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Module 3 - Evolution

Winter Biology

Chapter 22: “Evolution by Natural Selection”

Chapter 22.1: The Rise of Evolutionary Thought

  • People describe the theory of evolution by natural selection as revolutionary.
    • Overturning an existing idea about how nature works; replace it with a radically different idea.
  • Advance of theory of evolution represents a profound scientific revolution.

Plato and Typological Thinking

  • Plato claimed every organism was a perfect essence or type created by God, unchanging.
  • Small variations are trivial deviations from perfect essence.
  • Typological thinking - a divine being creates each type of organism.

Aristotle and the Scale of Nature

  • One of Plato’s students, Aristotle, organized typological thinking into a scale of nature.
    • Begins with minerals and plants, rises through higher plants, invertebrates, vertebrates, humans.
  • Aristotle’s ideas remained popular into the 1700s.

Lamarck and the Idea of Evolution as Change through Time

  • 1809, biologist Jean-Baptiste de Lamarck proposed evolution - species are not static but change through time.
  • Lamarck claimed simple organisms evolve by moving up Aristotle’s scale over time.
  • Lamarckian evolution always produces larger and more complex species.
  • Claimed that species change via inheritance of acquired characters - phenotype changes in response to challenges posed by the environment.

Darwin and Wallace and Evolution by Natural Seelction

  • Lamarck eventually abandoned the linear and progressive way of life.
  • Darwin claimed that variation among individuals in a population is chief in understanding the nature of species - population thinking.
  • Theory of evolution by natural selection is sigificant because:
    1. Overturns the idea that species are static and unchanging.
    2. Replaces typological thinking with population thinking.
    3. Was scientific.
  • Descent with modification - species change through time, and species are related by common ancestry.

Evidence for Change Through Time

  • Fossil - trace of any organism that has lived in the past.
  • Fossil record - all fossils that have been found on Earth.
  • Extant species - secpies living today.
the vastness of geologic timeJames Hutton principle of uniformitarianism - geological processes occuring today are similar to those of the past. Geologic time scale - a sequence of named intervals called eons, eras, and periods that represent major events in Earth history. Radioactive decay functions as a “natural clock”. Using radiometric dating, Earth is about 4.6 billion years old.
extinction changes the species present over timeSome species have gone extinct; large, distinctive terrestrial animals would have been found if they were not extinct. Species are not static, immutable entities. 99% of all species that have lived are extinct.
transitional features link older and younger speciesA transitional feature - trait in fossil species that is intermediate between an ancestral and derived species. Sequences of transitional features document changes in evolution. Individual fossils of transitional reforms are not necessarily direct ancestors of later species, though.
vestigial traits are evidence of change over timeVestigial trait - reduiced or incompletely developed structure that has no function. A vestigial trait may have been beneficial, but would be less beneficial depending on environmental factors.
species can be observed changing todayEvolution can be observed in the scale of days, weeks, and months.

Evidence of Descent from a Common Ancestor

similar species are found in the same geographic areaMany finches that were similar ocurred in the same geographic area; finches were similar because they had descended from the same common ancestor. DNA sequence comparisons enable researchers to place species on a phylogenetic trees, confirming this.
formation of new species from preexisting species can be observed todayMany contemporary populations are undergoing speciation - a process that results in the formation of new species. Powerful evidence that species living today are descendants of species that lived in the past.
related species share homologiesHomology - the study of likeness, a similarity that exists in species due to common ancestry (e.g. human hair and dog fur). Theory of evolution predicts homology would occur.

Levels of Homology

geneticSimilarity in DNA, RNA, or amino acid sequences.
developmentalSimilarity in embyronic form or developmental processes.
structuralSimilarity in adult organismal structures, morphology.

Evolution’s Internal Consistency - The Importance of Independent Datasets

  • Internal consistency - independent sources agree on supporting predictions made by a theory.
  • For examples, whales seem to have evolved from terrestrial animals.
    • fossil record
    • phylogeny of fossil cetaceans
    • relative dating - order of species agrees with phylogeny
    • absolute dating - agrees with relative dating
    • phylogeny of living whales and dolphins - hippos are related to whales
    • vestigial hip and hindlimb bones - occur in some dolphin embyros too
  • No single observation or experiment proves evolution; data from many different sources can be more consistent with one theory rather than another.

Chapter 22.3: The Process of Evolution: How Does Natural Selection Work?

  • Darwin did not just recognize evolution; but to recognize natural selection in explaining the pattern of descent with modification.

