Half-Yearly Notes Flashcards

Reproductive System

Male Reproductive System

• Function: Produces, matures, and transports sperm for fertilization.
• Components:
  • Testes:
    • Location: Housed in the scrotum outside the body.
    • Function:
      • Produces sperm through spermatogenesis.
      • Secretes testosterone.
    • Hormone (Testosterone):
      • Produced by Leydig cells in the testes.
      • Functions:
        • Responsible for male secondary sexual characteristics (deep voice, muscle growth, facial hair).
        • Regulates sperm production and sex drive.
  • Spermatogenesis (Sperm Production):
    • Occurs in the seminiferous tubules inside the testes.
    • Spermatogonia (stem cells) differentiate into mature sperm over several stages.
    • Supported by Sertoli cells, which provide nourishment and regulate the process.
  • Epididymis:
    • A long, coiled tube behind the testes.
    • Sperm mature and are stored here.
    • Gains motility and fertilization ability.
  • Vas Deferens:
    • A muscular tube that transports mature sperm from the epididymis to the urethra.
    • Peristalsis (muscular contractions) pushes sperm along during ejaculation.
  • Accessory Glands:
    • Add fluids to form semen, which nourishes and protects the sperm.
    • Seminal Vesicles:
      • Contribute to around 60% of semen.
      • Secretes fructose-rich fluid to provide energy for sperm.
    • Prostate Gland:
      • Adds alkaline fluid that neutralizes vaginal acidity, helping sperm survive.
    • Bulbourethral (Cowper’s Glands):
      • Secretes mucus-like fluid to lubricate and cleanse the urethra before ejaculation.
  • Urethra and Penis:
    • Urethra:
      • Carries semen through the penis and out of the body.
    • Penis:
      • Delivers sperm into the female reproductive system during intercourse.

Female Reproductive System

• Function: Produces eggs, facilitates fertilization, and supports the development of a baby during pregnancy.
• Components:
  • Ovaries:
    • Small almond-shaped organs on either side of the uterus.
    • Functions:
      • Release one mature egg (ovum) roughly every 28 days (ovulation).
      • Produce female sex hormones:
        • Estrogen: Develops female secondary sexual traits and regulates the menstrual cycle.
        • Progesterone: Prepares the uterus for pregnancy and maintains the uterine lining.
  • Fallopian Tubes (Oviducts):
    • Narrow tubes connecting the ovaries to the uterus.
    • Fertilization occurs here.
    • Lined with cilia to move the egg toward the uterus.
  • Uterus:
    • A hollow, muscular organ where the fertilized egg implants and develops.
    • Endometrium (uterine lining): Thickens to support the embryo; sheds during menstruation if there’s no fertilization.
  • Cervix:
    • The lower, narrow part of the uterus.
    • Produces cervical mucus, opens slightly during ovulation, and dilates during childbirth.
  • Vagina:
    • Muscular canal that receives the penis during intercourse and acts as the birth canal during delivery.

Fertilization and Early Development

• Fertilization:
  • Occurs in the fallopian tube within 24 hours of ovulation.
  • One sperm penetrates the outer layer of the egg.
  • The sperm and egg nuclei fuse, forming a zygote with a full set of chromosomes (46 total).

Stages of Development: Zygote to Fetus

• Zygote:
  • Definition: A single cell formed when the nucleus of the sperm fuses with the nucleus of the egg.
  • Chromosomes: Contains 23 from the mother and 23 from the father.
  • This is the first cell of a new human and marks the beginning of development.
  • It is totipotent: it has the potential to form any cell type in the body

Mendel’s Work

Who was Gregor Mendel?

• Gregor Mendel (1822 - 1884) was an Austrian monk and scientist.
• He is known as the “Father of Genetics” due to his groundbreaking work on inheritance.
• Mendel conducted experiments on pea plants in the mid-1800s in his monastery’s garden.
• His work remained largely unnoticed until the early 1900s when it was rediscovered and recognized as foundational to genetics.

Mendel’s Experiments

• Mendel studied how traits are passed from parents to offspring by breeding thousands of pea plants.
• He chose pea plants because:
  • They have distinct characteristics (e.g., flower color, seed shape, plant height).
  • They reproduce quickly and can self-pollinate or be cross-pollinated.
  • They have easily observable traits with only two variations (e.g., purple or white flowers).

