Lecture 7 - Modes of Reproduction

Two primary modes of reproduction

1. Asexual

2. Sexual

Asexual reproduction

Offspring inherit DNA from a single parent.

• Common in plants, unicellular animals, and some aquatic animals.

• Produces genetically identical descendants (clones).

Types of asexual reproduction

To name only a few:

1. Vegetative propagation (reproduction)

2. Parthenogenesis

3. Budding

4. Binary fission

Vegetative reproduction (propagation)

A form of asexual reproduction where a new plant grows from a vegetative part of the parent plant, rather than from the seeds or spores.

• This could be the root, stem, or leaf.

Parthenogenesis

A form of asexual reproduction where an egg develops into a new organism without being fertilized by a sperm.

Benefits of asexual reproduction

• Quick reproduction

• No need for a mate

• Can produce many offspring rapidly.

• Pass down 100% of genes.

• Can switch to asexual reproduction. 

Costs of asexual reproduction

Many lineages are recently evolved, suggesting short survival (go extinct). These groups either die out or switch back to sexual reproduction.

This could be because of no genetic variation, making it hard to adapt to changes, diseases, and predators.

Sexual reproduction

Offspring inherit DNA from male and female gametes; one haploid cell (n) from each parent. 

• Common in plants and animals.

• Produces genetically unique descendants via genetic recombination.

Benefits of sexual reproduction

• Genetic recombination = increased genetic variation.

• Purging of deleterious mutations.

• Recombination “repairs” the genome.

• Makes purifying selection more efficient.

Costs of sexual reproduction

• Meiosis takes longer than mitosis.

• Increased risk of predation/disease.

• Wasted time and energy searching for mates and competing for sex.

Red Queen hypothesis

The hypothesis that species must constantly evolve to survive because other species (predators, parasites, competitors) are also evolving, leading to an ongoing evolutionary arms race.

• Asexual species may struggle to keep up, while sexual reproduction provides genetic variation that helps populations adapt quickly.

What are the 3 patterns of mating between males and females?

1. Monogamy

2. Polygamy

3. Promiscuity

Monogamy

A pattern of mating where individuals mate with one partner for life.

• Least common.

• Strong, lasting social bonds.

• Shared parental care.

• Extra-pair copulations

Extra-pair copulations

Matings that occur outside an individual’s primary pair bond, often in socially monogamous species, leading to extra-pair paternity.

Polygamy

A pattern of mating where individuals mate and bond with multiple partners.

Two types:

1. Polygyny: one male mates with multiple females.

   • Male dominance hierarchies.

   • Guards territory and nesting sites.

   • High variation in male mating success.

2. Polyandry: one female mates with multiple males.

   • Rare in vertebrates.

   • Fairly common in insects.

Promiscuous

A pattern of mating where individuals mate with multiple partners and form no bonds.

• Most common. All plants, most animals.

• Females generally raise young, if any parental care at all.

Every individual has 3 goals in life.

1. Consume resources and grow to maturity.

2. Reproduce successfully

3. Avoid being killed

The “Darwinian Demon”

States for maximum fitness an individual should:

1. Begin producing immediately.

2. Produce many large offspring.

3. Never age or die.

Why hasn’t natural selection produced a “Darwinian Demon”?

• Constraints: is it physically possible?

• Trade-offs

• Principle of allocation: natural selection will favor individuals that allocate resources optimally (maximum fitness).

• Parental care is costly: parents working hard to feed more young leads to decreased parental survival.

What is the trade-off between offspring number and size?

More offspring born at once = the smaller the size of the offspring.

Smaller offspring = lower survival probability.

Reproductive strategies

There are two types:

1. Semelparous: organisms reproduce only once in their lifetime.

   • Store resources for one reproduction.

   • Usually produces more offspring during the one birth.

   • Programmed death after reproduction; unpredictable environment.

2. Iteroparous: organisms reproduce multiple times over their lifetime.

   • More common.

   • Usually produce fewer offspring.

   • Favored in stable environments.

Predation risk

Affects life history strategies.

• Low predation: higher adult size, higher embryo weight, and fewer offspring.

• High predation: lower adult size, lower embryo weight, and more offspring.

Phylogram

Cladogram

Ultrametric tree

Gene vs species tree