Evolution and Natural Selection Vocabulary
Natural Selection
Natural selection is a process where individuals with certain traits survive and reproduce at higher rates due to those traits.
Acts on phenotypic variations in populations; some phenotypes increase or decrease an organism's fitness (ability to survive and reproduce).
Fitness is measured by reproductive success.
Environments change, causing selective pressures on populations.
Theory of Natural Selection
Based on two main observations:
Traits are heritable: Characteristics can be passed from parent to offspring.
Adaptations: inherited characteristics that enhance survival and reproduction.
More offspring are produced than can survive.
Leads to competition for limited resources, resulting in differential survival.
Traits that lead to survival ("favorable" traits) will accumulate in the population.
Populations evolve, not individuals.
Artificial Selection
Natural selection compared to artificial selection.
Artificial selection: selective breeding of domesticated plants and animals to encourage desirable traits.
Natural vs. Artificial Selection
Natural Selection:
Nature "selects" traits that are better suited for survival and reproduction.
Artificial Selection:
Humans select traits that are desirable.
Domestication of plants and animals.
Both processes lead to evolutionary change, but natural selection occurs in nature without human influence.
Population Genetics and Hardy-Weinberg Equilibrium
Population Genetics
Population: a group of individuals of the same species that live in the same area and interbreed to produce fertile offspring.
Gene pool: a population's genetic makeup, consisting of all copies of every type of allele.
Fixed allele: If there is only one allele present for a particular locus in the population.
Many fixed alleles = less genetic diversity.
Population's Allele Frequencies
A population's allele frequencies will change over time.
Remember: populations evolve, not individuals.
Microevolution: small-scale genetic changes in a population.
Evolution is driven by random occurrences:
Mutations.
Genetic Drift.
Migration/gene flow.
Natural selection.
Mutations
Mutations can result in genetic variation.
Can form new alleles.
Natural selection can act on varied phenotypes.
Mutation rates tend to be slow in plants and animals and fast in prokaryotes (faster generation time).
Mutations can be harmful, neutral, or beneficial (most are neutral to harmful).
Not all mutations lead to evolution.
Genetic Drift
Genetic drift: chance events that cause a change in allele frequency from one generation to the next.
Most significant in small populations.
Can lead to a loss of genetic variation.
Can cause harmful alleles to become fixed.
Does NOT produce adaptations.
Two types:
Bottleneck effect.
Founder effect.
Bottleneck Effect
Bottleneck effect: when a large population is drastically reduced by a non-selective disaster (floods, famine, fires, hurricanes, hunting, etc.).
Some alleles may become overrepresented, underrepresented, or absent.
Founder Effect
Founder effect: when a few individuals become isolated from a large population and establish a new small population with a gene pool that differs from the large population.
Leads to loss of genetic diversity.
Gene Flow
Gene flow: the transfer of alleles into or out of a population due to fertile individuals or gametes.
Alleles can be transferred between populations (e.g., pollen being blown to a new location).
Natural Selection
Reproductive success is measured by relative fitness.
The number of surviving offspring that an individual produces compared to the number left by others in the population.
Effects of natural selection can be measured by examining the changes in the mean of phenotypes.
Three modes of natural selection:
Directional selection.
Stabilizing selection.
Disruptive selection.
Modes of Natural Selection
Directional selection: Selection towards one extreme phenotype.
Stabilizing selection: Selection towards the mean and against the extreme phenotypes.
Disruptive selection: Selection against the mean; both phenotypic extremes have the highest relative fitness.
Sexual Selection
Sexual selection: explains why many species have unique/showy traits.
Males often have useless structures (e.g., colorful male peacock feathers) simply because females choose that trait.
Can produce traits that are harmful to survival.
Example: colorful feathers in male peacocks make them easier to spot by predators.
Hardy-Weinberg Equilibrium
A model used to assess whether natural selection or other factors are causing evolution at a particular locus.
Determines what the genetic makeup of the population would be if it were NOT evolving.
This is then compared to actual data.
If there are NO differences, then the population is NOT evolving.
If there ARE differences, then the population MAY BE evolving.
Hardy-Weinberg Principle
The frequencies of alleles and genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work.
This is a hypothetical situation where no evolution would take place. In real populations, the allele and genotype frequencies DO change over time.
Conditions for Hardy-Weinberg Equilibrium
Five conditions must be met to be in Hardy-Weinberg equilibrium:
No mutations
Random mating
No natural selection
Extremely large population size
No gene flow
If any of these conditions are not met, then microevolution occurs (i.e., mutation, gene flow, genetic drift, natural selection, and non-random mating).
