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EVOLUTION Unit 6 Biology


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-Father of Taxonomy, developed a system of binomial nomenclature (genus, species)
-Founded paleontology, the study of fossils
-Founded by Cuvier
- appearance of new species in the fossil records are due to sudden catastrophic events (floods,droughts, earthquakes)
-Geological Processes are so uniform that their rates and effects balance out through time
-EXAMPLE: Rates of Mountain building and erosion are balanced
-DARWIN rejected this but was influenced: if geological change results from low, continuous action, the now we have a time frame for evolution
-Believed that evolution responded to an organism's need to attain perfection
-Use and Disuse
-Inheritance of Acquired Characteristics
Use and Disuse
-Body organs used to cope with the environment become larger and stronger whereas those do not, deteriorate
Inheritance of Acquired Characteristics
Acquired modifications can be passed on to offspring
Charles Darwin
-Naturalist aboard HMS Beagle (1831)
-Observed great diversity in organisms in South America and Galapagos Islands
-Wrote long essay on origin of species and natural selection (1844)
-Recieved Alfred Wallace's own theory of natural selection in manuscript form (6/1858)
-Rushed publication of THE ORGIN OF SPECIES (1859)
-Darwin is considered the main author of the idea since he developed and supported n/s more extensively
Alred Wallace
-Indirectly rushed Darwin's publication of THE ORIGIN OF SPECIES (1859) because a year prior, Darwin had read Alfred's own theory of natural selection very similar to Darwin's
Concepts of Darwinism
1.) Common Descent
2.) Natural Selection
Common Descent
-All organisms related by descent from an unknown common ancestor
Natural Selection
1.) The struggle for existence leads to differential reproductive success --> Influenced by MALTHUS's ESSAY
2.) Individuals best-suited for the environment will survive and reproduce
3.) Unequal ability to survive will lead to changes in the population in favor of beneficial traits--LEADS TO GRADUAL "ADAPTATION" OF THE SPECIES TO THE ENVIRONMENT
-Wrote an essay stating human population will increase faster than food supply, thus competition is inevitable
survival and reproductive success
Industrial Melanism
Example of Natural Selection:
-Prior to Industrial Revolution, dark peppered moths were rare (predation by birds)- trees were covered by light lichen...
-Industrial Revolution killed lichen, thus tree darkened
-Dark colored moths became more common
-POPULATION EVOLVED, not individual moths
Signs of Evolution
1.) Biogeography
2.) Fossil Records
3.) Taxonomy
4.) Comparative Anatomy
A.) Homologous Structures
B.) Analogous Structures
C.) Vestigial Organs
5.) Comparative Embryology
6.) Molecular Biology
Geographic distribution of species
-EX: Islands have many endemic species which are closely related to species on the nearest mainland or neighboring island
EVEN 2 species on different islands with similar environments in different parts of the world will resemble the species of the close mainland rather than each other
Fossil Record
Age and relationships of fossils provide evidence of evolution consistent with biochemical and molecular biology.
EX: Prokaryotes have the most primitive genetic and biochemical processes, they also contain the oldest fossils)
Linnaeus taxonomic scheme shows how different taxonomic levels are related
Comparative Anatomy
many species have organs with similar structures but different functions. This suggests that evolution has modified an existing structure for a different function
Homologous Structures
Similar structures due to common ancestry
EX: Human hand, bat wing
Analogous Structurs
Structures similar in function but of different orgin
EX: bird wing, butterfly wing
Vestigial Organs
Rudimentary structures with little or no use to the organism
EX: Human appendix
Comparative Embryology
Closely related organisms go through similar stages in their embryonic development
EX: Gill slit in vertebrate embryos
View held that embryonic development of an individual organism (ontogeny) is a replay of the evolutionary history of the species (phylogeny)
-EXTREME VIEW: Rather, a series of similar embryonic stages exhibit similar characteristics, not a sequence of adult-like stages
Ernest Haeckel
Molecular Biology
The closer two species are related taxonomically, the higher percentage of commmon DNA
Modern Evolutionary Synthesis
A Genetic Variation on Variation and Natural Selection
-Variation is important for evolution (populations cannot evolve without it)
-Variation ensures the survival of a population (dramatic climate changes)
-THEORY EMPHASIZES: the importance of population as the units of evolution, the essential role of natural selection and gradualism
-1930s: Population Genetics was born providing a mathematical theory for microev
-1940s: A comprehensive theory of evolution was worked out called Modern Synthesis or Neo-Darwinism
Sources of Variation
Crossing Over
Gene Flow
localized group of organisms which belong to the same species
Biological Species
Gene Pool
the total aggregate of genes in a population at any one time. Within a gene pool, there are often two or more alleles for a gene. The roportion of these alleles in the gene pool is called the allele frequency
Population Genetics
