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

1. Pollen grain transported to stigma.
2. Pollen tube containing two sperm grows down the style.
3. Pollen tube enters the embryo sac within the ovule.
4. 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).