Population Genetics Flashcards
Aristotle – Main Ideas
- examined natural world for evidence of divine order
- can understand the world by looking at physical biological objects – reveal reality by observing the natural world
- Scala naturae (Chain of Being) – fixed
Aristotle
What is the idea of Scala naturae (Chain of Being)?
- hierarchical arrangement of forms
- species arranged linearly along a scale (God → man → mammals → egg-laying animals → insects → plants → non-living matter)
- formed basis for Western belief in fixity of species, each of which has a typical form
Carolus Linnaeus – Main Ideas
- binomial system
- proposed a nested system of relationships (as opposed to Scala naturae)
- Linnaean system
- defined species by focusing on reproduction
- believed in balance of nature
- eventually acknowledged limited formation of new species by hybridization
Carolus Linnaeus
What is the binomial system?
classification system for organisms
each organism a specific name and generic name
ie. humans → Homo sapiens
Carolus Linnaeus
What is the Linnaean system?
nested system of relationships (as opposed to Scala naturae)
groups organisms into:
kingdom – ie. animalia phylum – ie. chordata class – ie. mammalia order – ie. primates family – ie. Hominidae genus – ie. Homo species – ie. sapiens
Carolus Linnaeus
How did he define species by focusing on reproduction?
recognized fundamental difference between:
- interbreeding organisms (within a species)
- non-interbreeding organisms (different species)
Carolus Linnaeus
What were his beliefs in the balance of nature?
- each species has its place in a divine plan
- species would not change or go extinct
Comte de Buffon – Main Ideas
- degeneration
- change only happens within families
Comte de Buffon
What was his idea of degeneration?
believed Linnean hierarchy reflected common descent with divergence over time
- physical environment (somehow) changes organic particles
- new species form when animals migrate
- new environment then causes change to the species
Comte de Buffon
How did he describe his idea that change only happens within families?
- each family conforms to an internal mold
- species can change over time, but are limited to their original mold
Erasmus Darwin – Main Ideas
- wrote about laws of organic life
- transformation or transmutation
- all of life consists of ‘one living filament’ connecting all living forms to common ancestor
Erasmus Darwin
What was his idea of transformation or transmutation?
believed organisms constantly attempted to improve themselves by adapting to their environment
Erasmus Darwin
Was there a mechanism for transmutation?
no
- could have been mutations
- could have been environment inducing mutations
Jean-Baptiste Lamarck – Main Ideas
- first to put mechanism to evolutionary change – inheritance of acquired characters
- theory of transformism
- evolution is a perfecting process
- suggested a mechanism for this organic progression
Jean-Baptiste Lamarck
What is the theory of transformism?
- organisms progress through hierarchy of advancing forms (almost Scala naturae in reverse)
- nature has gradually complicated their structure – producing in succession every species of animal, beginning with the least perfect (or simplest) and ending with the most perfect
- at base of hierarchy, ‘simple’ organisms constantly arise by spontaneous generation
Jean-Baptiste Lamarck
What was the mechanism that he suggested for the organic progression of species?
first law: use or disuse of a structure leads to its development or diminishment
second law: acquired characters can be passed onto offspring
Thomas Malthus – Main Ideas
principle of overproduction
Thomas Malthus
What is the principle of overproduction?
- most organisms produce far more offspring than can possibly survive
- even when resources are plentiful, populations tend to grow geometrically until they outrun their food supply
- poverty, disease, and famine are inevitable, leading to ‘struggle for existence’
Charles Lyell – Main Ideas
- principles of geology
- uniformitarianism
- applied his views to the living world
Charles Lyell
What are the principles of geology?
(had major influence on Darwin and Wallace)
- believed earth is constantly changing
- processes that molded earth’s surface can be understood by modern-day events
Charles Lyell
What is uniformitarianism?
earth is subject to gradual, continuous change – but without progress or development, earth remains at steady state
Charles Lyell
How did he apply his views of the Earth to the living world?
initially believed that some members of all classes of organisms existed throughout the history of Earth
- BUT the change that occurred was abundance and location of species, and exact form of each species
- species have a real existence in nature, and at the time of its creation, each was endowed with attributes and organization by which it is now distinguished
Charles Darwin – Main Ideas
natural selection
Charles Darwin
What were his critical facts on natural selection?
