Ch6: Life Cycles

Why Is Reproduction Important for Life and Evolution?

• Every organism alive today exists because its ancestors successfully reproduced.

• This forms an unbroken chain going back to the earliest life.

• At its core, reproduction is the passing on of genes.

• Reproductive success is the ultimate measure of evolutionary fitness because organisms must reproduce for their genes to continue.

Quick idea:
Survive + reproduce → pass genes to next generation → continuation of life

How Do Prokaryotes and Eukaryotes Reproduce Asexually?

Both can reproduce asexually, but they use different processes.

Prokaryotes → Binary fission

• Copy their single circular DNA molecule.

• Parent cell splits into 2 daughter cells.

• Each daughter receives one copy of the genome.

Eukaryotes → Mitosis

• DNA is copied.

• Chromosomes are separated.

• Cell divides into 2 daughter cells with the same full set of chromosomes.

• Mitosis is more complicated because eukaryotes have:

  • Multiple chromosomes

  • Linear chromosomes

  • Chromosomes enclosed in a nucleus

Quick idea:
🦠 Prokaryotes → binary fission
🧬 Eukaryotes → mitosis

How Does Sexual Reproduction Differ From Asexual Reproduction?

Asexual reproduction

• Does not mix DNA from two parents.

• Convenient and allows rapid population growth.

• Produces less genetic diversity.

Sexual reproduction

• Mixes DNA from two parents.

• Uses meiosis to reduce chromosome number by half.

• Two cells then fuse → zygote.

• Produces genetically mixed offspring.

Quick idea:
Meiosis → chromosome number cut in half → cells from two parents fuse → zygote

Why Is Sexual Reproduction Important for Eukaryotes?

• Asexual reproduction can increase population size quickly, but it produces less genetic diversity.

• Less diversity makes it harder for a species to persist through changing conditions over long periods.

• Most eukaryotes therefore use sexual reproduction, either alone or along with asexual reproduction.

• Exclusively asexual eukaryotes are rare.

Prokaryotes can remain asexual because they still generate diversity through:

• High rates of mutation, helped by short generation times and generally poor DNA repair.

• Horizontal gene transfer.

Eukaryotes rely more heavily on the genetic mixing produced by sexual reproduction.

⭐ Key idea:
Sexual reproduction → genetic diversity → greater ability for populations to persist through changing challenges

How Common Is Sexual Reproduction in Animals?

• Sexual reproduction is almost universal among animals.

• Many animals can also reproduce asexually.

• Very few animal species reproduce only asexually.

• Most exclusively asexual animal species are evolutionarily young, meaning they abandoned sexual reproduction relatively recently.

How Does Meiosis Create Genetically Different Offspring?

During mitosis:

• DNA is copied.

• Chromosomes are separated.

• 1 cell → 2 genetically identical cells

• Chromosome number stays the same.

During meiosis:

1. Chromosomes are copied.

2. Different versions of chromosomes exchange DNA through crossing over.

3. Chromosomes are randomly distributed into two cells.

4. Each cell divides again.

5. 4 genetically different cells are produced.

6. Each has half the original number of chromosome sets.

The cells are genetically different because of:

• 🔀 Crossing over → chromosomes exchange DNA.

• 🎲 Random distribution → different chromosome versions end up in different cells.

Quick idea:

Mitosis:
1 cell → 2 identical cells → same chromosome number

Meiosis:
1 cell → 4 different cells → ½ chromosome sets

How Does Meiosis Work in Humans?

Human body cells normally have:

46 chromosomes = 2 sets of 23

Mitosis:

46 → 46 + 46

Produces 2 genetically identical cells, each with 46 chromosomes.

Meiosis:

46 → 23 + 23 + 23 + 23

Produces 4 genetically different cells, each with 23 chromosomes.

Then during fertilization:

23 from one parent + 23 from other parent → 46-chromosome zygote

⭐ This restores the full chromosome number while mixing DNA from two parents.

What Is Ploidy?

Ploidy = the number of chromosome sets in a cell.

• Monoploid = 1 set

• Diploid = 2 sets

• Triploid = 3 sets

• Tetraploid = 4 sets

There are two ways to describe chromosome sets:

• x = monoploid number

  • Based on the number of different chromosome types.

• n = haploid number

  • Based on the chromosome number after meiosis.

This textbook mainly uses n because it makes sexual life cycles easier to follow.

⭐ Most important rule:

Meiosis: 2n → 1n
→ ploidy is cut in half

Fertilization: 1n + 1n → 2n
→ ploidy is restored/doubled

How Is Ploidy Described in Humans?

Humans have:

2n = 2x = 46

This means:

• 2n → body cells have twice the chromosome number found after meiosis.

• 2x → body cells have two of each chromosome type.

• n = 23 → human gametes contain 23 chromosomes.

• x = 23 → humans have 23 different chromosome types.

So for humans, n and x happen to be the same number.

Quick idea:
Human body cell → 2n = 46
↓ meiosis
Gamete → 1n = 23
↓ fertilization
Zygote → 2n = 46

Why Aren't n and x Always the Same?

Some organisms have multiple copies of each chromosome type.

For example, the African clawed frog has:

2n = 4x = 28

• 2n → it still has twice the chromosome number found in its gametes.

• 4x → it has four copies of each chromosome type.

• Total = 28 chromosomes.

So an organism can be diploid relative to reproduction (2n) while having more than two copies of each chromosome type.

⭐ Key distinction:
n = chromosome sets relative to meiosis/reproduction
x = number of copies of each different chromosome type

The body cells of the African clawed frog Xenopus laevis have a ploidy of 2n = 4x = 28. How many different kinds of chromosomes does it have (that is, how many chromosomes make up each set)?

7

Correct! The frog is tetraploid (4𝘹) in the absolute sense, with 28 chromosomes, so it has four sets of chromosomes making up the 28: 28/4=7.

The body cells of the Lake Oku clawed frog, Xenopus longipes, are diploid relative to the products of meiosis and carry 108 chromosomes, which occur in sets of 9 (i.e., it has 9 different chromosome types). What is the correct chromosome designation for this frog?

