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Unit 7: Natural Selection


Evolution:changes over time
Populations and species can change
Population: group of members of the same species living in the same place at the same time (first level of evolution, micro), change over time to adapt to environment
Species: group of individuals that can mate and make fertile offspring → biological species concept (second level of evolution, macro) change over time makes new species, speciation
Individuals cannot evolve only populations can
Evolution is a change in proportions of genes
Early Evolution Views
Curvier and Fossils: Catastrophism
Curvier was one of the first paleontologists
Species used to exist that don’t anymore and species that exist now may have not existed in the past
Catastrophism: the reason you saw different fossils in different strata of rock because all of the species in certain times went extinct
Hutton and Lyell:
Gradualism (Hutton): earth is gradually changing over time
Uniformitarianism (Lyell): earth changing at a uniform rate
Darwin thought that if earth is changing, then species must be too
Lamarck: early theories on evolution
Use and disuse: only correct idea
Use it or lose it, parts of the body that are used heavily will be accentuated, but parts that are reduced will not be maintained (won’t waste energy)
Inheritance of acquired characteristics:
Characteristics created over an individual’s lifetime would be passed on
Bonzai trees would not reproduce as small trees
Natural Transformation of Species: each species naturally goes from simple to complex, no speciation and no extinction
CHARLES DARWIN: traveled to Galapagos Islands and other places
Came up with the idea of Natural Selection NOT the idea of evolution → On the Origin of Species on the Basis of Natural Selection
“Descent with modification”
Generation to generation change for a particular trait that takes place in a population, constantly changing to become better adapted to the environment → Natural Selection
Environment is the driving force of natural selection
Selection Pressures: control which traits are the most beneficial
E.g. food source, temperature, avoid predation
Theory of Natural Selection: differential reproductive success, different ability to pass on genes
I. Organisms overproduce offspring
Based on Charles Malthus’ idea that human pop will exceed resources
II. Offspring are variation in appearance and function and some of those variations are heritable → variation is the fuel for selection
Variation is created through genetic recombination (random fertilization, independent assortment, crossing over) originally from mutation
III. Environmental resources are limited and those varied offspring must compete for their share → struggle for existence
IV. Individuals best able to survive in the environment will survive and reproduce → differential inheritance of genes
Focus on Adaptation:
Adaptation to an environment causes changes within a population and the forming of new species, if changes build up and can no longer mate must be two new species
Unity of Life: all organisms are similar (common ancestor)
Diversity of Life: better adapted to unique environments
Match between organisms and environment
History of Life:phylogenetic tree tree with branches showing life’s diversity, and where the come from
Alfred Russel Wallace: sent his ideas of natural selection to Darwin, Darwin’s ideas were more developed so only Darwin published
Natural Selection in Action:
Beak depth in a finch changes as an environment changes
Drier years mean nuts they eat are harder → thick deep beaks survive
Wetter years means nuts are easy to eat → beak depth decreases
Drug and antibiotic resistance from bacteria and viruses
Peppered moths got polluted, trees became darker as covered by soot and other material, peppered moths became darker for camouflage
Artificial Selection: nature does not determine selected traits, but humans select them
Wild mustard creates brussel sprouts, cabbage and broccoli, by selecting for various traits
E.g. select for leaves, breed those to make kale
Evidence of Evolution:
Biogeography: geographical distribution of species
Modern species are found where they are because they come from ancestors in the hose regions
Fossil Record (palaeontology): deeper the rock fossil, older it is, can look progression of species over time
Comparative Embryology: different stages in embryonic dev.
