Sexual vs Asexual Reproduction

Reproduction

• the production of a new generation of individuals of the same species

→ it involves the transfer of genetic material from one generation to the next, ensuring the species survives over long periods of time

Asexual Reproduction

the production of an offspring from a single organism without the production of gametes

• genetically identical offspring - clones

• any genetic variation is a result of mutations

Binary Fission

• occurs in unicellular organisms such as bacteria and most protoctists

• involves dna replication then division of the cell into two daughter cells - identical to parent cell

• in eukaryotes fission involves in mitosis

• rapid pop growth

Binary Fission by planarian platyhelminths

• formation of waist - cross sectional weak point

• rupturing at the waist

• two pieces regrow the missing structures

Multiple Fission - Amoeba

• withdraws into a chitin like cyst - protects it from adverse conditions

• mitosis occurs multiple times

• when conditions are more favorable cyst wall ruptures and the new daughter cells amoebas are released

Budding

• a form of asexual reproduction where a new individual is formed from an outgrowth of the parent

• cells differentiate before bud detaches

• buds may remain attached to parent forming a colony - obelia sp.

• occurs in cnidarians, yeast

Strobilation

• the body or strobila spilts into many offspring

Vegetative Propagation in Plants

• where a bud grows and develops into a new plant - detaches from parent plant to lead an independent existence

• specilaized organs of propagation often develop - buds, buds can only occur on stems, they all contain a small part of a stem

  • eg bulbs, rhizomes, tubers

• these are known as perennating organs - store food for when conditions are favorable

Tuber

• underground storage organ formed from a stem or a root, swollen with food and capable of perennation

• survive one year - shrivel up next growing season as their contents r used

• have buds which may grow into a new plant during next growing season

Fragmentation and Regeneration

• fragmentation is the breaking of an organism into two or more parts, each of which grows to form a new individual

• regeneration - external influences break up the organism

• vegetative propagation is a type of fragmentation

Parthenogenesis

• development of offspring from unfertilized eggs

• can determine sex - honeybees

• Sometimes requires sexual behavior to trigger egg laying - even though no fertilization occurs

Advantages/Disadvantages of Asexual Reproduction

• Advantages

1. Only 1 parent is required - no investing time/energy in finding a mate, no need for specialized mechanisms, no wasteful prod. of gamete

2. Genetically Identical Offspring - successful gene combinations r preserved

3. Rapid Multiplication

4. Dispersal and Spread - asexual reproduction often enables efficient dispersal - mucor , spores

Disadvantages

1. No genetic variation

2. If spores r produced many will fail to find a suitable place for germination - waste

3. No sexual dimorphism/little parental care

4. no efficient dispersal mechanism - overcrowding and intraspecific competition

Sexual Reproduction

• production of an offspring by the fusion of two haploid gametes to form a diploid zygote which develops into a new organism

• act of fusion is called fertilization

Unisexual/Dioecious

• separate male and female individuals

Bisexual/Monoecious

• hermaphrodite

• obelia

• taenia - platyhelminthes

• most flowering plants

How is Genetic Variation Achieved and why is it advantageous

• choice of partner or mate

• crossing over

• random alignment → independent assortment

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Advantageous because it provides the ‘raw’ material on which natural selection can act.

different combinations of genes may give organisms an advantage - allow them to better adapt to environment, allow them to survive, hence pass on their genes

this drives natural selection - this can produce a new species aka speciation

Advantages vs Disadvantages of Sexual Reproduction

• Advantages

1. Sexual Dimorphism - allow sexes to adopt different roles and increasing success through parental care

2. Genetic variation

3. Increasing survivability - quality over quantity

4. Overcrowding is less likely

5. Partner selection is a mechanism of natural selection - strengthens the gene pool

• Disadvantages

1. cannot take place w out both sexes

2. evolutionary dead ends in smaller populations due to genetic variation

3. dispersal is slower

4. population growth may be slow

5. finding a mate takes energy and time

Sexual vs Asexual Reproduction

Sexual:

usually 2 parents

genetic variation

sexual dimorphism/parental care

gametes

less rapid increase in numbers

all plant/animal species

meiosis

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Asexual:

one parent

no genetic variation

no sexual dimorphism/parental care

no gametes

usually only occurs in organisms with a high mortality

depends on mitosis

Evolution of Internal Fertilization

• Fish and other sessile aquatic organisms such as oysters release gametes directly into the water

• release of gametes must be synchronized between male and female

• close proximity is also required at the time of gamete release

• amphibians must return to water to mate

• reptiles evolved internal fertilization- inserting the male gamete directly into the female reproductive tract

• this requires less gametes to be produced

• increases the chances of fertilization

• cleidoic egg allows the development of zygote

Cleidoic Egg

• Outer membrane - calcareous in birds, leathery in reptiles, provide protection from mechanical damage

• yolk sac - encloses yolk and develops from the embryonic gut. yolk contains nutrients which are eventually absorbed by the blood vessels. albumin is a protein food source.

• amnion; encloses the amniotic cavity, which becomes filled with amniotic fluid secreted by cells of the amnion. provides embryo with a fluid environment - as embryo grows this fuses w chorion

• chorion - prevents excessive water loss from amnion

• allantois - increases surface area for gas exchange between atmosphere and embryo and storing excretory products

Placental Mammals

• chorion and allantois form the placenta

Oviparous

• lay eggs

Viviparous

• animals which retain the embryo within the mother’s body during early developmental stages