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Chapters 22, 23, & 24 (Chapter 22: Descent with Modification: A…
Chapters 22, 23, & 24
Chapter 24
: The Origin of Species
Hybrid Zones
hybrid zone
- region, members of diff. species meet and mate, producing some offspring and mixed ancestry
typically located wherever habitats of interbreeding species meet
outcomes: can either become reproductively isolated from parents and form new species or reinforcement of barriers, fusion of species, or stability
Modes of Speciation
allopatric speciation
-gene flow is interrupted, pop. divided into geographically isolated subpopulations
sympatric speciation
- occurs in populations that live in same reproductive barriers
autopolyploid
- morethan two chromosome sets that are derived from a single species, allopolyploid - fertile, more than two chromosome sets resulting from two different species interbreeding
habitat differentiation
- subpopulation exploits a habitat or resource not used by parent population
sexual selection
- mate selectiveness, cichlid fish chose a certain color mate over others
polyploidy
- species originated from accident during cell division, results in extra sets of chromosomes
What is a Species
prezygotic barriers
- impede mating or hinder fertilization if mating does not occur
habitat isolation
- different habitats, rare encounters, apple fly and blueberry fly
temporal isolation
- breed at different times of day, matings seasons in summer and winter
behavioral isolation
- rituals attract mates unique to species, specific bird mating rituals
mechanical isolation
- mating is attempted, unsuccessful, morphological differences, snails with opposite shell spirals
gametic isolation
- sperm cannot fertilize egg of another species, sea urchins shoot out gametes into water, can only fertilize own species
postzygotic barriers
- prevent a hybrid zygote from developing into a viable, fertile adult
microevolution
- change in allele frequencies(by nat. selection, gen. drift, & gene flow) in a population over generations,
macroevolution
- broad pattern of evolution above the species level
species concepts
morphological species concept
- distinguishes a species by body shape and other structural features
ecological species concept
- defines a species in terms of ecological niche
biological species concep
t - a species is a group of populations whose members have the potential to interbreed in nature and produce fertile offspring
Chapter 22
: Descent with Modification: A Darwinian View of Life
The Darwinian Revolution
observations on the HMS Beagle voyage: gradualism (vs punctuated), uniformitarianism (vs catastrophism)- the earthquake & new fossils, changes in earth's crust over millenia (more than thousands),
descent with modification
- Darwin meant offspring were born with the better traits, thus gradually modifying the entire population, ancestral organisms
natural selection
observation #1: members of a population often vary in inherited traits, inference: the best at survival and reproducing leave more offspring
observation #2: produce more offspring than environment can support, many fail to survive and reproduce, inference: over generations, favorable traits will accumulate (essay by Malthus inspired the idea that organisms "overreproduce")
classification scheme influence: Darwin understood that species are related (evolutionary tree sketch)
artificial selection
- selection and breeding of individuals with desired traits, same concept but controlled by humans
individuals do not evolve. the genome changes over generations, not within a single individual, through the process of natural selection
natural selection acts on heritable traits (amplifies or diminishes) that differ among individuals in a population
The Evidence for Evolution
fossil records: can observe past evolutions along lineages, can discover new organisms and connections and fill in gaps in evolutionary trees
biogeography: study of geographic distribution of species, continental drift, originally neighbor species were separated at the split of Pangea and then evolved separately as they each adpated to their own continent
anatomical structures
homologous structures
- variations of a structure in separate species that was present in a
common ancestor
, the structure was passed down and maintained through many generations of changes (arms, wings, flippers -
different functions
)
vestigial structures
- remnants of features that served a function in the organisms's ancestors, the structure is passed down but does not affect survival, therefore does not get weeded out (human embryo tail)
analogous structures
by convergent evolution - independent evolution of similar features (
similar functions
) in
different lineages
(flying squirrel and sugar glider)
examples: soapberry bugs survive based on the length of their beak in regards to the depth of the fruit they eat, appearances of bacteria resistant to previously-used drugs showcases evolution
The Historical Context for Evolutionary Theory
adaptation
- inherited characteristics of organisms that enhance survival and reproduction in specific environments
evolution
- descent with modification, change in the genetic composition of a pop. from gen. to gen.
