Natural Selection

Key features

• Natural selection: The process by which individuals with more favorable traits are more likely to survive, reproduce, and pass on their alleles to future generations

• There is variation in heritable traits within a population

• Species produce more offspring than the environment can support

• Individuals w/ more favorable traits are more likely to survive + reproduce

This will lead to the accumulation of favorable traits over generations

Evidence of natural selection

• Homology: Structural similarities resulting from common ancestry. Not always as reliable because of analogous structures + convergent evolution (species independently develop similar structures due to similar environmental pressures)

• Fossil record: Shows extinct species + change over time, but not all organisms can be fossilized so the fossil record is biased

• Embryology can show similarities in structures very early in development

• Molecular data: (DNA, amino acids) directly shows differences between the genetics of organisms

Phylogeny

Phylogenetic trees

• Represent possible evolutionary relationships

• Sister taxa: Share an immediate common ancestor

• Basal taxon/outgroup: Diverges early in the group’s history, near the whole group´s common ancestor

• Monopholyetic group: Branch and every subsequent taxon

• Paraphyletic group: A monophyletic group that leaves out a taxon

• Polyphyletic group: Group of different taxa with a different common ancestor

  

Hardy-Weinberg + Population Genetics

HW Equilibrium

• Population: A group of individuals of the same species in the same area that produce viable and fertile offspring

• HW allele frequency: Dominant allele frequency + recessive allele frequency = 1 (p + q = 1)

• HW genotype frequency: Homozygous dominant frequency (p²) + Heterozygous frequency (2pq) + Homozygous recessive frequency (q²) = 1, (p² + 2pq + q² = 1)

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• REQUIREMENTS: No mutations, random mating, no natural selection, very large population, no gene flow

• Mechanisms of evolution are mutation, gene flow, non-random mating, genetic drift, and natural selection

  Population genetics

• Genetic drift: Change in allele frequency due to random chance

• Founder effect: Few individuals are isolated from the larger population, creating a new and less genetically varied gene pool (part of genetic drift)

• Bottleneck effect: Drastic population reduction due to a sudden environmental change, gene pool after the event will be different (part of genetic drift)

  

• Gene flow: movement of alleles among populations between gametes

• Fitness: An individual´s reproductive success/how many offspring it´s able to produce

• Directional selection: Conditions favor 1 end of the phenotypic range

• Disruptive selection: Conditions favor individuals at both ends of the phenotypic range

• Stabilizing selection: Conditions favor individuals in the middle of the phenotypic range

• Sexual selection: Natural selection for mating success

Reproductive Isolation: Existence of biological factors impeding members of 2 species from interbreeding and producing fertile offspring

Species???!

• Biological species concept: A species is a group of populations whose members can interbreed and produce fertile offspring

• Other species concepts are morphological (species based on morphology), ecological (classified by ecological niche), and phylogenetic.

Prezygotic Barriers- Block Fertilization

• Habitat Isolation: 2 species occupy different habitats and rarely/don’t encounter each other

• Temporal Isolation: Species breed at different times of day, seasons, or years

• Behavioral Isolation: Courtship rituals and other unique mating behaviors make species incompatible

• Mechanical Isolation: Morphological differences prevent successful mating

• Gametic Isolation: The sperm of one species can’t fertilize the egg of the other species

Postzygotic Barriers- Prevent hybrid zygote from developing into fertile/viable adult

• Reduced hybrid viability: The genes of the 2 species interact in a way that impairs the hybrid offspring’s development or survival

• Reduced hybrid fertility: Hybrid is sterile (unable to reproduce)

• Hybrid breakdown: The hybrid is able to reproduce, but the hybrid’s offspring are feeble or sterile

Allopatric Speciation- One species splits into 2+ due to a geographical divide that prevents gene flow

• As geo divide interrupts gene flow, intrinsic barriers (barriers that make 2 species sexually incompatible) such as genetic differences or sexual selection arise

  

Sympatric Speciation- Speciation in a population that is not geographically separated

• Polyploidy: Offspring receive an extra set(s) of chromosomes due to errors during cell division. Creates a reproductive barrier.

• Habitat differentiation: Species develop by using different resources in their geographical area and slowly separating from the initial species

• Sexual selection

• Adaptive Radiation: One species diversifies into multiple species to fill empty ecological niches

Chapters 23-24

23

• Radiometric dating uses half lives

• Mass extinctions pave the way for adaptive radiation because it frees up ecological niches

24

• Gram + bacteria has thick simpler walls high in peptidogylcan

• Gram - bacteria has a thin peptidoglycan wall between possibly toxic outer membrane and inner membrane

  

• Transduction: movement of genes between prokaryotes via phages (viruses that infect bacteria)

• Conjugation: Genetic material transferred directly between prokaryotes

• Transformation: A bacteria takes up a piece of DNA in its environment