Reproduction and Survival Strategies
1. Reproductive Strategies
Learning outcomes
- I can compare sexual and asexual reproduction.
- I can explain the advantages and disadvantages of different reproductive strategies.
- I can identify reproductive adaptations in animals.
- I can explain how reproductive strategies affect survival.
- I can analyze reproductive success in different environments.
Why Do Animals Reproduce?
Every individual animal eventually dies, but populations and species can continue because organisms reproduce.
Reproduction produces new individuals and passes genetic information from one generation to the next.
Animals use a remarkable variety of reproductive strategies. These strategies influence:
- How many offspring are produced.
- How much energy parents invest in reproduction.
- How much genetic variation exists among offspring.
- How offspring develop.
- How likely offspring are to survive.
- How quickly populations can increase.
- How populations respond to environmental change.
Two fundamental forms of reproduction are:
- Sexual reproduction
- Asexual reproduction
Neither strategy is automatically better. Its success depends on the organism and the environment in which it lives.
Sexual Reproduction
Sexual reproduction involves the combination of genetic material from two gametes.
In animals, these gametes are usually:
- Sperm
- Egg cells
When their nuclei combine during fertilization, they produce a zygote.
A simplified sequence is:
Sperm + egg → fertilization → zygote → embryo → offspring
The offspring receives genetic information from both parents.
Gametes
Gametes are specialised reproductive cells.
In most animals:
- Sperm are relatively small and mobile.
- Eggs are relatively large and contain resources that support early development.
Gametes contain only one set of chromosomes.
When sperm and egg nuclei combine, the normal chromosome number is restored in the zygote.
Genetic Variation in Sexual Reproduction
One major feature of sexual reproduction is that offspring are genetically different from one another and from their parents.
Variation arises partly because:
- Meiosis produces genetically different gametes.
- Gametes from different individuals combine.
- Fertilization is generally a random process.
As a result, each offspring usually receives a unique combination of alleles.
This genetic variation can be extremely important when environments change.
Advantages of Sexual Reproduction
Sexual reproduction can provide several advantages.
Genetic Variation
Offspring are genetically varied.
Adaptation to Changing Environments
If environmental conditions change, some individuals may possess characteristics that increase their chances of survival and reproduction.
Resistance to Disease
Genetic variation can reduce the likelihood that every individual in a population will respond identically to a particular pathogen.
Evolution
Variation provides the raw material upon which natural selection can act.
Over many generations, this can contribute to evolutionary change.
Disadvantages of Sexual Reproduction
Sexual reproduction can also involve substantial costs.
Animals may need to:
- Find a mate.
- Compete for mates.
- Produce specialised reproductive structures.
- Perform courtship behaviours.
- Spend time and energy mating.
- Risk injury or predation while reproducing.
In many species, only part of the adult population directly produces offspring.
Sexual reproduction can therefore be slower than some forms of asexual reproduction.
Asexual Reproduction
Asexual reproduction involves reproduction from a single parent without the fusion of sperm and egg nuclei.
Offspring produced asexually are usually genetically very similar to the parent, although mutations can still introduce variation.
Asexual reproduction occurs in several groups of animals.
Examples can be found among:
- Hydra.
- Some flatworms.
- Sea stars.
- Some insects.
- Some crustaceans.
- Some reptiles.
Budding
In budding, a new individual develops as an outgrowth from the parent's body.
Hydra can reproduce in this way.
A small bud forms through cell division.
The bud grows and develops structures such as tentacles.
Eventually, it may separate from the parent and live independently.
One parent can therefore produce offspring without finding a mate.
Fragmentation
Some animals can reproduce when part of the body separates and develops into another individual.
This is called fragmentation when it functions as a reproductive strategy.
It is often associated with regeneration, the ability to regrow missing structures.
Some flatworms and other invertebrates can reproduce through forms of fragmentation.
It is important to distinguish regeneration from reproduction: repairing a damaged body part does not automatically mean that a new organism has been produced.
Parthenogenesis
Parthenogenesis is reproduction in which an embryo develops from an unfertilized egg.
It occurs in some:
- Insects.
- Crustaceans.
- Reptiles.
- Other animals.