Mendel’s Methodology

1. Controlled Pollination
   • Mendel prevented self-pollination by removing the male reproductive parts (stamens) of flowers.
   • He cross-pollinated plants with different traits by transferring pollen manually.
2. Monohybrid Crosses (Inheritance of One Trait)
   • Mendel first bred plants that had opposite versions of a trait (e.g., tall x short).
   • All offspring (F1 generation) showed only one of the parent traits (e.g., all were tall).
   • When F1 plants were self-pollinated, the F2 generation showed a 3:1 ratio (three tall for every short).
3. Dihybrid Crosses (Inheritance of Two Traits)
   • Mendel studied how two traits were inherited together (e.g., seed color and seed shape).
   • The results showed traits were inherited independently, leading to the law of independent assortment.

Mendel’s Key Discoveries

1. Law of Segregation
   • Each organism has two copies of a gene (one from each parent).
   • These copies (alleles) separate during gamete (sex cell) formation.
   • Offspring inherit one allele from each parent.
2. Law of Independent Assortment
   • Different traits are inherited independently of each other.
   • This applies only if genes are on different chromosomes or far apart on the same chromosome.
3. Dominant and Recessive Traits
   • Some traits (dominant) mask the appearance of others (recessive).
   • In his experiments, tall plants were dominant over short, and yellow seeds were dominant over green.

Why is Mendel Important?

• His experiments introduced the concept of genes (though he didn’t use the term).
• His laws explain genetic inheritance in plants, animals, and humans.
• His work helped in agriculture, medicine, and understanding genetic disorders.

Summary

Gregor Mendel, known as the Father of Genetics, conducted experiments on pea plants in the mid-1800s to study how traits are inherited. He used controlled pollination to track how traits were passed from one generation to the next. Through monohybrid crosses and dihybrid crosses, he discovered the Law of Segregation and the Law of Independent Assortment. He also identified that each gene has a dominant allele and a recessive allele.

Punnett Squares

What is a Punnett Square?

• A Punnett Square is a diagram used to predict the possible genetic outcomes of a cross between two organisms.
• It shows how alleles (versions of a gene) are inherited from each parent.
• It is useful for understanding dominant and recessive traits, probabilities of inheritance, and genetic variation.

Monohybrid Crosses

• Definition: A monohybrid cross is a genetic cross involving one trait with two alleles.

• Example - Tall (T) vs Short (t) Pea plants

  • T (Tall) is dominant over t (Short).
  • A cross between two heterozygous tall plants (Tt x Tt) results in the following Punnett Square:

  	T	t
  T	TT (Tall)	Tt (Tall)
  t	Tt (Tall)	tt (Short)

  • Results:
    • 75% ($\frac{3}{4}$) Tall (TT or Tt)
    • 25% ($\frac{1}{4}$) Short (tt)
    • Genotypic ratio 1:2:1 (1 TT: 2 Tt: 1 tt)
    • Phenotypic Ratio: 3:1 (3 Tall: 1 Short)

Dihybrid Crosses

• Definition: A dihybrid cross involves two different traits inherited independently.

• Example - Seed Colour and Seed Shape

  • Yellow (Y) is dominant over Green (y)
  • Round (R) is dominant over Wrinkled (r).
  • A cross between two heterozygous plants (YyRr x YyRr) results in the following Punnett Square:

  	YR	Yr	yR	yr
  YR	YYRR	YYRr	YyRR	YyRr
  Yr	YYRr	YYrr	YyRr	Yyrr
  yR	YyRR	YyRr	yyRR	yyRr
  yr	YyRr	Yyrr	yyRr	yyrr

  • Results:
    • 9 Yellow, Round (YYRR, YYRr, Yyrr, YyRr)
    • 3 Yellow, Wrinkled (YYrr, Yyrr)
    • 3 Green, Round (yyrr, yyRr)
    • 1 Green, Wrinkled (yyrr)
    • Phenotypic Ratio - 9:3:3:1

Key Findings:

• This follows Mendel’s Law of Independent Assortment (traits are inherited independently).
• The phenotypic ratio is 9:3:3:1, showing how different traits combine in offspring.

Why are Punnett Squares Important?

• They help predict genetic traits in offspring.
• They explain dominant vs recessive inheritance.
• They are used in genetics for breeding, medicine, and understanding hereditary diseases.

Cell Division

Mitosis:

• Definition: Mitosis is the process that produces two identical daughter cells from a single parent cell.
• It is used for growth, repair, and asexual reproduction.
• The daughter cells are diploid ($2n$), meaning they have the same number of chromosomes as the original cell.

Key Features of Mitosis

• Produces two identical daughter cells.
• Daughter cells are diploid ($2n$) (same chromosome number as the parent).
• Used for growth, repair, and asexual reproduction.

