Evolution Notes

Evolution as Change in Heritable Characteristics

• Evolution is defined as the change in the heritable characteristics of a population over time.
• Heritable traits are essential for evolutionary change.
• Populations are groups of organisms of the same species living in a specific area.

Lamarckism

• Jean-Baptiste de Monet Lamarck (1744-1829) was the first to propose a mechanism of evolution.
• Lamarck's ideas included:
  • Use and Disuse: Structures used frequently strengthen and enlarge, while unused structures weaken and deteriorate.
  • Example: Giraffe ancestors stretching their necks to reach leaves, resulting in elongated necks over time.
  • Inheritance of Acquired Characteristics: Physical changes during an organism's lifetime are inherited by offspring.
  • Example: Elongated necks acquired by parent giraffes are passed to offspring, leading to a long-necked species over generations.

Darwinian Evolution

• Charles Darwin (1809-1882) proposed natural selection as the mechanism of evolution.
• Natural selection involves:
  • Heritable variations (not acquired during an organism's lifetime).
  • Variations that benefit an individual’s survival and reproduction.
  • Beneficial variations passed to offspring.
  • Over generations, the frequency of beneficial variations increases in the population.

Paradigm Shift

• Darwin's natural selection superseded Lamarck's inheritance of acquired characteristics.
• Physical changes during an organism's lifetime are not passed to offspring because germline cells (involved in reproduction) are distinct from somatic cells (rest of the body).
• Only changes affecting the germline can be passed on.
• This shift exemplifies a paradigm shift in science, a fundamental change in understanding a phenomenon.
• Thomas Kuhn described scientific progress as a cycle, challenging the idea of steady accumulation of new ideas in The Structure of Scientific Revolutions (1962).

Evidence for Evolution from Genetic Sequences

• Mutations are random alterations in the genetic code during DNA replication.
• Mutations change the base sequences of DNA, potentially affecting amino acid sequences in proteins.

Sequence Similarity

• Comparing DNA or protein sequences between organisms shows that more similar sequences indicate closer relationships.

Genetic Divergence

• Over time, mutations accumulate in DNA sequences, leading to changes in amino acid sequences of proteins.
• These changes trace evolutionary relationships.
• Example: Tracing rapid evolution of HIV and influenza.

Comparative Genomics

• Comparing genomes of different organisms identifies conserved regions resulting from common ancestry.
• Example: Genomic analysis of conserved sequences provides evidence for LUCA living near hydrothermal vents.

Cladistics Analysis

• Aligning and comparing DNA or protein sequences across multiple species allows constructing cladograms.
• Cladograms visually represent evolutionary relationships.
• Example: Classification of all organisms into three domains based on rRNA base sequences.

Evidence for Evolution from Selective Breeding

• Artificial selection occurs when humans deliberately breed crop plants and domesticated animals with specific phenotypic traits.
• Examples include:
  • Meat and milk production in cattle.
  • Meat and egg production in chickens.
  • Wool production in sheep.
  • Working tasks in horses.
  • Companion animals like cats and dogs.
  • Increased food yield in crop plants.
  • Fiber production in cotton.

Artificial Selection Demonstrates Evolution

• Artificial selection shows that evolution occurs, often resulting in astounding phenotypic variation over short periods due to intense selection pressure.
• Examples: domestic dogs, pigeons, brassicas, wheat.

Example of Artificial Selection: Teosinte to Corn

• Corn resulted from artificial selection of teosinte in Mexico around 9,000 years ago.
• Humans selected for:
  • Fewer, larger ears (the part of the plant that contains the kernels and is wrapped in a husk).
  • Softened and enlarged kernels (seeds of the plant).
  • Reduced stem branching to produce one central stalk.

Variations in Domestic Dogs

• Dogs were first domesticated at least 14,000 years ago from a gray wolf ancestor.
• Approximately 400 breeds have been bred from this single ancestral species through selective breeding.
• Selection was based on physical and behavioral characteristics.
• Five ancient dog breeds:
  • Mastiff-type: Originally from Tibet, dating back to the Stone Age.
  • Pointer-type: Bred for hunting small game.
  • Sheepdog: Originated in Europe and bred for stock protection.
  • Greyhound: One of the oldest breeds, originating in the Middle East.
  • Wolf-type: Developed in snow-covered habitats in Alaska, northern Europe, and Siberia.

