Biology · Inheritance
Asexual and Sexual Reproduction
Some organisms reproduce on their own, and others need a partner. This lesson is for separate Biology only: Combined Science students do not need it. It compares asexual and sexual reproduction in a range of organisms, including the number of live offspring per birth and how quickly the organisms can reproduce, against the need for variation in a population under environmental pressure.
Learning Objectives
- 1Describe asexual and sexual reproduction, and the differences in the offspring.
- 2Explain some of the advantages and disadvantages of asexual and sexual reproduction in a range of organisms.
- 3Use data on the number of live offspring per birth and the speed of reproduction.
- 4Explain why variation in a population is important when there are environmental pressures.
Retrieval practice
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1
What is mitosis?
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Cell division that produces two genetically identical cells.
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2
What is a gamete?
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A sex cell, such as a sperm or an egg cell.
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3
What is variation?
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Differences between individuals of the same species.
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4
What is a clone?
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A genetically identical copy of an organism.
Two ways to reproduce
In asexual reproduction one parent produces offspring, without gametes. Cells divide by mitosis, so the offspring are genetically identical to the parent, and to each other. In sexual reproduction two parents each make gametes, by meiosis, and the gametes fuse at fertilisation. The offspring are genetically different from each other and from the parents. Neither method is better in every situation, and many organisms use one, or both, depending on the conditions. The decision depends on the number of offspring that can be produced, how quickly the organism can reproduce, and the need for variation in a changing environment.
Asexual and sexual reproduction
Asexual reproduction
- One parent
- No gametes and no fertilisation
- Cells divide by mitosis
- Offspring are clones
Sexual reproduction
- Two parents
- Gametes made by meiosis fuse at fertilisation
- Offspring differ from each other and from the parents
- Produces variation in the population
Clones and variation
Asexual reproduction gives clones. Sexual reproduction gives variation.
Number of offspring and speed of reproduction
The two methods differ in how many offspring are made, and how fast.
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Asexual: many, and fast
A bacterium can divide every 20 minutes in good conditions, so one cell can give rise to millions in a day. A strawberry plant sends out runners, and a potato plant makes new tubers.
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Sexual: fewer, and slower
An organism has to find a mate, make gametes, and bring up the young. Most mammals have a small number of live offspring per birth, such as one baby in a human or an elephant, or several in a litter of mice.
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Time between generations
A human takes about 9 months to make a baby, and about 15 years or more to mature, but a bacterium may reproduce in 20 minutes.
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Energy
Sexual reproduction uses a lot of energy, in making gametes, courtship and rearing the young.
Advantages and disadvantages of asexual reproduction
Asexual reproduction works best when the environment is stable.
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Advantages: no mate
A single organism can reproduce by itself, which helps plants that cannot move, and organisms that live far apart.
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Advantages: fast
Many offspring are made quickly, so a population can grow rapidly, and colonise a new habitat.
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Advantages: well adapted
The offspring are clones of a parent that has survived, so they are well adapted to the same environment.
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Disadvantages: no variation
All of the offspring have the same genes, with only occasional mutations, so there is little genetic variation.
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Disadvantages: vulnerable
If the environment changes, or a new disease arrives, there may be no individuals that can survive, and the whole population can be wiped out.
Advantages and disadvantages of sexual reproduction
Sexual reproduction produces variation, which helps a species to survive a change.
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Advantages: variation
The offspring are all different, so if there is an environmental pressure, such as a disease, a new predator or a change in climate, some are likely to survive.
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Advantages: adaptation
The survivors reproduce, so over many generations the species can adapt, which makes natural selection possible.
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Disadvantages: slow
Fewer offspring are produced, and more slowly, than by asexual reproduction.
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Disadvantages: energy
A mate must be found, and gametes made, which costs time and energy, and the young may need care.
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Disadvantages: good combinations lost
A set of alleles that work well in one parent may be split up in the offspring.
Comparing the growth of two populations
Bacteria divide once every 20 minutes. Calculate how many bacteria there will be after 2 hours, from one cell, and compare this with a mammal that has one baby every 2 years.
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- 1 Number of divisions 2 hours is 120 minutes, so \(120 \div 20 = 6\) divisions.
- 2 Bacteria \(2^6 = 64\) cells.
- 3 Mammal One offspring in 2 years, so none in 2 hours.
- 4 Compare The bacteria grow quickly, but they are identical, whereas the mammal makes only a few offspring, which are genetically different.
AnswerThere will be 64 bacteria, because \(2^6 = 64\). The mammal will have only one offspring in the same time, and will not have more for 2 years. Asexual reproduction can make a population grow much faster.
Organisms that use both
Some organisms can reproduce in either way, and use the one that suits the conditions.
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Strawberry plants
They make runners, which are asexual, when conditions are good, and flowers and seeds, which are sexual, which give variation.
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Fungi
Many make spores by mitosis to spread quickly, and reproduce sexually when conditions get difficult.
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Aphids
In summer they reproduce asexually, so that numbers grow quickly, and in autumn they reproduce sexually, to give variation and eggs that survive the winter.
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Malaria parasites
They reproduce asexually in the human liver and blood, and sexually in the mosquito.
Case study
The Irish potato famine
Potatoes are grown from pieces of the parent plant, which is a form of asexual reproduction, so the plants in a field are clones. In the 1840s the potatoes grown in Ireland came from only a small number of types. When a pathogen that causes potato blight arrived in 1845, none of the plants were resistant, because they were all genetically identical. The crop failed in several years running, and about a million people died, and another million left Ireland. It shows the danger of having no variation when there is an environmental pressure such as a new disease.
Summary and exam focus
- Asexual reproduction needs one parent, and produces genetically identical offspring.
- Sexual reproduction needs two parents, and produces genetically different offspring.
- Asexual reproduction is faster and makes more offspring, but there is no variation.
- Sexual reproduction is slower, but the variation helps a species to survive environmental pressures.
- Some organisms use both methods, depending on the conditions.
Exam focus
Evaluate the advantages and disadvantages of asexual and sexual reproduction for a species that lives in an environment that is changing. (6 marks) (6 marks)
Give at least two advantages and two disadvantages of each, using ideas about the number of offspring, the speed of reproduction and variation. Say that when the environment changes, variation is needed, so sexual reproduction is more likely to ensure that some survive, and give a conclusion.
Key terms
The words this lesson expects you to use. Each one is linked from the first place it appears above.
- Asexual reproduction
- Reproduction involving one parent, with no gametes, producing genetically identical offspring.
- Sexual reproduction
- Reproduction involving the fusion of gametes from two parents, producing genetically different offspring.
- Clone
- A genetically identical copy of an organism.
- Environmental pressure
- A factor such as disease, predators or a change in climate, which affects survival.
- Offspring
- The young produced by a parent organism.
- Runner
- A stem that grows along the ground and makes new plants, as in strawberries.
- Mitosis
- Cell division that gives two genetically identical cells.
- Meiosis
- Cell division that gives four genetically different gametes.
- Colonise
- To spread into and settle in a new habitat.
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