Most animals look extreme similar in the womb, closer species are more similar embryonic development
Comparative Anatomy: anatomical similarities between species, to determine common ancestry → evidences evolution
Focuses on homology (w/ bones often), similarity from common ancestry
Homologous structures: same structure but different function in different species, only different because of adaptation
Analogous structures: structures that look similar, adapted to the same environment, but do not share a common ancestor, or similar structures
Vestigial Structures: leftover pieces of from ancestors that used to serve a function when it was necessary
Coccyx (tailbone) when humans had tails
Molecular Biology: study molecules amino acids and DNA that are shared in common between species, distant species have less DNA base pairs in common


Change in ratios (in phenotypes/genotypes, change in proportion of alleles, one of those alleles is being selected for) in a population over time
Individuals cannot evolve, only populations can, generation after generation, it adapts better to the environment
Population Genetics: combines evo with genetics to measure change taking place in a population
Gene Pool: all alleles in a populations
Genetic Variation: crucial for natural selection and evolution
Measured by average heterozygosity, contains both alleles for a particular gene
Natural Selection: selects for advantageous alleles within a population
Evolution: when a population’s gene structure changes
Gene structure: ratio of dominant alleles to ratio of recessive alleles AA:Aa:aa
Hardy-Weinberg Theorem: measure the ratio of alleles and genotypes in a population
Genetic structure of a non evolving population remains constant, in Hardy-Weinberg equilibrium
p^2+2pq+q^2=1(100% of alleles, p + q =1) equation can measure ratio of genes in equilibrium
p^2: frequency of AA
2pq: frequency of Aa
q^2: frequency of aa
P: frequency of A allele
Q: frequency of a allele
If frequencies change over time then evolution has occurred, always begin by finding q^2, easy because only recessive expressing people have aa
Measure frequency at time A and then at time B, if you find a difference in the ratios, then quantified evolution must be occurring
Equilibrium Requirements: pop will not evolve in equilibrium can only use the equation in this theoretical state
Very large pop.
Genetic Drift: when a small random change has a large effect on a small population frequencies, doesn’t affect large pops
Bottlenecking: when an event drastically reduces population size (disaster, disease event), will not reflect same frequencies of same population → microevolution, chance creates new frequencies
Founder Effect: small subgroup moves away from large pop. will have a different ratio of genotypes of alleles than original → also microevolution, random events will change this pop. drastically
NOT adaptive (doesn’t make population better)
Isolation from other populations
Gene Flow: when two different populations begin mating, reduce the differences between the populations (become similar) → microevolution, still evolving, movement of alleles betwee pops
No net mutations
Mutations are the raw source where new variation is created (not the major source of variation though, that would be recombination)
Random mating
Nonrandom mating/Sexual Selection: just as important to drive evolution as natural selection, not adaptive, just about mating
Intrasexual Selection: happens within a sex (male competition for the right to mate with females, males with better genes get to pass them on) males get big and strong
Intersexual Selection:between sexes (female choice, choose which male they allow to mate with them) males usually look fabulous to please the female (best genes correspond with best colors sometimes, color of mane corresponds with testosterone levels in lions, so lionesses pick the best color)
Both create sexual dimorphism or differences between the sexes of a species
NO natural selection
Major force that causes evolution (better adaptation to the environment) ONLY ADAPTIVE MECHANISM (helps organisms to be better at environment, all the others are just ways the change)
Darwinian Fitness: indicator of how strong an individual’s genes are, based on children's production, more children == more fitness
Measured 0-1, others’ fitness is compared to that of the most fit individual
Modes of Selection:
Stabilizing Selection: average individual is favored (e.g. average birth weight 7 lbs)
Directional Selection: one extreme is favored over the other, giraffes with the longest necks survived and reproduced
Disruptive Selection: both extremes are favored could lead to two new species being formed
Genetic Variation: natural selection would get rid of all variation (high rate of extinction because of this) but the environment keeps changing so variation is preserved
Without different forms, one cannot be selected for over the other
Quantitative Characters; vary along a continuum (not discrete in an either or basis tall or short, black or blue etc.), has many different possible phenotypes
Mutation: important source of variation, raw source, but only mutations that take place in germlines pass on (only really effective for bacteria and viruses)
Sexual Recombination: during meiosis, crossing over, independent assortment and random fertilization
What prevents natural selection from extinguishing the bad allele?
