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- Genes – sequences of DNA that specify proteins
- Each gene can have2(+) forms called alleles
- change in allele frequency in organisms over
Mechanics of evolution (four main
causes/mechanisms of evolution)
- Mutation – Random change in dna sequence.
- Genetic drift – Process in which chance events determine which alleles are passed
- on to the next generation. Affects small pops more.
- Gene flow – transfer of alleles from one pop to another via mvmt of individs (or
- migration) or gametes (pollen)
Mutation – Random change in dna sequence.
- Ultimate source of new alleles. Provides raw
- material for evolution (genetic variation)
- Too rare to cause significant change in allele
- Process in which chance events determine which alleles are passed on to the next generation. Affects small pops more.
- Fixation – if alleles disappear from a pop or
- become fixed. (freq =1.0)
- Can reduce genetic variation in the pop and
- capacity to evolve.
- Can increase freq of harmful alleles and reduce
- survival and reproduction.
- transfer of alleles from one pop to another via mvmt of individs (or migration) or gametes (pollen)
- Pops become +genetically similar
- Can introduce new alleles into a pop.
3 conditions for evolution
- Individs in a pop must share phenotypic variation in a train
- Variation I the trait influences ability of individs to survive and reproduce
- Variation must be heritable
3 types of selection:
- Stabilizing selection (bell curve that gets
- narrower, birth weight)
- Disruptive selection (bell curve to 2 humped
- Directional selection (bell curve that shifts to
- either extreme, white foxes/snow)
bell curve that gets narrower, birth weight
bell curve to 2 humped camel
bell curve that shifts to either extreme, white foxes/snow
natural selection is the only mechanism that consistently causes adaptive evolution.
Why isn’t natural selection perfect?
- Gene flow can limit local adaptation
- Envs always changing
- Constraints on adaptation evolution (ecological trade offs, evolutionary hist, lack of gen. variation.
record of events relating to an org’s growth, devel, reproduction, survival (death).
ex of life hist traits
- Age, size, sexual maturity
- Freq of reproduction
- Survival and mortality rates
Life history strategy
- overall pattern in avg timing and nature of life hist events that characterize a species.
- Phenotypic variation caused by genetic and/or env variation
- Life history traits can vary among individs w/in species (ex age at puberty)
life hist strategies can be maintained by
- Genetic variation
- Phenotypic plasticity – one genotype produces dif phenotypes under dif env conditions.
- May result in continuous variation, or discrete morphs with no intermediate forms.
- one genotype produces dif phenotypes under dif env conditions. May result in
- continuous variation, or discrete morphs with no intermediate forms.
- Ex: carnivore tadpoles grow faster&
- metamorphose earlier than omnivore toad tadpoles. Favored in ephemeral ponds that dry up quickly.
- Omnivores grow slowly, metamorphose later. Favored in ponds that persist longer and are in better condition, increasing
- juvenile survival.
r – selection
- intrinsic rate of ↑of a pop. →fast pop growth.
- Occurs in envs w/ low pop density.
- Favors good colonizers.
- Beneficial in harsh, unpredictable, ephemeral, or disturbed habitats.
- Short lifespan, rapid devel, early maturation, +#, smaller offspring, ↓parental investment.
- Ex: insects, small vertebrates, weedy plant
K – selection
- carrying capacity for a pop (max pop size an env can support)
- In pops at/near carrying capacity, K-selection leads to slow pop growth rates. Occurs in envs with high pop density. Favors good competitors for limited resources.
- Long lifespan, slow devel/maturation low # of larger offspring, +parental investment.
- Beneficial in stable, predictable habitats; “climax communities”
- Ex: long-lived reptiles, large mammals,
- long-lived trees
Life hist trade-offs are balanced by natural
selection to maximize lifetime reproductive success.
4 main qs
- What determines distribution of species? (geo area where individs are present)
- What determines abundance of orgs? (pop size)
- What factors promote pop growth?
- What factors limit pop growth?
4 mian factors that affect pop size
Nx Dx Sx Qx Lx
- Nx = # of individs alive during ageinterval x
- Dx = individs that die during ageinterval x
- Sx (survival rate) = proportion of individs (Nx+1)/Nx
- Qx = (dx/Nx) or1-Sx
- Lx = (Nx/N0)
3 types of life tables
- Cohort life table
- Age-at-death-observed life table
- Static life table
Cohort life table
- fate of a group of individuals born during same period
- (=cohort) is followed from birth to death.
- **Survivorship is directly observed**
- but data is difficult to collect for mobile or
- long-lived species. Better for short-lived, sessile orgs.
Age-at-death-observed life table
- estimates a cohort life table based on known ages of death. Dead individs assumed to represent a cohort.
- Ex: 12k robin nestlings werebanded, 200 dead were recovered over the next 5 years.
- Dead individs must be a random sample of the pop
- Pop size must be stable
- Age-specific survival rates must remain constant
- estimates cohort life table based on survival of individs of dif ages during a single time period (snapshot).
- Must know age of individs
- Ex: count # individs of dif ages in a pop. 100 newborns, 40 1yr olds, 25 2yr ect
- Problem – useful only if you can determine ages of org (tree rings, tooth wear).
- pop size stable, age-specific survival rates must remain constant.
Survivorship curve, 3 general types
plot of # of individs from a hypothetical cohort that will survive to specific ages. Based on lx values.
- 3 general types:
- Type I: high survival until old age. Humans,
- large mammals
- Type II: chance of survival remains constant throughout life. Birds, some turtles, fish.
- Type III: marine invertebrates, insects, many plants
- Life table data can also be used to project
- future age structure, size, and growth rate of a population.
- Age structure – proportions of pop in each age class (pyramid graphs by age/gender over time)
Geometric population growth rate ʎ
- ratio of pop size at time t+1 (Nt+1) to current pop size t(Nt)
- If age-specific survival and fecundity rates remain constant, pop will ultimately grow at fixed rate.
- Leads to stable age distribution same proportion of individs in each age class from year to year
- Any factor that changes survival/fecundity can change pop growth rate.
How does population size influence genetic
Smaller populations are more susceptible to genetic drift, and will often become fixated. Although fixation can happen to larger populations as well, it is less frequent than with smaller p populations, and fixation of a large population takes much longer.
How does the starting allele frequency influence genetic drift?
- The farther away from 0.5 allele frequency a population is, the sooner
- it is likely to become fixated. If one particular allele already has a higher
- frequency, it can quickly take the place of the other allele. Since populations
- with starting alleles of .5 have more of a balance of allele frequencies, they
- are less likely to become fixated so quickly, and it would take many generations for them to become that way if they did.
Do loci always become fixed at the same value (p = 0.0 or p = 1.0) [CTH1] for a given starting allele frequency? Why or
why or why not>
Loci may get fixed at either extreme because drift is a random process.[CTH1]
How does the rate of evolution differ when selection operates against a dominant allele vs. when selection operates against a
- When selection acts against the dominant allele, the population rapidly becomes
- fixed at p = 0. With selection against the recessive allele, selection slows at
- you approach p = 1.0