2n = 12x = 108

• Yes! The frog is diploid = 2n with 108 chromosomes in 12 sets of x = 9: 108/9 = 12 = dodecaploid. Such large ploidies are rare in animals, but occur occassionally among amphibians and fish, and are very common in plants (especially agricultural strains)

What Is the Basic Pattern of Sexual Reproduction?

Sexual reproduction alternates between haploid (1n) and diploid (2n) stages.

Three processes control this cycle:

• Meiosis: 2n → 1n

  • Cuts ploidy in half.

• Fertilization: 1n + 1n → 2n

  • Two haploid cells fuse.

  • Restores the diploid state.

• Mitosis: ploidy stays the same

  • Produces genetically identical copies of cells.

  • Used for growth and increasing cell number.

Different eukaryotes follow this same general pattern, but when meiosis happens and how fertilization occurs can differ.

Quick idea:
2n → meiosis → 1n → fertilization → 2n
⭐ Mitosis = no change in ploidy

How Do Animals Reproduce Sexually?

Animals reproduce through the fusion of haploid gametes.

• Adult parents are diploid (2n).

• Specialized germ cells undergo meiosis to produce haploid (1n) gametes.

• Germ cells are located in reproductive organs called gonads.

• Female → produces eggs

• Male → produces sperm

• Egg + sperm fuse during fertilization → diploid (2n) zygote.

• The zygote undergoes mitosis and cell differentiation to grow into a multicellular adult.

  • Cell differentiation = cells become specialized for different jobs.

Animal life cycle:

Diploid adult (2n)
↓ meiosis
Gametes (1n): egg + sperm
↓ fertilization
Zygote (2n)
↓ mitosis + differentiation
Diploid adult (2n)

⭐ Key thing: In animals, gametes are haploid, while the zygote and adult are diploid.

How Is Sex Determined in Animals?

Animals can have genetic or environmental sex determination.

Genetic sex determination

Sex depends on the chromosomes an individual inherits.

• XY system

  • XX = female

  • XY = male

• X0 system

  • XX = female (or rarely hermaphrodite)

  • X = male

• ZW system

  • ZW = female

  • ZZ = male

• Haplodiploid system

  • Fertilized egg (2n) → female

  • Unfertilized egg (1n) → male

Quick memory trick:
XY → male has different letters
ZW → female has different letters

How Can the Environment Determine an Animal's Sex?

In some animals, sex is determined by external conditions rather than chromosomes.

Examples include:

• 🌡 Temperature → incubation temperature determines sex.

• 👥 Social interactions → sex can depend on:

  • Chemical cues from other members of the species

  • Social hierarchy

  • Certain bacteria present during development

In some species, environmental cues can even cause an individual to change sex during its lifetime.

What Does Hermaphroditic Mean?

Hermaphroditic animals produce both eggs and sperm.

• Each individual can produce both types of gametes.

• Therefore, individuals do not have to develop strictly as male or female.

⭐ Big picture: Animal sex determination is highly variable—different species use chromosomes, ploidy, environmental conditions, or both reproductive functions in one individual.

How Does the Haplodiploid Sex Determination System Work in Hive Insects?

Haplodiploidy is a genetic sex-determination system found in hive-forming insects such as bees, wasps, and ants.

The key difference is whether an egg is fertilized:

• 🐝 Fertilized egg → diploid (2n) → female

• 🐝 Unfertilized egg → haploid (1n) → male

⭐ Easy memory:
Fertilized = Female = 2n
Unfertilized = Male = 1n

How Does a Queen Produce Workers and Drones?

A typical reproductive pattern is:

1. A diploid female (2n) mates with a haploid male (1n).

2. She becomes a queen and stores the male's sperm.

3. She can then choose whether her eggs are fertilized:

   • Fertilizes egg → 2n female worker

   • Doesn't fertilize egg → 1n male drone

Quick idea:

Queen's egg
→ + sperm → 2n female 🐝
→ no sperm → 1n male 🐝

What Are the Roles Within the Colony?

• 👑 Queen → produces eggs and stores sperm from the male.

• 👷 Female workers (2n) → come from fertilized eggs.

• ♂ Male drones (1n) → come from unfertilized eggs and may reproduce with future queens or perform other roles depending on the species.

• In some species, only the queen lays eggs.

• In others, certain workers become foundresses and also contribute eggs.

⭐ Most important thing to know:
Haplodiploid system = females are diploid (2n), males are haploid (1n).

Which of the following statements is correct about the hive insects?

Females have both a mother and a father, but males have only a mother

Yes! The females are produced from an egg that has been fertilized by a sperm, but the males are produced from an egg that has not.

Do male ants and bees produce sperm by meiosis or mitosis?

• By mitosis! Males in the haplodiploid system are already haploid, so they do not need to reduce their chromosome sets during sexual reproduction, but simply copy them all. This means that their sperm are all genetically identical to themselves (clones).

Why Might Haplodiploidy Promote Cooperative Hive Behavior?

In haplodiploid insects, the female sisters in a hive can be very genetically similar.

This happens because:

• Their father is haploid (1n).

  • He has only one set of alleles to pass on.

  • Sisters can therefore inherit the same paternal alleles.

• Their mother is diploid (2n).

  • Sisters can also inherit many of the same alleles from her.

This creates an unusually high genetic relatedness among sisters.

How Could High Genetic Relatedness Lead to Altruism?

Altruism = sacrificing or helping for the benefit of others.

Because hive members are closely related, helping relatives survive and reproduce can also help shared genes continue into future generations.

This is called kin selection = natural selection favoring behaviors that help genetic relatives.

Quick idea:

Haplodiploidy → sisters highly related → kin selection → more cooperation & altruism → hive acts like a superorganism

⭐ Superorganism = a colony whose members cooperate so closely that they function almost like the cells of one large organism.