three key observations by Darwin in
On the Origin of Species
:
unity & shared characteristics of life
the rich diversity of life
the ways organisms are suited for their environments
other views:
Aristotle - viewed species as fixed & could be arranged on a ladder of complexity, linear hierarchy,
scala naturae
(scale of nature), reflected religious views of creation (species designed perfect)
Linneaus - created nested classification system that grouped similar species into general categories, based on "pattern of creation", species designed for a particular purpose
Lamarck - compared living organisms with fossil forms, lines of descent, use and disuse (giraffe necks), inheritance of those stronger modifications during its lifetime (modern understanding includes genetics)
"Theory" of Evolution
evolution is not "just a theory" because there is evidence to support it, it is a pattern and process
theory
- accounts for observations and explains and integrates a variety of phenomena, can be accepted through evidence and testing
Chapter 23
: The Evolution of Populations
Mutation and Sexual Recombination
mutations and effect on evolution
mutations that change somatic cells do not get passed to offspring, don't affect evolution, mutations in gametes do
most point mutations are harmless or if so, removed through natural selection most times, rarely mutations can be to advantage for the organism,
genetic variability
transposons in bacterial DNA could duplicate a mutation to different locations throughout the DNA sequence
sexual recombination
- unique combination of alleles from each parent in an offspring of sexual reproduction
The Hardy-Weinberg Principle
species
- class of individuals with common attributes, can produce fertile offspring
gene pool
- a population's genetic makeup, copies of every type of allele at every locus in all members of the pop.
population
- group of individuals of same species, live in same area, interbreed, produce fertile offspring
meiosis and random fertilization only shuffle and reshuffle the alleles in the population without introducing new ones
Conditions for Hardy-Weinberg Equilibrium
no natural selection: causes changes in allele frequencies(favors certain ones), the equation measures allele frequencies and given frequencies cannot be allowed to change
extremely large population size: genetic drift occurs in small populations
random mating: any inbreeding or patterns in mates would prevent random mixing of gametes
no gene flow: gene flow alters allele frequencies
no mutations: the entire gene pool would be changed
Genetic Variation, the Substrate for Natural Selection
nucleotide variability
- genetic variation at nucleotide level, doesn't always affect phenotype, average heterozygosity is greater
variation in introns(noncoding segments of DNA) is ineffective
variation in exons(coding) most times does not affect amino sequence of a protein for a gene
quantitative characters
can be a blend of discrete characters(dominant and recessive), greater variation & unique phenotype (ex: pink , not red or white)
natural selection acts on individuals, yet evolution (the impact of natural selection) defines the change in a
population
over time, occurs through the inheritance of genes (Mendel)
cline - gradation of a specific trait in a population
Natural Selection, Genetic Drift, and Gene Flow
genetic drift
: chance events are much more likely to affect small populations and cause changes in allele frequencies
founder effect
- few individuals isolated from larger pop., establish new population, gene pool differs from source pop.
bottleneck effect
- a sudden change in environment reduces size of population, pop. grows back with overrepresented and/or underrepresented alleles
gene flow: genes pass between populations from occasional occurrences of interbreeding and the different alleles eventually mix with the other alleles in the separate populations
mutations are rare, very small effect on allele frequencies over generations
Modes of Selection
directional selection
- conditions favor individuals exhibiting one extreme of a phenotype range, shift frequency curve in one direction, environment change or new habitat
disruptive selection
- conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes, intermediate finch beaks don't match either seed, less fit
stabilizing selection
- acts against both extreme phenotypes and favors intermediate variants, weights of babies, average is best
Sexual Selection
intrasexual selection
- selection within the same sex, one sex competes directly for mates, rams
intersexual selection
- one sex is choosy in selecting mates, peacocks, showcases overall good genes
Balancing Selection
frequency-dependent selection
- fitness of a phenotype depends on how common it is in population, the frequencies are always changing
heterozygote advantage
- individuals at a particular locus have greater fitness than both kinds of homozygotes, natural selection maintains two or more alleles at the locus, genotype, maintains genetic variation
relative fitness
- contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals