T4 - Microevolution Flashcards
What is microevolution?
A chance in allele frequency in a population or species across generations
Focus is on variatino within populations/species and evolutionary change over shorter time periods
What is macroevolution?
Evolution above the species level; variation among species and on questions releated to diversification (origin of new species and higher order groups) acorss relatively long periods of time
The result of microevolution writ large (longer term and higher taxonomic consequences)
What does microevolution requires? What are the processes that can cause microevolution (4) ?
Microevolution requires genetic variation (more than one allele segregating at a locus in a population)
Processes that can cause microevolution; mutation, gene flow, genetic drift, natural selection
What is random mating?
In a population is also called panmixia - where some species can be panmictic
Ex. Different eel species migrate to a large lake to mate with other eel species and random mating occurs, then the eels migrate back to their original lakes
What is non-random mating?
Affects how alleles are organized into genotypes (alters genotype frequencies of homozygosity and heterozygosity)
under the assumption that all individuals still mate (≠ sexual selection), allele frequencies are not affected ∴ it is not a mechanism of evolution on its own
(if one indiv. mates with others more compared to another that does not mate at all)
Why is mating often non-random?
- relatives may mate more/less often than expected by chance (inbreeding and outbreeding respectively)
- indivs may self-fertilize more or less often by chance
- indivs may mate more often with indivs that are more or less similar to them in phenotype than expected by chance (assortative/disassortative mating or positive/negative assortative mating)
What is the difference between positive and negative assortative mating?
Positive = similar in phenotype
Negative = differs in phenotype
What is inbreeding ? What does this cause ?
When mating takes place between related indivs and the resulting offspring are inbred
Causes an increase in the frequency of homozygotes across the genome (at all loci) and a deviation from the HW expected genotype frequencies
What are the effects of inbreeding?
Effects can be ephemeral; one or a few generations of random mating can restore the HW expected genotype frequencies
- By taking away inbreeding possibilities
- Homozygotes if inbred can ONLY produce homozygotes
What is an inbreeding depression?
The increase in homozygosity as a result of inbreeding tends to decrease fitness
Inbreeding depression = the decrease in fitness
- can exacerbate the loss of genetic variation (allelic diversity) that can occur in small populations due to genetic drift
What are some Mendelian causes of inbreeding depressions?
1) Dominance hypothesis;
- alleles that decrease fitness (deleterious alleles) tend to be partially to completely recessive
- these alleles are held at a low frequency in a population due to NS, so when found primarily in heterozygotes, their phenotypic effects are masked
- increasing homozygosity will increase the phenotypic expression of these alleles ∴lowering fitness
2) Heterozygote advantage;
- at some loci, heterozygotes have a higher fitness than either homozygote
- inbreeding decreases heterozygosity ∴lowering fitness
How have plants/animals evolved traits allowing for inbreeding avoidance?
- kin recognition
- dispersal (spreading out of its small region)
- delayed maturation/reproductive suppresion
- extra-pair copulations
How have hermaphrodites/monoecious species evolved traits allowing for inbreeding avoidance?
- self-incompatibility
- physical/temporal separation of reproductive organs
What is outbreeding? What are the results of it?
The mating between indivs that are less related than would be expected by random mating within the population
Tends to increase heterozygosity, a common outcome is an increase in fitness relative to non-outbred indivs (heterosis/hybrid vigour)
What is a mutation? What are some important characteristics?
Mutation = change in the genetic info (nuceotide sequence) of an organisms DNA
- Arises from errors during DNA replicatioin/recombination
- can create new alleles and is ultimate source of genetic variation
- can also impact the frequency of an allele it might create additional copies of
- are random in occurence
- transmissable (heritable) if in the germ line
- can have variable effects on an organisms fitness
What are two main types of mutation?
Small scale (point mutation);
Substitution = when a single nucleotide is replaced with another
- silent mutation = when amino acid (∴the product) is not affected
- replacement (missense mutation) = when the amino acid (∴the product) is affected
Insertion/deletion = one or more nucelotides are added/removed
- Can alter the reading frame (frameshift), which can produce severe consequences
Large scale; mutations in chromosomal structure (translocations, inversions, chromosomal loss, duplications)
What are some impacts of mutation?
- Most new mutations are deleterious (reducing fitness)
- Beneficial (advantageous) mutations are much less common
- Many are likely neutral or nearly neutral = having little to no effect on fitness; these are mostly lost due to drift but few will drift to fixation
- The combination of mutation and drift can maintani substantial neutral genetic variation in a population
What is gene flow?
AKA migration
The movement of alleles between populations
- introduces and removes alleles from a given population
- rates of gene flow are greater than mutation rates so the effect of gene flow on allele frequencies is generally much greater than those of mutation
- homogenizes populations - reducing genetic differences between them
What is local adaptation?
Occurs when a population adapts to its local environment; populations inhabiting different local environments may therefore diverge due to the differences in the selection they each experience
What effect does gene flow have on local adaptation?
Gene flow can hinder local adaptation by constantly introducing maladaptive alleles from other populations (outbreeding depression = reduced fitness from matings between populations)
- Gene flow can also promote adaptation by helping spread beneficial alleles among populations
What is genetic drift?
Occurs due to sampling variation = the difference between the value in a finite sample compared to the true value of a population
What are the effects of genetic drift?
- causes random change in allele frequency across generations
- the magnitude of change is inversely related to population size (smaller populations = larger changes due to drift -> stronger drift)
- drift reduces genetic variation because alleles are lost
- can overwhelm selection such that deleterious alleles may rise in frequency and even fix
- causes populations to diverge from one another (in absence of gene flow)
What are population bottlenecks?
A severe (generally rapid) decrease in population size which reduces genetic variation and enhances genetic drift
- can be caused by founder event = when a small group of indivs colonizes a new geographic area, isolated from other populations
What is Darwinian fitness?
The absolute contribution of an indiv to the next generation where its reproductive success is measured as the number of offspring it produces
What is relative fitness?
An indivs contribution to the next generation relative to that of other indivs
What are the components of fitness and the types of selection?
- Survivorship/viability (probability of surviving)
- Fecundity (# of eggs produced)
- Mating/fertilization success (sexual selection of ones mating sucsess)
How would one detect natural selection?
- Direct measurement (observational studies)
- Manipulating putative selective agent
What is a problem that arises from correlated traits?
Because traits are often correlated (pleiotropy/physical linkage), direct selection on one trait can produce a correlated (indirect) response in another trait
Ex. Typicaly, finches with longer beaks will also have thicker beaks compared to others with short beaks and thinner beaks
However because of this, we cannot assume that;
- because a trait changed it must have been a direct target of selection
- selection favoured a change in the direction observed
- because a trait didn’t evolve, selection must not have acted on it
What is frequency dependent selection? What are the 2 subtypes?
when the fitness of a phenotype depends on its frequency in the population
i.e. a trait may be beneficial if its in low frequency and harmful at high frequencies
- Positive frequency-dependance= directional selection for a phenotype strengthens as phenotype becomes more common (Mullerian mimicry)
- Negative frequency-dependance = directional selection for a phenotype is stonger when the phenotype is less common (selection favouring rare types)
- this type is a form of balancing selection = one that actively maintains genetic variation
- Ex. MHC molecule (an immune sys pathogen) allows us to recognize cells within the body as our own; if an infection were to adapt to this and evade our body, this could be harmful because we cannot fight it; if this had a high frequency, viruses could learn to create the same pathogen to reproduce
What is heterozygote advantage?
When the heterozygote at a locus has a higher fitness than either homozygote
- preserves genetic variation
- ex. sickle cell anemia
