Domain 1: Evolution

1. NATURAL SELECTION

1.1: Natural selection is a major mechanism of evolution

Charles Darwin

Pre-Darwin Lyell: Geology, Uniformitarianism! very old earth. Malthus: Exponential Population Growth LaMarck: Evolution. Inheritance of acquired characteristics (wrong, but still evolutionary)

The Voyage of the HMS Beagle

Natural Selection

Observation 1: Variation

No two organisms are completely alike.

Observation 2: Reproduction…

…And overproduction

Inference 1: Differential “fitness” in the environment due to variations.

The “struggle for existence”

Inference 2: Over the span of geological time (billions of years), inheritance of adaptations will lead to evolution of the population.

Fundamental Conclusions 1.  To develop the diversity of life seen on

the Earth today, the Earth has to be incredibly old.

2.  If organisms evolve from pre-existing organisms, then all organisms should share a universal “common ancestor”

“tree thinking”

Unsettled by Darwin 1.  Origin of Life 2.  Origin of species 3.  Nature of variation/inheritance

2. THE MODERN SYNTHESIS

1.1: Natural selection is a major mechanism of evolution

The “Modern Synthesis” Connects Darwinian evolution to genetics and modern understanding of inheritance.

Where Traits come from:

Trait

Variation comes from Mutation Mutation: A change in a DNA sequence. Happens spontaneously and unavoidably.

Mutations create “alleles” Alleles: Different versions of the genes for a trait.

Evolution Defined: Evolution: Changes in allele frequencies over time.

Ex. Galapagos Finches Grant and Grant: Studied the finch population on an isolated island in the Galapagos. Measured the beak dimensions of all birds on the island every year for decades.

Connected changes in beak dimensions to fluctuations in the environment (precipitation, seed sizes)

Evolution Misconception Alert! Misconception: Individuals evolve. Evolution is a “population level” phenomenon. Individuals DO NOT evolve! The evolution of a population emerges from the individual fitness of members of that population. As they survive and reproduce or not, the frequencies of alleles in the next generation will change accordingly.

1. HOW NATURAL SELECTION WORKS.

1.2: Natural selection acts on phenotypic variations in populations.

Genotype The alleles that an individual has for a particular trait. 2 Types: Homozygous: Two copies of the same allele. Heterozygous: Two copies of different alleles for each trait.

Phenotype The trait that an individual shows. Genotype determines Phenotype!

Alleles control the production of proteins and proteins determine traits.

Trait

Dominant & Recessive Some alleles (“dominant”) will control phenotype over other alleles (“recessive”) when both are present. Ex. Eye color (simplified)

Two alleles: B (dominant) and b (recessive) Two phenotypes: Brown eyes and blue eyes

Eye Color Genetics: 3 possible genotypes:

BB Bb bb Homozygous Heterozygous Homozygous Dominant Recessive

2 possible phenotypes: Brown eyes Blue eyes Heterozygotes have BROWN eyes.

Evolution Misconception Alert! Misconception: Dominant = “better” Dominant alleles are NOT “better” than recessive alleles Dominant and recessive have nothing to do with their effect on fitness. They only refer to how they contribute to phenotype expression.

Phenotype and Fitness

Different phenotypes will be more or less fit, depending upon the requirements of the environment.

“Fitness”: Ability to contribute genes to the next generation (reproduction).   The environment determines fitness.

Fitness changes with the environment

Ex. Pesticide Resistance

Human Impact on Variation Humans are able to impact variation in other organisms by controlling which individuals are able to reproduce. Artificial selection: When reproductive success is determined by human requirements

Ex. Dog Breeds

Ex. Food Crops

1. OTHER EVOLUTIONARY FORCES

1.3: Evolutionary change is also driven by random processes.

Genetic Drift

Random, non-selective, changes in allele frequency due to chance.

Has a larger effect on smaller populations, since each individual is more of the total alleles.

Founder Effect The descendants of a small, founding population have different allele percentages than the population the founders came from.