Darwin’s Inspiration

  • Darwin spent decades exploring and documenting the diversity of plants and animals.
  • Gave Darwin a strong foundation fo patterns of evolution.
  • Darwin turned to pigeon breeding.
    • Darwin crossbred pigeons and observed how characteristics were passed on to offspring.
    • Certain individuals with desirable traits were selected to reproduce, manipulation a population by artificial selection.

Darwin’s Four Postulates

  • Darwin distilled evolution by natural selection into four postulates:
    1. Variation exists among organisms that make up a population.
    2. Some of the trait differences are heritable.
    3. Survival and reproductive success are highly variable. More offspring are produced than can survive.
    4. Individuals that survive best and produce the offspring are not random.
  • Natural selection occurs when individuals with certain heritable traits produce more surviving offspring than individuals without those traits.
  • Evolution by natural selection occurs when a heritable variation leads to differential reproductive success.

Biological Definitions of Fitness, Adaptation, and Selection

  • Fitness - the ability of an individual to produce surviving, fertile offspring relative to that ability in other individuals.
  • Adaptation - a heritable trait that increases the fitness of an individual in a particular environment.
  • Selection - the passive process of differential reproduction as a result of heritable variation.

Chapter 22.4: Measuring Natural Selection in Populations Today

  • The theory of evolution is testable.

Case Study 1: How Did Mycobacterium tuberculosis Become Resistant to Antibiotics?

  • Mycobacterium tuberculosis is the bacterium that causes tuberculosis (TB).
  • TB was fought off in two respects:
    1. Advances in nutrition made fighting off M. tuberculosis easier.
    2. Antibiotics allowed physicians to stop infections.
  • In late 1980s, strains of M. tuberculosis became resistant to antibiotics.
  • Found a point mutation in the rpoB gene for drug-resistant strain.
    • rpoB gene codes for a component of RNA polymerase, essential for the survival and reproduction of bacterial cells.
    • Missense mutation caused a change in amino acid sequence and a change in its shape.
    • Rifampin - antibiotic used to treat tuberculosis - works by binding to the RNA pol of M. tuberculosis and interfering with transcription.
    • Mutated bacterial cells continue to produce offspring because the drug cannot bind effectively to the mutant RNA pol.
  • Resistance to a variety of insecticides, fungicides, antiviral drugs, herbicides, etc. have evolved in hundreds of species.

Case Study 2: Why Do Beak Sizes and Shapes Vary in Galapagos Finches?

  • Daphne Major of the Galapagos; finches crack seeds with their beaks.
  • Beak size and shape, as well as body size, are traits with heritable variation.
  • Daphne Major underwent a droubt; 84% of the medium ground finch population disappeared.
    • This was a natural experiment - allow researchers to compare treatment groups.
  • Finches with large and deap beaks were more likely to crack fruits efficiently enough to survive.
    • Natural selection led to an increase in average beak depth in the population.
  • Two transcription factor genes that are important:
    • ALX1 gene for beak shape.
    • HMGA2 gene for beak size.
  • Signaling molecules like BMP4, CAM, and beta-catenin may play a role in expression patterns of finches.

Chapter 22.5: Debunking Common Misconceptions about Natural Selection and Evolution

Natural Selection Does Not Change Individuals

  • Individuals do not change - the population does.
  • Natural selection is not Lamarckian Inheritance.
    • Evolution is not driven by inheritance of acquired characters.
    • Individuals do not change when they are selected; they simply produce more or fewer offspring.
  • Individuals do not adapt.
    • Some individuals do change in response to changes in the environment.
    • Acclimatization - change in an individual’s phenotype in response to a change in natural environmental conditions.
    • These changes are not passed onto offspring because the alleles themselves have not changed, and do not cause evolution.

Natural Selection Is Not Goal Directed

  • Mutations occur by chance, not “on purpose”.
    • Mutations create mutant alleles randomly due toe rrors in DNA synthesis and happened to be advantageous.
  • Evolution is not progressive.
    • There is nothing predetermined or absolute about a trait becoming more x.
    • Complex traits are routinely lost or simplified because of natural selection.
  • There is no such thing as a higher or lower organism.
    • Lamarck’s idea of evolution involved a higher or lower organism.
    • Species cannot be higher or lower than the other, though.

