Biology 2 - Unit 1 (Evolution)

What is biological evolution

• Evolution = descent with modification

• Organisms change over generations.

• All life on Earth shares a common ancestor.

Types of Evolution

• Microevolution → small-scale evolution

  • A change in allele frequency within a population from one generation to the next.

  • Ex: A population of animals gradually has more individuals with a certain trait.

• Macroevolution → large-scale evolution

  • Different species descend from a common ancestor over many generations.

  • Ex: birds evolving from dinosaur ancestors

Main ideas about species before Darwin

Aristotle — “Old World” view

• Species are fixed.

• This means species do not change over time.

Lamarck

• Use & disuse: using a trait more can make it stronger; not using it can make it weaker.

• Inheritance of acquired traits: traits an organism develops during its lifetime can be passed to its offspring.

• Increasing complexity: organisms become more complex over time.

• No extinction: Lamarck did not believe species went extinct.

Natural selection

• organisms with helpful traits are more likely to survive and reproduce, causing those traits to become more common over time.

Darwin’s Voyage on the HMS beagle

• Darwin noticed that species varied globally, locally, and over time.

• These observations helped Darwin develop his Theory of Natural Selection.

Charles Lyell (1830) influence on Darwin

• Earth is older than people previously thought.

• Slow changes over a long time can cause major changes.

• This provided enough time for natural selection to occur.

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Thomas Malthus (1798) influence on Darwin

• More individuals are born than the environment can support.

• This creates competition/struggle for existence.

• Competition creates selective pressure.

• Influenced the idea of “survival of the fittest.”

4 key principles of Darwin’s theory

1. Variation and Adaptation

2. More individuals are born than can survive

3. Variations allow some individuals to reproduce more often than others

4. Descent with Modification

Variation and Adaptation

• Variation: Individuals have different physical traits due to mutations and sexual reproduction.

• Adaptation: A heritable trait that helps an organism survive and reproduce in its environment.

More individuals are born than can survive

• Members of a population compete for:

  • Food

  • Space

  • Other limited resources

• Selective pressure: An environmental factor that reduces an organism’s ability to survive and reproduce.

• Selective pressures can be:

  • Biotic → living factors

  • Abiotic → nonliving factors

Variations Allow Some Individuals to Reproduce More Often than others

• Some organisms are more “fit” → they survive and reproduce more often.

• “Survival of the Fittest”

• Fittest does NOT mean the trait is the “best version.”

Descent with Modification

• Offspring are slightly different from their parents.

• Beneficial traits can be passed to offspring.

• These traits become more common in each generation.

• Over time, this can lead to changes in species.

important limits of natural selection

• Natural selection can only choose from traits that already exist.

• Evolution is limited by what an organism inherited from its ancestors.

• Adaptations are often trade-offs → a trait may help in one way but have a downside in another.

• Chance + the environment + natural selection all work together to shape evolution.

How would Darwin explain how cheetahs evolved to run faster

• Some cheetahs were naturally faster than others.

• Faster cheetahs were better at catching prey, so they survived and reproduced more.

• Their fast-running traits were passed to their offspring.

• Over many generations, cheetahs became faster through natural selection.

How would Lamarck explain cheetahs evolving to run faster?

• Cheetahs used their legs more to chase prey.

• This made their legs stronger and faster.

• They passed these acquired traits to their offspring.

• Over generations, cheetahs became faster and faster.

Do we see evolution happening today?

• Yes! Evolution is not just something that happened in the past, it is happening right now.

• Examples:

  • Antibiotic resistance

  • Urban bedbugs

  • Silent crickets in Hawaii

What the age of earth and fossil record tell us about evolution

• Fossil: remains, traces, or other evidence of past life.

• Scientists use radioactivity to determine the age of rocks and fossils.

• Fossils show:

  • Increasing diversity of life over time.

  • How modern species evolved from extinct ancestors.

  • Transitional life forms with both ancestral and modern characteristics.

• Example: the evolution of the modern horse.

What is a transitional fossil?

• An organism with a combination of ancestral and descendant traits.

• Evolution does not happen like a simple ladder.

• A transitional fossil is not necessarily the direct ancestor of a modern organism.

• It shows a mix of older and newer characteristics.

Why is Tiktaalik a transitional fossil?

• 375 million years old lobe-finned fish.

• An evolutionary cousin of the ancestor of tetrapods.

• Had both fish-like features and tetrapod-like features:

  • Fins + scales

  • Neck + strong limb bones

  • Flattened body

  • Could breathe air

What are homologous structures?

• Similar structures in different related organisms.

• Inherited from a common ancestor.

• They can have different functions.

• Result of different selective pressures and descent with modification.

Homologous vs. Analogous Structures

• Homologous: related organisms, same evolutionary origin, function may differ, common ancestor.

  • Example: human arm & whale flipper

• Analogous: unrelated organisms, different evolutionary origin, function often similar, not necessarily a common ancestor.

  • Example: bird wing & insect wing

What are vestigial structures?

• Remnants of structures that were useful in ancestors but have little or no clear function today.

• They provide evidence of a common ancestor.

• They remain because they don’t affect fitness, so natural selection doesn’t eliminate them.

Comparative Embryology

• Closely related organisms often have similar stages of development.

• Embryos use similar genes and patterns to develop tissues and organs.

• Pharyngeal gill slits are an example.

• These similarities suggest organisms share a common ancestor.

Biogeography

• the study of where organisms live now and where they and their ancestors lived in the past.

• The locations of living and fossil species help show how modern organisms evolved from their ancestors.

What are the two patterns of evolution

• Divergence: Closely related species become different because of different selective pressures.

  • May end up in different parts of the world due to continental shifts.

• Convergence: Distantly related species become similar because of similar selective pressures.

How molecular biology supports evolution

• Same genetic code in all organisms → suggests a common ancestor.

• Related species have similar genes and proteins.

• Examples: Hox genes and Cytochrome C.

If an individual develops a new trait during it’s lifetime, has the population

evolved?

no because individuals do not evolve genetically during their lifetime, populations evolve across generations

Gene pool

all of the alleles present in a population

Hardy-Weinberg principle

• Acts as a null model

• Shows when a population stays unchanged

• Helps determine if evolution is occurring

• Looks for changes in the gene pool

What are the five events that take place to drive the

evolutionary process?

1. Mutation

2. Gene flow

3. Genetic drift

4. Natural selection

5. Non random mating

Genetic Drift

• random change in allele frequencies caused by chance

• Genetic drift has a stronger effect in small populations.

• Situations that lead to genetic drift:

• Bottleneck Effect

• Founder Effect

ex: If a population has 50 rabbits and 10 have a gene for white fur, but a random event kills most of the white rabbits, the gene for white fur becomes less common.

Bottleneck Effect

• A population experiences a catastrophic event:

• wildfire

• hurricane

• disease

• habitat destruction

Most individuals die regardless of genotype. The survivors represent only a random sample of the original gene pool.

Founder Effect

• A small group leaves a population and establishes a new population.

• The new population's allele frequencies reflect the random genetic composition of the founders.

Examples:

• Island populations

• Colonization

• Isolated populations

Why does genetic drift have a bigger effect on small populations?

• Small populations have fewer individuals.

• Random events can remove a large part of the gene pool.

• This causes bigger changes in allele frequencies.

Non random mating

• Random mating means individuals choose mates independently of genotype/phenotype.

But in nature:

• Mating is often NOT random.

• Examples:

Mate choice

Sexual selection

Assortative mating

Disassortative mating

Assortative Mating

Positive

• Individuals mate with those that are similar

• Examples:

Humans, Eastern Blue Birds

Negative

• Individuals mat with those that are different

• Examples:

Humans, Scarlet Lilly Leaf Beetle

Sexual selection mating

• The tendency of females to choose males that have certain traits

Can be behavioral, structural, or biochemical in nature

Example:

Lions-> males have manes

Peacocks males have blue & green

feathers and females are brown

Why would a trait evolve if it makes survival more difficult?

• Increased mating success can outweigh decreased survival

Mutations

• a random change in the DNA sequence that creates a new heritable genetic variation

• Possible Outcomes:

  New allele

  No phenotypic effect ⟶ we do not see

  the new mutation

  Harmful effect

  Beneficial effect

Gene Flow

• movement of alleles between populations

• Response to environmental pressures:

• Immigration ⟶ movement into a population

• Emigration ⟶ movement out of a population

Gene flow tends to

• Increase genetic variation within a population

• Make populations more genetically similar to one another

• Introduce alleles into populations

mechanism #5- Natural selection

• Individuals with helpful, heritable traits survive and reproduce more.

• Over generations, those traits become more common.

• Peppered moths: Dark moths became more common after pollution darkened tree bark because they were harder for predators to see.

4 Requirements:

• Variation

• Heritability

• Different survival/reproduction

• Change over generations

What does a bell curve show about variation in a population?

• Shows the different variations of a trait.

• Peak = average phenotype

• Most individuals have the average trait.

• Ends = extreme phenotypes

Directional Selection

• Favors individuals with a trait at one extreme.

• Over time, that trait becomes more common.

• Example: After pollution darkened trees, dark-colored peppered moths were harder to see, so they survived and reproduced more.

Stabilizing Selection

• Favors the average phenotype.

• Example: Robin eggs that are an average size are more likely to survive. Very small or very large eggs are less successful.

Disruptive Selection

• Favors both extreme phenotypes.

• Example: In rabbits, very light and very dark fur may help them blend into different environments, while medium-colored rabbits stand out more.