Information taken from PowerPoints and readings. Vocab and summary information
Evolution
The change in frequency of heritable traits of a population over time
Lamarck’s Transmutation Theory
First theory of transmutation. Consisting of the complexifying force and the adaptive force
Complexifying force
Organisms move toward “ever greater perfection” i.e. vertebrates, humans
Adaptive force
2 rules by which species adapt to their environments:
1) Use and disuse: if a part or trait is used, it will enlarge or improve
2) Inheritance of acquired characteristics: once a trait is modified, this modification is inherited in the next generation
Malthusian Crunch/Catastrophe
When population growth outpaces subsistence availability, causing famine or war, resulting in poverty and depopulation.
Carrying Capacity
Resource limitation (food, space) that sets the absolute upper bound of population size
Density-dependent factors
Disease, competition, predation, etc.
Density-independent factors
Storms, volcanoes, droughts, fires, etc.
Adaption via natural selection
1) Variation: all populations of an organism vary
2) Inheritance: offspring inherit phenotypic traits from their parents
3) Differential reproductive success: some variants, those best adapted to their environments, leave more offspring than others
The Darwinian Revolution
All living beings are connected by common ancestry
Major mechanism of change is natural selection, with no conscious designer
Humans are not special creations
Variation is the norm
Blending Inheritance
The dominant theory of inheritance at the time of publishing The Origin. Offspring of two phenotypes are expected to display an intermediate phenotype
Genetic Swamping Argument
Proposed by Fleeming Jenkin in 1867: argued that (1) a new species (sport) is rare, and (2) even if the sport had an advantage, the trait would be lost over generations as it was swamped by the wildtype via blending
Blending results in 50% of variation being lost with each generation.
Darwin’s Theory of Pangenesis
A quick solution theory of particulate inheritance to resist the genetic swamping argument: information from different parts of the body is sent to the reproductive system via gemmules, enabling inheritance of acquired characters. Gemmules had affinity for each other and aggregated into the sexual elements/buds.
Weismann’s Germ Plasm Theory
Very early on in the development of animals, the germ line is set aside from the rest of the animal (soma). No inheritance of acquired characters—deviations in soma give rise to temporary non-hereditary variations. Called the Weisman barrier
Hugo De Vries
Tested Darwin’s pangenesis theory by breeding plants: noticed a reproducible 3:1 ratio, determined that tiny material elements are being inherited unchanged (called them pangenes). Identified unique bud sport of primrose, calling it a mutation.
RA Fisher
A biological mathematician who resolved conflict between Mendelians and Biometricians, showing that Mendelian inheritance proves natural selection.
Hardy-Weinberg Equation
p²+2pq+q²
p = frequency of allele A
q = frequency of allele B
Hardy-Weinberg Equillibrium
A state in which a population of organisms is not evolving (i.e. allele frequencies are not changing from generation to generation). Must meet these requirements:
No non-random mating
No natural selection
No genetic drift (random variation)
No immigration or emigration
No mutation
De Vries’ Theory of Mutationism
New species are created in a single macromutation, natural selection only acts to destroy the most harmful mutations
The Genetic Code
Triplet codons are translated to amino acids in proteins, or are messages to stop translation
Types of mutations
1) Point Mutations
Silent, Nonsense (change of protein length), Missense (change of amino acid)
2) Indel mutations
Insertions or deletions (can shift reading frame)
Mc1r Gene
Located in chromosome #16. Codes for MC1R receptor protein that helps stimulate genes involved in the synthesis of eumelanin. A cytosine → thymine mutation at position 199 changes arginine → cystine, a missense mutation creating lighter coat color in mice.
Light colored mice have a repressed receptor on the Mc1r gene.
CCR5-delta32 Mutation
Deletion of 32 bp of CCR5 receptor. 2 copies of this alleles results in HIV immunity, because CCR5 is a one receptor that enables the HIV virus to enter helper T cells.
Gene duplication
Occurs when a mutation (ex. ectopic recombination) accidentally copies a gene, resulting in 2 identical copies. Serves as raw material: mutations to one of the copied genes won’t affect phenotype, so copy genes can act as origins of novelty.
Created cone snail venom toxins, antifreeze glycoproteins in fish
Lederberg & Lederberg Experiment
Demonstrated that mutations are random: bacteria that survived antibiotic treatment had preexisting antibiotic resistance that was selected for.
Parthenogenesis
Asexual reproduction that involves the development of a female gamete without fertilization
Weismann’s Diversity Hypothesis
Sex exists because it generates diversity that is critical for adaptation, that outweighs the cost of sexual reproduction
Muller’s Ratchet
Sex purges the genome of deleterious mutations that would otherwise accumulate in asexual genomes; allows repairs of mutation via copy from wildtype alleles.
Proved by Graham Bell when he made protozoans sad by making them virgins
Red Queen Hypothesis
Genetic diversity generated by sex helps protect against parasitic infection
Permanent linkage
Beneficial and deleterious alleles cannot be unlinked from one another: entire genomes must be the smallest units of selection
Crossing over
Discovered by Thomas Morgan: traits can become unlinked due to genetic recombination during sexual reproduction
Can also happen in the middle of a gene to create a novel allele (raw material)
Ruby-in-the-Rubbish effect
via sexual recombination, individual alleles can be sorted, allowing individual alleles to contribute to fitness
In yeast, sexual populations are able increase frequency of beneficial alleles and decrease frequency of deleterious alleles. Asexual populations must increase both
Gene flow
Sex enables alleles to be changed between populations, can enable adaption (ex. EPAS1 gene passed from Denisovans to Tibetans)
Benefits of sex
Lose the bad: eliminate negative mutations (rachet) and alleles (ruby-in-the-rubbish)
Gain the good: obtain beneficial alleles from crossover and gene flow
Arms race: help to battle with parasites (Red queen hypothesis)
Horizontal Gene Transfer (HGT)
Gene transfer that occurs between living beings of the same species or different species, without transfer via reproduction
Can be adaptive in the same way that gene flow can lend adaptive genes to different populations. Plays a large part in antibiotic resistance
Syncytin
A virus-originating gene encoding a protein made only by cells in the placenta that enables placenta cells to fuse together to create the syncytiotrophoblast
Transferred at least six times into mammalian lineages: placental fusion is more efficient with syncytin than with mammalian genes
Bdelloid rotifers
Group of animals that have been asexual for 80 mya
Transposons
Segments of DNA that can move around to different positions in the genome of a single cell (‘jumping genes’), discovered by McClintock
Agouti Mice
When a retrotransposon inserts near the coding side of the agouti gene, the agouti gene’s expression is disrupted and codes for yellow pigmentation (and obesity)
Transposons and Pregnancy
Mammalian genomes have ‘tamed’ transposon start site fragments to remaining next to hundreds of progesterone responsive genes, without the transposition ability. Enables the coordination of bodily functions during pregnancy
Symbiogenesis
Lynn Margulis’ idea that symbiosis was the driving force of evolution, after hypothesizing the theory of endosymbiosis
Endosymbiosis
Mitochondria formed after a prokaryote that had evolved into an early eukaryote engulfed and kept one or more proteobacteria cells.
Microbiome
The ecosystem of microbes inside/outside of animal bodies and guts. Microbial genes outnumber human genes 100:1.
Help to digest food, build essential nutrients, and defend against bacterial infections
Microbiome and HGT
Gut bacterium Bacteroides plebius contains the same gene as in Japanese people due to symbiosis in digesting seaweed. People can pass on bacterium and bacterium genes to offspring—heritable trait!
Origins of Variation
HGT: adaptive genes can be transferred among organisms, especially bacteria
Transposons: jumping genes can sometimes jump into a spot that is beneficial, especially in regulatory regions
Mutualism: organisms can acquire variations by associating with other organisms that possess those variations