Imagine a fictional species called ardviks. Among the ardviks, Asel has an allele that promotes selflessness (altruism), whereas Ednok does not. Who is more likely to benefit from interactions between them?

• Yes. Ednok will benefit from the help of Asel, at Asel’s expense, with nothing given in return. Ednok’s self-centeredness allele will be selected for in this scenario, whereas Asel’s altruism allele will be selected against

Asel and Berlof are siblings, and both have inherited the altruism allele from their parents. Are they each likely to be more or less successful in reproduction when they interact and help each other, or when they do not?

When they help each other

• That’s right. Asel and Berlof are more likely to be successful when they help each other. Because they both carry the altruism gene, its passage to the next generation is thus boosted when either helps the other. This is kin selection

Suppose that receiving help from others results in much greater reproductive success than does going it alone, such that those who help each other successfully raise on average 2 times more offspring than those who do not, even despite the cost of helping. Consider a situation where the siblings Asel and Berlof both have the altruism allele and help each other, but the siblings Ednok and Ralo do not and are each on their own. Will the altruism allele be selected for in this scenario?

Yes, the altruism allele will be passed on more effectively than its counterpart

• Exactly! If Asel and Berlof pass their alleles to twice as many offspring on average, then their shared altruism gene will more successfully flood the market in the next generation.

What would happen if most of the individuals in the ardvik population never actually reproduce themselves—would kin selection still cause the altruism gene to be selected for?

• Yes, it sure would! A non-reproducing altruist still helps their siblings successfully pass on their shared alleles. Kin selection works any time the benefits of associating in family units exceed the costs on average among them.

How Does Kin Selection Promote Cooperation Among Relatives?

Kin selection occurs when closely related individuals interact and affect each other's reproductive success.

• Family members share many of the same alleles.

• If an allele encourages individuals to help their relatives, those relatives may reproduce more successfully.

• Because relatives share that allele, helping them can cause the allele to be passed on more frequently.

• Therefore:

  • Helpful/cooperative alleles → more likely to be selected for.

  • Alleles encouraging relatives to harm each other → may reduce reproduction → more likely to be selected against.

Quick idea:
Relatives share genes → helping relatives reproduce → shared genes get passed on → cooperation is favored

How Can Kin Selection Produce a "Hive Mind"?

In hive insects, kin selection may have strongly reinforced genetically based behaviors involving:

• Cooperation

• Selflessness

• Altruism

Individual insects perform relatively simple behaviors, but when thousands of individuals interact, their combined behavior can produce complex, intelligent outcomes.

This makes the colony behave almost like one large organism, called a superorganism or "hive mind."

The text compares this to the brain:

Simple individual insects → interact → complex hive behavior

Individual neurons → interact → complex intelligence/consciousness

⭐ Key idea: Complex behavior can emerge from a large network of individually simple interactions.

What Is the Difference Between Oviparity and Viviparity?

Animal eggs contain much more than DNA. They provide important materials needed for development, including:

• Mitochondria → energy production

• Signaling molecules → help direct development

• Yolk → nourishment

After fertilization, the developing offspring becomes an embryo.

There are two major ways embryos can develop:

🥚 Oviparity

• Mother lays eggs.

• Embryo develops outside her body.

• Generally less costly to the mother.

• Offspring face greater environmental risks.

🤰 Viviparity

• Embryo develops inside the mother's body.

• Results in live birth.

• Provides a safer, more controlled environment for development.

• More demanding on the mother's health and resources.

Quick idea:
🥚 Oviparity = egg laid outside
🤰 Viviparity = develops inside → live birth

How Did the Amniotic Egg Allow Vertebrates to Reproduce on Dry Land?

Eggs developing outside the body on land face two major problems:

• 💧 Drying out

• 🥚 Physical damage/crushing

To reproduce successfully on land, eggs need water-preserving and protective structures.

Some invertebrates use water-preserving cocoons.

Among vertebrates, fully dry life cycles became possible through the evolution of the amniotic egg.

The amnion:

• Is a membrane surrounding the embryo.

• Holds amniotic fluid.

• Acts somewhat like a water balloon, keeping the embryo surrounded by fluid even though the egg is on dry land.

⭐ The amnion is one of four extraembryonic structures unique to amniotes.

Quick idea:
🌊 Embryos need water → amnion holds fluid around embryo → embryo can develop on dry land 🌎

Which Vertebrates Have Fully Dry Life Cycles?

The amniotes are:

• 🦎 Reptiles

• 🐦 Birds (a type of reptile)

• 🐭 Mammals

They share an ancestor that evolved the amniotic egg, allowing reproduction without returning to external water.

All amniotes use internal fertilization.

Among them, egg-laying (oviparity) occurs in:

• All birds

• Most reptiles

• Monotremes → the small mammal group containing platypuses and echidnas

⭐ Big picture:
Internal fertilization + amniotic egg → reproduction independent of external water → fully dry vertebrate life cycle

Why Was the Amniotic Egg Important for Life on Land?

About 350 million years ago, amphibians were the dominant land vertebrates, but they still depended on water for reproduction.

• Amphibian eggs were shell-less and jelly-like.

• If laid in dry conditions, the eggs would dry out.

• Therefore, amphibians had to remain in or near freshwater to reproduce.

The evolution of the amniotic egg changed this.

• It was shelled and waterproof.

• It could be laid on dry land.

• This allowed amniotes to live a fully terrestrial life cycle.

⭐ Think of the amniotic egg as a "portable pond"—it brings a watery environment onto land with the developing embryo.

How Does the Protective Shell Help the Amniotic Egg Survive on Land?

The protective shell is:

• Tough but flexible

• Resistant to drying out

• Protective against:

  • Predators

  • Bacteria

  • Physical damage

However, the shell is also porous.

• Tiny pores allow oxygen (O₂) to enter.

• They allow gases to move so the embryo doesn't suffocate.

Quick idea:
🥚 Shell = keeps water in + protects embryo + still allows gas exchange

What Do the Chorion and Allantois Do?

The chorion and allantois work together to help with gas exchange.

Chorion:

• Protective membrane.