Diploidy: hides genetic variation in the form of carrier recessive alleles through heterozygotes, two alleles for every gene, contain two alleles except only express one
Balanced Polymorphism: ability of nature to maintain diversity/variation/shape
Heterozygote Advantage: selected as the best option (like sickle cell anemia, and malaria trait, if you heterozygous, Sickle Cell Trait, doesn’t have SCA but resistant to malaria), keeps both alleles in the population
Hybrid Vigor: breed homozygous dom and homozygous rec to have better quality
Frequency-dependent selection: what trait is successful depends on how many organisms have that trait, more of the trait == bad
Fish either latch onto prey on right side or left side. If left side more successful, then prey will being defending that side, so right side now becomes more successful.
Natural Selection cannot fashion perfect organisms
A population can only modify preexisting traits, locked into ancestry
Not all evolution is adaptive, things can change because of outside events


The formation of new species
Anagenesis (phyletic evolution): where one species transforms into a new species, old species no longer exists
Cladogenesis (branching evolution): new species buds off another species, increases biodiversity (number of species in an environment)
Biological Species Concept: group of individuals that can produce fertile offspring, reproductive isolation
Speciation: formation of new species
Allopatric: MAJOR way to make species when a physical barrier (e.g. grand canyon, newly formed river, etc. each new environment has different selection pressures) prevents two populations from being able to mate, two populations become different that they cannot mate anymore → breakup of Pangaea
Prevents gene flow
Adaptive Radiation: spreading out of new species from a common ancestor
Sympatric: anything other than a geographic barrier separates new populations
Chromosomal Changes: nondisjunction in meiosis
Polyploidy (in plants onlybecause animal gametes with different numbers cannot fertilize): whole extra sets of homologous chromosomes in gametes (do not separate), diploid plants can become tetraploid → can no longer mate, but can self-fertilize and propagate a new species
Nonrandom Mating: sexual selection, subgroup of a population decides to only mate with one type of individual
Habitat Differentiation: two groups within a population begin to exploit different resources within the environment, What obtain different food sources, e.g. females prefer certain males at certain level, and in deep water like certain males, create new species (a set of different groups with sexual selection in a population)
Reproductive Barriers: prevent different species from mating
Prezygotic barriers: prevent mating (stops zygote from mating, if two species hit the horny bing bong) between species or hinder fertilization of ova if members of different species attempt to mate
Habitat Isolation: two species live in different habitats, can’t meet up to mate
Temporal Isolation: mating seasons of two different species differs
Behavioral Isolation: (mainly birds) have unique mating rituals, if ritual not performed by male, female will not receive the mating
Mechanical Isolation: when physical reproductive parts of organisms do not line up
Gametic Isolation: when sperm of one species is physically unable to fertilize the egg of another (need same number of chromosomes, which is unique to the species)
Postzygotic barriers: occurs if two species do mate and make a hybrid offspring → prevent species from mating which maintains biodiversity
Reduced Hybrid Fertility: (MOST IMPT.) most hybrids are sterile, mules
Reduced Hybrid Viability: fail to complete development or have birth defects prevent them from mating
Hybrid Breakdowns: first gen of hybrids might be fertile, but next generation will NOT be viable or fertile (more important with plants)
Hybrid Zones: region where members of different species meet and mate to make hybrids (closely related species of course), would normally be separated
Can lead to:
Reinforcement: strengthening reproductive barriers, species become more and more distinct (don’t mate successfully)
Fusion:weakening reproductive barriers. Hybrid zone increases in size, two species are fusing together (to become one)
Stability: continued formation of hybrids, constant unchanging size
Take biological species concept into question → fusing species?
Patterns of Evolution: pull species in different directions
Divergent Evolution: one parent or ancestral species diverges into two new species, since new species evolve from common ancestor, will have homologous structures
Convergent Evolution: two unrelated species evolve to share similar traits, NO COMMON ANCESTRY, because they are separately adapted to the common environment → have analogous structures different structure but looks the same, natural selection favors a similar shape
Parallel Evolution: when two unrelated species share similar trends in evolution, do NOT live in the same environment, look similar but do NOT live together
Coevolution: one species evolves due to the evolution of another species (adapting predator to prey, host to parasite)
Speciation Speed:
Punctuated Equilibrium: can also be very abrupt, drastic change in environment changes one species very fast
Gradual Evolution: evolution occurs in slow changes accumulating over time, creates intermediate species in transition
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