involves the study of changes in allele frequencies
change in the population's relative allele frequencies in small scale evolutionary change
Hardy-Weinberg Theorem
-the frequences of alleles in the gene pool will remain constant unless acted upon by other agents
EX: the relative frequency of sickle cell anemia should stay the same every generation unless some force like natural selection acts to change it
-describes the genetics of NON evolving populations
Conditions for Hardy Weinberg Equilibrium
1.) Very large population
2.) Random Mating
3.) No mutations
4.) Isolation from other populations
5.) No Natural selection
Causes of Microevolution
1.) Genetic Drift
2.) Gene Flow
3.) Mutation
4.) Nonrandom mating
5.) Natural Selection (ADAPTIVE)
Genetic Drift
-changes in the gene pool of a small population due to chance (if a population is small, catastrophic events have greater impacts)
-Reduces overall genetic variability in a population
-Two situations for genetic drift to be important: BOTTLENECK EFFECT and FOUNDER EFFECT
Bottleneck Effect
Genetic Drift which results in drastic reduction to population size
EX: 1890s Pop. of northern elephant seals was reduced to just 20 due to hunters. By chance, some individuals survive (bottleneck)- small surviving population is unlikely to represent the genetic make up of the original
-Reduces overall genetic variability since some alleles may be completely absent
Founder Effect
Genetic Drift which results when a few individuals colonize a new habitat
-all descendants of those few individuals will be genetically similar
EX: Galapagos finches are all descended from a few S. American mainland finches
Gene Flow
the migration of fertile individuals or the transfer of gametes BETWEEN POPULTIONS
- Gene flow tends to reduce between-pop. differences which ahve accumulated (by n/s or genetic drift)
-extensive gene flow can eventually group neighboring populations into a single population
Mutation rates- one per 10^5 or 10^6 Gametes
-A single mutation takes a very long time to affect the gene pool of a large population
Nonrandom Mating
increases the number of homozygous and decreases heterozygous loci in a population but does not in itself alter frequences of alleles in a population's gene pool.
EX: p^2, q^2 and 2pq change but not p and q, only the way they are combined
TYPES of RANDOM MATING: 1.) Inbreeding, 2.) Assortive Mating
Mating with closely related individuals
-Self-fertilization ( common in plants) is the most extreme example of inbreeding
Assortive Mating
individuals mate with partners that are like themselves in certain phenotypic characters
Natural Selection (Micro-Ev)
due to selection, alleles are passed on to the next generation in disproportionate numbers relative to their frequencies in the present generation
-Variability in a population makes it possible for natural selection to occur
-natural selection is the bases if ADAPTIVE changes in evolution. As the allele frequency changes, the population as a whole becomes better adapted
Preserving Genetic Variation
Natural selection may eliminate unfavorable genotypes... Mechanism that preserve/restore variation:
2.)Balanced Polymorphism
3.) Frequency-dependent selection
hides much genetic variation from selection by the presence of recessive alleles in heterozygotes
-the more rare the recessive allel, the greater its protection by heterozygosity (a greater portion are hidden in heteros by a dominant allele)
-This type of protection maintains a large pool of alleles which may be beneficial if conditions change
Balanced Polymorphism
The ability of natural selection to maintain diversity in a population results from:
1.) Heterozygote advantage
2.) Balancing Selection
Heterozygote Advantage
Heterozygote Advantage (Hybrid Vigor)- heterozygotes have greater reproductive success
EX: Heterozygotes for sicke cell anemia are resistant to malaria, much higher frequency of heteros in Africa where malaria is present)
Balancing Selection
patchy environments where different phenotypes are favored in subregion sof a populations geographic range
EX: Beak sizes in finches (Small and large favored)
Frequency Dependent Selection
reproductive success of any morph declines if that phenotype becomes too commmon in the population
EX: Batesian Mimicry: Butterfly has certain type of protective coloration that resembles noxious butterfly species, if there are too many mimics, birds will encouter more good-tasting mimics and would not associate the color pattern with bad taste
Modes of Natural Selection
The frequency of heritable characteristics in a population may be affected in one of 3 different ways by natural selcection, depending on which phenotypes are favored
1.) Stabilizing Selection
2.) Directional Selection
3.) Disruptive/Diversifying Selection
Stabalizing Selection
Occurs when extreme phenotypes are eliminated and the intermediate phenotype is favored
EX: Human birth weights are in the 3-4kg range, extreme birth weights have greater infant mortality
Directional Selection
Occurs when extreme phenotype is favored and the distribution shifts in that direction
EX: A shift or dark-colored peppered moths from light colored correlated with increasing population
Diversifying/Disruptive Selection
Occurs when extreme phenotypes are favored and can lead to more than one distinct form