Meiosis

• Definition: Meiosis is a special type of cell division that produces gametes (sperm and egg cells).
• It reduces the chromosome number by half, creating four genetically different haploid ($n$) cells.
• Meiosis consists of two divisions (Meiosis I and Meiosis II), each with similar stages to mitosis.

Key Differences:

Pedigrees

What is a pedigree?

A pedigree is a diagram that shows the inheritance of a genetic trait over multiple generations in a family. It is used to track how a trait is passed down and to determine whether it follows dominant, recessive, or sex-linked inheritance.

Symbols in a Pedigree Chart

Types of Inheritance Patterns in Pedigrees

1. Autosomal Dominant Inheritance
   • Affects both males and females equally.
   • Does not skip generations (an affected person must have at least one affected parent).
   • If a person has the dominant allele (AA or Aa), they will show the trait.
   • Example: Huntington’s disease, Marfan syndrome
   • How to Identify a Pedigree:
     • At least one affected parent must pass on the trait.
     • If a child is affected, at least one parent is also affected
2. Autosomal Recessive Inheritance
   • Affects both males and females equally.
   • Can skip generations (affected individuals may have unaffected parents).
   • Only individuals with two recessive alleles (aa) will show the trait.
   • Carriers (Aa) do not show symptoms but can pass the gene to offspring.
   • Example: Cystic fibrosis, Tay-Sachs disease
   • How to Identify a Pedigree:
     • Two unaffected parents can have an affected child (if both are carriers).
     • If both parents are affected, all children will also be affected.
3. Sex-Linked Inheritance (X-Linked Recessive)
   • More males are affected than females (because males have only one X chromosome).
   • Can skip generations, as females can be carriers (XAXa).
   • Males only need one affected X chromosome (XaY) to have the trait, while females need two XaXa
   • Example: Hemophilia, Color blindness
   • How to identify in a pedigree:
     • More males than females are affected.
     • Affected mothers pass the trait to all their sons.
     • Affected fathers do not pass the trait to their sons (since sons inherit Y from their fathers).

How to Read a Pedigree Chart Step-by-Step

1. Determine if the trait is dominant or recessive:
   • If every affected child has an affected parent → Dominant
   • If unaffected parents have affected children, → Recessive
2. Check if the trait is sex-linked:
   • More males affected → Likely X-linked
   • Equal male/female ratio → autosomal
3. Look for carriers (if recessive):
   • If parents are unaffected but have an affected child, they must be carriers.

Key takeaways

• Pedigrees help track genetic traits through generations.
• Autosomal dominant traits appear in every generation.
• Autosomal recessive traits can skip generations and have carriers.
• X-linked traits affect more males than females.

DNA Replication

What is DNA Replication?

DNA replication is the process by which a cell makes an exact copy of its DNA. It is crucial for cell division, ensuring that each cell has a complete set of genetic information. This process occurs during the interphase in Meiosis and Mitosis.

Simplified Steps of DNA Replication:

1. Unwinding the DNA:
   • The double-stranded DNA molecule unzips, and the two strands separate, creating two single strands.
2. Base Pairing:
   • Each single strand of DNA acts as a template.
   • The cell adds new bases (A, T, C, G) to the template strand. Each base pairs with its complement:
     • A (adenine) pairs with T (thymine)
     • C (cytosine) pairs with G (guanine)
   • This forms two new strands of DNA, one for each original strand.
3. Result of Replication:
   • After replication, you end up with identical DNA molecules.
   • Each molecule has one original strand and one newly made strand.
   • This process is called semi-conservative replication because half of the DNA is from the original strand and half is new.

Steps of DNA Replication:

1. Initiation:
   • Helicase: The enzyme helicase unwinds the double-stranded DNA molecule by breaking the hydrogen bonds between the complementary base pairs. This creates a structure known as the replication fork, where the DNA strands are separated.
   • Single-strand binding proteins (SSBPs): These proteins bind to the separated strands to keep them from re-annealing (rejoining).
   • Primase: The enzyme primase synthesizes a short RNA primer that provides a starting point for DNA replication. DNA polymerase can only add nucleotides to an existing strand, so the primer is necessary for starting replication.
2. Elongation:
   • DNA Polymerase III: This enzyme adds new nucleotides to the 3’ end of the RNA primer, forming a new DNA strand in the 5’ to 3’ direction. The new strand is complementary to the template strand.
     • The strand that is synthesized continuously towards the replication fork is called the leading strand.
     • The strand synthesized in the opposite direction (away from the replication fork) is called the lagging strand, and it is synthesized in short segments called Okazaki fragments.
   • DNA Polymerase I: After the leading strand is synthesized, DNA polymerase I removes the RNA primers and replaces them with DNA nucleotides.
   • DNA Ligase: DNA ligase seals the gaps between the Okazaki fragments, completing the synthesis of the lagging strand.
3. Termination:
   • Once the entire DNA molecule is copied, the replication process is complete.
   • The newly formed DNA molecules consist of one original (template) strand and one newly synthesized strand.