Natural Selection Does Not Lead to Perfection

  • Traits are not always adaptive.
    • Vestigial traits do not increase the fitness of individuals.
    • These structures are not adaptive, but exist because they are present in the ancestral population.
  • Fitness trade-offs exist.
    • A fitness trade-off is a compromise between two traits that cannot be optimized simultaneously.
  • Traits are genetically constrained.
    • A number of genetic constraints on genetic selection are placed.
    • For instance, genetic correlation - selection on one trait causes change in another trait - like pleiotropy.
    • Lack of genetic variation is also important.
  • Traits are historically constrained
    • For instance, the bones found in your middle ear evolved from bones from the jaw of mammalian ancestors.
    • These bones are not the “perfect” solution, but they are historically constrained.
  • Traits are environmentally constrained.
    • Natural selection occurs in the context of a changing environment.
  • Natural selection is not the only process of evolution.

Chapter 22: “Evolutionary Processes”


  • Understanding evolution - change in heritable traits - is essential to understanding ourselves and the organisms around us.
  • Fruits of modern synthesis was population genetics - the study of processes that change allele and genotype frequencies in populations.
  • Evolution is now understood to be driven by 4 processes:
    • Natural selection increases the frequency of certain alleles.
    • Genetic drift causes allele frequencies to change randomly.
    • Gene flow occurs when individuals leave one population, join another, and breed.
    • Mutation modifies allele frequencies by introducing new alleles.
  • Fundamental ideas:
    • Natural selection is not the only process responsible for evolution.
    • Each of the four evolutionary processes has different consequences for genetic variation and fitness.

Chapter 23.1: Null Hypothesis - The Hardy-Weinberg Principle

  • 1908; G. H. Hardy and Wilhelm Weinberg published major results.
    • Common believed that changes in allele frequency were simply the result of sexual reproduction (meiosis and random fusion of gametes).
    • Do dominant alleles increase in frequency when gametes combine at random?
  • Hardy and Weinberg analyzed frequencies of alleles when many individuals in a population mate to produce offspring.

The Gene Pool Concept

  • Hardy and Weinberg imagined a scenario in which all alleles from gametes produced in each generation go into a gene pool.
  • Hardy and Weinberg calculated what would happen if pairs of gametes were randomly combined in this gene pool.
  • For example, consider a gene pool with frequencies 0.7 A and 0.3 a.
 0.7 A0.3 a
0.7 A0.49 AA0.21 Aa
0.3 a0.21 Aa0.09 aa
  • Hardy-Weinberg principle makes two claims:
    1. If frequencies of alleles A and a are given by p and q, then the frequencies of genotypes AA, Aa, and aa are given by p^2, 2pq, and q^2.
    2. When alleles are merely transmitted via meiosis and random combinations of gametes, their frequencies do not change over time.
      • Allele frequencies are the same in the next generation. (percent of offspring gametes that have gamete A can be calculated as 0.49 + (0.21/2) + (0.21/2) = 0.7.)
  • Given a set of parental allele frequencies, the Hardy-Weinberg principle predicts what genotype frequencies and allele frequencies will occur.
    • However, allele frequencies do often change over time in real populations.

The Hardy-Weinberg Principle Makes Important Assumptions

  • Five important assumptions about how populations and alleles behave:
    1. Random mating. Individuals are not allowed to choose a mate.
    2. No natural selection. All members of the parental generation survive, regardless of genotype.
    3. No genetic drift (random allele frequency changes). The model assumes alleles are picked by their exact frequencies p and q, and not by chance.
    4. No gene flow. No new alleles are added by immigration or lost through emigration.
    5. No mutation. No new alleles are introduced to the gene pool.
  • Nonrandom mating can cause genotype frequencies to change, not allele frequencies; they need to work in tandem with other forces to change allele frequencies.
  • The other four factors are processes of evolution - they change allele frequencies.

Chapter 23.2: Nonrandom Mating

  • In the Hardy-Weinberg model, gametes are picked from the gene pool at random and paired to create offspring genotypes.
  • Matings between individuals in nature may not be random, though.
  • Inbreeding - mating between relatives.

How Does Inbreeding Affect Allele Frequencies and Genotype Frequencies?

  • Focus on alleles A and a with equal frequencies of 0.5.
  • Individuals self-fertilize - the most extreme form of inbreeding.
  • Homozygous parents produce homozygous offspring; heterozygous parents produce a 1:2:1 ratio of homozygous and heterozygous offspring.
  • Consider several rounds of self-fertilization across a population.
    • The homozygous proportion of the population increases with each generation, while the heterozygous population is halved.
GenerationAA HomozygoteAa Heterozygoteaa Homozygote
  • Two fundamental points about inbreeding are demonstrated:
    • Inbreeding increases homozygosity.
    • Inbreeding does not itself cause evolution since allele frequencies do not change in the population.

