Mechanisms of Evolution:


| Ch. 24 - The Origin of Species | Ch. 22 - Descent with Modification: A Darwinian View of Life | | Ch. 13 - Meiosis and Sexual Life Cycles | Ch. 23 - Evolution of Populations

Ch. 24 - The Origin of Species


Must Know:
- micro/macroevolution
- concept of species
- prezygotic/postzygotic barriers
- allopatric/sympatric speciation
- autopolyploid/allopolyploid
- punctuated equilibrium and gradualism

Speciation:
- process by which new species arise

Microevolution:
- change in genetics of population between generations
- adaptations confined to single gene pool
ex. a population becoming taller

Macroevolution:
- change above species level
- used to define higher taxa
ex. appearance of feathers

Biological Species Concept:
- defines species as population/group of pops. whose members can interbreed in nature
- produce viable, fertile offspring
- unable to with members of other pops.

Reproductive Isolation:
- existence of biological barriers that impede two species from having viable, fertile hybrids

Gene Flow:
- genetic additions to and subtractions from a population from movement of fertile individuals/gametes
- genetic exchange across populations

Prezygotic barriers:
- prevent mating and hinder fertilization between 2 diff. species
1. Habitat Isolation: two species don't live in the same habitat
2. Behavioral Isolation: other species don't respond to unique mating signals
3. Temporal Isolation: breed at different times of day, seasons, years
4. Mechanical Isolation: anatomically incompatible
5. Gametic Isolation: gametes unable to fuse to form zygote

Postzygotic barriers:
- prevent fertilized egg from developing into a fertile adult
1. Reduced Hybrid Viability: genetic incompatibility causes development to cease
2. Reduced Hybrid Fertility: offspring is sterile
3. Hybrid Breakdown: two species can produce viable, fertile hybrids, but when hybrids mate, offspring are weak/sterile

Allopatric Speciation:
- two populations are geographically isolated
- geological events like emergence of mountain range, land bridge, evaporation of lake

Sympatric Speciation:
- small part of population becomes new population without geographical separation
- autopolyploid plants through nondisjunciton in meiosis, ex. have 4n instead of 2n, can't mate with 2n
- switching to new habitat, food source, etc.
- polyploid speciation uncommon in animals

Adaptive Radiation:
- many new species arise from single common ancestor
- a few organisms go to new distant area
- environmental changes cause extinctions, opening up ecological niches

Gradualism:
- proposes species descended from common ancestor
- gradually diverge as they acquire unique adaptations

Punctuated Equilibrium:
- periods of apparent stasis punctuated by sudden changes in adaptations



Ch. 22 - Descent with Modification: A Darwinian View of Life


Must Know:
- Lamarck vs. Darwin
- evidence for evolution
- homologous/analogous structures
- adaptations, variation, time, reproductive success, heritability

Historical Background:
Scala Naturae:
- Aristotle believed life forms could be arranged on scale increasing in complexity
Old Testament:
- Perfect species individually designed by God
Carolus Linnaeus:
- similar species grouped into increasingly general categories
- pattern of creation
- developed taxonomy
- binomial nomenclature, two-part naming
Georges Cuvier:
- opposed evolution
- catastrophism, events in past occured suddenly, by different mechanisms than today
- explained boundaries between strata, location of diff. species
Charles Lyell:
- uniformitarianism, geological processes have not changed over time
- earth must be very old
Jean-Baptiste de Lamarck:
- early theory of evolution
- use and disuse: parts of body used extensively become larger, stronger
- Inheritance of acquired characteristics: things acquired during lifetime could be passed on
- recognized evolution, explanation flawed
Charles Darwin:
- Natural Selection: process in which individuals with certain characteristics survive/reproduce at higher rate than others
- Adaptations: characteristics that enhance ability to survive and reproduce in specific environments

Artificial Selection:
- selection by humans, ex. dog breeding

- individuals do not evolve, populations evolve

Evidence for Evolution:
1. Direct Observations
- ex. predation of wild guppies = drably colored males, drug resistant viruses/bacteria

2. Fossil Record
- show evolutionary changes over time and origin of organisms

3. Homology and Convergent Evolution
- Homology: characteristics in related species have underlying similarity though different functions, homologous structures are anatomical signs of evolution, ex. bird's wing and whale's fin
- Embryonic homologies: similarities in early stages of animal development, ex. post-anal tail in vertebrates
- Vestigial Organs: remnants of structures important for ancestors, ex. appendix in humans
- Molecular homologies: shared at molecular level
- Convergent Evolution: distantly related species resemble one another, adapted to similar challenges, not because evolved from common ancestor
- Analogous Structure: result of convergent evolution

4. Biogeography:
- geographic distribution of species
- species in discrete area more closely related than to species in distant area, ex. desert animals in S. America more closely related to local animals than those in deserts of Asia
- Continental drift: break up of Pangaea explains similarity of species on continents distant today
- Endemic species: found at certain locations but nowhere else, ex. marine iguanas in Galapagos


Ch. 13 - Meiosis and Sexual Life Cycles


Must Know:
- asexual/sexual reproduction
- meiosis and fertilization
- homologous chromosomes
- diploid/haploid
- 3 differences between mitosis and meiosis
- crossing over, independent assortment, random fertilization

- offspring acquire genes by inheriting chromosomes

Genes:
- segments of DNA that code for basic units of heredity
- transmitted from one generation to next