• Allows O₂ to move inward.

• Allows CO₂ to move outward.

Think of it as the doors 🚪 of the egg.

Allantois:

• Helps manage the movement of O₂ and CO₂.

• Stores waste produced by the embryo.

Think of it as the lobby that handles what comes in and goes out.

Quick idea:
Chorion → gas exchange
Allantois → gas handling + waste storage

What Does the Amnion Do?

The amnion is the membrane surrounding the developing embryo.

• Contains amniotic fluid.

• Keeps the embryo moist, preventing desiccation.

• Allows the embryo to essentially develop in its own small aquatic environment.

• The fluid also protects the embryo from physical impacts, acting like a shock absorber.

⭐ Amnion = "portable pond" around the embryo 💧

What Adaptations Allowed the Amniotic Egg to Survive on Land?

The important structures work together to create a protected, watery environment:

🥚 Protective shell → prevents drying/damage while allowing gases through
🫁 Chorion → gas exchange
♻ Allantois → gas movement + waste storage
💧 Amnion → fluid around embryo; prevents drying + cushions it

Big picture:
Protection + gas exchange + waste storage + water retention → embryo can develop on dry land

What Major Groups Evolved From the Early Amniotes?

Amniotes eventually split into two major evolutionary groups:

• Synapsids → lineage containing mammals

• Sauropsids → lineage containing reptiles, birds, and dinosaurs

This means animals as different as walruses, rattlesnakes, and parakeets share ancestry through early amniotes.

Quick idea:

Early amniotes
→ Synapsids → mammals
→ Sauropsids → reptiles + dinosaurs + birds

⭐ Main takeaway: The amniotic egg freed vertebrate reproduction from dependence on external water, allowing amniotes to expand farther into terrestrial environments.

What Are the Three Major Types of Mammals?

All mammals share one defining feature: mothers nurse their young with milk.

Nursing requires:

• A large amount of parental investment

• Long-term care of offspring

• Producing and supporting a relatively small number of offspring

There are three major mammal groups based largely on how their young develop:

1. Monotremes 🥚

• Lay eggs (oviparous).

• Still nurse their young with milk.

• Only platypuses and echidnas remain today.

2. Marsupials 🦘

• Use viviparity (live birth).

• Give birth to a relatively undeveloped embryo.

• The offspring finishes developing in an external pouch on the mother.

3. Placental mammals 🐘

• Use viviparity.

• Young develop inside the uterus for much longer.

• A placenta provides nourishment during development.

• Offspring are more developed before birth.

Quick progression:

Monotremes → egg laying
Marsupials → short internal development + pouch
Placentals → longer internal development + placenta

Why Might Viviparity Have Been Beneficial for Mammals?

Mammals already invest heavily in their offspring because they nurse them with milk.

Therefore, the additional cost of keeping offspring inside the body during development may have been worthwhile because it provides the offspring with a protected, wet environment.

According to the text:

Nursing already requires high investment → live birth adds protection → viviparity provides reproductive benefits

The text describes marsupials as replacing most monotremes and placental mammals later replacing many marsupials.

What Is the Placenta?

The placenta is an adaptation of placental mammals that allows offspring to be nourished while developing inside the uterus.

It allows the young to remain inside the mother and develop for a longer period before birth.

⭐ Placenta → supports longer internal development

What Is the Difference Between the Estrus and Menstrual Cycles?

The endometrium is the preparatory lining of the uterus.

If fertilization does not occur:

• Estrus cycle: endometrium is maintained or periodically reabsorbed.

• Menstrual cycle: endometrium is shed from the body and replaced.

According to the text, the menstrual cycle occurs in primates and may help periodically remove pathogens introduced into the uterus.

Quick idea:
Estrus → reabsorb
Menstrual → shed

Why Can Animals Have Complex Mating Behaviors?

Animals can:

• Move in complex and responsive ways.

• Use internal fertilization.

• Strongly select their mates.

Many mating behaviors are innate, meaning they are instinctive and genetically hardwired, although learning can also influence them.

This has produced a huge variety of mating rituals and mating systems.

What Is Sexual Dimorphism?

Sexual dimorphism = males and females of the same species look different from one another in characteristics beyond what is directly necessary for reproduction.

These differences often result from sexual selection.

For example:

• Male and female animals may differ in size, coloration, horns, feathers, etc.

What Are Secondary Sex Characteristics?

Secondary sex characteristics are traits that:

• Differ between males and females

• Are not directly involved in producing or transferring gametes

Examples from the text:

• 🐏 Male sheep → larger horns associated with fighting over females.

• 🦚 Male peacocks → long, colorful tails preferred by females.

⭐ Secondary sex characteristic = sex-related difference that isn't the actual reproductive machinery

What Is Runaway Selection?

Runaway selection occurs when sexual selection pushes a trait to become extremely exaggerated, usually because of consistent mate preferences.

Example: 🦚 peacock tails

• Females prefer large, colorful tails.

• Males with those tails have a reproductive advantage.

• This can cause the trait to become increasingly extreme.

• Even though the tail can be a practical disadvantage, the mating advantage keeps it around.

Runaway selection can continue until the trait's practical disadvantages become greater than its sexual advantage.

Quick idea:

Mate preference → trait gives mating advantage → trait becomes more extreme → runaway selection

⭐ Key distinction:
Sexual dimorphism = males and females differ
Secondary sex characteristic = the specific non-gamete trait that differs
Runaway selection = sexual preferences drive a trait toward an extreme

Do we humans exhibit any secondary sex characteristics?

• We sure do! Body size, hip width, chest development, voice pitch, and degree of facial hair are examples of secondary sex characteristics in humans. (Genitalia are primary sex characteristics, since they directly produce and convey gametes.)

Why Are Females Often Choosier About Mates Than Males?

According to the text, eggs and offspring are much more costly to produce than sperm.

This creates a general pattern:

• Females → typically need resources to reproduce successfully → tend to be more selective about mates.

• Males → primarily need access to females → tend to be more competitive for mates.