EX: British snails: inforest areas, predators feed on snails with light bands, in low vegetation areas, feed on snails with dark shell that lack light bands
Sexual Selection*
Darwin viewed sexual selectin as a seperate selection process leading to sexual dimorphism. Usually secondary sexual characteristics have not adaptive advantage other than attracting mates
Sexual Dimorphism
Distinction between the secondary sexual characteristics of males and females, usually the male is "showier"
splitting of one species into two or more species or the transfoormation of one species into a new species over time; speciation in sthe final result of changes in gene pool allele and genotypic frequences, leads to macroevolution (origin of new taxonomic groups)
Origin of new taxonomic groups
accumulation of changes leads one species to transform into another
branching evolution budding off one or more NEW species from a parent species (promotes diversity by increasing number of species)
Species defined by anatomical features (Linnaeus' hierarchical classification KPCOFGS)
Biological species
members are reproductively isolated from all other such groups (alt. def. formulated by Mayr 1942)
Mechanisms of Reproductive Isolation
Reproductive Barrier
Reproductive Barrier
Any factor that impedes two species from producing fertile hybrids, thus contributing to reproductive isolation
1.) Prezygotic Barriers
2.) Postzygotic Barriers
Prezygotic Barriers
act to prevent mating or fertilization
1.) Ecological Isolation
2.) Temporal Isolation
3.) Behavioral Isolation
4.) Mechanical Isolation
5.) Gametic Isolation
Postzygotic Barriers
act after fertilization to prevent the hybrid zygote from developing into a fertile adult
1.) Hybrid Inviability
2.) Hybrid Sterility
3.) Hybrid Breakdown
Ecological Isolation
Species live in different habitats and rarely come into contact
EX: Two garter snake species live in the same geographical area but one is terrestrial and the other is aquatic
Temporal Isolation
Two species breed at different times of the day, season or year
EX: Brown trout and rainbow trout live in the same streams but brown trout breed in the fall, and rainbow in spring
Behavioral Isolation
Many species use special signals to attract mates. These tend to be highly specific for particular species
EX: Males of different species of firefuly use different flashing patterns to attract females
Mechanical Isolation
Structural differences in genitalia or flowers prevent copulation or pollen transfer
EX: Many plant species have flowers which have evolved shapes that are specific to the insects or birds that pollinate them
Gametic Isolation
Even when gametes of different species meet, cross-feritilization rearely occurs. Molecules on the outside of the eggs and sperm may function in gamete recognition
eX: Different species of mussels release their eggs and sperm into the water clumn for external fertilization. Cross-fertlization does not occur.
Hybrid Inviability
Hybrid zygotes fail to develop or reach sexual maturity
EX: Frogs of genus RANA occasionally form hybrids but the zygotes cannot complete development
Hybrid Sterility
Hybrid zygotes develop but the adults are sterile
EX: The mule is a cross between a horse and a donkey, mules cannot breed with either species
Hybrid Breakdown
Hybrid adults are fertile, but the offspring of hybrids are feeble or sterile
EX: Certain cotton species can form hybrids, but the offspring of hybrids die as seeds or grow into defective plants
Modes of Speciation
A new species is formed when a population becomes genetically isolated from other populations of the parent species. The mechanisms of mutation, genetic drift, natural selection, etc. cause the isolated population to evolve differences from the parent species.
Allopatric Speciation
Geographical processes may isolate a population from its parent species. A mountain range may emerge to separate valleys; a lake may subside to form several smaller lakes; etc.
EX: Death Valley California: isolated springs that used to be connected, each spring harbors its own speces of pupfish
EX: Island chains represent a diversity of new habitats which are isolated from one another.
Adaptive Radiation
Allopatric speciation can occur repeatedly leading to the formation of many new species.
EX: Over 500 species of Drosophilia have evolved on Hawaiian Islands
Sympatric Speciation
A new species arises within the range of the parent species. Reproducive isolation evolves without geographic isolation
-Sympatric speciiation is likely to involve a special GENETIC MECHANISM.
EX: in plants, nondisjunction f all the chromosomes can produce polyploidy (2n=3n)
Rates of Speciation
Therea re two models for how evolution proceeds. One model says changes are teady and gradual, the other says speciation occurs relatively quickly (punctuated), speerated by long periods of no change (stasis)
1.) Gradualism
2.) Punctuated Equilibrium
Morphological change occurs at a slow and steady pace. New speciations form at the same pace
Punctuated Equilibrium
Niles Eldredge and Stephen Jay Gould 1972: Speciation occurs relatively quickly (few thousand years) and between speciations are long periods of little change.
death of a species
-vitalto the process of evolution- a niche is left unoccupied that can be filled by another species
EX: mass extinction of dinosaurs left many niches open for mammals to evolve into

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