Gametes:
- reproductive cells that transmit genes
- sperm and ova
- haploid (n)
- half number of chromosomes as somatic cell
- 22 autosomes and single sex chromosome (X or Y)

Locus:
- location of a gene on a chromosome

Asexual Reproduction:
- single parent passes all genes to offspring
- mitosis
- exact copies of parent's genome
- clone, genetically identical

Sexual Reproduction:
- two parents contribute genes
- greater genetic variation

Life Cycle:
- generation to generation sequence from conception to production of offspring

Somatic Cells:
- any cells that are not gametes
- 46 chromosomes

Karyotype:
- picture of complete set of chromosomes arranged in homologous pairs from largest to smallest

Homologous Chromosomes- each carry genes that control same inherited characteristics
- same gene at same locus
- similar length, staining pattern, and centromere position
- one homologous chromosome from each pair inherited from each parent
- half of 46 chromosome set from mother and father

- human females have XX, males have XY

Autosomes:
- non-sex chromosomes

Fertilization:
- combination of sperm and egg
- haploid gametes from each parent fuse
- restores diploid number of chromosomes

Zygote:
- fertilized egg
- result of fertilization
- diploid (2n)

external image meiosis2cropped.jpg
external image Image_5.GIF
Meiosis:
- cell division that reduces chromosome number by half
- results in 4 daughter cells
Synapsis:
- joining of homologous chromosomes along their length
- results in Tetrad
- precisely aligned gene by gene

Processes that Contribute to Genetic Variation:

1. Crossing Over:
- DNA exchanged between homologous chromosomes
- prophase I
- criss-crossed regions called Chiasmata form
- hold homologous together until anaphase
- all four chromatids different

2. Independent Assortment:
- metaphase I
- homologous chromosomes line up on metaphase plate
- can face either direction
- 50-50 chance that the new cell will get either homologous chromosome
- 2^23 combinations possible

3. Random Fertilization:
- each egg and sperm is different
- each combination is unique
- 2^23 X 2^23 combinations for human sperm and egg



Ch. 23 - Evolution of Populations


Must Know:
- mutation and sexual production in genetic variation
- conditions of Hardy-Weinberg Equilibrium
- Hardy-Weinberg equation, calculating allelic frequencies, testing whether population is evolving

Microevolution:
- change in allele frequences over generations
- evolution on smallest scale

Mutations:
- only source of new genes and alleles
- only mutations in cell lines that produce gametes can be passed on

Point Mutations:
- changes in one base in a gene
- can have significant impact on phenotype
- ex. sickle-cell disease

Chromosomal Mutations:
- delete, disrupt, duplicate, rearrange many loci
- almost always harmful

- most genetic variation due to sexual recombination of already existing alleles
- sexual reproduction rearranges alleles into new combinations every generation

Population Genetics:
- study of how populations change genetically over time

Gene Pool:
- all alleles at all loci in all members of a population

Fixed Allele:
- all members of population are homozygous for same allele
- one allele exists at that locus in the population
- greater # of fixed alleles, lower species' diversity

Hardy-Weinberg theorem:
- describes population that is not evolving
- frequencies of alleles and genes in population's gene pool remain constant
- unless acted upon by forces other than Mendelian segregation and recombination of alleles
- unlikely conditions will be met

Conditions for Hardy Weinberg Equilibrium:
1. No mutations
2. Random mating
3. No natural selection
4. Population size must be extremely large (no genetic drift)
5. No gene flow (emigration/immigration, transfer of pollen, etc.)

Hardy-Weinberg Equation:
p^2 + 2pq + q^2 = 1
p + q = 1
p = frequency of dominant allele
q = frequency of recessive allele

3 Major Factors That Alter Allelic Frequences:

1. Natural Selection:
- individuals with variations better suited to environment produce more offspring
- alleles passed on in proportions different than in present generation

2. Genetic Drift:
- unpredictable fluctuation of allelic frequencies
- smaller population, greater chance
- random, nonadaptive change
- Founder Effect: a few individuals become isolated from larger pop., establish new pop. whose gene pool not reflective of source
- Bottleneck Effect: sudden change in environment drastically reduces size of pop., few survivors that pass through restrictive "bottleneck" have gene pool that no longer reflects original

3. Gene Flow:
- population gains/loses alleles
- movement of fertile individuals or gametes
- makes different populations more similar

Relative Fitness:
- reproductive success
- contribution an individual makes to next generation gene pool compared to that of other individuals

external image 4160464772_8e32a863a0_o.jpg
- natural selection acts directly on phenotype, indirectly on genotype, alters frequency of heritable traits in 3 ways:

1. Directional Selection:
- individuals with one extreme phenotypic range favoured, shifting curve toward this extreme

2. Disruptive Selection:
- favours individuals on both extremes rather than intermediate phenotypes

3. Stabilizing Selection:
- against both extremes, favours intermediate phenotypes

Diploidy:
- recessive alleles are hidden from selection

Heterozygote advantage:
- ex. sickle-cell disease: homozygous dominant more susceptible to malaria, homozygous recessive have sickle-cell disease, heterozygous protected from malaria and don't have sickle-cell disease

Why Natural Selection Can't Produce Perfect Organisms:
1. selection only edits existing variations
2. evolution is limited by historical constraints
3. adaptations are often compromises
4. chance, natural selection, environment interact