Download Unit 2 PPT 4 (Costs and benefits of sexual reproduction)

Costs and benefits of sexual reproduction:
The disadvantages and advantages of
sexual reproduction
CfE Advanced Higher Biology
Unit 2: Organisms and Evolution
SQA mandatory key information
• Disadvantages of sexual reproduction – males unable
to produce offspring, only half of each parent’s
genome passed onto offspring. Benefits outweigh
disadvantages due to increase in genetic variation in
the population.
• Successful asexual reproduction strategies – to
include examples of vegetative cloning,
parthenogenesis and horizontal gene transfer.
Key concepts 1
• Compared to asexual reproduction, sexual
reproduction appears to have two disadvantages.
First, half of the population is unable to produce
offspring – this is known as the paradox of the
existence of males. Second, by mixing the genetic
information between two individuals, each parent
disrupts a successful genome and only passes on half
to each offspring.
• Given that sexual reproduction is so widespread, the
benefits must outweigh these disadvantages. The
benefit lies in the greater genetic variation within
sexually reproducing organisms. This genetic
variation provides the raw material required to keep
running in the Red Queen’s arms race between
parasites and their hosts.
Key concepts 2
• 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.
Parthenogensis is more common in cooler climates
with low parasite diversity. For organisms that
reproduce principally by asexual reproduction, many
have mechanisms for horizontal gene transfer
between individuals, such as the plasmids of bacteria
and yeast.
The problem
“Sex is the queen of problems in evolutionary
biology. Perhaps no other natural phenomenon has
aroused so much interest; certainly none has sowed
as much confusion. The insights of Darwin and
Mendel, which have illuminated so many mysteries,
have so far failed to shed more than a dim and
wavering light on the central mystery of
sexuality......”
Graham Bell (1982) ‘The Masterpiece of Nature:
The Evolution and Genetics of Sexuality’.
What are the disadvantages of sexual
reproduction?
There are two main disadvantages:
1. In any population where sexual reproduction is the
reproductive strategy, only half of the population
are able to produce offspring.
2. Sexual reproduction involves the mixing of genetic
information between two individuals with each
parent disrupting a successful genome and only
passing half onto each offspring.
The paradox of the existence of males
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 males.
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
In order to produce offspring, sexual
reproduction requires the successful
genomes of both parents to be
halved and then combines these
halves during fertilisation. 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 and asexual reproduction
Asexual reproduction
Sexual reproduction
Asexual populations rapidly outgrow sexual ones as males in BLUE do not
reproduce offspring.
Asexual reproduction
• Asexual reproduction, unlike sexual reproduction does not
involve the fusion of gametes, the process of meiosis or any
disruption of a successful parental genome in order to
produce offspring.
• It can be a very successful reproductive strategy, producing a
large number of offspring, much more rapidly than through
sexual reproduction.
• Asexual reproduction uses the
process of
mitotic cell division.
• It is a useful strategy particularly
in
very narrow, stable niches or
when
recolonising disturbed habitats.
Sea anemone with asexually
reproduced offspring
Asexual reproduction – Eukaryotes
Mechanisms of asexual reproduction in Eukaryotes
Budding – eg
certain species of hydra
and coral. Offspring
grow out from the
parent’s body.
Hydra
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.
Parthenogenesis – continued…
• The progeny of parthenogenesis can be either haploid or diploid.
• If haploid, the offspring develop into adults that produce eggs or sperm
without meiosis.
• It occurs in certain species of bees, wasps, and ants.
• In vertebrates, parthenogenesis is observed in roughly one in a thousand
species.
• It has been observed in certain species of rock lizard, geckos and a
Komodo dragon.
• Parthenogenesis is more common in cooler climates with low parasitic
diversity.
Other examples of
parthenogenesis are
stick insects and
Komodo dragons
Asexual reproduction unicellular organisms
Many organisms reproduce principally by asexual reproduction, such as the
unicellular organisms in the archaea, bacteria and the protists.
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
turfacea
Example of bacteria
domain –
Staphylococcus aureus
Finally...
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 transfer:
•Transformation
•Transduction
•Conjugation