SEXUAL AND
ASEXUAL
REPRODUCTION
Outcomes
2.11 Using examples from living organisms discuss the
importance of asexual and sexual reproduction to their
growth and survival.
5.1 Contrast the advantages and disadvantages of
asexual reproduction with those of sexual
reproduction.
5.2 Compare external fertilization with internal
fertilization.
5.3 Describe fertilization in the earthworm.
5.4 Compare the amniotic egg of reptiles and birds with
the structures which form in the uterus of a pregnant
mammal.
Asexual reproduction
• An organism capable of asexual reproduction is able
to produce offspring in the absence of a mate. In
asexual reproduction, the offspring is a clone of the
parent and therefore results in low genetic variation
in the species as a whole.
Asexual Reproduction:
Copies And Clones
Modes of Asexual Reproduction:
• Budding – a new individual develops from an
outgrowth on the body of an organism
•
Ex. Hydra – a simple multicellular animal related to the
jellyfish. When growing conditions are favorable, hydra
grow one or more extensions on the sides of their bodies.
When they are sufficiently large, the buds detach and
begin to live freely as new, genetically identical
individuals.
• Roots or shoots – a plant will send out a new root or
shoot that develops into a new plant
•
Ex. Strawberry
• Fragmentation – all species of fungi are thought to be
capable of this. A part of the growing mass of fungi
breaks off and continues to grow independently
Advantages and
Disadvantages
Advantages
1. Large numbers of offspring are reproduced very
quickly from only one parent when conditions are
favourable.
2. Large colonies can form that can out-complete other
organisms for nutrients and water.
3. Large number of organisms mean that species may
survive when conditions or the number of predators
change.
4. Energy is not required to find a mate
Advantages and
Disadvantages
Disadvantages
1. Offspring are genetic clones. A negative mutation
can make asexually produced organisms susceptible to
disease and can destroy large numbers of offspring.
2. Some methods of asexual reproduction produce
offspring that are close together and complete for food
and space.
3. Unfavourable conditions such as extreme
temperatures can wipe out entire colonies.
Sexual Reproduction:
Adding Variety
• Each parent contributes a copy of half its genetic
information. This process of combining genetic
information from two individual results in offspring
that differ genetically from their parents and from
each other
• Sexual reproduction involves two key processes.
The formation of the haploid sex cells or gametes, which
contain genetic information from the parents
• Fertilization where the two sex cells join to produce a
zygote – the first cell of a new individual
•
Advantages and Disadvantages
Advantages
1. Sexual reproduction is a process that involves
organisms that are of the same kind and results to the
formation of gametes.
2. When the gametes fuse together, it can completely
mixed and combine all the genetic material of both
organisms.
3. The combination of the genes helps the organisms to
evolve and adapt in a healthier way in order to produce
healthy type of species.
4. The combination of the genes assures that there
would be a lesser chance for the newly formed
organisms to acquire diseases and other defects while
on the formation and developing process.
Advantages and Disadvantages
Disadvantages
1. This sexual reproduction process will just be evident
if both the gametes of two organisms will be fused
together. When gametes of the organism will not be
fused, it is an indication that sexual reproduction will
not take place.
2. There is only limited number of organisms that are
capable of undergoing the gestation process and most
are the female species or organisms.
3. Organisms are not guaranteed that right after the
mating process they will completely produced new
organisms since there is no assurance that both the
gametes of the two organisms will fuse completely
together during the mating process.
4. Sexual reproduction takes longer than before they
are able to produce new organisms, which is called the
offspring.
External Fertilization
External fertilization is limited essentially to animals
living in aquatic environments. The flagellated sperm
must have fluid in which to swim, and the eggs lack a
protective coat or shell (so that sperm can penetrate
and fertilize them), and would dry out in the air.
Almost all aquatic invertebrates, most fish, and many
amphibians use external fertilization. However,
shedding eggs and sperm into the water is an
uncertain method of fertilization: many of the sperm
never locate an egg, and many eggs are never
fertilized, even if both types of gametes are shed at
the same time and in the same place, as is usually the
case. Consequently, animals using external
fertilization generally release vast numbers of eggs
and sperm at one time.
Internal Fertilization
Most land animals, both invertebrate and vertebrate,
use internal fertilization. In effect, the sperm cells are
provided with the sort of fluid environment that is no
longer available to them outside the animals’ bodies.
The sperm can remain aquatic, swimming through the
film of fluid present on the walls of the female
reproductive tract. Once fertilized, the egg is either
enclosed in a protective shell and released by the
female, or held within the females’ body until the
embryonic stages of development have been
completed. Internal fertilization requires close
physiological and behavioral synchronization of the
sexes, which involves extensive hormonal control
Types of Internal Fertilization
•
Oviparous – Lay egg outside of the body
•
•
•
•
Reptiles and Class Aves (birds) and some mammals use what is called
an Amniotic Egg.
Allows for reproduction and development of the egg away from water
Much larger, self contained with a leathery, calcified shell
There are four membranes used for nourishment and protection:
•
•
•
•
Amnion – fluid filled sac which cushions and protects the embryo. It
also allows for exchanged of CO2 and O2 through the shell
Yolk Sac – contains stored food
Allantois – Holds wastes produced by embryo
Chorion – lines inside of cell and encloses the other three membranes
•
Ovoviviparous – live off of yolk supply, but develops in a pouch
on the mother’s back, some snakes develop this way.
•
Viviparous – Development within the womb
•
•
•
Placenta – developed to help exchange materials between mother
and young
Present only during pregnancy from both maternal and fetal tissues
Born after a period of time, can be single or multiple
Compare and Contrast
Oviparous
Viviparous
External vs. Internal
Fertilization
• External – egg cells are laid, male moves over and
fertilizes. Frogs and fish have females that lay their
eggs in the water, the male swims over and fertilizes,
the sperm needs the water to swim to the egg
• Internal – Sperm put into the body, egg is fertilized
inside the body. Does not mean that embryo
develops within the body, but fertilization takes place
inside.
Earthworm Fertilization
• One of the most interesting aspects of earthworms is
their sexuality. Earthworms are simultaneous
hermaphrodites, meaning worms have both male
and female reproductive organs. During sexual
intercourse among earthworms, both sets of sex
organs are used by both worms. If all goes well, the
eggs of both of the mates become fertilized. You can
imagine this is a highly efficient way of ensuring the
survival of the species
• Earthworms can also reproduce themselves if need
be. They can regenerate new segments if they lose a
few. Most earthworms are better at regenerating tails
than heads, but some can. They don't reproduce
asexually, however; only half (and likely the head half)
of an earthworm split in two will regenerate into a full
worm once again