Evidence for Evolution from Homologous Structures

• Homologous structures include:
  • Anatomical structures: Similarities from a common ancestor (e.g., pentadactyl limb in tetrapods).
  • Embryological patterns: Similarities in structures and timing during embryonic development (e.g., embryonic gills).
  • Metabolic reactions: Similarities in enzymes and metabolic reactions (e.g., glycolysis).
  • Molecular sequences: Similarities in DNA and amino acid sequences (e.g., similar eye development gene Pax6).
  • Vestigial structures: Remnants of functional structures in ancestors (e.g., pelvis and leg bones in snakes and whales).

Pentadactyl Limb

• Found in tetrapods (amphibians, reptiles, birds, and mammals).
• Same general bone structure inherited from a common ancestor.
• Components:
  • One bone proximal to the body: Humerus (forelimb) / Femur (hindlimb).
  • Two bones distal to the body: Radius and ulna (forelimb) / Tibia and fibula (hindlimb).
  • Group of little bones: Wrist = carpals (forelimb) / Ankle = tarsals (hindlimb).
  • Digit bones at the tips of the limb: Metacarpals and phalanges (forelimb) / Metatarsals and phalanges (hindlimb).
• Evolved into modified forms adapted to different functions.
• Well-documented in the fossil record; horse species lost side toes, the middle toe evolved into a hoof.

Divergent vs. Convergent Evolution

Divergent Evolution

• Organisms from a common ancestor accumulate differences over time due to different selective pressures, eventually forming new species.
• Homologous structures are inherited from a common ancestor but evolve in diverse ways for different functions.
• A homologous structure is a pattern of similarity modified through natural selection.

Adaptive Radiation

• Typically involves the evolution of many species from a common ancestor.
• Rapid and associated with ecological opportunity.
• Focuses on diversification to fill different niches.
• A macro-scale evolutionary process.

Divergent Evolution

• Can involve just two species or populations diverging.
• Can be slow and gradual over time.
• Focuses on increasing differences between related species.
• A micro- or macro-scale evolutionary process.

Connection Between Adaptive Radiation and Divergent Evolution

• Adaptive radiation is often divergent evolution on a large scale, involving many species diversifying from a common ancestor.
• Divergent evolution can occur without adaptive radiation, such as when two populations of a single species become geographically isolated and evolve different traits without filling new ecological niches.

Convergent Evolution and Analogous Structures

• Determining if a trait is homologous or analogous involves analyzing traits of the common ancestor of two species.
• If the common ancestor had the trait and passed it to descendants, the trait is homologous.
  • Example: Tetrapod ancestor passing the pentadactyl limb to descendant species.
• If the common ancestor did not have the trait, the trait is likely analogous and independently evolved.
  • Example: Flight in bats and birds.
• Be clear about which trait is being considered; the same species may have both analogous and homologous traits.
  • Example: Flight is analogous between bats and birds, while the pentadactyl limb structure is homologous.

Analogous Structure

• Wings of a butterfly, bird, and bat are analogous because they share a common function (flying), but were not inherited from a flying ancestral species.
• Analogous traits are not shared by all descendants of the common ancestor.

Streamlined Body Shape

• Sharks, dolphins, and penguins have analogous streamlined body shapes.
• The similarity results from the same selection pressures of a swimming carnivore niche, not common ancestry.
• A streamlined shape includes a rounded head that tapers to the tail, reducing friction drag with water.

Tendrils

• Grape and pea plants both create tendrils for support, but the structure is analogous and independently evolved.
• The tendril of the grape arises from the stem, while the tendril of the pea arises from the leaf.

Convergent Evolution

• Different species independently evolve similar traits or behaviors in response to similar environmental selection pressures.
• Analogous structures from convergent evolution were not inherited from a common ancestor.

