Evolution Notes

Evolution

Unit 6

Overview

This unit covers abiogenesis, the concept of organisms changing over time (evolution), and the evidence supporting evolution.

Abiogenesis

Definition

Abiogenesis is the idea that living things can originate from non-living matter.

Spontaneous Generation

For a long time, people believed in spontaneous generation, the idea that animals could come from non-living sources. Examples include:

• Frogs developing from falling rain.
• Mice arising from sweaty underwear.
• Flies coming from decaying meat.

History of Abiogenesis

Francisco Redi (1668)

• Italian physician and poet.
• Attempted to disprove spontaneous generation with an experiment:
  • Left some jars of meat uncovered and other jars sealed with a cloth.
  • Fly maggots grew on the uncovered meat but not on the covered meat.

John Needham (1745)

• English clergyman.
• Wondered if spontaneous generation applied to microorganisms.
• Experiment:
  • Prepared various broths and showed that they contained microbes.
  • Boiled the broths, eliminating the microbes.
  • Left the stoppers loose to allow air (the "vital force") to enter.
  • After a few days, microbes reappeared, suggesting spontaneous generation from the non-living broth.

Lazzaro Spallanzani (1745)

• Italian naturalist.
• Disagreed with Needham, claiming he did not seal the jars well enough and microbes could have come from the air.
• Repeated Needham’s experiment with changes:
  1. Boiled the flasks longer.
  2. Sealed them airtight by fusing the glass tops shut.
  • Result: No growth occurred in any flask.
• Second experiment:
  • Timed boilings with some jars left with leaky seals (as Needham did) and some sealed airtight.
  • Hypothesized that more boiling should lead to less life.
  • Showed that boiling did not damage the broth’s ability to support life and growth depended only on the seal.

Louis Pasteur (1859)

• French chemist.
• Participated in a contest sponsored by the French Academy of Sciences to prove or disprove spontaneous generation.
• Experiment:
  • Used swan-necked flasks, which allowed air in but trapped dust (and microbes).
  • Boiled the broth to kill initial microbes.
  • No growth occurred, even after many days, addressing concerns about damaging the "active principle" (life can still survive/grow).
  • Tipped the flask, allowing the nutrient broth to contact the dust, which carried microbes into the broth, resulting in growth.
• Pasteur effectively disproved abiogenesis, and scientists accepted that it did not exist. This raised the question of how life began on Earth.

Formation of Simple Organic Molecules

Life requires the formation of simple organic (carbon-based) molecules and their organization into macromolecules of life (carbohydrates, lipids, proteins, nucleic acids).

• Earth’s primitive atmosphere composition
  • Water vapor - 0.5%
  • Nitrogen - 3%
  • Carbon dioxide - 95%
  • Methane - 0.1%

The Primordial Soup Theory

Alexander Oparin

• Theorized that energy from sunlight and lightning triggered chemical reactions to produce simple compounds in the atmosphere. These products rained down on the oceans (the "soup").

Miller and Urey Experiment

• Proof for the "soup" theory.
  • Circulated a mixture of water vapor (steam) with ammonia, methane, and hydrogen.
  • Subjected the mixture to electrical sparks (lightning) and heated and cooled the mixture to simulate daily temperature fluctuations.
  • After one week, they noted the presence of amino acids, sugars, and organic compounds.

Sydney Fox

• Found that heating solutions of amino acids could form "protocells."
• Protocell: A primitive membrane-enclosed structure that can carry out growth, division, and metabolism.

The First Cells

• Heterotrophic Prokaryotes: Simple, and anaerobic (no free oxygen).
• Autotrophs evolved and began to perform photosynthesis.
• The byproduct of photosynthesis led to aerobic respiration (more energy = more complexity) and the formation of Ozone (O_3) to protect developing eukaryotes.
• Endosymbiont Hypothesis.

Evolution

Definition

Evolution is the change in a population over time. As the environment changes, species either adapt or go extinct.
Adaptation is an inherited characteristic that increases an organism's chance of survival.

Causes of Extinction

• Asteroid Strikes
• Invasive Species
• Lack of Food
• Loss of Habitat
• Better-Adapted Competition
• Climate Change
• Lack of Genetic Diversity
• Pollution
• Disease
• Human Predation

Mass Extinctions

A mass extinction is a sharp spike in the rate of extinction of species caused by a catastrophic event or rapid environmental change.
Scientists have identified five mass extinctions in Earth's history, each leading to a loss of over 75% of animal species.

1. Ordovician-Silurian Extinction (440 Million Years Ago): 86% loss. Caused by glaciation and heating events.
2. Late Devonian Extinction (365 Million Years Ago): 80% loss. Caused by land plants developing deep roots, releasing nutrients into oceans, or a global cooling.
3. Permian-Triassic Extinction (252 Million Years Ago): 96% loss. The deadliest extinction, caused by volcanic activity in Siberia.
4. Triassic-Jurassic Extinction (201.3 Million Years Ago): 75% loss. Caused by volcanic eruptions or an asteroid/comet impact.
5. Cretaceous-Paleogene Extinction (66 Million Years Ago): 60-76% loss. Wiped out the dinosaurs, likely caused by an asteroid impact in the Yucatán Peninsula.
6. Holocene Extinction (11,700 Years Ago to Present): Possibly happening now, with a current extinction rate much greater than the "normal" rate due to human impact.

Evolution: Fact vs. Theory

Evolution is the change in the genetic makeup of populations over time (a fact).
The mechanism for evolution (how it happens) is a theory. The theory of natural selection is a well-supported, testable explanation of how.

Jean-Baptiste Lamarck (1744-1829)

First scientist to propose a mechanism for how organisms change (1809).

• Theory of Acquired Traits: Through time, a generation would use an organ or appendage more than previous ancestors and would pass those acquired traits on to their offspring.
• Lamarck’s Assumptions
  1. Organisms have a desire to change.
  2. Organisms can change shape by using or not using their bodies.
  3. Organisms can pass on acquired traits to their offspring.

Charles Darwin (1809-1882)

• English naturalist.
• H.M.S. Beagle (1831).
• Sailed to the Galapagos Islands.
• Observed many species and fossils.
• Devised his theory on the mechanism of evolution.
• Species of Interest
  • Finches
  • Tortoises
  • Blue-footed boobies

Charles Darwin Influenced by Others

• Thomas Malthus: Essay on the Principle of Population (1798).
  • Predicted that the human population would grow faster than the space and food supplies needed to sustain it.
  • Stated that the only checks on the human population were war, famine, and disease.
  • Darwin applied this concept to all organisms.
• James Hutton & Charles Lyell:
  • Uniformitarianism: The rate of geologic formation is slow, and the Earth is constantly changing.
  • Darwin theorized that if the Earth is older than some thought, that gives organisms time to change.
  • And if Earth is always changing, why can’t organisms?
• Farmers:
  • Within a population, there are natural variations.
  • Selective breeding (breeding the plants or animals with most desired traits).
  • This process of humans allowing only the best organisms to reproduce is called artificial selection.

Theory of Natural Selection

• In nature, there is a struggle for existence: Members of a species compete for food, space, and other necessities.
• Over time, natural selection causes a change in the characteristics of a population (adaptations).

"Survival of the Fittest"

• Fitness is the ability of an organism to survive and reproduce in its environment.
• Individuals in nature with characteristics best suited to their environment survive the struggle for existence.
• This principle is called survival of the fittest; a better saying is "Survival of the Best Adapted."

Gradual vs. Punctuated Evolution

• Gradual: Slow and steady change.
• Punctuated: Long, stable periods interrupted by brief periods of rapid change.

Patterns of Natural Selection

• Stabilizing Selection: Genetic diversity decreases as the population stabilizes on a particular trait. It usually gets rid of the extreme values of the character.
  • Example: Plant height – intermediate height is favored.
• Directional Selection: A single phenotype is favored, causing the allele frequency to continuously shift in one direction.
  • Examples: Peppered moth, antibiotic-resistant bacteria.
• Disruptive Selection: Favors the extremes of a range of selection.
  • Example: Seed-eating birds – big and small beaks are favored over medium beaks if the source of medium seeds goes extinct.

Niche

A niche is:

• The full range of physical and biological conditions in which an organism lives.
• The way in which the organism uses those conditions.
• An organism’s fitness is higher within its niche than outside of its niche.
• Ex: A penguin’s fitness is higher in Antarctica than anywhere else.

Specialized Adaptation

• Mimicry: Where one species resembles another more formidable species.
• Camouflage: An organism blends into its surroundings.

