PRINCE YVO M.
ALCANAR
V-STAR
MRS. EMELITA BASANES
Reproduction
One of the requirements for all living things is reproduction. To
carry on the species and pass down genetic traits from one
generation to the next, species must reproduce. Without
reproduction, a species could go extinct.
Reproduction can happen in two main ways:
Asexual Reproduction, which requires only one parent, and
Sexual Reproduction, which needs gametes, or sex cells, from a
male and a female made by the process of meiosis. Both have
advantages and disadvantages, but in terms of evolution, sexual
reproduction seems a better bet.
Sexual reproduction involves the coming together of genetics
from two parents and hopefully producing a more "fit"
offspring that can withstand changes in the environment if
necessary. Natural selection decides which adaptations are
favorable, and those genes are passed to the next generation.
Sexual reproduction increases the diversity within a population
and gives natural selection more to choose from in deciding
which is the best suited for that environment.
Four Ways of Sexual
Reproduction
Autogamy
The prefix "auto" means "self." An individual that can undergo
autogamy can fertilize itself. Known as hermaphrodites, these
individuals have fully functioning male and female reproductive
parts necessary to make both the male and female gametes for
that individual. They don't need a partner to reproduce, but
some may be able to reproduce with a partner if the
opportunity arises.
Since both gametes come from the same individual in
autogamy, the mixing of the genetics in other types of sexual
reproduction doesn't happen. The genes all come from the
same individual, so the offspring will show traits of that
individual. However, they aren't considered clones because the
combination of the two gametes gives the offspring a slightly
different genetic makeup from the parent's.
Organisms that can undergo autogamy include most plants and
earthworms.
Allogamy
In allogamy, the female gamete (usually called an egg or ovum)
comes from one individual and the male gamete (usually called
the sperm) comes from another individual. The gametes fuse
together during fertilization to create the zygote. The ovum and
sperm are haploid cells, meaning they each have half the
number of chromosomes found in a body cell, which is called a
diploid cell. The zygote is diploid because it's a fusion of two
haploids. The zygote can then undergo mitosis and eventually
form a fully functioning individual.
Allogamy is a true mixing of genetics from the mother and
father. Since the mother and father each gives only half the
chromosomes, the offspring is genetically unique from either
parent and even its siblings. This unification of gametes through
allogamy ensures different adaptations for natural selection to
work on. Over time, the species will evolve.
Internal Fertilization
Internal fertilization occurs when the male gamete and female
gamete fuse to undergo fertilization while the ovum is still
inside the female. This usually requires some sort of sexual
intercourse to happen between a male and female. The sperm
is deposited into the female reproductive system and the
zygote is formed inside the female.
What happens next depends on the species. Some species, such
as birds and some lizards, lay the egg and keep it incubated
until it hatches. Others, such as mammals, carry the fertilized
egg inside the female body until it's viable for live birth.
External Fertilization
As the name implies, external fertilization occurs when the
male and female gametes fuse outside the body. Most species
that live in water and many types of plants undergo external
fertilization. The female lays usually many eggs in the water and
a male sprays sperm over the top of the eggs to fertilize them.
Usually, the parents don't incubate the fertilized eggs or watch
over them, so the new zygotes must fend for themselves.
External fertilization usually is found only in water because the
fertilized eggs need to be kept moist so they don't dry out,
giving them a better chance for survival. Hopefully, they will
hatch and become thriving adults that eventually will pass
down their genes to their own offspring.