⭐ Quick idea:
Eggs/offspring = expensive → females choosy
Sperm = less costly → males competitive

This is a general trend, not a rule for every animal species.

What Is a Nuptial Gift?

A nuptial gift is a resource a male provides to a female as part of mating.

For example:

• Male insects may give females a nutritious food item.

• Females may choose the male that provides the largest gift.

This fits the general pattern of females responding favorably to males that can provide resources or support.

How Do Males Compete for Females?

Because males generally benefit from gaining access to females, they may compete by:

• Blocking rivals

• Scaring away rivals

• Attracting females

• Or doing both

These behaviors increase the male's chance of being chosen to reproduce.

What Is the Difference Between r-Selected and K-Selected Species?

Reproduction always has a cost, so organisms must balance how many offspring they produce with how much they invest in each one.

r-selected species (r-strategists)

Many offspring + little parental investment

• Produce many offspring at once.

• Invest little or nothing in raising them.

• Strategy is essentially to produce so many offspring that at least some survive on their own.

• Very common among invertebrates and fish.

Quick idea:
r = lots of offspring, little care

K-selected species (K-strategists)

Few offspring + lots of parental investment

• Produce few offspring at a time.

• Invest considerable resources into raising them.

• Common among many terrestrial vertebrates.

Quick idea:
K = fewer offspring, lots of care

⭐ Main contrast:

r-strategist → MANY babies → LOW investment per baby

K-strategist → FEW babies → HIGH investment per baby

What Does Semelparous Mean?

Semelparous organisms:

• Reproduce only once in their lifetime.

• Invest essentially everything into one major reproductive event.

• Then die.

Some r-strategists use this strategy.

Quick memory:
Semelparous = one reproductive event → then death

⭐ Big picture: Evolution favors reproductive strategies that successfully balance the cost of reproduction with the number and survival of offspring produced.

Are we humans best classified as r- or K-strategists?

K-strategists

That’s right! We typically have only one offspring per reproductive event, which we then invest heavily in rearing. We are actually an extremely K-selected species!

Where Do the Terms r-Selection and K-Selection Come From?

The terms come from variables used in population growth equations:

• r = per capita growth rate

  • How many new individuals are added to the population per existing individual over time.

• K = carrying capacity

  • The maximum population size that an environment can support.

Therefore:

• r-strategists → maximize reproduction (r) → population size may increase and decrease dramatically.

• K-strategists → populations tend to stay closer to the environment's carrying capacity (K).

Quick memory:
r = reproduction RATE
K = carrying Kapacity (spell it wrong to remember it!)

What Are the Major Animal Mating Systems?

Animals differ in how long they form reproductive relationships, called pair bonds.

There are four important patterns:

1. Promiscuity — most common

• Pairing lasts only for the mating act.

• Individuals can mate with multiple other individuals.

• Maximizes genetic diversity among offspring.

• Occurs in both r- and K-strategists.

• Becomes less practical when offspring require a lot of parental care.

⭐ Promiscuity = no lasting pair bond

---

2. Polygamy — one individual + multiple partners

• One member of one sex maintains pair bonds with multiple members of the other sex.

• Those partners generally remain mostly exclusive to that individual.

There are two major types:

Polygyny 👨 → 👩👩👩

• One male + multiple females

• Most common type of polygamy.

• Often organized around a resource controlled by the male.

Polyandry 👩 → 👨👨👨

• One female + multiple males

• Much less common.

⭐ Memory trick:
PolyGYNY → one GUY + multiple females
PolyANDRY → one female + multiple men

What Is Monogamy?

Monogamy = two individuals of opposite sex form a mutually exclusive or mostly exclusive pair bond.

• Often lasts for a breeding season.

• More likely when offspring require intensive parental care.

• Especially useful when one parent cannot successfully raise the offspring alone.

• Most likely when both male and female can contribute to raising the young.

Quick idea:

Offspring need lots of care → two parents are helpful → monogamy more likely

How Does Parental Care Relate to Mating Systems?

The amount of care offspring require can influence which mating system works best.

Less dependent offspring
→ promiscuity is more viable
→ parents don't need to stay together

Highly dependent offspring
→ lasting pair bonds become more useful
→ monogamy more likely when both parents are needed

⭐ Big picture:

Promiscuity = no lasting bond
Polygyny = 1 male + many females
Polyandry = 1 female + many males
Monogamy = 1 male + 1 female pair bond

Which animal mating system achieves the greatest level of genetic recombination?

Yes! In promiscuous systems both males and females mate with many others, resulting in the highest level of gene mixing among individuals.

Which system of polygamy is likely to result in the highest output of offspring?

Polygyny

• That’s right. Females produce the offspring, so a larger number of females with reliable access to resources typically results in greater total reproductive capacity. This is probably why polygyny is considerably more common than polyandry in animals.

In which animal group is monogamy most common?

Birds

• Correct! Monogamy tends to be selected for when offspring need rearing, and both parents are equally capable of rearing them. Monogamy is thus most common among nesting birds.

What Are the Three Main Types of Asexual Reproduction in Animals?

Although sexual reproduction is most common, many animals can also reproduce asexually.

The three main methods are:

1. Fragmentation (fission)

2. Budding

3. Parthenogenesis

⭐ Fragmentation and budding always produce clones.
Parthenogenesis may or may not produce clones.

How Does Fragmentation (Fission) Work?

Fragmentation/fission occurs when:

• A piece of the parent's body breaks off.

• The piece regenerates and grows into a complete new organism.

• The offspring is a clone of the parent.

The piece may break off because of physical trauma or intentionally as part of reproduction.

Common in:

• Sponges

• Cnidarians, especially corals

• Echinoderms

• Flatworms

Quick idea:
Parent breaks → piece regenerates → new clone

How Does Budding Work?

Budding occurs when a small version of the organism grows directly from the parent's body.

• The "bud" grows.

• It eventually separates from the parent.

• It becomes an independent organism.

• The offspring is a clone.

