Both asexual and sexual reproduction occur in the animal kingdom
In sexual reproduction, 2 meiotically formed haploid gametes fuse to
form a diploid zygote, which develops into an offspring.
Games are usually relatively large, nonmotile egg, and a small, motile
In asexual reproduction, a single individual produces offspring, usually
using only mitotic cell division.
Mechanisms of Asexual Reproduction
Many invertebrates can reproduce asexually by fission, in which a
parent is separated into 2 or more individuals of equal size
Or by budding, in which a new individual grows out from the
Or by fragmentation, in which the body is broken into several
pieces, each of which develops into a complete animal.
Regeneration is the regrowth of body parts, and is necessary for a
fragment to develop into a new organism.
In parthenogenesis, eggs develop without being fertilized
Sexual Reproduction: An Evolutionary Enigma
Asexual reproduction potentially produces many more offspring,
yet most eukaryotic species reproduce sexually.
The shuffling of genes in sexual reproduction may allow more
rapid adaptation to changing conditions or allow harmful genes to
be eliminated more easily.
Reproductive Cycles and Patterns
Periodic reproductive cycles may be linked to favorable conditions
or energy supplies.
Ovulation is controlled by a combination of environmental and
Animals may reproduce sexually, asexually, or both.
Freshwater crustacean Daphnia, like aphids and rotifers,
produces eggs that develop by parthenogenesis, as well as eggs
that are fertilized.
works in that chromosomes double after
meiosis, creating diploid offspring.
In hermaphroditism, each individual has functioning male and
female reproductive systems.
Mating results in fertilization of both individuals.
Fertilization depends on mechanisms that bring together sperm and eggs of
the same species
Fertilization is the union of sperm and egg
In External Fertilization, eggs and sperm are shed, and fertilization
occurs in the environment.