Parthenogenesis can allow rapid reproduction when mates are unavailable.
The genetics of parthenogenesis varies between species, so the offspring are not always genetically identical to their mother.
Advantages of Asexual Reproduction
Asexual reproduction can provide several important advantages.
Only One Parent Is Required
An isolated individual may still be able to reproduce.
Rapid Population Growth
Under suitable conditions, populations can sometimes increase quickly.
Less Energy Spent Finding Mates
Animals do not need to invest as much energy in courtship or mate searching.
Successful Genetic Combinations Can Be Preserved
If an organism is well adapted to a stable environment, asexual reproduction can produce offspring with similar characteristics.
Disadvantages of Asexual Reproduction
The major limitation is generally lower genetic variation.
If many individuals are genetically similar, they may also have similar vulnerabilities.
For example, a major environmental change or new disease could affect a large proportion of the population.
Lower variation can therefore reduce the range of characteristics available for natural selection.
This can become especially important in rapidly changing environments.
Sexual and Asexual Reproduction Compared
| Sexual Reproduction | Asexual Reproduction |
|---|---|
| Usually involves gametes from two parents | Requires one parent |
| Fertilization occurs | No fertilization |
| Usually produces high genetic variation | Usually produces less genetic variation |
| Mate finding may be required | Mate finding unnecessary |
| Can require substantial time and energy | Can sometimes be rapid |
| Particularly useful for generating variation | Can be effective in stable, favourable conditions |
Some organisms use both strategies, switching according to environmental conditions.
Reproductive Success
From an evolutionary perspective, reproduction is not simply about surviving.
Reproductive success refers to an organism's success in passing its genetic information to future generations through surviving, reproducing descendants.
An animal might live for many years but leave no offspring.
Another might have a shorter life but produce offspring that survive and reproduce.
The second individual may therefore have greater reproductive success.
Producing Many Offspring
Some animals produce enormous numbers of offspring.
Examples include many:
- Fish.
- Marine invertebrates.
- Amphibians.
- Insects.
In these strategies:
- Each offspring may receive relatively little parental investment.
- Mortality may be high.
- Producing many offspring increases the chance that at least some survive.
This strategy can be particularly effective where juvenile survival is unpredictable.
Producing Fewer Offspring
Other animals produce relatively few offspring but invest heavily in each one.
Examples include many:
- Mammals.
- Birds.
Investment can include:
- Protection.
- Feeding.
- Teaching.
- Transport.
- Grooming.
- Defending young from predators.
Producing fewer offspring can be successful when parental investment greatly increases each offspring's probability of survival.
A Reproductive Trade-Off
Organisms have limited resources.
Energy invested in one activity cannot simultaneously be used for everything else.
Animals must effectively divide resources among:
- Growth.
- Maintenance.
- Defence.
- Movement.
- Reproduction.
Within reproduction itself, there can be a trade-off between:
Number of offspring ↔ investment in each offspring
Producing thousands of offspring usually prevents extremely high investment in every individual.
Producing only one or two offspring can allow much greater investment in each.
Parental Care
Parental care includes behaviours that increase offspring survival after eggs are produced or young are born.
Examples include:
- Guarding eggs.
- Building nests.
- Feeding offspring.
- Carrying young.
- Protecting young from predators.
- Teaching feeding or hunting behaviour.
Parental care can substantially increase offspring survival, but it also requires time and energy.
External Fertilization
In external fertilization, sperm and eggs meet outside the parents' bodies.
This is common in many aquatic animals.
Examples include many:
- Fish.
- Amphibians.
- Marine invertebrates.
Aquatic environments are particularly suitable because water prevents gametes from rapidly drying out and allows sperm to move.
Advantages and Limitations of External Fertilization
External fertilization can allow many gametes to be released at once.
However, there are challenges:
- Many gametes never meet.
- Eggs may be eaten.
- Environmental conditions can damage developing embryos.
- Gametes can be dispersed.
Animals using this strategy often produce large numbers of gametes.
Timing can also be important.
If many individuals release gametes at approximately the same time, the probability of fertilization increases.
Internal Fertilization
In internal fertilization, sperm is transferred into the reproductive tract of the female, and fertilization occurs inside the body.
