Origins of Life on Earth

Origins of Life

• Earth formed about 4.6 billion years ago (bya).
• Early Earth was uninhabitable for life until 3.9 bya.
• The earliest fossil evidence dates back 3.5 bya.
• Cyanobacteria represents some of the oldest fossil evidence.

How Life Arose

• Early Earth consisted of inorganic molecules.
• These molecules could have formed organic molecules with free energy and abundant oxygen.
• Organic molecules may have been transported to Earth by meteorites or celestial events.

Experimental Data

• Alexander Oparin and J.B.S. Haldane hypothesized early Earth was composed of hydrogen, methane, ammonia, and water.
• Stanley Miller and Harold Urey tested this hypothesis in their lab.
• Miller and Urey found that organic compounds and amino acids can form under those conditions.
• The organic molecules formed may have acted as building blocks for macromolecules.

RNA World Hypothesis

• The RNA World Hypothesis proposes that RNA could have been the earliest genetic material.
• RNA may help to explain a pre-cellular stage of life.

Speciation

• Species: A group that can interbreed and produce viable, fertile offspring.
• Speciation results in the creation of new species and diversity in life forms.
• Geography impacts speciation.
• Allopatric and sympatric are two modes of speciation.

Allopatric Speciation

• A physical barrier divides a population, or a small part of it separates from its main population.
• Populations become geographically isolated.
• A physical barrier prevents gene flow among geographically isolated populations.
• Natural disasters often cause physical barriers that lead to allopatric speciation.

Sympatric Speciation

• A new species evolves while inhabiting the same geographic region as the ancestral species.
• The exploitation of a new niche often causes sympatric speciation.

Reproductive Isolation

• Speciation happens due to reproductive isolation.
• Prezygotic and postzygotic barriers are two types of reproductive isolation.
• Both prezygotic and postzygotic barriers maintain isolation and prevent gene flow between populations.

Prezygotic Barriers

• Prezygotic barriers prevent mating or hinder fertilization.
• There are five types of prezygotic barriers: habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, and gametic isolation.

Habitat Isolation

• Species lives in different areas or occupy different habitats within the same area.
• As an example, the mountain bluebird lives at high elevation and the eastern bluebird lives at low elevation in western North America

Temporal Isolation

• Species breed at different times of the day, year, or season.
• As an example, the western spotted skunk mates in late summer, while the eastern spotted skunk mates in late winter.

Behavioral Isolation

• Unique behavioral patterns and rituals separate species.
• For example, the blue-footed boobies will only mate after a courtship ritual.

Mechanical Isolation

• The reproductive anatomy of one species does not fit with the anatomy of another species.
• As an example, snails can have varying spirals on their shells that prevent mating.

Gametic Isolation

• Proteins on the surface of gametes do not allow for egg and sperm to fuse.
• The sperm and eggs of red and purple sea urchins are released in the water but cannot fertilize each other.

Postzygotic Barriers

• Postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult.
• Reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown are the three types of postzygotic barriers.

Reduced Hybrid Viability

• The genes of different parent species may interact in ways that impair the hybrid's development or survival.
• As an example, domestic sheep can fertilize domestic goats, but the hybrid embryo dies early on

Reduced Hybrid Fertility

• A hybrid develops into a healthy adult but is sterile.
• Reduced hybrid fertility usually results from differences in the number of chromosomes between parents.
• If a male donkey and a female horse mate, they produce a mule that is sterile.

Hybrid Breakdown

• The hybrid of the first generation may be fertile. However, when they mate with a parent species or one another, their offspring will be sterile.
• Farmers have tried crossing different types of cotton plants, but after the first generation, the plants do not produce viable seeds.

Micro and Macroevolution

• Speciation is a connection between the concepts of microevolution and macroevolution.
• Microevolution: change in allele frequencies within a single species or population.
• Natural and sexual selection, genetic drift and gene flow drive microevolution.
• Macroevolution: large evolutionary patterns involving adaptive radiation and mass extinction.
• Stasis: Lack of change over long periods of time.

Pace of Speciation

• Evolution and speciation can occur at different speeds.
• Punctuated equilibrium: Evolution occurs rapidly after a long period of stasis.
• Gradualism: Evolution occurs slowly over hundreds, thousands, or millions of years.

Divergent Evolution

• Divergent evolution: Groups with the same common ancestor evolve and accumulate differences forming a new species.
• Adaptive radiation: If a new habitat or niche becomes available, species can diversify rapidly.

Convergent Evolution

• Convergent evolution: Two different species develop similar traits despite different ancestors.
• Analogous traits are examples of convergent evolution.

Extinction

• Extinction: the termination of a species.
• Extinctions have occurred throughout the Earth's history, including 5 mass extinctions.
• Human activities have affected extinction rates.
• Anytime there is ecological stress, extinction rates can quicken.
• When a species goes extinct, it opens up a niche that another species can exploit.

Phylogeny

• Systematics: Classification of organisms and determining their evolutionary relationships.
• Taxonomy: Naming and classifying species.
• Phylogenetics: Hypothesis of evolutionary history.
• Phylogenetic trees show evolution.

Taxonomy

• Every organism has its own Taxonomic classification.
• Organisms typically have seven levels of classification.
• The first is kingdom, followed by Phylum, order, class, family, genus and finally Species.
• The last two levels, genus and species, are referred to in Latin.

