Ch. 19 & 20

Evolution II


II. Connections


1. Sympatric Speciation & Microtubules: Sympatric speciation is when a species may form within a home range of an existing species, in the absence of a physical barrier.  Reproductive isolation is key for speciation to occur.  Reproductive isolation maight happen within a few generations through polyploidy.  Polyploids originate through spontaneous induced hybridization between closely related species or doubling of the chromosome number.  Homologous chromosomes failed to separate at meiosis, when the microtubule spindle fibers did not pull apart the chromosomes, and it produced fertile polyploids.


2. Endosymbiosis & Atmospheric Gases: Endosymbiosis is a theory that states a symbiont species lives out its life inside a host species, and the interaction benefits one or both of them.  By this theory, eukaryotic cells evolved after the noncyclic pathway of photosynthesis emerged and permanently changed the atmosphere.  BY 2.1 billion years ago, certain prokaryotic cells had adapted to the concentration of some atmospheric gases, such as oxygen, and were already engaged in aerobic respiration.  The ancestors of eukaryotic cells preyed upon some aerobic bacteria and were parasitized by others.  At that time, endsymbiotic interections began.


3. Adaptive Radiation & Character Displacement: An adaptive radiation is a urst of divergences from a single lineage that leads to many new species.  It requires adaptive zones, or a set of niches that come to be filled by a group of usually related species.  Either the lineage enters a vacant adaptive zone or it competes with the resident species will enough to displace them.  Sometimes as a result of this competition, a species will develop morphological differences  to reduce the competition.  This is known as character displacement.


4. Proto-cells & Uracil: Proto-cells were transitional forms between simple organic compounds and the first living cells.  These were no more than membrane-bound sacs that contained systems of enzymes and other agents of metabolism, and that were self-replicating. It all started with spontaneous formations of lipids, carbohydrates, amino acids, proteins, and nucleotides under abiotic conditions.  Then, the formation of lipid spheres and protein RNA systems appeared.  Because the one difference between RNA and DNA is a single -functional group of uracil, the RNA thus evolved into DNA, using RNA to create enzymes and proteins.  Proto-cells emerged and the living cells.


III. Few Essentials  


1. Prezygotic and Postzygotic Reproductive Isolation Mechanisms

• Mechanical Isolation: individuals can't make or pollinate because of physical incompatibilities
• Temporal Isolation: Individuals of different species reproduce at different times
• Behavioral isolation: Individuals of different species ignore or don't get the required cues for sex
• Ecological Isolation: Individuals of a different species live in different places and never meet up
• Gamete Mortality: Gametes of different species are incompatible, so no fertilization
• Hybrid Inviability: Hybrid embryos die early or the new individuals die before they can reproduce
• Hybrid Sterility: Hybrid individuals can't make functional gametes

2. The gradual model of speciation holds that species originate by slight morphological changes over time spans.  The model fits with many fossil sequences.  The sequence reflects gradual morphological change.  The punctuation model of speciation offers a different explanation for patterns of speciation.  Most morphological changes are said to evolve in a relatively brief geologic period, within the tens to hundreds of thousands of years when populations are starting to diverge.  Directional selection, genetic drift, the founder effect, bottlenecks, or some combination of them favor rapid speciation.

3. Stanley Miller was the first to test the hypothesis that the simple compounds that now serve as the building blocks of life can form by chemical processes.  He put water, methane, hydrogen, and ammonia in a reaction chamber.  He kept circulating the mixture and zapping it with sparks to stimulate lightning.  In less than a week, amino acids and other small organic compounds had formed in the chemical brew.

4. Evidence to support emdosymbiosis

• rod-shaped bacterium infected culture of amoeba discoides
• cells grew slowly and became vulnerable to starving to death
• five years later, infected amoebas were harboring many bacterial cells, yet all thriving
• infection-free cells got a nucles from an infected cell
• more than 90% survived when a few bacteria were included with transplant
• infected amoebas had lost their ability to synthesize an essential enzyme
• depended on bacterium to make it for them 
• mitochondria resemble bacteria in size and structure
• chloroplasts resemble cyanobacteria

5. on separate sheet of paper

Ch. 17 & 18

II. Connections


1. Prophase 1 & Allele Frequency: Allele frequency is the abundance of one allele relative to others at a gene locus among individuals of a population.  Alleles are different molecular forms of a gene at a given locus.  Alleles are a cause of genetic variation among individuals of a population.  The frequency of an allele in a population is determined by how often the gene is inherited.  An example of when an allele arises is during crossing over in meiosis 1.  When tetrads start forming in prophase 1, some alleles can switch between chromosomes causing an individual to inherit a different gene for a trait.


