Chapter 22

• 1859 Darwin published The Origin of Species

• Current species are descendants of ancestors

• Evolution can be defined by descent with modification

• Darwin’s theory challenged traditional views that Earth was inhabited by unchanging species

  • Aristotle thought species were fixed & arranged them on a natural scale/ladder (scala naturae)

  • Old Testament said that species were designed by God —> therefore perfect & unchanging

• Linnaeus said that adaptations were evidence that God had designed each species for a specific purpose

• Linnaeus was the founder of taxonomy —> classiying organisms & developed the binomial format (ex: homo sapiens)

• Study of fossils helped to lay the groundwork for Darwin’s ideas

  • Fossils are in layers (strata)

    

• Cuvier — catastrophism —> each boundary between strata represents a catastrophe

• Hutton & Lyell thought that changes in Earth’s surface are because of slow continuous changes (uniformitarianism) —> strongly influenced Darwin’s thinking

  • uniformitarianism —> change is constant over time

• Lamarck hypothesized that species evolve through use/disuse of body parts & inheritance of acquired characteristics

  • Ex: changing hair color doesn’t pass on to your children

• After graduating from Cambridge university, Darwin went on a 5 year voyage journey around the world on the Beagle

  • During his travels on the Beagle, he collected specimens of South American plants and animals

  • He observed that fossils resembled living species from the same region, & living species resembled other species from nearby regions

  • There was an earthquake —> showed the strata —> fossils, evolution

  • Darwin was influenced by Lyell’s Principles of Geology and thought that the Earth was more than 6000 years old

  • Geographic distribution in the Galapagos

    • Adaptation in the finches

• In 1844 Darwin wrote an essay on natural selection

  • Natural selection —> process in which individ. w/ favorable inherited traits are more likely to survive and reproduce

• 1858, Wallace had developed a similar theory to Darwin’s

• Darwin quickly finished The Origin of Species and published it the next yr

  • Descent with Modification: organisms are related through descent from an ancestor that lived in the past

• Darwin believed in a tree w/ branches

  • Similar to the hierarchy of Linnaeus

• Artificial selection: modified other species by selecting & breeding individs. w/ desired traits

• Darwin had 2 observations

  1. Members of a pop. often vary in their inherited traits

  2. All species can produce more offspring than the environment can support, & many of these offspring fail to survive & reproduce

• Darwin’s inferences:

  1. Individs. whose inherited traits give them a higher probability of surviving & reproducing in a given environment tend to leave more offspring than other individs.

  2. The unequal ability of individs. to survive & reproduce will lead to the accumulation of favorable traits in the pop. over generations

• Darwin was influenced by Malthus —> who said that the population will increase faster than food supplies & other resources (competition for resources)

  • If some heritable traits are advantageous these will accumulate in a population over time, & will increase the frequency of individs w/ these traits

• Summary of natural selection:

  • Individs w/ certain heritable characteristics survive & reproduce @ a higher rate than other individs.

  • Natural selection increases the adaptation of organisms to their environment over time

  • If an environment changes over time, natural selection may result in adaptation to these new conditions —> new species!

• Individs. do not evolve, population evolve over time

• Natural selection can only increase or decrease heritable traits that vary in a pop.

• Adaptations vary w/ different environments

• Evidence for natural selection: drug-resistant bacteria

• Natural selection does not create new traits, but edits or selects for traits already present in the pop

• The local environment determines which traits will be selected for or against in any specific pop.

• Homology — similarity resulting from common ancestry

  • Homologous structures — anatomical resembles that represents variations on a structural theme present in a common ancestor

    

• Comparative embryology reveals anatomical homologies not visible in adult organisms

• Vestigial structures — remnants of features that served important functions in the organisms’ ancestors

  • Ex: appendix

• Evolutionary trees — hypotheses about the relationships among different groups

  • Homologies form nested patterns

  • Can be made using different types of data —> anatomical & DNA sequence data

• What could cause species to look more similar to one another?

  • Analogous traits —> species independently adapt to similar environments in similar ways

    • Ex: moths and another animal slowly turn black to blend in better

  • Convergent evolution —> evolution of similar features in different species

  • Although they look similar, they do not necessarily share a common ancestor

• Fossil record — shows changes through species over time

• Biogeography - geographic distribution of species

• Endemic— species not found anywhere else in the world

  • Islands often have endemic species

  • Species on islands gave rise to new species as they adapted to new environments

• Darwin’s theory has been backed up by many

Chapter 23

• Microevolution — change in allele frequencies in a population over generations

  • Mechanisms that cause allele frequency change:

    • Natural selection

    • genetic drift

    • gene flow

• Only natural selection causes adaptive evolution

• Genetic variation among individs. is caused by differences in genes or DNA

• In order for evolution to occur there needs to be variation in heritable traits