How Does Inbreeding Influence Evolution?

  • Even though it may not influence evolution directly, inbreeding can speed up the rate of evolutionary change.
    • Increases the rate natural selection eliminates recessive deleterious alleles (alleles that lower fitness) from a population.
  • Many recessive deleterious alleles are rare and exist in heterozygote individuals.
    • In heterozygote individuals, a deleterious effect has little effect, since a normal allele can usually produce enough functional protein to support a normal phenotype.
    • Inbreeding increases the frequency of homozygous recessive individuals, exposing the rare allele to selection.
  • Inbreeding depression - the decline in average fitness when homozygosity increases and heterozygosity decreases.
    • Inbreeding depression is a common problem in small populations.

Nonrandom Mating via Sexual Selection

  • Sometimes nonrandom mating occurs not due to inbreeding but because of sexual selection - an organism chooses a mate based on physical/behavioral traits.
  • Fundamentally different from inbreeding because it does lead to changes in allele frequencies and increases fitness - thus it is a form of natural selection.

Chapter 23.3: Natural Selection

  • Evolution by natural selection occurs when heritable variation leads to differential success in survival and reproduction.
  • Natural selection violates the assumptions of the Hardy-Weinberg principle.
  • Sexual selection is a subset of natural selection - both adapt, increasing fitness.
    • Sexual and ecological selection may favor different traits.

How Does Selection Affect Genetic Variation?

  • Biologists focus on genetic variation - the number and relative frequency of alleles in a particular population.
    • Lack of genetic variation is usually a bad thing.
  • Natural selection occurs in four main modes.
    • Directional selection changes the average value of a trait.
    • Stabilizing selection reduces variation in a trait.
    • Disruptive selection increases variation in a trait.
    • Balancing selection maintains variation in a trait.
  • Directional selection. The average phenotype of the population changes in one direction.
    • Tends to reduce the genetic diversity of populations.
    • When deleterious alleles decline in frequency, purifying selection occurs.
  • Stabilizing selection. Reduces both extremes in a population.
    • Has two important consequences:
      • Genetic variation in the population is reduced;
      • There is no change in the average value of the trait over time.
    • For example, babies that are too large or too small are more likely to die.
  • Disruptive selection. Instead of favoring phenotypes near the average value, disruptive selection favors extreme phenotypes.
    • Overall amount of genetic variation in the population increases.
    • Disruptive selection plays a role in speciation - the formation of new species.
  • Balancing selection. Occurs when no phenotype has a distinct advantage.
    • There is a balance among several phenotypes in terms of their fitness and frequency.
    • The following are most common:
      • Heterozygote advantage occurs when heterozygote individuals have higher fitness than homozygous individuals.
      • Frequency-dependent selection occurs when alleles are favored when they are rare but not when they are common.

Sexual Selection

  • Darwin was perplexed by seemingly nonadaptive traits like trains of peacocks.
  • intersexual selection - the selection of one individual of one sex by an individual of another sex.
    • Organisms may select traits that are honest signals of health and genetic quality.
  • intrasexual selection - selection within sex in which individuals of the same sex compete with one another to obtain mates.
    • territory - actively defended area where the owner has exclusive or semi-exclusive use.
    • Males monopolize matings with females by winning territorial battles.
  • Bateman-Trivers hypothesis - the fundamental asymmetry of sex.
    • Females invest a lot in few eggs - they protect this investment by being choosy about their mates.
      • Males that invest little in each sperm should be more willing to mate with almost any female.
    • If there are an equal number of males and females in a population, males will compete for mates.
    • If male fitness is limited by access to mates, any allele that increases a male’s attractiveness should increase in a population.
      • Sexual selection acts more strongly on males than on females.
    • Sometimes, males invest more in parental care for offspring, and there may be more females than males in a population.
      • Sexual selection varies in the context of different mating strategies (some females benefit from being promiscuous while others benefit from being monogamous).
      • Sexual selection can vary in different environmental contexts.
    • Fundamental asymmetry of sex is not valid across all species all the time.
  • Sexual dimorphism - “two forms”, can be explained by intersexual and intrasexual selection.
    • Traits can differ between the sexes of the same species.