Found in many:

• Sponges

• Cnidarians

• Flatworms

Quick idea:
Small bud grows on parent → separates → clone

What Is a Gemmule?

Some sponges produce a special form of budding called a gemmule.

A gemmule is:

• A cluster of embryonic and nutrient-storing cells

• Surrounded by a protective layer

• Able to act like a tough spore

• Capable of developing into a new copy of the parent

Gemmules are usually produced shortly before the sponge dies.

⭐ Gemmule = protected package of cells that can grow into a new sponge

What Is Parthenogenesis?

Parthenogenesis occurs when an unfertilized egg develops into an offspring.

🚫 No male fertilizes the egg.

It occurs routinely in some:

• Rotifers

• Tardigrades

• Roundworms

• Arthropods

It has also occasionally been observed in vertebrates such as:

• Fish

• Lizards

• Amphibians

• Birds

⭐ Parthenogenesis = egg → no fertilization → offspring

Does Parthenogenesis Always Produce a Clone?

No. It depends on how the egg was produced.

If the egg is produced by mitosis:

Mother → mitosis → egg → offspring

• Egg receives the mother's full DNA complement.

• Offspring is a clone of the mother.

If the egg is produced by meiosis:

Mother → meiosis → genetically different egg → offspring

• Meiosis produces cells with different combinations of the mother's chromosomes.

• Therefore, offspring are not necessarily genetically identical to the mother or each other.

• All of the genetic material still comes from one parent.

⭐ Mitosis parthenogenesis = clone
⭐ Meiosis parthenogenesis = genetic variation

How Is Parthenogenesis Used in Haplodiploid Insects?

Hive insects regularly use a form of meiotic parthenogenesis.

Remember:

Queen → meiosis → haploid egg (1n)

Then:

• Egg + sperm → 2n female

• Unfertilized egg → 1n male

So the haploid males develop through parthenogenesis because they develop from an egg without fertilization.

How Can Meiotic Parthenogenesis Still Produce Diploid Offspring?

In many animals using meiotic parthenogenesis, the offspring still becomes diploid (2n).

The text gives two ways this can happen:

• The egg skips a cell division during meiosis, so chromosomes remain together instead of separating.

• Two cells produced during meiosis fuse back together, restoring the diploid state.

Big Picture

Asexual reproduction in animals:

Fragmentation → body piece → new organism → clone

Budding → organism grows off parent → clone

Parthenogenesis → unfertilized egg → offspring → may or may not be clone

⭐ The big difference: Parthenogenesis can involve meiosis, so it can create some genetic variation even though there is only one parent.

How Do Fungi Switch Between Sexual and Asexual Reproduction?

Fungi can routinely switch between sexual and asexual reproduction, giving them a lot of reproductive flexibility.

Fungi are:

• Haploid (1n)

• Specifically monoploid (1x) → they have one copy of each chromosome type.

• Able to reproduce sexually without producing gametes.

⭐ Unlike animals and plants, fungi do not need specialized eggs and sperm for sexual reproduction.

Quick idea:
Fungi = haploid + no gametes + can switch between sexual and asexual reproduction

How Do Fungi Reproduce Sexually Without Gametes?

Instead of fusing specialized gametes, fungi achieve sexual reproduction through the union of haploid somatic cells.

• Somatic cells = ordinary body cells.

• Two haploid fungal body cells can come together as part of sexual reproduction.

⭐ Key difference:

Animals → haploid gametes unite

Fungi → haploid somatic cells unite

How Was Fungal Sex Used to Classify Fungi?

Before scientists had modern genetic techniques, fungal groups were often distinguished by their spore-bearing structures produced during sexual reproduction.

These structures were useful for identifying different fungal phyla.

Quick idea:
Different sexual/spore structures → helped scientists classify fungal groups

Fungi are haploid (1n) organisms, meaning their somatic cells carry half the complement of chromosomes that their zygotes do. More specifically, fungi are monoploid (1x) organisms, meaning that in that haploid state their cells carry just one chromosome set. Which two of the following are correct about fungi?

The zygotes are 2n

Their zygotes divide by meiosis to produce haploid cells

Yes! The cells are already haploid (and monoploid), so they cannot reduce their number of sets any further by meiosis. The zygotes, in contrast, need to cut their chromosome number in half to return to the typical somatic cell ploidy.

How Do Fungi Complete Their Sexual Life Cycle?

Because fungi are already haploid (1n), they do not need meiosis before fertilization.

Instead:

1. Two haploid (1n) somatic cells from different parents fuse.

2. This produces a diploid (2n) zygote.

3. The diploid zygote undergoes meiosis.

4. Meiosis produces haploid spores (1n).

5. Spores disperse and germinate.

6. They grow into new haploid fungi containing a genetic mixture from the parents.

Quick idea:

Haploid parents (1n)
↓ cell fusion
Diploid zygote (2n)
↓ meiosis
Haploid spores (1n)
↓ germination
New haploid fungi (1n)

⭐ Big difference from animals:
Animals: meiosis happens before fertilization to make gametes.
Fungi: meiosis happens after fertilization to make spores.

Do Fungi Have Males and Females?

No. Fungi have different mating types rather than males and females.

• Mating types look the same.

• They have different biochemical markers on their cell surfaces.

• One mating type can reproduce sexually with other mating types of its species.

• It cannot reproduce sexually with its own mating type.

• Because fungi don't produce eggs or sperm, their mating types aren't considered male or female.

⭐ Fungi = mating types, NOT male/female

What Is Dikarya?

More than 98% of known fungal species belong to two phyla:

• Ascomycota

• Basidiomycota

Together, these form the fungal subkingdom Dikarya.

This includes:

• Common yeasts

• Mushrooms

• Morels

• Puffballs

• Truffles

These fungi can produce complex fruiting bodies.

Why Is It Called Dikarya?

During sexual reproduction, two fungal cells fuse, but their nuclei do not immediately fuse.

Instead:

Parent 1 nucleus + Parent 2 nucleus → same cell, but nuclei remain separate

The cell therefore contains two genetically different nuclei, one from each parent.