• Phenotype is the combination between inherited genotypes and environmental influences

• Natural selection can only act on variation w/ a genetic component

• Average heterozygosity — what percent of individ/genes are likely to have both dominant and recessive

• Geographic variation — differences between gene pools of separate populations

  • Ex: cline — change in a trait geographically

    • Fish vary in a cold-adaptive allele along a temperature gradient

      • Due to natural selection

• New genes & alleles due to mutations

  • Only mutations in reproduction cells (gametes) can be passed to offspring

    • Changes in somatic cells will not be passed along

• Mutation rates are low in animals & plants, higher in viruses

• Sexual reproduction can shuffle existing alleles into new combinations

  • Recombination

    

    • Makes adaptation possible

• Population — localized group of individuals capable of interbreeding & producing fertile offspring

• Gene pool — all of the alleles for all loci in a population

  • Ex: all the lanternflies at GHS!

• If every individual in the population are the same, meaning the are homozygous for every same allele —> change is not possible — locus is fixed

  

• Calculating Allele Frequencies using Hardy-Weinberg:

  • The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for every heterozygous individual; the same logic applies for recessive alleles

  • p+q = 1

    • p = frequency of the dominant allele, q = frequency of the recessive allele

  • Hardy-Weinberg principle — describes population that is not evolving

    • If it doesn’t meet the criteria, it is evolving

    • p²+2pq+q²=1

      • p² and q² represent the frequencies of the homozygous genotypes & 2pq represents the frequency of the heterozygous genotype

        • p² is the frequency of the homozygous dominant genotype

        • q² is the frequency of the homozygous recessive genotype

    • In real populations, allele & genotype frequencies do change over time. This represents a hypothetical population that is not evolving

  • Conditions that must be met in order for Hardy-Weinberg equilibrium to hold true:

    1. No mutations

    2. Random mating — no sexual selection

    3. No natural selection

    4. Extremely large population size —> no genetric drift

    5. No gene flow —> no migration

  • You can have some loci be in Hardy-Weinberg equilibrium while others are not

• What causes changes in allele frequencies?

  • Natural selection

  • Genetic drift

    • Seen much more often in smaller populations

    • Ex: 50 Question test & you miss 1 vs. 5 question test & you miss 1 —> much larger affect

    • Random changes reduce genetic variation bc of losses of alleles

  • Gene flow —> migration/immigration

• Founder effect — when some individuals become isolated from a larger population, so sometimes the allele frequencies in the small founder population can be different from those in the larger parent population

  • Ex: some individuals of the population get separated from the parent population mb because of a natural disaster, and they aren’t able to connect with them. They evolve separately and adapt to their new environment. If you were to combine them again they would likely look different

• Bottleneck effect — sudden reduction in population size due to a change in the environment

  • Resulting gene pool may no longer be reflective of the original population’s gene pool

  • Ex: original population had red, blue, yellow, and green bugs. Bottleneck effect happens/natural disaster —> only red and blue bugs remain, which isn’t reflective of the original population with the other color bugs too. Then the species evolves with red, blue, and maybe purple bugs, but the yellow & green bugs die off.

  • By chance

  • Humans can have an impact with bottleneck effect —> understanding how we impact other species

• Effects of genetic drift:

  1. Significant in small populations

  2. Causes allele frequencies to change at random

  3. Can lead to a loss of genetic variation within populations

  4. Can cause harmful alleles to become fixed

• Gene flow — migration/immigration

  • Alleles can be transferred through the movement of fertile individuals or gametes

  • Gene flow tends to reduce variation among populations over time & can reduce the fitness of the population

• Evolution by natural selection involves both chance and “sorting”

  • New genetic variations arise by chance

  • Beneficial alleles are “sorted” and favored by natural selection

  • Only natural selection consistently results in adaptive evolution

• Relative fitness — contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals

  • “survival of the fittest” is misleading bc it implies direct competition among individs. —> reproduction is usually more subtle and depends on many factors

  • Selection favors certain genotypes by acting on the phenotypes of certain organisms

• Three modes of selection:

  • Directional selection: favoring one end of the phenotypic range

  • Disruptive selection: favors both extremes of the phenotypic range

  • Stabilizing selection: favoring the outlier

• Natural selection increases the frequencies of alleles that enhance survival and reproduction

• Bc the environment can change, adaptive evolution is a continuous process

• Genetic drift & gene flow do not consistently lead to adaptive evolution bc they can increase or decrease the match between an organism & its environment

• Sexual selection:

  • Can result in sexual dimorphism — physical differences between genders

  • Intrasexual selection — competition among one sex (usually males) for mates of the opposite sex

  • Intersexual selection — mate choice, when individuals of one sex (usually females) are choosy in selecting their mates

  • Male showiness due to mate choice can increase a male’s chance of attracting a female while decreasing his chances of survival

