Heredity and Evolution

Accumulation of Variation During Reproduction

Introduction

• Reproduction creates new individuals that are similar but a bit different.
• Variations occur even in asexual reproduction, but more in sexual reproduction.
• Example: Sugarcane plants show little variation, but humans show a lot of variation.

Accumulation of Variation During Reproduction

• Inheritance from parents gives a basic body design with some changes.
• Each new generation has differences from the previous one and new differences.
• Asexual Reproduction:
  • Example: One bacterium divides into two, then into four.
  • The bacteria are very similar with minor differences due to DNA copying errors.
• Sexual Reproduction:
  • Creates greater diversity.
  • Variations give different survival advantages.
  • Example: Heat-resistant bacteria survive better in a heat wave.

Heredity

• Reproduction results in similar individuals with variations.
• Rules of Heredity: Determine how traits are inherited.
• Inherited Traits:
  • Similarities and differences are seen in children.
  • Children have basic human features but don’t look exactly like their parents.

Remember, variations and inheritance are key to understanding how evolution works!

Rules for the Inheritance of Traits — Mendel’s Contributions

• Both parents contribute equally to a child’s genetic material.
• Each trait in a child has two versions (one from each parent).

Gregor Johann Mendel (1822-1884)

• Studied science and mathematics.
• Conducted experiments with pea plants.
• First to keep count of traits in each generation.

Mendel’s Experiments with Pea Plants

• Used contrasting traits: round/wrinkled seeds, tall/short plants, white/violet flowers.
• Crossed tall and short plants:
  • First generation (F1): All tall plants (no medium height).
  • Second generation (F2): 75% tall, 25% short.
• Concluded that traits are controlled by “factors” (genes):
  • Two copies of each gene (one from each parent).
  • Genes can be identical or different.

Dominant and Recessive Traits

• Tallness (T) is dominant, shortness (t) is recessive.
• TT and Tt are tall, tt is short.

Inheritance of Multiple Traits

• Crossed tall plants with round seeds and short plants with wrinkled seeds:
  • First generation (F1): All tall with round seeds.
  • Second generation (F2): Varied combinations:
    • Tall with round seeds
    • Short with wrinkled seeds
    • Tall with wrinkled seeds
    • Short with round seeds
• Traits like seed shape and plant height are inherited independently.

Mendel’s work helps us understand how traits are passed from parents to children, and how variations occur in each generation!

How do these Traits get Expressed?

• DNA and Genes:
  • DNA is the source of information for making proteins.
  • A section of DNA that provides information for one protein is called a gene.
• Example: Plant Height:
  • Plants have hormones that trigger growth.
  • The amount of hormone depends on an enzyme’s efficiency.
  • Efficient enzyme = tall plant.
  • Less efficient enzyme = short plant.
• Gene Contribution:
  • Both parents contribute equally to the child’s DNA.
  • Each trait is influenced by genes from both parents.

Chromosomes and Gene Sets

• Germ Cells:
  • Each germ cell has one set of genes.
  • Germ cells combine to restore the normal number of chromosomes in the progeny.
• Chromosomes:
  • DNA is in pieces called chromosomes.
  • Each cell has two copies of each chromosome (one from each parent).
  • During reproduction, germ cells take one chromosome from each pair.

Sex Determination

• Different Strategies:
  • Some species use environmental cues.
  • Example: Temperature affects sex in some reptiles.
  • Some animals can change sex (e.g., snails).
• Humans:
  • Sex is largely genetically determined.
  • Sex Chromosomes:
    • Women: XX
    • Men: XY
  • Inheritance Pattern:
    • All children inherit an X chromosome from their mother.
    • A child’s sex is determined by the father’s chromosome:
      • X from father = girl.
      • Y from father = boy.

Evolution

Variations During Reproduction

• Variations occur due to errors in DNA copying and sexual reproduction.
• These variations have consequences in a population.

An Illustration with Beetles

• Initial Population:
  • Group of 12 red beetles living in bushes with green leaves.
  • Crows eat these beetles, reducing their numbers.

Situation 1: Green Beetles

• A green beetle is born due to a variation.
• Green beetles blend in with green leaves and are not eaten by crows.
• More green beetles survive and reproduce.
• Result: Green beetles become more common.

Situation 2: Blue Beetles

• A blue beetle is born due to a variation.
• Blue beetles are as visible as red beetles, so crows eat them.
• An elephant crushes most beetles by accident, but a few blue beetles survive.
• The population grows again, now mostly blue beetles.
• Result: Blue beetles become more common due to accidental survival (genetic drift).