Ex. Amish Populations and polydactyly

Bottleneck Effect The survivors of a catastrophic decrease in a population may have a different allele frequency than the pre-bottleneck population

Ex. Modern Cheetahs are all genetically similar due to 2 bottlenecks

Gene Flow

Movement of alleles due to immigration and emigration

Example: Modern Human Migration

Sexual Selection Persistence of traits that signify fitness and aid in reproduction

Ex. Peacocks are male.

Can be intersexual or intrasexual

Evolution Misconception Alert!

Misconception: Evolution is “random”. Evolution is a change in allele frequency in a population. That change involves random forces (ex. Genetic drift) and selective processes (ex. Natural selection).

1. EVIDENCE OF EVOLUTION

1.4: Biological evolution is supported by scientific evidence from many disciplines, including mathematics.

Geological Evidence: Radiometric dating: Used to date geological formations and fossils. Establishes chronological history of Earth, and establishes Earth’s age at ~4.5 billion years.

Fossil Record: Establishes History of life on Earth.

Living organisms resemble fossilized forms.

Transitional Fossils: Show evolutionary progression between groups

Ex. Tiktaalik

Anatomical Evidence Similarities and differences in the anatomy (morphology) of organisms. Vestigial structures: structures that have lost their primary adaptive purpose Ex. Whale hind-limbs

Homologous structures: Structures present in a common ancestor, which have diverged during evolution. Ex. Vertebrate limbs

Analogous structures: Structures that have evolved multiple times in different lineages to fill similar adaptive needs. Ex. Wings

Chemical Evidence Similarities and differences in DNA and protein sequences.

Chemical evidence has been used to establish the evolutionary relatedness (“phylogeny”) of all life on Earth

Mathematical Modeling: Computational analysis: The ability to analyze large amounts of chemical sequence data to establish evolutionary relationships among organisms. Hardy-Weinberg Theory: The ability to quantify the amount of evolutionary change from generation to generation.

1. EVIDENCE OF COMMON ANCESTRY

1.5 Organisms share many conserved core processes and features that evolved and are widely distributed among many organisms today

The Universal Genetic Code

Common Metabolic Pathways Ex. Glycolysis

Cellular Morphology Note: Not to scale. Prokaryotic Cell:

“Plant-like” eukaryotic cell: “Animal-like” eukaryotic cell:

Endosymbiosis

2. MATH SKILLS: HARDY-WEINBERG THEORY

1.4: Biological evolution is supported by scientific evidence from many disciplines, including mathematics.

What is Hardy Weinberg Theory? Equations that enable us to determine how much a population is evolving from generation to generation. “Hardy-Weinberg equilibrium”: Refers to an idealized, non-evolving population. Five characteristics:

Characteristics of a non-evolving population:

1.  Large size (no genetic drift) 2.  Random mating (no sexual selection) 3.  Stable environment (no natural

selection) 4.  No immigration/emigration (no gene

flow) 5.  No mutations. No real population is in HW equilibrium.

Hardy-Weinberg Equations For a trait controlled by two alleles, where p is the dominant allele and q is the recessive allele: Gene Frequency:

p + q = 1 Genotype Frequency:

p2 + 2pq + q2 = 1

Sample Problem In pea plants, the allele for purple flowers is dominant to the allele for white flowers. If 99% of the plants in the population have purple flowers, determine the percentage of heterozygotes in the population.

Uses of HW Theory To determine how a population is evolving from generation to generation. To help to determine which evolutionary pressures are affecting a population more/less.

1. PHYLOGENY

1.6: Phylogenetic trees and cladograms are graphical representations (models) of evolutionary history that can be tested.

Cladograms Diagrams that group items together based on the number of common characteristics. 1.  Determine number of shared

characteristics. 2.  Arrange items as a tree showing most

commonality possible

Phylogenetic Tree A cladogram that represents evolutionary relationships. Use two types of data: 1.   Shared Derived Characters: Physical

traits that represent evolutionary history (homologous structures).

2.   DNA/Protein sequence Data: Differences in sequences accumulate as species evolve away from each other.

Ex. Vertebrate Phylogeny.