Conclusion and Main Points

  • Multiple forms of sexual and/or ecological selection can occur at the same time in a population.
  • Alleles responsible for adaptive morphological, physiological, and/or behavioral phenotypes increase in frequency over time. This violates assumptions of the Hardy-Weinberg principle.
  • Selection can change over time and space because it occurs in the complex of ecology, which is highly complex and dynamic.

Chapter 23.4: Genetic Drift

  • Genetic drift - a change in allele frequencies in a population due to chance.
    • Allele frequencies drift up and down over time randomly with respect to fitness.
  • Genetic drift impacts small populations especially largely.
  • Cause of genetic drift is known as sampling error - when allele frequencies from a subset are different from those in the total population.

Key Points About Genetic Drift

  1. Genetic drift is random with respect to fitness - the changes in allele frequency it produces are not adaptive.
  2. Genetic drift is most pronounced in small populations.
  3. Genetic drift can lead to the random loss or fixation of alleles over time.

Experimental Studies of Genetic Drift

  • Warwick Kerr and Sewall Wright experimented to analyze how genetic drift works in practice.
  • Studied shape of body bristles on Drosophila melanogaster; changes in body bristle shape do not impact fitness.
    • In 73% of the experimental populations, genetic drift had reduced the diversity of alleles for one of the genes to zero.

What Causes Genetic Drift in Natural Populations?

  • This random sampling process occurs in every population in every generation in every species that reproduces sexually.
  • Genetic drift can occur by any process or event that involves sampling, not just the sampling of gametes that occurs during fertilization or loss of unlucky individuals.
  • Founder effect - when a group of individuals immigrates to a new geographic area and establishes a new population, causing a change in allele frequencies.
  • Genetic bottleneck - a sudden reduction in the diversity of alleles in a population, like a typhoon.

Take-Home Messages

  • Genetic drift is neutral concerning fitness; the loss of genetic variation tends to be the problem.
  • Loss of genetic diversity due to genetic drift reduces a population’s ability to adapt via natural selection.

Chapter 23.5: Gene Flow

  • When an individual leaves one population, joins another, and breeds, gene flow (movement of alleles between populations) is said to occur.
  • Gene flow has one outcome - equalizing allele frequencies between source population and recipient population.
    • Gene flow homogenizes allele frequencies across populations.

Measuring Gene Flow between Populations

  • The presence or absence of gene flow has particularly important implications for the conservation of threatened and endangered species.
  • Captive breeding of fish is increasing as wild populations are being depleted; when captive-bred fish escape, gene flow occurs.

Gene Flow is Random with Respect to Fitness

  • Gene flow doesn’t always reduce fitness in the receiving population.
  • If the population has lost alleles due to genetic drift, then new alleles brought via gene flow can increase genetic diversity.
    • This gene flow may then increase the average fitness of individuals.
  • Gene flow moves alleles among populations.

Chapter 23.6: Mutation

  • Recall:
    • Natural selection favors certain alleles and leads to a decrease in overall genetic variation.
    • Genetic drift tends to decrease genetic diversity over time, as alleles are randomly lost or fixed in a population.
    • Gene flow can increase or decrease genetic variation.
  • New alleles come from mutation.
  • Mutations can occur in many ways:
Point mutationA change in a base pair of DNA, which may result in a new amino acid sequence.
Chromosome-level mutationA change in the number or composition of chromosomes. For instance, gene duplication can increase the number of copies of a gene.
Lateral gene transferTransfer of genetic information from one species to another (rather than the parent to offspring).
  • Mutation introduces new alleles into populations in every generation.
  • Mutation is random with respect to the affected allele’s impact on the fitness of an individual.
  • Mutation in coding sequences can produce:
    • beneficial alleles on rare occasions - alleles that increase fitness.
    • neutral alleles somewhat often - with no effect on fitness.
    • deleterious alleles often - which lower fitness.

Mutation as an Evolutionary Process

  • Mutation is not a significant mechanism of evolutionary change by itself - its rate is too small.
  • By itself, mutation is a slow evolutionary process.
  • However, it can have a large effect when combined with genetic drift, gene flow, and selection.

Experimental Studies of Mutation

  • Richard Lenski studied E. coli and bred them for over three decades - the only source of variation is mutation.
  • Researchers observed:
    • Silent mutations accumulated at a constant rate.
    • Beneficial mutations occurred at a higher initial rate, then a slower rate. (bacteria living in an unchanging lab environment.)
    • Deleterious mutations did not accumulate in populations.
  • Random patterns of cumulative mutations exist.