Key Enzymes Involved in DNA Replication:

• Helicase: Unwinds the DNA double helix by breaking hydrogen bonds between base pairs.
• Single-strand binding proteins (SSBPs): Keep the separated strands from rejoining.
• Primase: Synthesizes RNA primers to initiate DNA replication.
• DNA Polymerase III: Adds new nucleotides to form the growing DNA strand.
• DNA Polymerase I: Replaces RNA primers with DNA nucleotides.
• DNA Ligase: Joins Okazaki fragments to complete the lagging strand.

Result of DNA Replication:

• After replication, two identical DNA molecules are produced.
• Each DNA molecule consists of one old (template) strand and one new (daughter) strand.
• This method of replication is called semiconservative replication because each new DNA molecule conserves one original strand.

Mutations

What is a Mutation?

• A mutation is a change in the DNA sequence of an organism.
• Mutations can occur naturally or due to external factors like radiation, chemicals or viruses.
• They can happen during DNA replication or due to damage to the DNA

Types of Mutations

1. Gene Mutations, also called point mutations
   • Affect a signal gene.
   • Examples:
     • Substitution: One base is by another (e.g, A → G)
     • Insertion: An extra base is added.
     • Deletion: A base is removed.
   • Frameshift mutations (insertion/deletion) shift the reading frame and can severely affect the protein.
2. Chromosomal Mutations
   • Affect whole chromosomes or large sections.
   • Types:
     • Duplication - a section is copied.
     • Deletion - a section is lost
     • Inversion - a section is reversed.
     • Translocation - sections from different chromosomes are swapped.

Cause of Mutations

• Spontaneous: Errors during DNA replication.
• Induced: Caused by mutagens like:
  • UV radiation, X-rays,
  • Chemicals (e.g., tobacco smoke),
  • Viruses

Effects of Mutations

• Neutral: No impact on the organism.
• Beneficial: Lead to advantageous traits (e.g., disease resistance).
• Harmful: Causes genetic disorders or diseases (e.g., cancer, cystic fibrosis).

Examples of Genetic Mutations

Inheritance of Mutations

• Mutations in somatic cells (body cells) are not inherited.
• Mutations in germ cells (egg or sperm) can be passed on to offspring

Role of Genes and Environment

1. Examples of variation

   • Variation refers to differences in traits among individuals in a population. These variations can be genetic (inherited from parents or environmentally influenced by the surroundings.

   • Examples of Genetic Variation:

     • Eye color, hair type, and blood group in humans.
     • Different fur colors in rabbits affect camouflage.
     • Beak size in Darwin’s finches, affecting food accessibility

   • Examples of Environmental Variation:

     • Sun exposure affects human skin tone.
     • Malnutrition can stunt growth, even if genes code for tall height.
     • Training and exercise influence muscle development.

2. Twin Studies and Variation

   • Twin studies help scientists understand the influence of genes and the environment on traits.
   • Identical Twins (Monozygotic) - share 100% of their DNA; differences arise due to the environment.
   • Fraternal Twins (Dizygotic) - Share only 50% of their DNA; differences result from both genes and the environment.

3. Sources of Genetic Variation

   • Genetic variations arise because of differences in DNA, leading to different traits in a population.
     • Mutations - Random changes in DNA create new traits (e.g., sickle cell mutation provides malaria resistance).
     • Recombination (Crossing over in Meiosis) - During Prophase 1, homologous chromosomes exchange DNA with each other, leading to new gene combinations.
     • Independent Assortment (Meiosis) - In Metaphase 1, chromosomes line up randomly, ensuring different combinations of parental genes in gametes.
     • Random Fertilization - Sperm and egg combine randomly, making each individual genetically unique.
     • Gene Flow - Migration allows different populations to mix genes, increasing diversity

4. Sources of Environmental Variation

   • Environmental factors can influence traits even when genes remain the same.
     • Climate - Temperature and weather conditions impact fur thickness in animals.
     • Diet and Nutrition - A person’s height can be influenced by diet, even if they have genes for tallness.
     • Lifestyle and Culture - Education and experiences shape intelligence and skills.
     • Pollution and Toxins - Exposure to chemicals can affect health, even if genes are predisposed to good health.