This occurs in groups including:
- Reptiles.
- Birds.
- Mammals.
- Many insects.
Internal fertilization can increase the probability that sperm and egg meet and protects gametes from drying out.
It is particularly important for reproduction in terrestrial environments.
Eggs and Reproductive Adaptations
Animals have evolved many adaptations that protect developing embryos.
Bird and reptile eggs, for example, possess structures that allow development on land.
The amniotic egg includes protective membranes and usually a shell.
These structures help:
- Protect the embryo.
- Reduce water loss.
- Provide nutrients.
- Allow gas exchange.
- Store metabolic wastes.
The evolution of the amniotic egg was an important adaptation for reproduction away from water.
Courtship Behaviour
Animals often use courtship behaviours before mating.
These can involve:
- Sounds.
- Colours.
- Movement.
- Chemical signals.
- Gifts.
- Displays.
- Physical competition.
Courtship can help animals:
- Identify members of the same species.
- Determine whether a potential mate is receptive.
- Assess potential mates.
- Coordinate mating behaviour.
Courtship may be energetically expensive, but it can increase reproductive success.
Sexual Selection
Some characteristics increase an individual's chances of obtaining mates.
This can produce a form of natural selection called sexual selection.
Examples can include:
- Bright feathers.
- Large antlers.
- Complex songs.
- Courtship dances.
- Large body size.
These characteristics may increase mating success even when they require substantial energy or create other costs.
Competition for Mates
Members of the same sex may compete for access to mates.
Competition can involve:
- Physical contests.
- Territorial defence.
- Displays.
- Calls.
- Establishing dominance.
For example, antlers can be used in contests between male deer.
Winning a contest can increase access to mates, but fighting also consumes energy and creates a risk of injury.
Mate Choice
In many species, individuals select among potential mates.
Characteristics influencing mate choice can include:
- Appearance.
- Courtship behaviour.
- Territory quality.
- Ability to provide food.
- Health.
- Signals of physical condition.
Mate choice can influence which characteristics become more common over generations.
Reproductive Timing
Animals must often reproduce at the appropriate time.
Environmental cues can include:
- Day length.
- Temperature.
- Rainfall.
- Food availability.
For example, producing offspring when food is abundant can increase juvenile survival.
Reproductive timing therefore links an animal's life cycle to environmental conditions.
Reproductive Synchronization
In some species, many individuals reproduce during a relatively short period.
This is called synchronous reproduction.
Potential advantages include:
- Increased probability of fertilization.
- Large numbers of offspring appearing simultaneously.
- Matching reproduction with favourable environmental conditions.
For species using external fertilization, synchronization can be especially important because eggs and sperm must encounter one another outside the body.
Reproduction in Stable Environments
Imagine an environment that remains relatively stable for many generations.
An organism is already very well adapted to those conditions.
Asexual reproduction can sometimes be advantageous because:
- Reproduction can occur rapidly.
- No mate is necessary.
- Successful genetic combinations can be maintained.
However, environmental stability is rarely permanent.
Reproduction in Changing Environments
Now imagine that:
- Temperature changes.
- A new predator arrives.
- A new disease appears.
- Food availability changes.
A genetically varied population may contain some individuals better able to tolerate the new conditions.
Sexual reproduction continually generates new combinations of alleles.
This can increase the range of characteristics present within the population.
Worked Example: Disease Outbreak
Imagine two populations of the same type of animal.
Population A reproduces mainly asexually and has relatively low genetic variation.
Population B reproduces sexually and has higher genetic variation.
A new infectious disease appears.
If nearly all individuals in Population A have similar susceptibility, the disease could spread very effectively.
In Population B, genetic differences may mean that some individuals are more resistant than others.
Those individuals may be more likely to survive and reproduce.
This illustrates one possible advantage of genetic variation.
Reproduction in Unpredictable Environments
Environments can change unpredictably.
Conditions might vary in:
- Temperature.
- Rainfall.
- Food availability.
- Predator abundance.
- Disease exposure.
Strategies producing greater genetic diversity can become particularly important under these circumstances.
However, reproductive success still depends on many factors. Sexual reproduction does not guarantee survival, and asexual reproduction can remain highly successful in many environments.
