Unit 7 · Natural Selection

Natural selection ● (core concept) — A major mechanism of evolution. According to Darwin's theory, competition for limited resources results in differential survival: individuals with more favorable phenotypes are more likely to survive and produce more offspring, passing those favorable traits to subsequent generations.

Differential survival ● (core concept) — The outcome of competition for limited resources: individuals with more favorable phenotypes are more likely to survive and reproduce.

Evolutionary fitness ● (core concept) — Measured by reproductive success.

Biotic and abiotic environments ● (core concept) — Biotic and abiotic environments can fluctuate, affecting the rate and direction of evolution; different genetic variations can be selected in each generation.

Phenotypic variation ● (core concept) — Differences in observable traits within a population; natural selection acts on phenotypic variations in populations.

Selective pressure ● (core concept) — Environmental factors that favor certain phenotypes. Environments change and apply selective pressures to populations; some phenotypic variations increase or decrease an organism's fitness in particular environments.

Molecular variation and fitness ● (core concept) — Variation in the number and types of molecules within cells can provide populations a greater ability to survive and reproduce in different environments.

Selection examples ○ — Illustrative (suggested, not required) CED examples of phenotypic variation under selection: flowering time in relation to global climate change, sickle cell anemia, DDT resistance in insects.

Artificial selection ● (core concept) — The process by which humans affect variation in other species by selecting which individuals reproduce.

Genetic drift ● (core concept) — A change in allele frequencies attributable to a nonselective process occurring in small populations. It can allow a small population to diverge from other populations of the same species.

Bottleneck effect ● (core concept) — A type of genetic drift that occurs when a population size is reduced to a small number of individuals for at least one generation.

Founder effect ● (core concept) — A type of genetic drift that occurs when a population is separated from other members of the population; the frequency of genes and traits shifts based on the genes in the new founder population.

Gene flow ● (core concept) — The addition or removal of alleles from a population via migration. Gene flow between two populations prevents them from diverging into separate species.

Allele frequency ● (core concept) — The proportion of each allele in a population's gene pool. Changes in allele frequencies provide evidence for the occurrence of evolution in a population.

Hardy-Weinberg equilibrium ● (core concept) — A model for describing and predicting allele frequencies in a non-evolving population. Conditions: large population size, no migration, no new mutations, random mating, no natural selection. These conditions are never met, but they provide a valuable null hypothesis.

Hardy-Weinberg equation ● (core concept) — p² + 2pq + q² = 1 and p + q = 1, where p and q are the frequencies of the two alleles. Allele frequencies in a nonevolving population can be calculated from genotype frequencies.

Fossil ● (core concept) — Preserved remains or traces of organisms that document change over time. Fossils can be dated by the age of the rocks where they are found, the rate of decay of isotopes including carbon-14, and geographical data.

Morphological homology ● (core concept) — Similarities in body structure across species, including vestigial structures, that provide evidence of common ancestry.

Vestigial structure ● (core concept) — A reduced or nonfunctional remnant of a structure that was functional in an ancestor; provides evidence of common ancestry.

Molecular evidence for evolution ● (core concept) — A comparison of DNA nucleotide sequences and protein amino acid sequences provides evidence for evolution and common ancestry.

Common ancestry of eukaryotes ● (core concept) — Structural and functional evidence — membrane-bound organelles, linear chromosomes, and genes that contain introns — indicates the common ancestry of all eukaryotes.

Ongoing evolution ● (core concept) — All species have evolved and continue to evolve, evidenced by genomic changes over time, continuous change in the fossil record, evolution of resistance to antibiotics/pesticides/herbicides/chemotherapy drugs, and pathogens evolving to cause emergent diseases.

Evolution of resistance ● (core concept) — The evolution of resistance to antibiotics, pesticides, herbicides, or chemotherapy drugs — an example of evolution as an ongoing process in living organisms.

