Ochromonas sp., that was growing in the
second stage. Due to the failure of a pump, flagellates washed back into
the first stage. Within five days a colonial form of the Chlorella
appeared. It rapidly came to dominate the culture. The colony size ranged
from 4 cells to 32 cells. Eventually it stabilized at 8 cells. This
colonial form has persisted in culture for about a decade. The new form has
been keyed out using a number of algal taxonomic keys. They key out now as
being in the genus Coelosphaerium, which is in a different family from
Chlorella. "

http://www.talkorigins.org/faqs/faq-speciation.html

So, now we have a multicellular animal in which each cell can reproduce the
being. Say some of the cells lose the ability to reproduce but have some
useful function. Then you have the simple volvox type of arrangement which
leads to the division of labor among cells either sex cells or somatic cells.

Probably the best case is to let you read what Gilbert has to say about the
origin of sex.

"Sexual reproduction is one other invention of the protists that has had
profound effect on more complex organisms. It should be noted that sex and
reproduction are two distinct and separable processes. Reproduction
involves the creation of new individuals. Sex involves the combining of
genes from two different individuals into new arrangements. Reproduction
in the absence of sex is characteristic of those organisms that reproduce
by fission; there is no sorting of genes when an amoeba divides or when a
hydra buds off cells to form a new colony. Sex without reproduction is
also common among unicellular organisms. Bacteria are able to transmit
genes from one individual to another by means of sex pili. This
transmission is separate from reproduction. Protists are also able to
reassort genes without reproduction. Paramecia, for instance, reproduce by
fission, but sex is accomplished by conjugation. When two paramecia join
together, they link theiry oral apparatuses and form a cytoplasmic
connection. Each macronucleus (which controls the metabolism of the
organism) degenerates while each micronucleus undergoes meiosis followed by
mitosis to produce eight haploid micronuclei, of which all but one
degenerate. The remaining micronucleus divides once more to form a
stationary micronucleus, thereby creating a new diploid nucleus in each
cell. This diploid nucleus then divides to gie rise to a new micronucleus
and a new macronucleus as the two partners disengage. No reproduction has
occurred, only sex.
The union of these two distinct processes, sex and reproduction, into
SEXUAL REPRODUCTION, is seen in unicellular eukaryotes. Figure 13 shows
just one copy of each chromosome (like a mammalian gamete). The
individuals of each species, however, are divided into two mating groups:
plus and minus. When these meet, they join their cytoplasms together and
their nuclei fuse to form a diploid zygote. This zygote is the only
diploid cell in the life cycle, and it eventually undergoes meiosis to form
four new Chlamydomonas. Cells. Here is sexual reproduction, for
chromosomes are reassorted during the meiotic divisions and more
individuals are formed. Note that in this protist type of sexual
reproduciton, the gametes are morphologically identical-the distinction
between sperm and egg has not yet been made.
In evolving sexual reproduction, two important advances had to be
achieved. The first is the mechanism of MEIOSIS (Figure 14), where by the
diploid complement of chromosomes is reduced to the haploid state. (This
will be discussed in Chapter 22.) The other advance is the mechanism
whereby the two different mating types recognize each other. Recognition
occurs first on the flagellar membranes (Figure 15: Goodenough and Weiss,
1975; Bergman et al., 1975). The aggulutination of flagella enables
specific regions of the cell membranes to come together. These specialized
sectors contain mating type-specific components that enable the cytoplasms
to fuse. Following agglutination, the plus individuals initiate the fusion
by extending a 'fertilization tube.' This tube contacts and fuses with a
specific site on the minus individual. Interestingly, the mechanism used
to extend this tube-the polymerization of the protein actin-is also used to
extend processes of sea urchin eggs and sperm. In the next chapter, we
shall see that the recognition and fusion of sperm and egg occur in a
manner amazingly similar to that of these protists.
"Unicellular eukaryotes appear to have the basic elements of the
developmental processes that characterize the more complex organisms of
other phyla: cellular synthesis is controlled by transcriptional,
translational, and posttranslational regulation; a mechanism for processing
RNA through the nuclear membrane exists; the structures of individual genes
and chromosomes are as they will be throughout eukaryotic evolution;
mitosis and meiosis are perfected; and sexual reproduction exists,
involving cooperation between individual cells. Such intercellular
cooperation becomes even more important with the evolution of multicellular
organisms.