5. Role of Genes and Environmental Factors in Survival

   • Both genes and environmental factors interact to determine an organism’s ability to survive and reproduce.
     • Genetic Influence on Survival:
       • Genes determine inherited traits like speed, camouflage, or disease resistance.
       • Example: Some insects have a genetic mutation making them resistant to pesticides.
     • Environmental Influence on Survival:
       • Conditions like food availability, predators, and climate shape survival changes.
       • Example: The color of Arctic fox fur changes with seasons to blend into the surroundings.
     • Interaction of Genes and Environment:
       • Traits can be genetically inherited but activated or modified by the environment.
       • For example, identical twins may have a genetic predisposition to obesity, but diet and exercise affect their weight.

6. Role of Genes in Survival

   • Genes provide the blueprint for survival by determining physical and physiological traits.
     • Adaptations for Survival:
       • Thick fur in polar bears helps them survive cold temperatures.
       • Long necks in giraffes allow them to reach high food sources.
       • Venom in snakes helps them hunt and defend themselves.
     • Genetic Resistance to Diseases:
       • Some humans carry genes that make them resistant to malaria (sickle cell trait).
       • Bacteria develop antibiotics through mutations

7. Role of Environmental Factors in Survival

   • The environment plays a key role in determining whether genetic traits are beneficial.

     • Natural Selection:

       • If an environment changes, once beneficial traits may become disadvantageous.
       • Example: Dark-colored peppered moths became common during the Industrial Revolution when soot darkened trees.

     • Climate Influence:

       • Animals with thick fur survive in cold climates, while those with thin fur survive in warm regions.

     • Food Availability

       • Bird beak shapes evolve depending on the available food in an area.

     • Predator-Prey Relationships:

       • Faster prey are more likely to survive and pass on their genes.
       • Predators with better camouflage
         or speed catch more prey and reproduce.

Biotechnology - 4 Types

1. Dolly the sheep (Cloning/Genetic Engineering)
   • The first mammal was cloned from an adult somatic cell (1996, Scotland).
   • Cloned using a technique called somatic cell nuclear transfer (SCNT):
     • The nucleus from an adult sheep cell was inserted into an egg cell, and its nucleus was removed.
     • The egg developed into an embryo and was implanted into a surrogate.
   • Significance:
     • Proved that specialized cells can be reprogrammed to create an entire organism.
     • Advanced understanding of genetics, cell development, and cloning.
   • Impacts:
     • Opened doors for therapeutic cloning and research into organ regeneration.
     • Raised ethical concerns about cloning humans and animal welfare.
2. GMOs (Genetically Modified Organisms)
   • Organisms (usually crops or animals) whose DNA has been altered to express desirable traits.
   • Examples:
     • Bt cotton: Resists pests.
     • Golden rice: It contains vitamin A to help prevent deficiency.
   • Advantages:
     • Higher yield, pest drought resistance, reduced need for pesticides.
     • Improved nutrition in some cases.
   • Disadvantages:
     • Long-term health effects are still debated.
     • Environmental concerns (e.g., biodiversity loss, resistance in pests).
     • Ethical concerns about tampering with nature.
   • Impact:
     • Revolutionized agriculture and food production.
     • Helped scientists understand plant genetics and gene transfer techniques.
3. Vaccines
   • Vaccines stimulate the immune system to recognize and fight specific diseases.
   • Types of vaccines:
     • Traditional: Use weakened or inactivated pathogens (e.g., polio, measles).
     • mRNA vaccines: Contain instructions for cells to make a harmless piece of a virus (e.g., COVID-19 vaccine).
   • Advantages:
     • Prevent the spread of infectious diseases.
     • Reduce the severity of illness and deaths.
   • Disadvantages:
     • Some people may experience side effects.
     • Vaccine hesitancy and misinformation can limit effectiveness.
   • Impact:
     • Increased understanding of how the immune system functions.
     • Pushed biotechnology forward in disease prevention and treatment research.
4. IVF (In Vitro Fertilization)
   • A fertility treatment where an egg is fertilized by sperm outside the body and then is put back into the uterus.
   • It is used by couples with infertility issues or genetic concerns.
   • Process:
     • Hormone treatments to stimulate egg production
     • Eggs are collected and fertilized in a lab
     • The embryo is implanted into the uterus
   • Advantages:
     • Enables people to have biological children
     • Allows for genetic screening to avoid inherited diseases
   • Disadvantages:
     • Expensive and emotionally challenging
     • Ethical issues around embryo selection and unused embryos
   • Impact:
     • Advanced understanding of reproduction, fertilization, and embryo development
     • Led to developments in pre-implantation genetic diagnosis (PGD)