Key Concepts

• Natural Selection:
  • Variations giving survival advantages (e.g., green beetles) become common.
  • Environment (e.g., crows) selects for these traits.
• Genetic Drift:
  • Random events (e.g., elephant stepping) can change trait frequency.
  • Happens by chance, not by survival advantage.

Situation 3: Environmental Impact

• Bushes get a disease, reducing leaf material.
• Beetles have less food and become smaller in size.
• No genetic change occurs.
• When the disease is eliminated and food is plentiful again:
  • Beetles return to their normal size.
• Result: Environmental changes can affect traits temporarily without genetic changes.

Acquired and Inherited Traits

• Acquired Traits:
  • Traits developed during an organism’s lifetime (like reduced weight due to starvation) do not change DNA in germ cells.
  • Example: A beetle’s low weight from starvation isn’t passed to its offspring.
  • Cutting off a mouse’s tail doesn’t lead to tailless offspring.
• Inherited Traits:
  • Traits that are passed from parents to offspring through DNA.
  • Only genetic changes in germ cells are inherited by the next generation.
  • Charles Darwin’s work on evolution lacked the genetic understanding provided by Gregor Mendel’s experiments.

Speciation

• Micro-evolution:
  • Small, significant changes within a species.
  • Does not lead to new species.
• Formation of New Species (Speciation):
  • New species form when two populations can no longer reproduce with each other.
  • Example: Beetles living on different sides of a mountain range:
    • Beetles mostly reproduce within their own sub-populations.
    • Occasional beetles might move and introduce new genes to other populations.
    • Natural barriers, like a river, can further isolate populations.
• Genetic Drift and Natural Selection:
  • Genetic Drift: Random changes in gene frequencies in small populations.
  • Natural Selection: Different environments can select for different traits.
    • Example: In one area, eagles eliminate crows, so green beetles aren’t selected against. In another, many crows exist, so green beetles are favored.
• Result:
  • Isolated populations accumulate different genetic changes.
  • Eventually, these populations become so different that they can no longer interbreed.
  • Mechanisms:
    • Severe DNA changes, like different chromosome numbers.
    • Behavioral changes, like green females only mating with green males.

Together, these processes lead to the creation of new species through isolation and adaptation to their environments.

Evolution and Classification

Understanding Evolutionary Relationships

• Looking Back in Time:
  • Evolutionary relationships can be understood by tracing back characteristics.
  • Classification helps group organisms based on similarities.

Characteristics for Classification

• Characteristics:
  • Details of appearance or behavior.
  • Examples: Four limbs in animals, photosynthesis in plants.
• Levels of Classification:
  • Basic Unit: Cell, the fundamental unit of life.
  • Cell Type: Presence or absence of a nucleus (e.g., bacterial cells vs. nucleated cells).
  • Cellularity: Unicellular vs. multicellular organisms.
  • Function: Ability to perform photosynthesis.
  • Skeleton: Internal skeleton (vertebrates) vs. external skeleton (invertebrates).

Relationship and Common Ancestry

• Hierarchy of Characteristics:
  • More shared characteristics mean closer relationships.
  • Closer relationships indicate more recent common ancestors.
• Example:
  • Siblings are closely related (common parents).
  • Cousins are less closely related (common grandparents).

Building Classification Groups

• Small Groups:
  • Species with recent common ancestors form small groups.
• Super-Groups:
  • These small groups can combine into larger groups with more distant common ancestors.
• Ultimate Ancestor:
  • Theoretically, tracing back can lead to a single species at the beginning of evolutionary time.
  • Non-living material giving rise to life is a subject of many theories.

Understanding these principles helps us see how species are related and how life on Earth evolved.

Tracing Evolutionary Relationships

Identifying Common Characteristics

• Inherited Similarities:
  • Characteristics are similar because they come from a common ancestor.
  • Example: Mammals, birds, reptiles, and amphibians all have four limbs. This is a homologous characteristic.
• Homologous vs. Analogous Characteristics:
  • Homologous: Same structure, different functions (e.g., limbs in mammals and birds).
  • Analogous: Different structures, same function (e.g., wings of birds and bats).
  • Bird wings and bat wings are analogous because they serve the same purpose but have different structures.