Colonial eukaryotes: The evolution of differentiation

"One of evolution's most important experiments was the creation of
multicellular organisms. There appear to be several paths by which single
cells evolved multicellular arrangements; we will discuss only two of them.
The first path involves the orderly division of the reproductive cell and
the subsequent differentiation of its progeny into different cell types.
This path to multicellularity can be seen in a remarkable series of
multicellular organisms collectively referred to as the family Volvocaceae,
or the 'Volvocaceans.'
"The simpler organisms among the Volvocaceans are collections of numerous
Chlamydomonas-like cells; but the more advanced members of this group have
developed a second, very different, cell type. A single organism of the
Volvocacean genus Oltmannsiella contains four Chalmydomonas-like cells in a
row, embedded in a gelatinous matrix. In the genus Gonium (Figure 16), a
single cell divides to produce a flat plate of 4 to 16 cells, each with its
own flagellum. In a related genus -Pnadorina-the 16 cells form a sphere;
and in Eudorina, the sphere contains 32 or 64 cells arranged in a regular
pattern. In these organisms, then, a very important developmental
principle has been worked out: the ordered division of one cell to generate
a number of cells that are organized in a predictable fashion. Like most
animal embryos, the cell divisions by which a single Volvocacean cell
divides to produce an organism of 4 to 64 cells occur in very rapid
sequence and in the absence of cell growth.
"The next two genera of the Volvocacean series exhibit another important
principle of development: the differentiation of cell types within an
individual organism. The reproductive cells become differentiated from the
somatic cells. In all the Vovocacean genera mentioned above, every cell
can, and normally does, produce a complete new organism by mitosis (Figure
17A,B). In the genera Pleodorina and Volvox, however, relatively few cells
can reproduce. In Pleodorina californica, the cells in the anterior side
can reproduce. In P. californica, a colony usually has 128 or 64 cells,
and the ratio of the number of somatic cells to the number of reproductive
cells is usually 3:5. Thus, a 128-cell colony has 48 somatic cells, and a
64-cell colony has 24.
"In Volvox, almost all the cells are somatic, and very few of the cells
are able to produce new individuals. In some species of Volvox,
reproductive cells, as in Pleodorina, are derived from cells that
originally look and function like somatic cells before they enlarge and
divide to form new progeny. However, in other members of the genus, such
as V. carteri, there is a complete division of labor: the reproductive
cells that will create the next generation are set aside during the
division of the reproductive cells that are forming a new individual. The
reproductive cells never develop functional flagella and never contribute
to motility or other somatic functions of the individual; they are entirely
specialized for reproduction. Thus, although the simpler Volvocaceans may
be thought of as colonial organisms (because each cell is capable of
independent existence and of perpetuating the species), in V. carteri we
have a truly multicellurlar organism with two distinct and interdependent
cell types (somatic and reproductive), both of which are required for
perpetuation of the species. (Figure 17C). Although not all animals set
aside the reproductive cells from the somatic cells (and plants hardly ever
do), this separation of germ (reproductive) cells from somatic cells early
in development is characteristic of many animal phyla and will be discussed
in more detail in Chapter 7.
Although all of the Volvocaceans, like their unicellular relative
Chlamydomonas, reproduce predominantly by asexual means (as described
earlier) they are also capable of sexual reproduction. This involves the
production and fusion of haploid gametes. In many species of
chlamydomomonas, including the one illustrated in Figure 13, sexual
reproduction is ISOGAMOUS, since the haploid gametes that meet are similar
in size, structure, and motility. However, in other species of
Chlamydomonas-as well as many species of colonial Volvocaceans-swimming
gametes of very different sizes are produced by the different mating types.
This is called HETEROGAMY. But the larger Volvocaceans have evolved a
specilized form of heterogamy, called OOGAMY. This involves the production
of large, relatively immotile eggs by one mating type and small, motile
sperm by the other. Thus, the Volvocaceans include the simplest organisms
that have distinguishable male and female members of the species and that
have distinct developmental pathways for the production of eggs or sperm."
~Scott F. Gilbert, Developmental Biology, (Sinauer, Mass.: Sinauer
Associates, Inc., 1991), p. 13-18

glenn

Adam, Apes and Anthropology
Foundation, Fall and Flood
& lots of creation/evolution information
http://www.isource.net/~grmorton/dmd.htm