Speciation

Changes leading to the formation of new species.

• Populations evolve and become reproductively isolated from each other (members of two populations cannot interbreed).
• Behavioral isolation
• Geographic isolation

Behavioral Isolation

Capable of interbreeding but have different courtship rituals, circadian rhythms, etc.

Geographic Isolation

Populations are separated by geographic barriers (rivers, mountains, bodies of water, highways/manmade structures, etc.).
Note: Doesn’t always lead to new species.

Gene Flow (migration)

Gene flow: Any movement of genes (genetic info) from one population to another.
Examples: Corn, milkweed.

Genetic Drift

Genetic drift: Individuals in a population survive and reproduce due to random chance, NOT because they are more “fit,” as in natural selection.
Genetic drift decreases diversity within a population.

• Bottleneck effect: A random event dramatically decreases the population size (and, therefore, genetic diversity) for at least one generation.
• Founder effect: A few members of an original population colonize a new population. The founders may not accurately represent the genetic makeup of the original population. Genetic diversity may decrease.

Factors of Evolution

• Mutation
• Genetic Drift
• Migration
• Natural Selection

Patterns of Evolution

• Convergent Evolution: Unrelated organisms begin to resemble each other; similar environmental demands.
• Coevolution: Two species evolve in response to changes in each other; for example, plants and their animal pollinators.
• Adaptive Radiation: (Descent with Modification). Adaptive radiation is the process by which a single species evolves into several different forms that live in different ways. This is an example of divergent evolution.

Summary Table: Factors Affecting Genetics

Evidence for Evolution

Evidence for Macroevolution

• Fossil Record
• Homologous Structures of Living Organisms
• Vestigial Structures
• Similarities in Embryology
• Molecular Biology

Fossil Record

Fossils are the remains of ancient life.
When fossils are arranged according to their age, a progressive series of changes is seen.
By comparing fossils from older rock layers with fossils from younger rock layers, scientists can see how life on Earth has changed over time.
Hundreds of transitional fossils have been found, which show intermediate stages of evolution of modern species from species now extinct; this is an “incomplete record” with many gaps.

How Fossils are Formed

• Petrification: All elements except carbon removed; organic material replaced by minerals.
• Mold & Cast: Form of organism after it rots away.
• Carbonization
• Unaltered Preservation: Original state (amber).

Dating Fossils

• Law of Superposition: Each layer of rock is older than the layer above it. Fossils arranged by age relative to each other (“Relative Dating”).
• Radioactive Dating: Used to date layers in which fossils occur. Allows the time frame of when the fossil was formed by using radioactive isotopes and half-life.

Fossil Record Can Infer Evolutionary Relationships

Archaeopteryx was an ancient creature that shows a direct evolutionary link between dinosaurs and birds.

Anatomy

• Homologous structures: Parts of different organisms, often quite dissimilar, that develop from the same part of the embryo. Same structure but usually different function. Helps biologists group animals according to how recently they shared a common ancestor.
  • Phylogenetic trees show evolutionary relationships.
• Analogous structures: Similar in form & function, but are structurally quite different.
• Vestigial Structures: Traces of structures no longer used by an organism. May be a reduced version of homologous structures in other species. The presence of the organ does not affect its ability to survive and reproduce, so natural selection does not eliminate it.
  • Examples: wings on flightless birds, human coccyx and appendix.

Biochemistry & Molecular Biology

All organisms from bacteria to humans share many of the same biochemical details.

• The genetic code for protein-coding genes.
• All organisms use ATP.
• Plasma membranes of all organisms are structurally similar (phospholipid bi-layer).

Similarities in Embryology

All embryos develop similarly.

• Similar genes define their basic body plan.
• ‘Tails’ as embryos.
• Embryos of all vertebrates are especially similar; the same groups of cells develop in the same order and in similar patterns (homologous structures).
• Early in their development, embryos possess gill slits like those of fish.
• Later in development, the embryos develop a tail.
• Human fetuses have a fine fur (lanugo) during the fifth month of development.

Our Common Ancestor: Aegyptopithecus

• 1st Primates were creatures which descended from insectivores in the late Cretaceous.
• Aegyptopithecus was a small cat-sized primate that is the most likely common ancestor between modern apes and humans. (Divergent evolution).