- variability exists within species
- variant traits may be inherited (Darwin didn’t know how)
- composition of populations must change over time → evolution by natural selection
- as natural selection acts on geographically isolated populations, they become increasingly different from each other, leading to formation of first varieties within a species → separate species → genera, etc. (ever-branching process)
Charles Darwin
What did he draw from Malthus’s Principle of Overproduction?
- the principle implies that many individuals must die or fail to reproduce
- individuals slightly better suited to their environment must be more likely to survive
- therefore, some variants will be preserved over time more than others
Alfred R. Wallace – Main Ideas
- natural selection co-discovered
- (drawing from Lyell and Malthus) realized that self-acting process of natural selection would necessarily improve the race, because in every generation the inferior would inevitably be killed off and the superior would remain – fittest would survive
What is evolution?
change in form and/or behaviour of organisms between generations
descent with modification (Darwin definition)
What is change?
origin and alteration over generations of (ideas within society)…
- frequencies of genotypes within populations
- proportion of differentiated populations within species
- proportion of species with different traits within a lineage
What is natural selection?
process by which some individuals contribute more offspring to next generation as a consequence of carrying trait(s) favourable to survival or reproduction
What are some reasons why evolutionary change can occur?
chance bursts of reproduction, deaths and mutation
natural selection explains how undirected change can improve match between organism and its environment
What are the 3 things required for evolution by natural selection?
- individuals vary in some trait (variance)
- individuals with some trait values are more likely to live and/or reproduce (selection)
- parents have offspring with similar trait values (heritability)
What is selection?
differential survival or reproduction of different entities
- only time when there is variation in fitness (ability to survive and reproduce)
- some individuals are better able to survive than others
What is fitness?
average contribution per parent to next generation, including survival and reproduction
What contributes to changes in allele frequencies?
differences in fitness (W)
What do changes in allele frequencies do?
alleles cause individuals to survive at different rates
can measure these rates by fitness, which automatically leads to change
How does allele frequency change before and after haploid selection?
frequency after selection takes into account fitnesses of the alleles
Haploid Selection
What happens to allele frequencies if there is no selection?
allele frequencies are expected to remain the same after gametes come together (gamete union), and after they fall apart again (meiosis)
Haploid Selection
What does the equation for allele frequency after one generation assume?
- haploidy – only has one copy of each gene (A or a)
- only two alleles
- nothing else is going on – no mutation, or selection in diploid phase, etc.
What does it mean to reproduce asexually?
doesn’t have alternations between haploid and diploid – only has haploid selection regime
What are the differences between the equations for evolutionary change for sexual and asexual haploids?
identical equations, as long as there is no selection in diploid phase
Evolution by Natural Selection: Long Term
What are we assuming?
- environment is staying the same
- relative fitness of the two alleles are constant over time
How do we measure fitness when considering the spread of a new beneficial allele (A) in a population of wildtype alleles (a)?
typically measure fitness relative to the wildtype:
Wa = 1 WA = 1 + s
What is the selection coefficient (s)?
proportional increase in fitness caused by replacing allele a with A
- need to define reference, and measure all fitnesses relative to that value, before calling it a selection coefficient
- find selection curve by fitting data
What is selection?
proportional change of relative fitness
Is mean fitness always positive?
yes
Is mean fitness in offspring or parent generation higher?
offspring
What is mean fitness?
sum of fitness of each individual times its weighted frequency in population
If evolution were so easy and effective, why isn’t everything the same?
- organisms vary
- environments differ – fitnesses would be different, would be selected in different directions
- world is changing – not having same fitness when it started
What are methods of evolution NOT by natural selection?
- mutations arise – prevents from rising to perfection
- chance plays a role
- sex and recombination alter
- alleles favoured for effects on some traits may affect other traits (pleiotropy)
- neighbouring alleles in genome can be dragged with selected alleles (hitchhiking) – genes are not selected in isolation