2. Analogous Structures & Natural Selection: Analogous structures are dissimilar body parts that have become similar in structure, function, or both in lineages that are not closely related but were subjected to similar pressures.  They are different responses of different body parts to similar challenges.  These similar challenges are results of environment changes that hindered the populations' survival.  The population needed to develop these changes in their body parts so that they can survive and reproduce.  These populations' experienced natural selection. 


3. Gene Pool & Bottleneck: A bottleneck is a drastic reduction in population size brought about by severe pressure, such as contagious disease, habitat loss, or hunting.  Even if a moderate number of individuals survive a bottleneck, a genetic drift will occur, changing the allele frequencies at random.  With different allele frequencies, the population will have a different gene pool because now their pool of genetic resources is altered.


4. Balanced Polymorphism & Search Image: Balanced polymorphism is an outcome of natural selection against homozygotes, so that two or more alleles for a trait are bring maintained in the population.  A search image is when a predator develops a knowledge of their prey type, including what the prey looks like, what it smells like, etc.  Both of these terms refer to ways that the environment limits the weak traits in a population and accentuates the strong traits in a population.


III. Few Essentials


1. Natural Selection

1. Natural populations have an inherent reproductive capacity to increase in size over time.
2. No population can indefinitely grow in size, because it individuals will run out of food, living space, and other resources.
3. Sooner or later, individuals will end up competing for dwindling resources.
4. Individuals share a pool of heritable information about traits, encoded in genes.
5. Variations in traits start with alleles, slightly different molecular forms of genes that arise through mutations.
6. Some forms of traits prove better than others at helping an individual compete for resources survive, and reproduce.  In time, alleles for adaptive forms become more frequent relative to others alleles in the population.  They lead to increased fitness- an increase in adaptation to the environment as measured by the genetic contribution to future generations.
7. Natural selection is the outcome of differences in reproduction among individuals of a population that vary in shared traits.  Environmental agents of selection act on the range of variation, and the population may evolve as a result.

2. Most Important Events

1. 4,600-3,800 million years ago: Chemical, molecular evolution leads to origin of life (from proto-cells to anaerobic prokaryotic cells)
2. 2,500-544 million of years ago: Oxygen accumulates in atmosphere; origin of anerobic metabolism; origin of eukaryotic cells.
3. 410-360 million of years ago: Origin of vascular plants
4. 99-65 million of years ago: Origin of angiosperms
5. 65 million years ago: Asteroid impact; mass extinction of all dinosaurs and many marine organisms
6. 1.8 million years ago-present: Modern humans evolve; most recent extinction crisis is under way

3. An individual organism does not evolve, or develop a mutation in their DNA causing them to have certain traits, on their own.  An individual's genes are determined during meiosis, where genetic variation occurs through mutations.  These mutations in individuals are either favored or hindered by the environment the individuals in the population are living in.  If the individuals have a favorable trait, those individuals will survive and reproduce, passing the genes of the favorable trait onto the their offspring.  If the individuals have the not-favorable trait, they will not survive, thus eliminating that trait.  A population of individuals can evolve from having one trait to another due to the environment favoring one trait over another.

4. Phenotypic variation most often comes from natural selection.  In natural selection, the environment decides specific genes that are the most favorable in a particular situation, and those organisms are the ones that are able to survive. The phenotypes that are not favorable in that particular environment die off and are eliminated from that population. Phenotypic variation also occurs in basic genetic variation. 

5. Hardy-Weinberg Principle 

• equation: p + q = 1.0 
• The dominant allele=A 
• The recessive=a
• Their frequencies=p and q 
• freq(A) = p; freq(a) = q; p + q = 1. 
• If the population is in equilibrium= 
• freq(AA) = p² 
• freq(aa) = q² 
• freq(Aa) = 2pq  
• 5 conditions: 
• there must be no mutation
• the population is infinitely large
• the population is isolated from all other populations of the species
• mating is random
• all individuals survive and produce the same number of offspring.