Domains of Life: Bacteria, Archaea, and Eukarya

Domain Classifications

• Domains classify organisms into Bacteria, Archaea, and Eukarya based on specific characteristics.
• They can also classify websites into categories like .com, .org, and .edu.

The Three-Domain System

• Proposed in 1990, it consists of Bacteria, Archaea, and Eukarya.
• The Bacteria domain was formerly the Eubacteria Kingdom.
• The Archaea domain was formerly the Archaebacteria Kingdom.
• The Eukarya domain includes the plant, animal, protist, and fungi kingdoms.
• This system highlights similarities among eukaryotes and differences between eukaryotes, bacteria, and archaea.
• Domains accurately show relationships without replacing the six-Kingdom system.

Characteristics of the Domains

• Archaea: simple prokaryotes, thrive in harsh environments like hot temperatures and high pressure, making up 20% of Earth’s biomass.
• Bacteria: simple prokaryotes, inhabit diverse environments, including inside human bodies, composing about 13% of Earth’s biomass.
• Eukarya: organisms with eukaryotic cells, including the Protista, Fungi, Plantae and Animalia kingdoms.

Characteristics of Kingdoms

• Archaebacteria: unicellular prokaryotes in extreme environments, heterotrophic or autotrophic, asexual reproduction.
• Eubacteria: unicellular prokaryotes in various environments, heterotrophic or autotrophic, asexual reproduction.
• Protista: unicellular or multicellular eukaryotes, heterotrophic or autotrophic, sexual or asexual reproduction.
• Fungi: mostly multicellular (a few unicellular), heterotrophic decomposers with sexual (spores) and asexual reproduction.
• Plantae: multicellular eukaryotes, autotrophic, sexual and asexual reproduction.
• Animalia: multicellular eukaryotes, heterotrophic, mostly sexual reproduction (a few asexual).

Advancements in Classification

• Early classifications relied on physical characteristics.
• Modern classification uses advanced technology and DNA sequencing.
• DNA is now the primary basis for classifying organisms.
• DNA helps map evolutionary relationships and show common ancestry.
• Organisms sharing a recent common ancestor are more closely related.

Reclassification Example: Flightless Birds

• Genetic evidence shows ratites (ostriches, emus, kiwis, etc.) evolved from flying ancestors.
• Ratites lost flight multiple times, indicating it was beneficial for survival.
• Ratites once lived in Australia, South America, and Africa which was once the supercontinent Gondwanaland.
• Molecular evidence revealed tinamous, capable of flight now, are closely related to ratites.
• Land-bound life led to similar skeletal evolution in ratites, masking true relationships.
• Losing flight is an easier trait to lose than regain, benefitting land-adapted birds .

Taxonomy and Cladistics

• Taxonomy classifies and names species
• Cladistics focuses on evolutionary aspects of classification.
• Modern taxonomic groups are based on Carl Linnaeus' work in the 1700s.
• Binomial nomenclature gives each organism a two-word scientific name (Genus species).
• Linnaeus originally divided living things into plants and animals.
• Domains were added in the 1990s.
• There are 3 Domains and 6 Kingdoms.
• Domains contain the most organisms, species the least.
• Organisms become more similar in appearance towards the species level.

Cladistics

• Cladistics studies clades (ancestral species and descendants).
• Clades illustrate evolutionary relationships.
• Darwin's theory of evolution involves new heritable traits passed to offspring.
• New traits are called derived characteristics.
• Organisms sharing derived characteristics are more closely related.
• Cladograms show relationships through derived characteristics.

Genetic Variation

• Measures genetic differences within a population.
• Alleles are variations of a gene (e.g., eye color).
• Genetic diversity refers to the genetic variation of an entire species.
• Essential for natural selection.
• Natural selection increases or decreases allele frequency.

Causes of Genetic Variation

• Mutation: random, heritable changes in DNA.
• Random mating.
• Random fertilization.
• Crossing over of homologous chromosomes during meiosis.
• The last three factors lead to genetic recombination.
• Mutations and gene recombination create new DNA sequences and combinations.

Mutation Effects

• Mutations introduce genotypic and phenotypic variation.
• Some mutations are harmful and may be eliminated by natural selection.
• Beneficial mutations are passed on to future generations.
• A mutation is considered beneficial if it helps an organism survive and reproduce.
• Effects of mutations range from no effect to dramatic phenotypic changes.

Genetic Variation and Environment

• Enables some individuals to survive in changing environments.
• Those best suited pass traits to future generations.
• Low genetic diversity means organisms are susceptible to disease and lethal mutations (e.g., cheetahs).