Hardy-Weinberg Formulas
Two formulas:
p + q = 1
p = Frequency of the dominant allele in a population
q = Frequency of the recessive allele in a population
p^2 + 2pq + q^2 = 1
p^2 = Percentage of the homozygous dominant individuals
q^2 = Percentage of the homozygous recessive individuals
2pq = Percentage of the heterozygous individuals
Quick Check
If 20% of a population is homozygous recessive, that refers to q^2.
If the frequency of a dominant allele is 75%, that refers to p.
Solving Hardy-Weinberg Problems
Which formula you start with depends on the information you are given.
"Allele frequencies" refers to p and q.
Information about individual organisms or populations refers to p^2, 2pq, and q^2.
Most times you will use both formulas to complete the problem.
Usually you are given q and will need to find p, but that is not always the case.
Tips for Solving Problems
Always write down both equations.
Identify the information given (alleles or populations).
Solve for both p and q first, regardless of what the problem asks.
Use a calculator for square and square root functions.
Double-check your work.
Variations in Populations
The more genetic diversity in a population, the better it can respond to changes in the environment.
More likely to be individuals that can withstand changes.
Species with low genetic diversity are at risk of decline and/or extinction.
Example: The California Condor was reduced to 27 individuals due to poaching and poisoning, drastically lowering the gene pool. Restoration efforts have increased numbers, but diversity was lost.
Evidence of Evolution and Common Ancestry
Evidence of Evolution
Overwhelming evidence supports the theory of evolution.
Primary sources of evidence:
The fossil record.
Comparative morphology.
Biogeography.
Fossil Record
Fossils: remains or traces of past organisms.
Fossil record: gives a visual of evolutionary change over time.
Fossils can be dated by examining the rate of carbon-14 decay and the age of rocks where the fossils are found.
Gives geographical data for the organisms found.
Comparative Morphology
Comparative morphology: analysis of the structures of living and extinct organisms.
Homology: characteristics in related species that have similarities even if the functions differ.
Embryonic homology: many species have similar embryonic development.
Vestigial structures: structures that are conserved even though they no longer have a use (e.g., tailbone and appendix in humans).
Molecular homology: many species share similar DNA and amino acid sequences.
Homologous structures: characteristics that are similar in two species because they share a common ancestor (e.g., arm bones of many species).
Convergent Evolution
Convergent evolution: similar adaptations that have evolved in distantly related organisms due to similar environments.
Analogous structures: structures that are similar but have separate evolutionary origins (e.g., wings in birds vs. bats vs. bees).
Each species has wings, but the wings did not originate from a common ancestor.
Structural Evidence
Structural evidence indicates common ancestry of all eukaryotes.
Many fundamental cellular features and processes are conserved across organisms.
Cellular examples:
Membrane-bound organelles.
Linear chromosomes.
Introns in genes.
Biogeography
Biogeography: the distribution of animals and plants geographically.
Example: Species on oceanic islands resemble mainland species.
Example: Species on the same continent are similar and distinct from species on other continents.
Populations continue to evolve
Genomes chnage
Examples:
Antibiotic resistance in bacteria
*Insect resistance to pesticides
*Pathogens cause emerging (new) diseases
Phylogeny
Systematics
Systematics: classification of organisms and determining their evolutionary relationships.
Taxonomy: naming and classifying species.
Phylogenetics: hypothesis of evolutionary history.
Use phylogenetic trees to show evolution.
Determining Evolutionary Relationships
To determine evolutionary relationships, scientists use:
Fossil records
DNA
Proteins
Homologous structures
Phylogenetic Trees
Phylogenetic trees: diagrams that represent the evolutionary history of a group of organisms.
Similar to cladograms, except trees show the amount of change over time measured by fossils.
Cladograms
Each line represents a lineage
Each branching point is a node
Nodes 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 the species
Two clades that emerge from the same node are sister taxa.
A lineage that evolved from the root and remains unbranched is the basal taxon.
Synapomorphy
Synapomorphy: a derived character shared by clade members.
Derived characteristic: similarity inherited from the most recent common ancestor of an entire group.
Ancestral characteristic: similarity that arose prior to the common ancestor.
Outgroup
Many cladograms and trees include an outgroup.
A lineage that is the least closely related to the rest of the organisms.
Groups
Monophyletic group: includes the most recent common ancestor of the group and all of its descendants (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
If there are conflicts among characters, use the principle of parsimony.
Use the hypothesis that requires the fewest assumptions (DNA changes).
Speciation and Extinction
Speciation
Species: a group able to interbreed and produce viable, fertile offspring.
Speciation: formation of new species, resulting in diversity of life forms.
Modes of Speciation
Geography impacts speciation.