Phylogenetic tree ● (core concept) — A diagram showing hypothetical evolutionary relationships among lineages that can be tested. Phylogenetic trees show the amount of change over time, calibrated by fossils or a molecular clock. They can be constructed from morphological similarities of living or fossil species and from DNA and protein sequence similarities, represent hypotheses that are constantly revised based on evidence, and can be used to illustrate speciation that has occurred.

Cladogram ● (core concept) — A diagram showing hypothetical evolutionary relationships among lineages. Unlike phylogenetic trees, cladograms do not show a time scale or the evolutionary difference between groups.

Shared derived character ● (core concept) — Traits gained or lost during evolution that can be present in more than one lineage and indicate common ancestry; informative for constructing phylogenetic trees and cladograms.

Outgroup ● (core concept) — The lineage least closely related to the remainder of the organisms in a phylogenetic tree or cladogram.

Most recent common ancestor ● (core concept) — The nodes on a phylogenetic tree represent the most recent common ancestor of any two groups or lineages.

Molecular clock ● (core concept) — A method using steady rates of molecular change to calibrate the amount of evolutionary change over time in phylogenetic trees.

Molecular data in phylogenies ● (core concept) — Molecular data typically provide more accurate and reliable evidence than morphological traits in the construction of phylogenetic trees or cladograms.

Speciation ● (core concept) — The formation of new species; occurs when two populations become reproductively isolated from each other.

Biological species concept ● (core concept) — A commonly used definition of a species for sexually reproducing organisms: a group capable of interbreeding and exchanging genetic information to produce viable, fertile offspring.

Reproductive isolation ● (core concept) — The separation of populations so they cannot exchange genes. Various pre-zygotic and post-zygotic mechanisms maintain reproductive isolation and prevent gene flow between populations.

Sympatric speciation ● (core concept) — Speciation occurring in populations with geographic overlap.

Allopatric speciation ● (core concept) — Speciation occurring in populations that are geographically isolated.

Reproductive isolating mechanisms ● (core concept) — Pre-zygotic and post-zygotic mechanisms that maintain reproductive isolation and prevent gene flow between populations.

Punctuated equilibrium ● (core concept) — A pattern in which evolution occurs rapidly after a long period of stasis.

Gradualism ● (core concept) — A pattern in which evolution occurs slowly over hundreds of thousands or millions of years.

Divergent evolution ● (core concept) — Evolution in which adaptation to new habitats results in phenotypic diversification.

Adaptive radiation ● (core concept) — A period of especially rapid speciation as new habitats become available.

Convergent evolution ● (core concept) — Evolution in which similar selective pressures result in similar phenotypic adaptations in different populations or species.

Speciation examples ○ — Illustrative (suggested, not required) CED speciation examples: Hawaiian Drosophila, Caribbean Anolis, apple maggot Rhagoletis.

Genetic diversity and survival ● (core concept) — The level of genetic variation in a population affects its ability to withstand environmental pressures: populations with little genetic diversity are at risk of decline or extinction, while genetically diverse populations are more resilient because they are more likely to contain individuals that can withstand the pressure. Alleles that are adaptive in one environmental condition may be deleterious in another because of different selective pressures.

Genetic diversity examples ○ — Illustrative (suggested, not required) CED examples of genetic diversity and selective pressure: California condors, black-footed ferrets, prairie chickens, potato blight, corn rust, and antibiotic resistance in bacteria.

Origin of life timeline ● (core concept) — Geological evidence reinforces models of the origin of life on Earth. Earth formed approximately 4.6 billion years ago; the environment was too hostile for life until about 3.9 bya, and the earliest fossil evidence for life dates to 3.5 bya. Together this gives a plausible range of dates for the origin of life.

RNA world hypothesis ● (core concept) — The hypothesis that RNA was the earliest genetic material, based on three assumptions: genetic continuity was assured by RNA replication, base-pairing is necessary for replication, and genetically encoded proteins were not involved as catalysts.