Gene Flow

• Gene flow exchanges genes between populations.
• Occurs when organisms move into a population, bringing new genes.
• It increases the genetic variation of the receiving population.
• Lack of gene flow can cause populations to evolve differently.
• Pollen dispersal in plants and male lions forming new prides are examples of gene flow.
• Maintained gene flow combines gene pools, reducing genetic variation.

Genetic Drift

• Genetic drift randomly changes the frequency of an allele in a population.
• Results in changes in genetic diversity due to random chance.
• Differs from natural selection, which is based on traits enabling survival.
• Genetic drift is determined by random chance, not selection pressure.
• Can lead to elimination of an allele from a population by chance.

Bottleneck Effect

• Magnified by natural events that randomly kill a large portion of a population.
• Reduces variation in the gene pool of a population.
• Survivors' genes become the genes of the entire population.

Founder Effect

• Occurs when a portion of a population leaves to start a new one.
• Or when a population is divided by a physical barrier.
• The gene pool changes to reflect the founding population's genes.

Negative Effects of Genetic Drift

• Loss of genetic variation reduces the ability to adapt to changing environments.
• Lethal alleles in heterozygous individuals may become more common.

Natural Selection

• Organisms better adapted to their environment survive and produce more offspring.
• Conditions include overproduction of offspring, inherited variation, and struggle for survival.
• Survivors with beneficial adaptations produce offspring that inherit the adaptation.

Origins of Life

• Life began when molecules of nonliving matter reacted chemically in early Earth history.
• The Miller-Urey experiment showed organic molecules could be created from inorganic substances.
• Rocky structures on the ocean floor may have held basic organic molecules.
• Early single-celled organisms were prokaryotes that did not need oxygen.
• Some evolved the ability to photosynthesize, releasing oxygen.
• Eukaryote evolution involved endosymbiosis (one organism living within another).
• Early mitochondria and chloroplasts may have been small prokaryotic cells taken up by larger prokaryotes.

Evolution

• Theory of evolution: all species evolved and diversified from a common ancestor.
• Provides direction for predictions about living things.
• Supported by the fossil record, comparative anatomy, embryology, biogeography, molecular biology, and observed evolutionary change.
• Natural selection explains how species change over time.
• Charles Darwin described the mechanism for evolution.
• Observations on the Galápagos Islands showed similar but distinct species.
• Darwin proposed island species were modified from a mainland species.
• Varied beaks of finches helped them acquire specific types of food.

Natural Selection Details

• Individuals with favorable traits survive environmental changes, reproduce, and pass on traits.
• Leads to evolutionary change with the favorable trait becoming predominant.
• Darwin observed long-necked tortoises in the Galapagos that could reach more leaves.
• Natural selection is an inevitable outcome of inheritance, overproduction of offspring, and competition for resources.
• Characteristics are inherited and better represented in the next generation.
• Natural selection leads to greater adaptation to the local environment.

Evidence of Evolution: The Fossil Record

• Fossils are preserved remains or traces of past organisms.
• Fossils are found in sedimentary rock layers.
• Certain fossils are associated with certain rock layers, creating a geological timescale.
• The fossil record includes all known fossils and their placement in rock layers.
• Provides solid evidence that organisms from the past are not the same as those today.

Comparative Anatomy

• Scientists compare anatomy between species.
• Homologous structures are similar in structure and origin but may differ in function.
• Examples include bones in forearms of vertebrates.
• Comparative anatomy compares body parts to understand adaptive changes.
• Fossils, along with comparative anatomy, create an anatomical record.
• By comparing anatomies, scientists can infer the lineage of species.

Comparative Embryology

• Vertebrate embryos in early development closely resemble each other.
• As development proceeds, characteristic traits become more apparent.
• Vertebrate embryos (including humans) show a tail at some point in their early development.
• Comparative embryology compares embryos of different species.

Biogeography

• Studies where species are geographically distributed.
• Distribution patterns are explained by evolution and land mass movement.
• Groups evolved before the breakup of Pangaea are distributed worldwide.
• Groups evolved after the break appear uniquely in regions of the planet.
• Australia has great marsupial diversity and endemic species due to long isolation.

Molecular Biology

• Compares DNA and amino acid sequences between organisms.
• Greater similarity indicates closer relatedness.
• Relatedness is reflected in the universal nature of DNA, similar DNA sequences, genetic codes, replication and expression of DNA.

Explore the three domains of life: Bacteria, Archaea, and Eukarya. Understand their characteristics, habitats, and importance in Earth's ecosystems. Learn how this classification system highlights evolutionary relationships.