Two modes of speciation:
Allopatric speciation.
Sympatric speciation.
Allopatric Speciation
Allopatric speciation: A physical barrier divides a population, or a small population is separated from the main population.
Populations are geographically isolated, which prevents gene flow.
Often caused by natural disasters.
Sympatric Speciation
Sympatric speciation: A new species evolves while still inhabiting the same geographic region as the ancestral species.
Usually due to the exploitation of a new niche.
Speciation occurs due to reproductive isolation.
Two types:
Prezygotic barriers
Postzygotic barriers
Both types maintain isolation and prevent gene flow between populations.
Prezygotic Barriers
Prezygotic barriers: prevent mating or hinder fertilization.
Five types:
Habitat isolation
Temporal isolation
Behavioral isolation
Mechanical isolation
Gametic isolation
Habitat Isolation
Habitat isolation: Species live in different areas, or they occupy different habitats within the same area.
Example: The mountain bluebird lives at high elevation, while the eastern bluebird lives at low elevation.
Temporal Isolation
Temporal isolation: Species breed at different times of the day, year, or season.
Example: The western spotted skunk mates in late summer, while the eastern spotted skunk mates in late winter.
Behavioral Isolation
Behavioral isolation: Unique behavioral patterns and rituals separate species.
Example: The blue-footed boobies will only mate after a courtship ritual.
Mechanical Isolation
Mechanical isolation: The reproductive anatomy of one species does not fit with the anatomy of another species.
Example: Snails can have varying spirals on shells, which prevent mating.
Gametic Isolation
Gametic isolation: Proteins on the surface of gametes do not allow for the egg and sperm to fuse.
Example: The sperm and eggs of red and purple sea urchins are released in the water, but they cannot fertilize each other.
Postzygotic Barriers
Postzygotic barriers: prevent a hybrid zygote from developing into a viable, fertile adult.
Three types:
Reduced hybrid viability
Reduced hybrid fertility
Hybrid breakdown
Reduced Hybrid Viability
Reduced hybrid viability: The genes of different parent species may interact in ways that impair the hybrid's development or survival.
Example: Domestic sheep can fertilize domestic goats, but the hybrid embryo dies early on.
Reduced Hybrid Fertility
Reduced hybrid fertility: A hybrid can develop into a healthy adult, but it is sterile.
Usually results due to differences in the number of chromosomes between parents.
Example: A male donkey and a female horse can mate to produce a mule, but mules are sterile.
Hybrid Breakdown
Hybrid breakdown: The hybrid of the first generation may be fertile, but when they mate with a parent species or one another, their offspring will be sterile.
Example: 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 bridge between the concepts of microevolution and macroevolution.
Microevolution: Change in allele frequencies within a single species or population (natural and sexual selection, genetic drift, gene flow).
Macroevolution: Large evolutionary patterns (adaptive radiation, mass extinction).
Stasis: no change over long periods of time
Pace of Speciation
Evolution and speciation can occur at different speeds.
Punctuated equilibrium: when evolution occurs rapidly after a long period of stasis.
Gradualism: when evolution occurs slowly over hundreds, thousands, or millions of years.
Evolution and Speciation
Divergent evolution: groups with the same common ancestor evolve and accumulate differences resulting in the formation of a new species.
Adaptive radiation: if a new habitat or niche becomes available, species can diversify rapidly
Convergent evolution: two different species develop similar traits despite having different ancestors.
Extinction
Extinction: the termination of a species.
Extinctions have occurred throughout Earth's history (5 mass extinctions).
Human activity has affected extinction rates.
Anytime there is ecological stress, extinction rates can quicken.
If a species goes extinct, it opens up a niche that can be exploited by a different species.
Origins of Life on Earth
Origins of Life
Earth formed approximately 4.6 billion years ago (bya).
Early Earth was not suitable for life until 3.9 bya.
Earliest fossil evidence is 3.5 bya (Cyanobacteria).
How Did Life Arise?
Early Earth contained inorganic molecules.
These could have synthesized organic molecules due to free energy and abundant oxygen.
Organic molecules could have also been transported to Earth via meteorites or other celestial events.
Experimental Data
Oparin and Haldane hypothesized that early Earth was primarily composed of hydrogen, methane, ammonia, and water.
Stanley Miller and Harold Urey tested the hypothesis in their lab.
Stanley Miller and Harold Urey Experiment
They found organic compounds and amino acids formed.
Miller and Urey hypothesized that the organic molecules that formed served as the building blocks for macromolecules.
RNA World Hypothesis
RNA World Hypothesis: proposes that RNA could have been the earliest genetic material.
Helps to explain the pre-cellular stage of life.