Speciation

• Speciation is the process by which a population of one species diverges to become two distinct species.
• A single population is a group of organisms of the same species living in an area.
• Members become reproductively isolated through geographic, behavioral, or temporal means.
• Over time, physical and/or behavioral differences accumulate through differential selection, leading to different species.
• Speciation is often depicted using a branching line diagram.

Cladograms

• Depict hypothesized evolutionary relationships.
• Common ancestral species are inferred at branch points (“nodes”).
• New species evolving from the ancestor inferred at the tips of the branches.

Speciation and Divergent Evolution

• Speciation through divergent evolution has resulted in an immense diversity of life on Earth.
• However, life forms continue to share some structures and processes due to common ancestry.

Divergence During Speciation

• Speciation usually happens gradually, with populations becoming more and more different.
• Genetically diverging populations may be hard to distinguish as unique species as the speciation process is occurring because there is a continuum from merely somewhat restricted gene flow within the ancestral species and the complete reproductive isolation of the two resulting species.
• There is a taxonomic “grey zone” during the periods of time between there clearly being 1 species and there clearly being two resulting species.

Gradual Change is Not Speciation

• Gradual evolutionary change in a species over time is not speciation.
• Alterations of a single species over time eventually produce physically, morphologically, and/or genetically distinct populations from ancestors.
• Throughout the change, there is only a single evolutionary species.
• This type of change is often the result of directional selection.

Total Number of Species on Earth

• Speciation increases Earth's species count, while extinction reduces it.
• Estimates suggest over 99% of all species that ever existed are extinct.
• At least five times in the last 500 million years, mass extinctions eliminated 75-90% of all species.

Processes of Speciation

• Speciation occurs from reproductive isolation and differential selection.

Reproductive Isolation

• Step in speciation, populations stop interbreeding.
• Inbreeding causes mixing of genes whereas speciation depends on separation of gene pools.
• Reproductive isolation can be geographic, behavioural, or temporal.

Gene Flow

• Gene flow is the movement of genes into or out of a population.
• No gene flow leads to isolated gene pools.
• Less gene flow increases the likelihood of speciation.
• Geographic features are common barriers to gene flow.

Differential Selection

• Isolated populations experience different selection pressures.
• These pressures include competition for resources, climate differences, predators, and food sources.

Natural Selection

• Natural selection results in different traits in each population.
• Over time, each population becomes more and more different.
• If this results in the populations not being able to interbreed the populations are now different species.

Example: Chimpanzee and Bonobo

• Approximately 1.8 million years ago, ancestors of bonobos and chimpanzees were a single species in the Congo rainforest.
• Speciation occurred; bonobos and chimpanzees are now separate species.

Reproductive Isolation : Chimpanzee and Bonobo

• The Congo River is one of the world's deepest rivers.
• The river formed a geographic barrier.
• It led to allopatric speciation.
• The ancestor of chimpanzees and bonobos is thought to have inhabited the areas north of the Congo River.
• When the Congo flooded water, the north and south populations of apes were geographically isolated and not able to interbreed.
• The apes' inability to swim across the world's deepest river has maintained the divide.

Differential Selection : Chimpanzee and Bonobo

• The northern population needed to compete with gorillas for food and territory whereas the southern population did not.

Chimpanzees vs Gorilla

• North of the river, the ancestors of chimpanzees had to compete with ancestral gorillas for both food and territory.
• Fighting skills improved their chances at survival, chimpanzees were evolutionarily selected for aggressive tendencies.
• Today, chimpanzees are patriarchal - with the group being led by a single alpha male.
• Chimps have been observed hunting, using tools, and exhibiting lethal aggression.

Bonobos

• South of the river, the ancestors of bonobos did not have to compete with ancestral gorillas for food or territory.
• With additional access to food, the bonobos had no particular use for aggression.
• The bonobo ancestors could travel in larger, more stable parties, and form strong social bonds.
• Instead of violence being a key skill for survival, friendship and cooperation became the most advantageous traits.
• Today, bonobos groups are female dominant, with females forming tight bonds through same-sex socio-sexual contact that is thought to limit aggression.
• In the wild, they have not been seen to cooperatively hunt, use tools, or exhibit lethal aggression.