Study of Mutation in Natural Populations

  • Pea aphids feed on plant sap and can be red or green.
  • Aphids produce enzymes that allow them to synthesize their own carotenoid pigments to produce their color.
    • DNA had moved from the genome of a fungus to the genome of a recent ancestor of the pea aphids.
    • Evidence of lateral gene transfer.
  • Cartenoids are part of a family of molecules with similar pigments.
    • After fungal genes for carotenoid enzymes were transferred to aphids, they underwent further mutations.
    • A deletion produced green pea aphids, which can only synthesize yellow and green carotenoid pigments, whereas red pea aphids could synthesize yellow+green+red carotenoid pigments.

Take-Home Messages

  1. Mutation is the ultimate source of genetic variation. Mutations create new alleles.
  2. Mutations are random with respect to fitness. Mutations just happen - there is no notion of mutations being created out of some purpose.
  3. If mutation did not occur, evolution would eventually stop. There would be no variation to act on.
  4. Mutation alone is usually inconsequential in changing allele frequencies at a particular gene. WHen combined with natural selection, genetic drift, and gene flow, mutation becomes an important evolutionary process.

Introduction to Evolution Textbook Notes: Chapter 1.4-1.5, BioSkills 13 (Phylogenetic Trees)

BioSkills 13: Reading and Making Phylogenetic Trees

  • Phylogenetic trees show hypothesized evolutionary relationships between species and other taxa.
  • Taxon - group of organisms, like a population, species, or larger group.
rootThe most ancestral population, where the tree originates.
branchesRepresents population through time.
nodes/forksOccur when hypothetical ancestral groups split into multiple groups. Each node represents the most recent common ancestor of descendant populations that emerge from it.
tips/terminal nodesTree’s endpoints, representing a taxon of organisms living today or in the past.
monophyletic group / lineage / cladeConsists of an ancestral species and all of its descendants.
traitAn ancestral trait is a characteristic existed in an ancestor, a shared derived trait (synapomorphy) can be a characteristic that is gained or lost.
outgroupA taxon that is known ot have diverged from the rest of the taxa. Outgroups establish if a trait is ancestral or derived.

Chapter 1.4: Life Evolves

What is Evolution?

  • Darwin and Wallace’s theory made two claims:
    • Species are related by common ancestry.
    • Characteristics of species can be modified from generation to generation.
  • Evolution - change in characteristics of a population over time.
  • Population - defined as a group of individuals of the same species living in the same area t the smae time.

What is Natural Selection?

  • Two conditions of natural selection:
    1. Individuals within a population vary in heritable characteristics.
    2. Certain versions of these traits affect the fitness of an individual.
  • Natural selection acts on individuals, evolutionary change occurs in populations.
  • Speciation - natural selection causes populations of one species to diverge and form a new species.
  • Fitness - an individual’s ability to produce viable offspring.
  • Adaptation - a heritable trait that increases the fitness of an individual relative to individuals lacking the trait.

Chapter 1.5: The “Tree of Life” Depicts Evolutionary History

  • Biologists should be able to construct a tree of life.

Using Genetic Sequences to Understand the Tree of Life

  • Carl Woese attempted to undersand the phylogeny - genealogical relationships - of all organisms.
    • phylogeny translates to tribe-source.
  • Genetic variation can be analyzed via mutations in DNA and RNA; a phylogenetic tree can be produced.
  • LUCA - last universal common ancestor of cells.
  • Tree of Life implies that there are three fundamental groups of organisms: bacteria, archaea, and eukarya.
  • Eukaryotes - cells have a nucleus.
    • Eukaryotes are usually multicellular.
  • Prokaryotes - do not have a nucleus (bacterial and archaeal cells).
    • Bacteria and archaea are usually unicellular.

How Should We Name Branches on the Tree of Life?

  • Taxonomy - the effort to name and classify organisms.
  • Taxon (plural: taxa) - any named group at any level of classification.
  • Domain - taxonomic category encompassing the bacteria, archaea, and eukarya.
  • Phylum (plural: phyla) - major lineages within each domain.
    • Within eukaryotic lineage, there are about 30-35 phyla.
  • Scientific latin names consist of two parts:
    • Genus (plural: genera) - a closely related group of species.
    • Species name - identifies the organism’s species.
    • Each scientific name is unique.