evolution

• Evolution refers to changes in populations, species or groups; i.e. changes in allele frequencies in populations over time.
• Microevolution is evolutionary change within a population. Also over short periods over time. Microevolution happens due to mutation, selection, gene flow, gene drift, and nonrandom mating
• Macroevolution refers to patterns of evolution in groups of related species over broad periods of geological time. These patterns determine phylogeny (evolutionary relationships among species and groups of species), and can be used to establish a phylogenetic tree.
• The Lamarck theory focuses on body parts forming/weakening due to use and disuse, and these traits could be passed down In other words, he thought body parts can develop with increased usage, and unused body parts would weaken (both false). Also, such traits cannot be passed on; only genetic changes can be passed on. Lamarck also believed that each generation became more complex, which is also incorrect.
• Darwin’s theory focused on natural selection, i.e. survival of the fittest
• Descent with modification is a part of Darwin’s theory and occurs via natural selection Over time and generations, traits that provide reproductive advantages become more common within the population
• Fossils provide evidence for evolution because they prove the existence of extinct species. They’re often found in sediment layers. Deeper fossils represent oldest specimens. Fossil types include actual remains, petrification, imprints, molds, and casts
• Biogeography provides evidence for evolution because unrelated species in different parts of the world look alike when found in similar environments. Biogeography is geography to describe the distribution of species. The evidence above suggests continental drift – the supercontinent Pangea slowly broke apart into 7 continents
• Embryology provides evidence for evolution because there are similar stages of development among related species, which establishes evolutionary relationships (phylogeny). Gill slits and tails are found in fish, chicken, pig, and human embryos.
• Comparative anatomy provides evidence for evolutionary relationships and includes: Homologous structure: similar body parts in different species, from common ancestor.Analogous structure: similar body parts in different species, from evolving independently, as adaptation to their environments. Analogous structures are also called homoplasies, e.g. cacti and euphorbs Note on distinguishing analogous vs homologous: ask yourself, same class (e.g. both mammals)? If not, likely analogous and not homologous.Molecular biology provides evidence for evolution because i...
• Comparative biochemistry provides evidence for evolution because organisms with a common ancestor have common biochemical pathways
• Natural selection is responsible for producing adaptations that increase an individual’s fitness
• Fitness is the ability to survive and produce fertile offspring If a lot offspring are produced but die before maturing, fitness is low.
• Stabilizing selection happens when the population favors some intermediate trait. Image from OpenStax, CC BY 4.0
• Directional selection happens when one extreme trait is favored. The allele frequency leans towards one direction. Ex: industrial melanism, selection of dark-colored (melanic) varieties in various species of moths (peppered moth) as a result of industrial pollution. Image from OpenStax, CC BY 4.0
• Disruptive selection occurs when the environment favors extreme or unusual traits while selecting against intermediate traits. Ex: Short and tall are favored while average is not. Below, listed as diversifying selection Image from OpenStax, CC BY 4.0
• Sexual selection has two aspects: Members of one sex choose to mate with the opposite sex (intersexual selection)Members of the same sex compete for mating opportunities (intrasexual selection)
• When females choose superior males, they are increasing the fitness of their offspring They invest greater energy to maximize quality.
• Males increase fitness of offspring by maximizing quantity. Male competition favors traits like musculature, horns, large stature, etc.
• Male competition and female choice often leads to sexual dimorphism (differences in appearance of males and females). This essentially becomes a form of disruptive selection, where the extremes are favored.
• Artificial selection is a form of directional selection carried out by humans when they breed favorable traits.
• Sources of variation: Mutation: introduces a new allele. Sexual reproduction: genetic recombination (crossing over, independent assortment, random joining of gametes). Diploidy: presence of two copies of each chromosome. In heterozygous conditions, recessive allele is stored for later generations ⟶ more variation maintained in the gene pool. Outbreeding: mating with unrelated partners ⟶ mixing different alleles ⟶ new allele combinations. Balanced polymorphism: maintenance of different phenotypes in population.
• Heterozygote advantage is when heterozygote genotypes have an advantage over either homozygous genotypes. Ex: Sickle cell has genotypes AA, AS, and SS. AS is 14% of the population in Africa because it has resistance against malaria.
• Frequency-dependent selection (minority advantage) is when the frequency of phenotypes depends on the relative amount compared to other phenotypes. In this type of selection, the least common phenotypes have a selective advantage. Common phenotypes are selected against. Rare phenotypes will increase in frequency, then eventually be selected against, and this process repeats. For example, predators (who have a search image of common phenotypes) allow rare phenotypes to get away; then the rare eventually become common, and the cycle repeats.
• Neutral variation is variation without selective value i.e. the variation doesn’t affect fitness, e.g. fingerprints in humans
• Geographic variation is variation of a species dependent on climate or geographic conditions.
• A cline is a gradient of variation of a phenotype due to geographic variation e.g. variation spanning north to south would be a north-south cline
• Causes of changes in allele frequencies: Natural selection: increase/decrease of allele frequencies due to environment. Gene flow: introduction/removal of alleles from population when individuals leave or enter the population. Genetic drift: random increase/decrease of allele by chance. Founder effect: allele frequencies in group of migrating individuals are (by chance) not the same as that of their population origin. Bottleneck: when population undergoes a dramatic decrease in size (natural catastrophe, etc) ⟶ vulnerable to genetic drift. {{c2::Nonra...
• Genetic equilibrium (Hardy-Weinberg equilibrium) is when allele frequencies remain constant for many generations and the population does not evolve.
• The Hardy-Weinberg equilibrium requires the following conditions: no mutation, all traits are neutral, population must be isolated, a large population, mating is random, no net migration. All traits are neutral ⟶ there is no natural selection Isolated population ⟶ no gene flow Large population ⟶ no genetic drift
• Hardy-Weinberg equilibrium math: Allele frequencies for each allele are represented by p and q Frequency of homozygous alleles are represented by p2 and q2 The frequency of heterozygote alleles is pq + pq = 2pq
• Rules for Hardy-Weinberg frequencies: All allele frequencies sum to 100%: p + q = 1All individual genotype frequencies sum to 100%: p2 + 2qp + q2 = 1 Both must be true for Hardy-Weinberg equilibrium!
• Speciation is formation of a new species Speciation begins when gene flow ceases between two sections of a population.
• A species is a group of individuals capable of interbreeding.
• Allopatric speciation is when a population is split by a geographic barrier and divides into new species.
• Sympatric speciation is when new species form even though there is no geographic barrier.
• Balanced polymorphism is natural selection due to polymorphism. Results in maintenance of different phenotypes in a population. Ex: different color in insects, one color can camouflage to different substrate, and the other that can't will be eaten. Only insects with same color can mate (isolated from other subpopulations). This belongs under sympatric speciation
• Polyploidy is having more than two sets of chromosomes Ex: like 3n, 4n in plants In the case of nondisjunction, there can be two viable diploid gametes and two sterile gametes with no chromosomes ⟶ tetraploid 4n zygote formed ⟶ repeat with diploid gametes male/female ⟶ reproductive isolation with normal gametes. Plants, can be autopolyploid or allopolyploid. This belongs under sympatric speciation
• Hybridization is when two closely versions of a species mate and produce offspring. This happens in a geographic region called the hybrid zone. This belongs under sympatric speciation
• Adaptive radiation is rapid evolution of many species from a single ancestor; it occurs when an ancestral species is introduced to an area where diverse geographic/ecological conditions are available for colonization. Ex: Galapagos finches