Fossils

• What Are Fossils?:
  • Preserved traces of living organisms.
  • Formed when organisms’ bodies are preserved in environments that prevent decomposition (e.g., insect in hot mud).
• Dating Fossils:
  • Relative Dating: Fossils found closer to the surface are newer than those found deeper.
  • Isotope Dating: Measuring ratios of different isotopes in the fossil material to determine age.
• Example of Fossil Formation:
  • 100 million years ago: Invertebrates die and are buried in sand.
  • Millions of years later: Dinosaurs die and are buried in mud above invertebrate fossils.
  • Even later: Horse-like creatures die and are fossilized above dinosaur fossils.
  • Erosion: Exposes older fossils as layers are worn away.

Evolution by Stages

Complex Organs Develop Bit-by-Bit

• Intermediate Stages:
  • Complex organs like the eye develop gradually over generations.
  • Even a simple or rudimentary eye can be useful and provide a fitness advantage.
  • Example: Eyes in insects, octopuses, and vertebrates are different but all serve the purpose of seeing.
• Changing Functions:
  • Some features evolve to serve new purposes over time.
  • Example: Feathers initially provided insulation but later became useful for flight.
  • Birds are closely related to dinosaurs, some of which had feathers.

Artificial Selection and Evolution

• Wild Cabbage Example:
  • Humans have cultivated wild cabbage into different vegetables by selecting specific traits.
  • Cabbage, broccoli, cauliflower, kohlrabi, and kale all come from the same ancestor but were selectively bred for different features.
• DNA Comparisons:
  • Comparing DNA of different species shows how much they have changed and helps trace evolutionary relationships.

Evolution Should Not Be Equated with ‘Progress’

• Multiple Branches:
  • Evolution creates multiple branches; it’s not about one species replacing another.
  • New species emerge, but older ones may still survive depending on the environment.
• No ‘Better’ Species:
  • New species are not necessarily better than old ones.
  • Humans did not evolve from chimpanzees; both share a common ancestor.
• Diversity and Adaptation:
  • Evolution is about creating diversity and adapting to the environment.
  • More complex designs have emerged over time, but simpler designs like bacteria still thrive in extreme conditions.
• Humans Are Not the Pinnacle:
  • Humans are just one part of the evolving spectrum of life, not the pinnacle of evolution.

Human Evolution

Tools for Studying Human Evolution

• Methods Used:
  • Excavating fossils
  • Time-dating fossils
  • Studying DNA sequences

Diversity in Human Forms

• Human ‘Races’:
  • People used to classify humans into races based on skin color: yellow, black, white, brown.
  • Modern evidence shows no biological basis for races; all humans are one species.

Origins in Africa

• African Roots:
  • All humans originated from Africa.
  • The earliest Homo sapiens can be traced back to Africa.
  • Genetic evidence supports that we all have African roots.

Migration Patterns

• Spread Across the Planet:
  • A few hundred thousand years ago, some humans left Africa while others stayed.
  • Migrants spread to various parts of the world:
    • From Africa to West Asia, Central Asia, Eurasia, South Asia, East Asia.
    • Reached Australia via Indonesia and the Philippines.
    • Crossed the Bering land bridge to the Americas.

Movement and Mixing

• Complex Journeys:
  • Migration was not linear; people moved back and forth.
  • Groups sometimes separated and sometimes mixed again.
  • Movement included traveling in and out of Africa.
  • Human evolution was a mix of accidental changes and survival efforts.

Chapter Summary:

• Variations during reproduction can be inherited.
• Variations may help individuals survive better.
• Sexually reproducing individuals have two gene copies for the same trait.
  • If the copies are not identical, the dominant trait is expressed, and the other is the recessive trait.
• Traits in one individual may be inherited separately, leading to new trait combinations in offspring.
• Sex determination varies in species.
  • In humans, a child’s sex depends on whether the father’s chromosome is X (for girls) or Y (for boys).
• Variations may give survival advantages or contribute to genetic drift.
• Changes in non-reproductive tissues from environmental factors are not inheritable.
• Speciation can happen with variation and geographical isolation.
• Evolutionary relationships are traced through the classification of organisms.
• Tracing common ancestors shows that non-living material must have given rise to life at some point.
• Evolution can be studied through living species and fossils.
• Complex organs may evolve due to the survival advantage of intermediate stages.
• Organs or features may adapt to new functions during evolution.
  • For example, feathers may have evolved for warmth and later adapted for flight.