BIO 115
Human evolution
Human evolution
Set of flashcards Details
| Flashcards | 288 |
|---|---|
| Language | English |
| Category | Biology |
| Level | University |
| Created / Updated | 23.01.2021 / 24.01.2021 |
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When will a trait evolve?
- if there is correlation between a phenotypic trait and the number of offspring
- if there is correlation between the phentope of a trait in parents and their offspring
Positive selection
one allele has a higher fitness than another
Relative fitness
e.g. Individual with fitness W=2
- advantage if population fitness = 1 -> rel. fitness = 2
- disadvantage if population fitness = 4 -> rel.fitness = 1/2
Selective sweeps
- beneficial mutations sweep through a population drastically -> changing allele frequencies
- s: measure of strength of selection that favours beneficial allele
- s>0: beneficial; s<0: deleterious
When is genetic variation maximized?
- if the allele frequency at a locus is 0.5
- is low when near 0 or 1
How many generations for mutation to increase from 10 - 90%?
- approximately 4/s generations
When do beneficial allele frequencies spread fastest?
- Allele is dominant -> fastest spread
- Allele is recessive -> slowest spread
- Allele is neither -> intermediate spread
Mutations that are deleterious / decrease fitness
- Selection is very ineffective at removing these mutations from a population
- many diseases are recessive
Examples: Strong natural selection
- Domestiacation of cows -> ancestors cannot digest lactose -> lactase persistence mutation spread quickly among humans (areas with no domestication of cows -> persistence uncommon)
- Golden gene (SLC24A5) -> less sunlight in northern regions of the earth -> whitening of skin to get more vitamin D from UV light
How do selective sweeps eliminate polymorphisms?
- genetic diversity is reduced close to the beneficial mutation -> neutral sites are preserved (hitchhike)
How can selection preserve variation?
If there is a mutation present that is advantageous if the environment changes -> standing genetic variation
Definition: Balancing selection
- maintaining genetic variation
Examples: Balancing selection
- Malaria: an allele that protects against malaria is most frequent where the disease is common
Definition: Polymorphic equilibrium
- overdominance, when heterozygous A1A2 is most frequent allele
Examples: Selection and allele frequency changes
- Two alleles: A1 and A2
- Positive selection: fitness highest in A2A2 -> fixation (loss of genetic variation)
- Overdominance: fitness highest in A1A2 -> polymorphic equilibrium (maintains diversity)
- Underdominance: fitness lowest in A2A2 -> A2 gets fixed or lost (depends on initial freq)
Measure of genetic differentiation: F_ST (genetic differentiation between populations based on variance in allele frequencies)
F_ST = 0 (no differentiation)
F_ST = 1 (complete differentiation)
Definition: Genetic drift
- happens between generations even without natural selection
- random change in allele frequencies (mutations)
- random fluctuations larger in smaller populations
- causes genetic variation to be lost
Genealogy of genes
- used to find the most recent common ancestor (MRCA) -> lineages of two gene copies merging
What effects genetic drift?
- population size
- changes in size
- age demography
- unequal reproduction
Genetic drift and effective population size
- N_e: number of individuals that would give the same strength of random drift as actual population
- 2*N_e -> Average time to common ancestor
Genetic diverstiy in humans compared to great apes (GA)
- much lower in humans while census size is much larger
- the branch (in tree) is much longer in apes -> larger genetic diversity present
Four reasons why human genetic diversity is lower than in great apes (GA)
- very recent speciation event -> Homo sapiens is new kid on the block
- genetic diversity and size of ancestral population
- demographic events
- stochastic events
Reasons against human races
- differences between clusters are very small -> no effect with high power
- to justify races atleast 25-30% between group differences -> observed 3-5%
- groups found with PCA or other methods -> geographical regions
-> race is a social construct not a biological one!
How is cultural inheritance transmissed?
- horizontally, vertically, and oblique -> by teaching, imitation etc.
Characteristics of gene-culture co-evolution
- faster, stronger and broader range of conditions than conventional evolutionary dynamics
Examples: Gene-culture co-evolution
- learning, social transmission and culture -> genes responsible for learning are changed/developped
- lactase persistence (behaviour for agriculture is cultural) -> gene for human lactase persistence
-. intelligence <- language-facilitiating genes
- handedness <- genes for lateralization of hand preference
- cooperation <- genes predisposing individuals to cooperate in team
- incest taboos <- genes predisposing individuals to avoid incest
- sexual behaviour, sexual selection <- genes for skin, hair etc.
- sex-biased infanticide and parental investment <- sex-ratio distorter genes
- niche construction <- genes related to metabolism, immunity and pathogen construction
How is geographic variation in orang-utans explained?
- Neither genetics nor the environment explain the differences -> hence it is the culture that differs
Kummer's statement on culture/behaviour
"The behaviour of two groups with the same gene pool and the same type of habitat can differ only by culture."
Examples: Cultural species
- Great apes, dolphins
Example: Gene culture co-evolution in animals
- Dolphins use maringe sponge to protect beak for food acquisition (tool use) -> most spongers belong to the same maternal lineage -> cultural transmisson of tool
Process of gene-culture co evolution
- culture selects functional genes
- culture shapes netural genetic variation
- culture reduces genetic diversity
- culture may drive early phases of speciation
Examples: Inferred cultural selection pressure
- Digestion of milk <- Dairy farming
- Detoxification of plant secondary compounds <- domestication of plants
- Immunity, pathogen response <- Dispersal
- Energy metabolism <- Dispersal
Why studying the evolution of cooperation?
- Individuals vary in their traits
- Carriers of certain traits have more offspring than others
- Traits are passed on to offspring
Who does natural selection favour?
Individuals that maximize their fitness.
What is life history theory? (and its Focus)
- Life history studies the changes organisms undergo from conception to death
- focus particularly on the schedule of reproduction and survival
- Explains the phases of the life cycle as a consequence of evolution by natural selection
Who developed a general theory of life history that interrelates many aspects of ontogeny and reproduction across a wide range of organisms?
Charnov (1993)
What is an external factor that cannot be controlled by the organism?
Death (TIME is therefore the most important limitation)
Why selection does not produce eternal lives? There is one reason that cannot be completely controlled
accidental death cannot be completely controlled (it is an external factor)
- examples: Predation, parasites and accidents.
Considering an accidental death rate of 1 % a day, the probability of surviving after one year is only --?
2.6%
(The more extrinsic mortality there is in an organism’s environment (predation, parasites, accidents), the faster will be its life history)
Scheduling production - Individuals have to choose:
- Growing for longer means (2 options)
1) more energy (larger body size) to put into reproduction and therefore better quality offspring (TIME invested in GROWTH)
2) On the other hand growing for longer means less time to invest into reproduction (LENGTH OF PRODUCTIVE SPAN)
