Biological Principles: Life Cycles, Reproduction, Evolution
Posted on Jul 11, 2025 in Biology
Biological Processes and Concepts
Fungi Life Cycle
- Haploid (n): Single set of chromosomes.
- Plasmogamy: Haploid cells from two different mycelia fuse to form a heterokaryotic cell (two or more nuclei).
- Dikaryotic (n + n): Cell containing two distinct haploid nuclei.
- Karyogamy: The nuclei fuse to form a diploid (2n) zygote.
- Diploid (2n): Double set of chromosomes.
- Meiosis: Haploid (1n) spores are formed.
Plant Life Cycle
- Alternation of two generations.
- Two multicellular phases: haploid (n) and diploid (2n).
Plant Reproduction and Morphology
Flower Structure
- Four Whorls: Arranged from outside to inside.
- Sepals: Form the calyx, protect the flower bud, attach to the stalk, usually green.
- Petals: Form the corolla, attract pollinators, vary in size, shape, and color.
- Stamens: Male parts, consisting of an anther and a filament.
- Pistil: Female parts, consisting of a stigma, style, and ovary. The ovary contains one or more ovules.
Reproductive Morphology
- Hermaphroditic: (> 85%) Perfect flowers possess both male and female parts (bisexual), on the same plant.
- Monoecious: (5-6%) Imperfect flowers (either male or female parts, unisexual) on the same plant.
- Dioecious: (6-7%) Imperfect flowers (either male or female parts, unisexual) on different plants.
Gametophyte Formation
- Anther: Microspore mother cells (2n) → microspores (n) → pollen grains (n, male gametophytes).
- Ovule: Megaspore mother cell (2n) → megaspore (n) → embryo sac (n, female gametophyte).
Pollination
- Transfer of pollen from anther to stigma.
- Carried out by animals (especially insects) or by wind.
- Many flowers attract only specific pollinators (coevolution).
Animal-Pollinated Flowers
- Attraction: Petal color and shape, scent.
- Rewards: Nectar, pollen.
Wind-Pollinated Flowers
- Small, greenish, and odorless.
- Petals greatly reduced or absent.
- Produce lots of pollen.
- Examples: Grasses, some trees.
Double Fertilization
- Pollen grain transported to stigma.
- Pollen tube containing two sperm grows down the style.
- Pollen tube enters the embryo sac within the ovule.
- One sperm + egg → zygote (2n); one sperm + two central cell nuclei → endosperm (3n).
Fruit and Seed Development
- Ovules develop into seeds.
- Ovary or groups of ovaries develop into fruit.
Functions of Fruits
- Protect seeds.
- Aid in seed dispersal by wind, water, gravity.
- Aid in seed dispersal by animals (attached to fur or feathers, or eaten and excreted).
Animal Reproduction and Development
Animal Reproductive Strategies
- Monoecious Animals:
- Individuals have both male and female parts (hermaphrodites).
- Usually practice cross-fertilization (although sessile or sedentary species may self-fertilize).
- Sequential hermaphrodites change sex (e.g., male to female – protandry; female to male – protogyny).
- Examples: Most sponges, flatworms, some annelids.
- Dioecious Animals:
- Separate male and female individuals.
- Sex determination can be genetic (chromosomes) or environmental (temperature).
- Sexual dimorphism may result from sexual selection.
- Examples: Most cnidarians, mollusks, roundworms, arthropods, echinoderms, vertebrates, some annelids.
Fertilization Types
- External Fertilization:
- Takes place outside the female’s body.
- Both eggs and sperm are released into the environment.
- Usually occurs in water (protects eggs from drying out).
- Survival rate is low (predation), so many offspring must be produced that mature rapidly.
- Internal Fertilization:
- Takes place inside the female’s body.
- Male inserts sperm into the female’s body.
- Most common in terrestrial animals (protects eggs from drying out).
- Produces fewer offspring, but the survival rate is higher.
Spawning Methods
- Broadcast Spawning:
- Large numbers of males and females gather to release gametes at the same time.
- Higher chances of successful fertilization.
- Higher genetic diversity in offspring.
- Often triggered by water temperature or day length.
- Pair Spawning:
- One male and one female coordinate their release of gametes.
- All offspring have the same parents.
Developmental Strategies
- Ovoviviparous: Fertilized egg develops inside the mother, nourished by yolk.