This is called a:

• Dikaryotic cell = cell containing the two separate nuclei.

• Heterokaryotic cell = another term emphasizing that the nuclei are genetically different.

⭐ Easy memory:
Di = two
kary = nuclei
→ Dikaryotic = two nuclei in one cell

Important: This is not yet a diploid zygote because the two haploid nuclei are still separate. They must eventually fuse to create the diploid nucleus.

1/2

A circle of mushrooms sometimes pops up in a lawn or meadow, often appearing overnight. These circles have been whimsically named fairy rings, as though the mushrooms were put there to be used as stools by fairies during a meeting they held. (Mushrooms are also referred to as toadstools). Why do these mushrooms grow in a ring?

Because the dikaryotic mycelium grows evenly outward

• That’s right. The dikaryotic mycelium that forms underground from the fusion of two parents grows outward and eventually upward to form fruiting bodies. In ground with uniform nutrients the growth occurs evenly in all directions, in an expanding circle.

What Happens to the Dikaryotic Cell After Two Fungi Mate?

Remember, a dikaryotic cell contains two separate haploid nuclei, one from each parent.

Instead of immediately fusing those nuclei, the fungus can:

1. Keep the two nuclei separate.

2. Divide the dikaryotic cell repeatedly by mitosis.

3. Produce many more dikaryotic cells.

4. Eventually fuse the two nuclei → diploid (2n) zygote.

5. The diploid zygote undergoes meiosis → haploid (1n) spores.

⭐ This allows the fungus to delay meiosis and spore production until conditions are favorable.

How Is a Fruiting Body Formed?

In multicellular fungi:

Dikaryotic cell
↓ repeated mitosis
Dikaryotic mycelium
↓ grows
Fruiting body 🍄

The fruiting body can contain millions of dikaryotic cells.

Eventually:

Two nuclei in each cell fuse → diploid → meiosis → spores

Millions of spores may then be released together in a "rain" of spores.

⭐ Fruiting body = structure that eventually produces and releases huge numbers of spores.

Why Is the Dikaryotic Stage Beneficial to Fungi?

The dikaryotic stage gives fungi a lot of reproductive flexibility.

Fungi can reproduce:

• Sexually → fusion with another mating type.

• Asexually → fragmentation or spores produced by mitosis.

Their haploid, dikaryotic, and even diploid forms can continue dividing asexually.

The major advantage is that a fungus can:

Mate once → create genetically recombined cells → copy those cells millions of times by mitosis → wait for good conditions → produce/release millions of spores

⭐ So fungi can take advantage of a sexual opportunity now but wait until the best environmental conditions to release offspring.

How Can Fungi Switch Between Sexual and Asexual Reproduction?

Fungi can move very smoothly between the two modes.

Sexual:
Two haploid cells fuse → genetic mixing → eventually meiosis → spores

Asexual:

• Fragmentation, OR

• Mitosis → spores

Because different stages can keep dividing by mitosis, fungi aren't forced to immediately finish the sexual life cycle after mating.

Quick idea:
Sex when opportunity arises + asexual reproduction whenever useful = highly flexible life cycle

What Is Parasexual Reproduction?

About 20% of Dikarya fungi can reproduce through parasexual reproduction.

The unusual part:

🚫 They do NOT use meiosis.

Normally:

Diploid (2n) → meiosis → haploid (1n)

In parasexual fungi:

Diploid (2n) → gradually lose chromosomes → haploid (1n)

The exact mechanism is poorly understood, but it appears to involve multiple rounds of modified mitosis that gradually eliminate an entire chromosome set.

Why Isn't Parasexual Reproduction Considered True Sexual Reproduction?

Even though there is genetic mixing, these fungi never use meiosis.

Therefore, technically, the process doesn't qualify as sexual reproduction.

Parasexual = "nearly sexual" or "sex-adjacent"

⭐ Most important distinction:

Sexual reproduction:
2n → MEIOSIS → 1n

Parasexual reproduction:
2n → chromosome loss through modified mitosis → 1n

Both can ultimately produce genetically mixed haploid offspring, but they get there in different ways.

How Is Plant Sexual Reproduction Different From Animals and Fungi?

Plant reproduction combines features seen in both animals and fungi.

Plants produce:

• Haploid spores (1n) through meiosis

• Haploid gametes (1n) through mitosis

⭐ This is important: plants do NOT produce their gametes directly through meiosis.

Quick comparison:

Animals: meiosis → gametes
Fungi: meiosis → spores
Plants: meiosis → spores, then mitosis → gametes

Plants can also reproduce asexually, usually through:

• Fragmentation

• Budding

What Is Polyploidy?

Polyploid = a cell containing more than two complete sets of chromosomes.

For example:

• 3x = triploid

• 4x = tetraploid

• 8x = octaploid

Polyploidy can occur when chromosomes fail to separate correctly during cell division.

Plants tolerate extra chromosome sets much better than animals and fungi, so polyploid plants are relatively common.

Why Is Polyploidy So Common in Plants?

When plants accidentally inherit extra chromosome sets:

• They are often still able to survive.

• Different plant species with different ploidies can sometimes interbreed.

• Their offspring can develop new polyploid states.

Humans have also deliberately hybridized crops, contributing to very high ploidies in agricultural plants.

For example, some crops have ploidies greater than 8x.

An extreme example is the black mulberry:

2n = 22x = 308 chromosomes

Meaning it has 22 copies of each chromosome type in its diploid state!

Why Does Sexually Sustainable Ploidy Need an Even Number of Chromosome Sets?

During meiosis, chromosomes need to be divided evenly into complete sets.

An even number can divide evenly:

4x → meiosis → 2x + 2x ✅

But an odd number causes problems:

3x → meiosis → can't evenly divide complete chromosome sets ❌

Therefore, plants with odd-numbered ploidies can exist, but they are usually sterile because they can't successfully produce viable haploid reproductive cells.