Switching Reproductive Strategies
Some animals can reproduce both sexually and asexually.
For example, some populations of aphids can use parthenogenesis during favourable conditions.
This allows populations to increase rapidly.
Sexual reproduction can occur at other stages of the life cycle.
Using different strategies at different times can combine some advantages of both reproductive modes.
Reproduction and Population Growth
Reproductive strategy can strongly influence how quickly a population changes.
Consider an organism that:
- Matures rapidly.
- Produces many offspring.
- Reproduces frequently.
Its population may increase rapidly when conditions are favourable.
Another organism that:
- Matures slowly.
- Produces few offspring.
- Provides extensive parental care.
will generally have a lower maximum rate of population increase.
However, each offspring may have a greater probability of surviving.
Offspring Survival
Producing offspring does not automatically produce reproductive success.
Offspring must survive long enough to contribute to future generations.
Factors affecting survival include:
- Food availability.
- Predation.
- Disease.
- Competition.
- Parental care.
- Environmental conditions.
- Genetic characteristics.
Therefore, reproductive strategy involves more than simply maximizing the number of offspring produced.
Reproductive Adaptations
Animals possess many adaptations related to reproduction.
Structural Adaptations
Examples include:
- Reproductive organs.
- Protective eggshells.
- Structures used in courtship.
- Structures used in competition.
Physiological Adaptations
Examples include:
- Hormonal control of reproduction.
- Production of gametes.
- Internal fertilization.
- Pregnancy.
Behavioural Adaptations
Examples include:
- Courtship.
- Nest building.
- Territorial behaviour.
- Parental care.
- Migration to breeding sites.
Reproductive success often depends on several types of adaptation working together.
Worked Example: Sea Turtle
A sea turtle may lay a large number of eggs on a beach.
The mother does not remain to provide extensive parental care after nesting.
Young turtles face many dangers, including:
- Predators.
- Heat.
- Dehydration.
- Difficulty reaching the ocean.
Mortality is high, but producing many offspring increases the chance that some will survive to adulthood.
Worked Example: Elephant
Elephants use a very different strategy.
They:
- Produce relatively few offspring.
- Have long pregnancies.
- Provide extensive parental care.
- Protect their young.
- Have long developmental periods.
Considerable resources are invested in each offspring.
This can greatly increase the survival probability of individual young.
Worked Example: Frog
Many frogs reproduce using external fertilization.
Adults may release large numbers of gametes into water.
This strategy is suited to aquatic reproduction because:
- Sperm can move through water.
- Eggs remain moist.
- Many offspring can be produced.
However, eggs and tadpoles may experience high mortality.
Some frog species compensate with parental care, demonstrating that even related species can evolve very different reproductive strategies.
Reproductive Strategy and Natural Selection
Reproductive strategies evolve through natural selection.
Characteristics that increase successful reproduction can become more common over generations.
The important outcome is not simply:
Who produces the most offspring?
Instead, evolutionary success depends more closely on:
Which individuals leave surviving descendants that themselves reproduce?
This explains why very different reproductive strategies can persist.
Producing thousands of poorly protected offspring can work.
Producing one highly protected offspring can also work.
Success depends on the environmental and ecological context.
Trade-Offs in Reproductive Strategies
There is rarely a reproductive strategy with every possible advantage.
Animals face trade-offs such as:
Many offspring ↔ high investment per offspring
Rapid reproduction ↔ extensive parental care
Mate competition ↔ risk and energy expenditure
Large reproductive investment ↔ resources available for survival
Genetic similarity ↔ genetic diversity
Evolution acts on these trade-offs over many generations.
Common Mistakes
Saying Sexual Reproduction Always Requires Two Separate Animals
Sexual reproduction involves the fusion of gametes. Some organisms can produce both types of gametes, although many animal species have separate male and female individuals.
Saying Asexual Offspring Are Always Completely Identical
They are generally genetically very similar, but mutations and some forms of asexual reproduction can produce differences.
Saying Asexual Reproduction Has No Advantages
It can be extremely effective because it requires only one parent and can permit rapid reproduction.
Saying Sexual Reproduction Is Always Better
Sexual reproduction provides important genetic variation but also carries costs. The effectiveness of either strategy depends on environmental conditions.
