5. Costs and benefits of sexual reproduction

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Transcript 5. Costs and benefits of sexual reproduction

Costs and benefits of sexual and
asexual reproduction:
CfE Advanced Higher Biology
Unit 2: Organisms and Evolution
2.3 Variation and sexual reproduction
a) Costs and benefits of sexual and asexual
b) Meiosis forms viable gametes
c) Sex determination
Key areas
• Describe the costs and benefits of sexual and
asexual reproduction.
• Compare sexual and asexual reproduction.
• Describe advantages and disadvantages to both
• Link the processes to the red queen hypothesis.
• Define parthenogenesis and give examples of
organisms that use this process.
• Identify the use of horizontal gene transfer in
asexual reproduction.
Which organisms use sexual or asexual
Sexual and asexual reproduction
• How do we define each?
• Sexual- offspring arise from the
combination of male and female gametes
after the process of fertilisation.
• Asexual- offspring arise from a single
organism and share the DNA of that parent
only. Offspring can be classed as clones.
• What are the advantages and disadvantages
of both?
Sexual reproduction has 2 major
• 1. Only half of a genome is passed on from each
parent. This means that a “successful” parent
can only pass on half of their “successful”
genes, as their genetic information is mixed
with the genetic information of another
• 2. Only half of the population produce offspring
(males don’t reproduce). This is known as the
paradox of the existence of males.
The paradox of the existence of
In any sexually reproducing species, half of any
offspring produced will be male and half female.
However, it is only the females who are able to
reproduce the next generation.
In an asexually reproducing species, all offspring
are able to reproduce and would be expected to
increase in number at twice the rate of a sexually
reproducing species.
The paradox of the existence of males
• The fact that the males in a sexually
reproducing organism are not able to
reproduce would appear to put sexually
reproducing species at a disadvantage.
• In terms of the number of offspring they
are able to produce and, they would be
outcompeted by asexual species.
This is known as the
paradox of the existence of
Sexual and asexual reproduction
Asexual reproduction
Sexual reproduction
Asexual populations rapidly outgrow sexual ones as males in BLUE do not
reproduce offspring.
Genome disruption
• Sexual reproduction requires that the genomes of both parents
(male and female) are disrupted in order to half the number of
chromosomes present in each gamete. In all sexually
reproducing eukaryotes, this involves the production of
gametes through the process of meiosis.
Human male genome
• By its very nature, only half
of each of these genomes
will be passed on and which
alleles are present in each
gamete is random as a
result of crossing over and
independent assortment.
The benefits of sexual reproduction
• As sexual reproduction is so widespread across the eukaryotic
kingdoms, the benefits must outweigh the disadvantages.
The benefits:
• Greater genetic variation which allows species to maintain a large
gene pool.
• Genetic variation allows organisms to cope with selection pressures
and evolve to changing environmental situations.
• It provides the raw material required to keep running in the Red
Queen’s ‘arms race’ between parasites and their hosts.
It is important to remember that evolution itself does not have a goal
or a direction.
Sexual Reproduction
Key Concepts
• Disadvantages of sexual reproduction: males unable
to produce offspring; only half of each parent’s
genome passed onto offspring disrupting successful
parental genomes.
• Benefits outweigh disadvantages due to increase in
genetic variation in the population.
• This genetic variation provides the raw material
required to keep running in the Red Queen’s arms
race between parasites and their hosts.
Asexual reproduction
• Asexual reproduction avoids both of the disadvantages of
sexual reproduction, but does not give the same amount of
• It can be an effective reproductive strategy, particularly
in very narrow, stable niches or when recolonising
disturbed habitats.
• This type of reproduction allows offspring to be produced
quickly (allowing quick colonisation) in the absence of a
Sea anemone with asexually
reproduced offspring
Asexual reproduction- Prokaryotes/ Unicellular
• In prokaryotes, cell division produces new individuals. To increase the
flow of genetic information horizontal gene transfer via plasmids is
often used.
Mechanism of asexual reproduction – Fission
In the process of fission, the parent cell is replaced by two daughter
cells as it literally splits into two cells of approximately equal size.
Example of Archaea
domain - Halobacter
Example of the Protist
Kingdom - Acathocystis
Example of bacteria
domain –
Staphylococcus aureus
Asexual reproduction – Eukaryotes
Mechanisms of asexual reproduction in Eukaryotes
Vegetative cloning – in
plants. eg. production of
rhizomes (ginger) or runners
(strawberry) or bulbs
(daffodils) or tubers (dahlia)
Parthenogenesis – a type
of asexual reproduction
in which an unfertilised
egg develops into a new
individual eg . fire ants.
• In eukaryotic plants there are many
successful forms of vegetative
– Plantlets on runners e.g. strawberry
– Leaf edge plantlets e.g. Mexican hat
– Offsets e.g water lettuce (pistia)
– Tubers e.g sweet potato
• In eukaryotic animals another
successful asexual reproductive
strategy is Parthenogenesis.
• Parthenogenesis involves
producing an individual from an
unfertilised egg, the word is
derived from the Greek for
‘virgin birth’.
• Individuals produced can
therefore be haploid or diploid.
• Adults produced are usually haploid and
their cells don’t undergo meiosis in
forming new eggs.
• Parthenogenesis is more common in
cooler climates with low parasite
Examples of parthenogenesis
In social insects e.g. bees, wasps or ants, males are
produced from unfertilised eggs and so are haploid
individuals. Sterile workers and reproductive workers
(Queens) are produced from fertilized eggs.
• Some species of Whiptail Lizards go through
mating behaviours and hormone cycles similar to
that of organisms that undergo sexual
reproduction. The desert grassland whiptail
lizard is an all female species.
• They reproduce by parthenogenesis. The
chromosomes within their eggs double after
meiosis, thus allowing them to develop into
lizards without fertilisation occurring.
Examples of parthenogenesis
Daphnia have the ability
to switch between sexual
and asexual reproduction.
The method is often
related to the season;
asexual occurring when
conditions are favourable
and sexual occurring
during times of
environmental stress.
Horizontal gene transfer
For the many organisms for whom asexual reproduction is the
principal method of reproduction, many have mechanisms for
horizontal gene transfer between individuals, such as the plasmids of
bacteria and yeast.
Main mechanisms of
horizontal gene
Asexual Reproduction
Key Concepts
• Asexual reproduction can be a successful reproductive
strategy, particularly in very narrow, stable niches or
when recolonising disturbed habitats.
• In eukaryotes, examples of asexual reproduction
include vegetative cloning in plants and parthenogenic
animals that lack fertilisation.
• Parthenogenesis is more common in cooler climates
that are disadvantageous to parasites or regions of low
parasite density/diversity.
• Organisms that reproduce principally by asexual
reproduction often have mechanisms for horizontal
gene transfer between individuals, such as the
plasmids of bacteria and yeast.
Homework Task
• Using the scientific paper take notes about the
evolution of sexual and asexual reproduction
(reference your notes).
• The whole paper will need to be read to help you
answer the questions.
• Further reading:
• http://www.trueorigin.org/sex01.asp
• http://www.nature.com/scitable/topicpage/sexual
Past paper questions