Evolutionary Relationships

• Scientists use fossil records, DNA, proteins, and homologous structures to determine evolutionary relationships.

Phylogenetic Trees

• Phylogenetic trees are diagrams displaying the evolutionary history of a group of organisms.
• They are similar to cladograms, but trees show the amount of change over time measured by fossils.

Cladograms

• Each line represents a lineage.
• Each branching point is a node which represent common ancestors.
• Nodes and all branches from it are called clades.
• Species in a clade have shared derived features.
• The root is the common ancestor of all species.
• Two clades emerging from the same node are sister taxa.
• A lineage evolving from the root and remaining unbranched is the basal taxon.
• Synapomorphy: a derived character is shared by clade members.
• Similarity inherited from the most recent common ancestor of an entire group represents derived characteristics.
• Similarity that arose prior to the common ancestor represents a ancestral characteristic.
• An outgroup is a lineage that is the least closely related to the rest of the organisms.

Types of Groups

• Monophyletic group: The most recent common ancestor of the group and all of its descendants, forming a clade.
• Paraphyletic group: Includes the most recent common ancestor of the group, but not all its descendants.
• Polyphyletic group: Does not include the most recent common ancestor of all members of the group.

Parsimony

• The principle of parsimony states that if there are conflicts among characters, use the hypothesis that requires the fewest assumptions specifically regarding DNA changes.

Evolution

• Evolution: occurs when the genetic makeup of a population changes over time, also known as descent with modification.
• Darwin proposed the idea of natural selection to explain the pattern of descent with modification/evolution.

Natural Selection

• Natural selection occurs through a process where individuals with certain traits tend to survive and reproduce at higher rates than other individuals because traits.
• Natural selection acts on phenotypic variation in populations.
• Some phenotypes will increase or decrease an organism’s fitness, which is the ability to survive and reproduce.
• Fitness can be measured by reproductive success.
• Environments can change, causing selective pressures on populations.

Traits Are Heritable

• Traits are heritable i.e. they can be passed from parent to offspring.
• Adaptations: traits that enhance survival and reproduction.

Differential Survival

• Differential Survival: The traits that lead to survival traits will accumulate in the population over time.
• More offspring are produced than can survive, this leads to competition for limited resources:
• Populations evolve, NOT individuals.

Artificial Selection

• Artificial selection refers to the selective breeding of domesticated plants that encourages desirable traits.

Natural Selection vs Artificial Selection

• Natural Selection involves NATURE that selects traits that are better suited for survival and reproduction. Artificial Selection includes HUMANS that selects traits that are desirable and involves Domestication of plants and animals.

Population Genetics

• Population: A group of individuals of the same species living in the same area that interbreed to produce fertile offspring.
• Gene pool: A population’s genetic makeup. Consists of all copies of every type of allele.

Alleles

• If only one allele is present for a particular locus in the population it is fixed. Many fixed alleles result in less genetic diversity.
• Mutations can result in genetic variation. Some can form new alleles. Further more natural selection can act on varied phenotypes.
• Mutation rates tend to be high for plants and slow for animals. Further more more rates tend to be fast in prokaryotes than compared to animals and plants.
• Mutations can be harmful, neutral, or beneficial

Genetic Drift

• Genetic drift encompasses chance events that cause a change in allele frequency from one generation to the next, and is most significant to small populations.
• Genetic drift can lead to a loss of genetic variation, cause harmful alleles to become fixed and DOES NOT produce adaptations
• Bottleneck effect: occurs when a large population is drastically reduced by a non-selective disaster.
• Founder effect: occurs when a few individuals become isolated from a large population to establish a new small population.

Population Genetics and random occurrences

• A population’s allele frequencies will change over time.
• Microevolution takes place small scale genetic changes happen in a population.
• Evolution is driven by random events like Mutations, Genetic Drift, Migration/gene, and flow Natural selection

Gene Flow

• Gene Flow occurs through The transfer of alleles into or out of a population due to fertile individuals or gametes.
• Further more Alleles are transferred between populations such as pollen being blown to a new location

Natural Selection Fitness

• Natural selection involves reproductive success that is then measured by relative fitness. Such as number of surviving offspring that include an individual produces compared to the number left by others in the population.

Effects of Natural Selection

• Effects of natural selection are directly measure by examining the changes in the mean of phenotypes. Directional selection, Stabilizing selection, and Disruptive selection are often measures by their effects of natural selection.
• Sexual selection is another type of natural selection often shown among unique/showy traits among many species.

Hardy Weinberg Equilibrium

• Hardy Weinberg Equilibrium is model used to assess whether natural selection or other factors are causing evolution at a particular locus. It Determines what the genetic makeup of the population would be if it were NOT evolving

Formulas for Hardy Weinberg Equilibrium

• p + q = 1
• p2 + 2pq + q2 = 1
• Percentage ofThe homozygous dominant is shown through
• p*2
• Percentage of the heterozygous is shown through
• 2pq
• Percentage of the homozygous recessive is shown through
• q2
• The frequencies of alleles and genotypes in a pop will remain Constant from generation to generation is only Mendelian segregation and recombination of alleles are at work.