  • Female preferences evolve over time, but the
    good genes” are related to male health —> both the male trait & female preference for that trait should increase in frequency

• Neutral variation — genetic variation that does not have an advantage or disadvantage

• Diploidy — genetic variation in the form of hidden recessive alleles

  • Heterozygotes can carry recessive alleles that are hidden from the effects of selection

• Balancing selection — natural selection results in more than 2 phenotypic forms in a population

  • Includes heterozygote advantage — when heterozygotes have a higher fitness than do both homozygotes

    • Ex: malaria resistance

    • increases the chance a deleterious recessive allele will stay in the gene pool

    • Natural selection tends to maintain 2+ alleles at the locus

  • Frequency-dependent selection: fitness of the phenotype declines if it becomes too common in the population

    • Selection can favor whichever phenotype is less common in a population

• Natural selection can’t make perfect organisms

  1. Selection can act only on existing variations

  2. Evolution is limited by what's already present

  3. Adaptations are compromises

  4. Nothing is static — chance, natural selection, & the environment all work together

Chapter 24

• Speciation — origin of new species

  • Connects Microevolution and Macroevolution together

    • Microevolution — changes in allele frequency

    • Macroevolution — broader patterns of evolutionary change above the species level

      • Families, classes, along those lines

  • Evolutionary theory connects those 2 together & also explains how new species originate & how populations evolve

• Biological species concept emphasizes reproductive isolation

  • Group of populations whose members have the potential to interbreed in nature & produce viable fertile offspring; they do not breed successfully w/ other populations

  • Gene flow between populations holds the phenotype of a population together

• Reproductive isolation — biological barriers (factors) that restrict two species from producing viable fertile offspring

  • Can be classified by whether factors act before (prezygotic) or after fertilization (postzygotic)

• Hybrids — offspring of crosses between different species

  

• One definition doesn’t necessarily fit everything —> reason why there are so many definitions

• Biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes)

• Emphasizes absence of gene flow

  • However, gene flow can occur between distinct species

    • Ex: grizzly bears + polar bear = grolar bear

• Morphological species concept — shape & structural features

  • Can apply to both sexual & asexual species (but is subjective)

  • Ex: golden retriever & Chihuahua —> shape the same, but fish is not classified as the same

• Ecological species concept — ecological niche

  • Applies to both sexual & asexual —> emphasizes the role of disruptive selection

  • Ex: species clean teeth

• Phylogenetic species concept — species being the smallest group of individuals on a phylogenetic tree

  • Applies to both sexual & asexual, but it’s difficult to determine how different it needs to be in order for it to be classified as separate species

  • Ex: share ancestor, wolf & dog

  

• Polyploidy — presence of extra sets of chromosomes due to mistakes in cell division

  • Much more common in plants than in animals

• Autopolyploid — individual w/ more than 2 chromosome sets, derived from 1 species

  

• Allopolyploid — species w/ multiple sets of chromosomes derived from different species

• Hybrid zones — members of different species mate & produce hybrids

• Hybrids typically have reduced fitness compared with parent species

  

• Reinforcement — hybrids are less fit than the parent species over time, the rate of hybridization decreases

  • Reproductive barriers are stronger for sympatrics than allopatrics

• Fusion — reduce reproductive barriers

  • If hybrids are fit as parents, there can be substantial gene flow between species

  • If gene flow is great enough, the parent species can fuse into a single species

    • Ex: pollution in a lake makes it hard for females to see the different color fish so they fused together

• Stability — there is a continuation of forming hybrids but extensive gene flow from outside the hybrid zone can overwhelm selection for increase reproductive isolation inside the hybrid zone

• Speciation can occur fast or slow & can result from changes in few or many genes

  • A lot of questions left to answer

• Fossil record includes examples of species that appear suddenly, unchanged for a while, & then apparently disappear

• Punctuated equilibria — all of a sudden changes & then remains unchanged

  • Contrasts w/ gradualism

• Macroevolution is speciation + extinction events

Other notes

Adaptive radiation

convergent evolution — distantly related organisms independently evolve similar traits to adapt to similar necessities

transitional fossil

Kin selection — individuals will favor their own offspring over other individuals in the species

• Kills some individuals but saves relatives with shared genes

all organisms share a common ancestor because all living things use nucleic acids for their genetic code

sources of genetic variation in populations:

• Recombination during fertilization

• Crossing over in meiosis

• Mistakes in DNA replication

Morphological species concept & phylogenetic species concept

Hardy-Weinberg practice problems

Sickle-cell allele if it is present & if its homozygous dominant or recessive it is dangerous, but if it heterozygous its safe. It also means than if you are in environments that are low in oxygen it hurts you more than other people

Adaptations are compromises

100 birds, 64 of the birds are brown, 36 are red (recessive homozygous) find the p value = 0.64

Transitional structure question