Confusing Fertilization With Mating
Fertilization specifically means the fusion of gamete nuclei. Mating refers to reproductive behaviour that may lead to fertilization.
Assuming More Offspring Always Means Greater Reproductive Success
Producing offspring is useful only if enough survive and ultimately contribute to future generations.
Thinking All Animals Provide Parental Care
Many animals provide little or no care after eggs are produced.
Saying External Fertilization Means Reproduction Is Uncontrolled
Animals can coordinate spawning through behaviour and environmental cues, greatly increasing the chance of fertilization.
Check Your Understanding
1. Distinguish between sexual and asexual reproduction.
2. Explain why sexual reproduction usually produces genetically varied offspring.
3. Give two advantages and two disadvantages of sexual reproduction.
4. Give two advantages and two disadvantages of asexual reproduction.
5. Describe budding and give an example of an animal that reproduces this way.
6. What is parthenogenesis?
7. Compare internal and external fertilization.
8. Explain why external fertilization is particularly common in aquatic environments.
9. Why do many animals using external fertilization produce large numbers of gametes?
10. Explain the trade-off between offspring number and parental investment.
11. Give three examples of reproductive adaptations.
12. Explain how parental care can increase reproductive success.
13. Why might genetic variation be particularly valuable in a changing environment?
14. Compare the reproductive strategies of a sea turtle and an elephant.
15. A population lives in an environment that suddenly becomes much more variable. Explain why genetic variation among offspring could influence the population's long-term survival.
Key Terms
- Reproduction – process by which organisms produce new individuals.
- Sexual reproduction – reproduction involving the fusion of gametes.
- Asexual reproduction – reproduction without fusion of gametes.
- Gamete – reproductive cell containing one set of chromosomes.
- Sperm – typically small, mobile male gamete.
- Egg – typically larger female gamete containing resources for early development.
- Fertilization – fusion of gamete nuclei.
- Zygote – cell produced by fertilization.
- Genetic variation – genetic differences among individuals.
- Budding – asexual reproduction in which a new organism develops as an outgrowth of the parent.
- Fragmentation – reproductive process in which a body fragment can develop into another individual.
- Regeneration – regrowth or replacement of missing or damaged structures.
- Parthenogenesis – development of offspring from an unfertilized egg.
- External fertilization – fertilization occurring outside the parent's body.
- Internal fertilization – fertilization occurring within the reproductive tract.
- Parental care – parental behaviour that increases offspring survival.
- Courtship – behaviour associated with attracting and selecting mates.
- Sexual selection – selection associated with differences in mating and reproductive success.
- Reproductive success – success in passing genetic information into future generations through surviving descendants.
- Reproductive strategy – combination of characteristics and behaviours involved in reproduction and offspring survival.
- Reproductive investment – time, energy, and resources devoted to reproduction and offspring.
- Natural selection – process in which heritable characteristics affecting survival or reproduction influence which traits become more common over generations.
Key Takeaways
- Animals use a wide range of reproductive strategies.
- Sexual reproduction involves the fusion of gametes and usually produces genetically varied offspring.
- Asexual reproduction requires only one parent and generally produces less genetic variation.
- Genetic variation can increase the range of responses available to a population when environmental conditions change.
- Asexual reproduction can allow rapid reproduction without the need to find a mate.
- Budding, fragmentation, and parthenogenesis are forms of asexual reproduction found in animals.
- Fertilization can occur internally or externally.
- External fertilization is especially common in aquatic environments.
- Internal fertilization protects gametes from drying out and increases opportunities for fertilization on land.
- Animals vary greatly in the number of offspring produced and the investment made in each offspring.
- Producing many offspring can compensate for high juvenile mortality.
- Producing fewer offspring allows greater parental investment in each one.
- Courtship, mate competition, parental care, reproductive timing, and specialised reproductive structures can all increase reproductive success.
- Reproductive strategies involve trade-offs because animals have limited energy and resources.
- Different environments favour different combinations of reproductive characteristics.
- Reproductive success depends not simply on producing offspring, but on producing descendants that survive and reproduce.
- There is no single reproductive strategy that is optimal in every environment.