⭐ Even ploidy → can divide evenly during meiosis
⭐ Odd ploidy → meiosis problems → usually sterile

How Does Polyploidy Produce Seedless Watermelons?

Humans intentionally take advantage of this sterility.

For example:

Tetraploid plant (4x) × Diploid plant (2x)

Their reproductive cells contribute:

2x + 1x → 3x offspring

So the offspring is triploid (3x).

Because 3 is odd, its chromosomes cannot divide properly during meiosis.

Triploid watermelon → cannot produce viable reproductive cells → cannot successfully produce seeds → seedless watermelon 🍉

Even though these plants are sterile, humans can continue the valuable strains through asexual propagation.

⭐ Easy chain to remember:

4x × 2x → 3x → meiosis doesn't work properly → sterile → seedless

What Is Alternation of Generations in Plants?

Alternation of generations means a plant's life cycle alternates between two multicellular generations:

• 🌿 Sporophyte = diploid (2n)

• 🌱 Gametophyte = haploid (1n)

Each generation produces the next one.

How Does the Alternation of Generations Life Cycle Work?

Follow the cycle:

1. Sporophyte (2n)
↓ MEIOSIS
2. Spores (1n)
↓ grow by MITOSIS
3. Gametophyte (1n)
↓ MITOSIS
4. Gametes (1n)
↓ FERTILIZATION / SYNGAMY
5. Zygote (2n)
↓ grows by MITOSIS
6. Sporophyte (2n) 🔄

⭐ Then the cycle repeats.

What Does the Sporophyte Do?

The sporophyte is the diploid (2n) generation.

• Usually multicellular.

• Produces haploid spores (1n).

• Uses meiosis to make those spores.

Sporophyte → SPORES

Easy memory: both start with spor-.

What Does the Gametophyte Do?

The gametophyte is the haploid (1n) generation.

• Multicellular.

• Produces haploid gametes (1n).

• Uses mitosis, NOT meiosis, to make gametes.

Gametophyte → GAMETES

Easy memory: both start with gamet-.

Why Does the Gametophyte Use Mitosis Instead of Meiosis?

Because the gametophyte is already haploid (1n).

It doesn't need to cut its chromosome number in half.

1n gametophyte → mitosis → 1n gametes

In contrast:

2n sporophyte → meiosis → 1n spores

⭐ This is one of the most important distinctions to memorize:

MEIOSIS makes SPORES.
MITOSIS makes GAMETES.

Where Are the Sporophyte and Gametophyte Found?

In some marine algae, the sporophyte and gametophyte exist as completely separate organisms.

All other plants also have both generations, but one may remain attached to the other and can be very small or difficult to see.

Super Simple Memory Chain

SPOROPHYTE (2n)
→ meiosis →
SPORES (1n)
→ mitosis →
GAMETOPHYTE (1n)
→ mitosis →
GAMETES (1n)
→ fertilization →
ZYGOTE (2n)
→ mitosis →
SPOROPHYTE (2n) 🔄

The sporophytes of a certain species of plant have a ploidy of 8x. What will the ploidy of its gametophytes be?

4x

How Did Sporophytes and Gametophytes Change as Plants Became Better Adapted to Land?

All plants have alternation of generations, meaning they have both:

• Sporophyte (2n) generation

• Gametophyte (1n) generation

However, in most land plants, these don't look like two separate organisms. Instead, one generation grows dependently on the other, almost like an appendage or organ.

⭐ Importantly, the sporophyte and gametophyte are genetically distinct even when they're physically attached.

What Is the Evolutionary Trend From Algae to Seed Plants?

As plants became better adapted to dry land:

• Gametophyte → became smaller and less dominant

• Sporophyte → became larger and more dominant

The progression is:

🌊 Algae
→ sporophyte and gametophyte often separate organisms

🌿 Bryophytes
→ large/dominant gametophyte
→ small/inferior sporophyte grows from it

🌱 Pteridophytes
→ large/dominant sporophyte
→ small/inferior gametophyte

🌲🌸 Seed plants (gymnosperms + angiosperms)
→ large/dominant sporophyte
→ microscopic gametophytes grow on it

Which Generation Becomes Dominant as Plants Move Onto Land?

The sporophyte becomes increasingly dominant.

Algae → both can be separate
↓
Bryophytes → GAMETOPHYTE dominant
↓
Pteridophytes → SPOROPHYTE dominant
↓
Seed plants → SPOROPHYTE extremely dominant; gametophyte microscopic

⭐ Main trend to memorize:

Better adaptation to dry land → bigger sporophyte + smaller gametophyte

Or:

Algae → Bryophytes → Pteridophytes → Seed plants
Gametophyte ↓ ↓ ↓
Sporophyte ↑ ↑ ↑

What Is the Simplest Form of Alternation of Generations?

In the simplest form:

• Sporophyte (2n) and gametophyte (1n) are completely separate organisms.

• They mainly differ in:

  • Their ploidy

  • What they produce

Sporophyte (2n) → spores
Gametophyte (1n) → gametes

This pattern occurs in some algae.

How Does Chara Survive When Its Water Habitat Dries Up?

In the freshwater alga Chara:

• Female gametophyte retains the egg.

• Fertilization occurs on the plant.

• The resulting zygote becomes coated in sporopollenin.

Sporopollenin = a water-resistant biopolymer that prevents desiccation (drying out).

This allows the zygote to act like a survival spore during dry periods.

Quick idea:
Retained egg → fertilization on plant → zygote + sporopollenin → survives dry periods

How Do Bryophytes Reproduce?

In bryophytes:

• Gametophyte is dominant.

• Gametophyte retains the egg.

• After fertilization, the sporophyte grows on the gametophyte.

• Sporophyte produces sporopollenin-coated spores.

• Sporopollenin allows spores to disperse through air.

BUT bryophytes still depend heavily on water because:

• 💧 Sperm must splash/swim to the egg.

• They lack vascular tissue, so they can't efficiently transport water and other materials.

• This keeps them small and restricted to moist environments.

⭐ Bryophytes = dominant gametophyte + need water for sperm