Ex. Complete Phylogeny

Phylogenetic Tree Construction 1.  Determine similarities among

organisms (character table works well). 2.  Arrange organisms in a tree diagram

showing simplest possible evolution. Maximum parsimony: All else being equal, a trait is assumed to evolve once and be present in all descendants

SKILL: Create a tree- Selected Vertebrates

Animal Opposable Thumb

4-chamber heart

Amniotic egg

lungs Spinal column

Chimpanzee

1 1 1 1 1

Mouse 0 1 1 1 1

Turtle 0 0 1 1 1

Frog 0 0 0 1 1

Fish 0 0 0 0 1

Lamprey 0 0 0 0 0

Character Table:

Trees are Hypotheses Continual revision:

As more data is gathered, the phylogenetic relationships among organisms are continually revised. Role of computers:

Computer analysis is needed to determine the similarities in large amounts of DNA/protein sequence information.

1. SPECIATION CONCEPTS

1.7: Speciation and extinction have occurred throughout the Earth’s history.

What is a species? “Biological Species”: A group of organisms that are capable of successfully reproducing. It’s testable, but simplistic. And it is limited in application.

Speciation Rate Gradualism: species are the product of slowly accumulating, small evolutionary changes. Punctuated equilibrium: species undergo long periods of very little change, followed by rapid, large evolutionary changes.

Ex. Major Extinctions.

Adaptive Radiation One species evolves in to many species that occupy open niches. Ex. Lake Cichlids, Mammals, Galapagos Finches.

1. SPECIATION PROCESS

1.8: Speciation may occur when two populations become reproductively isolated from each other.

Reproductive Isolation Speciation occurs when a population can no longer interbreed with any other population. Allopatric: Happens due to physical separation. Sympatric: Happens while occupying the same area.

Species Barriers Pre-Zygotic: Post-Zygotic: Physical Reduced Viability Temporal Reduced Fertility Behavioral Hybrid Breakdown Mechanical Chemical

Ex. Mules

Ex. Apples

Ex. Fruit Fly Food Speciation.

1. ONGOING EVOLUTION OF ORGANISMS

1.9 Populations of Organisms Continue to Evolve

Evolution is Ongoing Evolution continues to happen. Ex. Pesticide Resistance

Ex. Rock Pocket Mouse

Analysis of Evolution Mathematical modeling (e.g. HW Equilibrium) and genetic analysis can be used to investigate evolution as it occurs in real-time over generations.

1. ORIGIN OF LIFE

1.10: There are several hypotheses about the natural origin of life on Earth, each with supporting scientific evidence.

Origin Hypotheses Hypotheses must be testable. Many thoughts about the origin of life are not testable. Two major hypothesis for life on Earth. 1.   Panspermia: Life from extraterrestrial

life. 2.   Abiogenesis: Life from non-life.

Requires 4 major milestones to occur.

1. Development of Biological Molecules

2. Development of Proto-cells

3. Information Molecule Evolution

4. Reproduction

The “RNA World” A Hypothetical pre-DNA state of life. Based on RNA’s dual ability to store information AND catalyze reactions.

Evolution of Metabolism “Heterotroph Hypothesis”: Glycolysis ! Photosynthesis ! Aerobic Cellular Respiration.

Endosymbiosis Prokaryote ! Eukaryote

Multicellularity Multicellularity opens previously inaccessible niches. Many organisms have unicellular and multicellular stages of their life cycles.

History of Life on Earth

1. EVIDENCE FOR THE ORIGIN OF LIFE

1.11: Scientific evidence from many different disciplines supports models of the origin of life.

Geology Radioisotope Dating: Allows estimates of events during evolutionary history.

~65 – 70 mya

Ex. Banded Iron Formations

Ex. Fossil Fuels

Miller-Urey Experiments Simulated “Early Earth” conditions (no O2). Created Simple Biological Molecules

Commonalities among all organisms suggests common ancestry.

It is the simplest explanation for the evidence. DNA Stores Information in all cells on Earth.

The Genetic Code is universal in all cells

Glycolysis is universal in all cells

All life is organized into cells These cells contain fundamental structural similarities Note: Not to scale.

Prokaryotic Cell:

“Plant-like” eukaryotic cell: “Animal-like” eukaryotic cell:

A Universal Phylogenetic

Tree

Image Credits All images taken from wikimedia commons. Exceptions slide 23: Image from Grant & Grant, 2002. Slide 96: Image and Diagram from M. Nachman