- Viviparous: Fertilized egg develops inside the mother, embryo receives extra nourishment from the mother.
- Oviparous: Egg laid outside the mother’s body, development occurs externally.
Mating Systems
- Monogamy: One male, one female.
- Polygyny: One male, multiple females.
- Polyandry: Multiple males, one female.
- Polygynandry: Multiple males and females.
Evolutionary Biology Fundamentals
Evolution: The Unifying Theory of Biology
- Based on “The Origin of Species.”
- All living things adapt to their environment through evolution.
- Species evolve, not individual organisms.
Microevolution
- Small-scale changes in the genetic makeup of a population from one generation to the next.
Macroevolution
- Large-scale changes over long periods of time, resulting in new species and higher taxonomic groups.
Population
- A group of individuals of the same species living in the same area at the same time.
Gene Pool
- Includes all alleles at all gene loci in all individuals of a population.
- Described in terms of genotype and allele frequencies.
Causes of Microevolution
- Mutation:
- Changes in an organism’s DNA.
- Ultimate source of new alleles.
- Natural selection can only act on existing variation.
- Adaptive value of a mutation depends on current environmental conditions.
- Gene Flow (Migration):
- Movement of individuals into or out of a population, leading to movement of alleles.
- Reduces differences among populations, can prevent speciation.
- Nonrandom Mating:
- Mate selection based on genotype or phenotype, not by chance.
- Example: Assortative mating, where individuals tend to mate with others of the same phenotype, increasing homozygotes in frequency.
- Genetic Drift:
- Random changes in allele frequency due to chance.
- Depends on which members live to reproduce.
- More likely in small populations.
- More likely to lose rare alleles.
Natural Selection and Speciation
Natural Selection
- Individuals with more favorable traits for their environment are more likely to survive and reproduce.
- This leads to more offspring in the next generation.
- Over time, these favorable traits become more common in the population (adaptation).
- Requires heritable variation and limited resources (not all offspring survive to reproduce).
Sexual Selection
- Favors characteristics that increase the chances of getting a mate.
- Males often compete for access to females.
- Females often choose males based on attractiveness.
Types of Sexual Selection
- Intersexual Selection: One sex, typically males, displays a certain trait or behavior to attract and mate with the opposite sex.
- Intrasexual Selection: Occurs between members of the same sex (e.g., male-male competition).
Population Genetics Concepts
- Bottleneck Effect: A population suffers a catastrophic loss, and only a few survivors go on to produce future generations, leading to reduced genetic diversity.
- Founder Effect: A small population breaks away or becomes isolated from a larger population. The new gene pool may differ from the original gene pool by chance alone.
Polygenic Traits
- Most traits are polygenic, controlled by two or more genes.
- Have a range of phenotypes resembling a bell-shaped curve (e.g., human height).
Types of Natural Selection on Phenotypes
- Directional Selection: Occurs when an extreme phenotype is favored; the distribution curve shifts in that direction.
- Stabilizing Selection: Occurs when an intermediate (average) phenotype is favored; the distribution curve becomes narrower. This is the most common form of selection.
- Disruptive (Diversifying) Selection: Occurs when two or more extreme phenotypes are favored; the distribution curve splits, which may cause speciation.
Biological Species Concept
- Species are defined as groups of organisms capable of interbreeding and producing fertile offspring.
- Only applies to sexually reproducing organisms.
Reproductive Isolation
- Different species are reproductively isolated from one another.
- Achieved through reproductive barriers (prezygotic and postzygotic).
Prezygotic Barriers (Types of Isolation)
- Habitat isolation
- Temporal isolation
- Behavioral isolation
- Mechanical isolation
- Gamete isolation
Postzygotic Barriers
- Zygote mortality
- Hybrid sterility
- Reduced F2 fitness
Speciation
- The process where one species splits into two or more new species.
Types of Speciation
- Allopatric Speciation: Occurs in geographically isolated populations.
- Parapatric Speciation: Occurs in adjacent populations.
- Sympatric Speciation: Occurs in overlapping populations.
Adaptive Radiation
- Many new species evolve from a single ancestral species.
- Example: Darwin’s Finches
- Populations became established on different islands (founder effect).
- Each population adapted to a particular habitat on its island (natural selection).
- Beak shapes adapted for different food sources.
- Finches will not mate with individuals with the wrong beak shape (behavioral isolation).