Watson-Crick Model

Introduction

• DNA stands for Deoxyribonucleic Acid.
• It is the molecule that carries the genetic instructions for all living things.
• In 1953, James Watson and Francis Crick proposed a model explaining the structure of DNA, based on data from scientists like Rosalind Franklin and Maurice Wilkins.

Key Features of the Watson-Crick Model

1. Double Helix Structure
   • DNA is shaped like a twisted ladder or spiral staircase, called a double helix.
   • The two strands wind around each other.
2. Components of DNA
   • Each strand of DNA is made of nucleotides.
   • A nucleotide has three parts:
     • A phosphate group,
     • A sugar molecule (deoxyribose),
     • A nitrogenous base (adenine, Thymine, Cytosine, or Guanine).
3. Base Pairing
   • The nitrogenous bases pair in a very specific way:
     • Adenine (A) pairs with Thymine (T)
     • Cytosine (C) pairs with Guanine (G).
   • These pairs are called complementary base pairs.
   • Base pairs are held together by hydrogen bonds:
     • A-T has two hydrogen bonds,
     • C-G has three hydrogen bonds (making it slightly stronger)
4. Anti-Parallel Strands
   • The two strands run in opposite directions.
   • One strand runs 5’ to 3’ and the other runs 3’ to 5’ (this refers to the direction of the sugar-phosphate backbone).
5. Sugar-Phosphate Backbone
   • The sides of the ladder (the “rails”) are made of alternating sugar and phosphate groups.
   • This backbone is strong and protects the genetic information inside.
6. Importance of the Model
   • Watson and Crick’s model explained how DNA can copy itself (replication) and how it can store information.
   • It’s… shown that the sequence of bases acts as a code for building proteins, which control cell activities.

Supporting Evidence

• Rosalind Franklin’s X-ray diffraction images showed an X-shaped pattern, indicating a helical structure.
• Without Franklin’s data, Watson and Crick might not have figured out the correct shape.

Summary

• DNA’s double helix structure allows it to be compact, stable, and easily copied.
• The specific base-pairing ensures accurate transmission of genetic information from one generation to the next.

Natural Selection and Darwin’s work

Charles Darwin

• Charles Darwin (1809 - 1882) was an English naturalist.
• He is most famous for his Theory of Evolution by Natural Selection.
• His key observations came from his voyage on the HMS Beagle (1831 - 1836), especially from the Galapagos Islands.

Darwin’s Observations

• Different species of animals and plants were adapted to their environments.
• Depending on their food source, finches on different islands had different beak shapes.
• Fossils looked similar to modern animals but had slight differences

The Theory of Natural Selection

Natural selection is where organisms better suited to their environment survive and reproduce, passing on their traits. The steps of Natural Selection:

1. Variation
   • Individuals in a population vary in their traits (e.g., size, color, speed).
   • Variation can be inherited from parents.
2. Overproduction
   • Most species produce a lot of offspring; however, only a limited number of them can survive due to limited resources such as food, water, and shelter.
3. Competition
   • Organisms compete for resources
   • Only some survive and reproduce.
4. Survival of the Fittest
   • Those with traits best suited to the environment are more likely to survive and have offspring.
   • “Fitness” means reproductive success.
5. Adaptation
   • Over many generations, beneficial traits become more common.
   • This can lead to evolution, where a population changes over time.

Important ideas Darwin Proposed

• Common Ancestry: All species are related through common ancestors.
• Gradualism: Evolution happens slowly over a long time.
• Speciation: Given enough time, natural selection can create new species.

Example: Galapagos Finches

• Finches with strong, thick beaks survived better on islands with hard seeds.
• Finches with slender, sharp beaks survived better on islands with insects.
• Over time, different species of finches evolved, each suited to their environment.

Impact of Darwin’s Work

• Darwin’s book, “On the Origin of Species” (1859) changed how scientists viewed life.
• His theory explained biodiversity using natural processes, without needing supernatural explanations.

Summary

• Darwin’s theory of natural selection explains how species evolve.
• Organisms with traits best suited to their environment survive, reproduce, and pass