10 Microevolution To Speciation Flashcards
Problems with Darwin’s model of natural selection (3)
Natural selection —> if variation exists in populions, some of this variation is heritable and some variants survive better than others. Leads to next generation of population being biased to variants. Over many generations - adaptation made.
Natural selection should drive any beneficial mutation to fixation
Process gets rid of genetic variation
Blending inheritance destroys variation that exists
Natural selection required variation but blending inheritance gets rid of it - as does the process of natural selection
Adding in mutations and heredity (2)
Mutation generates variation, permits novelty
Diploidy, segregation maintains variation in absence of selection
Types of mutation
Silent mutations / Missense mutations / Nonsense mutations
Exon mutations may affect protein sequence and functions OR protein presence
Mutations that effect transcription factors
Repressor - silencing element (prevents transcription)
Activator - enhancer elecmet (activated transcription)
Mutations in repressor / promoter / activator regions affect temporal and spatial patterns of gene expression
What mutation events have large effects
Gene duplication / novel genes / endosymbiosis / changes in karyotype / polyploidy
Does eveolution use small or large effect mutations?
Most selection on mutations of small effects allow for gradual and slow change overtime
Why are macro mutations important in microbes
Analysis in microbial genomes —> widespread transfer of genetic material between microbes —> ‘cassettes’ of genes introduces = complete function transferred
Macro mutations in eukaryotes
Symbiosis —> eukaryotes and photosynthetic eukaryotes —> genome duplication (Hox gene complexity and jawed fish)
Influence of population size on mutations and adaptation
Smaller population size —> mutations less likely to happen —> slower adaptation rate
Mutation occurs in 1/1billion individuals / generation
Selection acts on phenotype, underpinned by genotypes (simple)
Simple genetic basis —> underpinned by genes of 1 or few loci (can measure frequnecy directly / segregation patterns known / can use maths to model changes and track allelic variants)
Selection acts on phenotype, underpinned by genotypes (complex)
Complex genetic basis —> underpinned by variation at many loci (quantitative traits / cant track individual loci / can track overall phenotype change using heritability)
CALCULATIONS ON LECTURE VIDEO
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Quantitative traits, many aspects of phenotype are the sum…
Of many genetic loci (model evolution using heritability)
Which allele can be selected for when rare
Dominant
Paradox of genetic diversity - Natural selection should (2)
Drive any beneficial mutation to fixation (positive selection)
Drive any deleterious mutation out of population (purifying selection)
In theory these should get rid of genetic variation
Deleterious variants
Selected against (subject to purifying selection - should be removed)
Neutral variants
Doesn’t affect fitness (not subject to purifying selection)
Advantageous variants
Selected for (subject to positive selection)
Why does genetic disease persist
On average each person carries 4 lethal/deleterious mutations but vast majority are recessive so have little phenotypic effect unless homozygous and most individuals have different lethal mutations so each genetic disease is rare
Alleles causing genetic disease present because mutation is recurrent. Selection removed them, but does so slowly when rare (recessive). Therefore there are many different lethal mutations, all rare —> founder effects make particular genetic disease more common.
Neutral mutations not subject to selection - why more diversity in large populations?
Neutral mutations don’t affect phenotype - outsider of genie regions (in introns) / third base pair positions so no AA change or affect phenotype in insignificant way - AA with same property coded for
Process of genetic drift - every neutral mutation has a chance of spreading into the population
Eventually every mutation will fix or be lost (but can hang around for a long time in large populations - fixed faster in smaller populations)
Why is there still genetic diversity in naturally selected traits?
Classic model of natural selection - beneficial mutation (positive selection) mutation increases in frequency and everyone has it in the end
Balanced polymorphism
Segregational variation - heterozygous advantage
Example - sickle cell anaemia (West Africa) - have oxygen carrying blood and resistant to malaria
Environmental variation with gene flow
Spatially diversgant selection —> Point mutation (lactase persistence)
Mutations onlu spread in human populations where cattle domesticated, lack of fixation in these population: incoming genes / presence in other populations: move,net out of pastoralist societies
Frequency dependant selection
Some traits are good when theyre rare - eg. Predators learn to detect common prey
Processes that maintain adaptive genetic diversity (6)
- Natural selection
- Gene flow
- Mutation
- Genetic drift
- Sexual selection
- Balancing selection
Natural selection
Favors traits that enhance survival and reproduction. It can maintain genetic diversity if different alleles are advantageous in different environments or circumstances
Gene flow
Movement of alleles between populations. It can introduce new genetic variation or maintain diversity by preventing populations from becoming too genetically similar
Mutation
Source of new genetic variation. Although mutations are generally rare, they provide new alleles that could be advantageous under certain environmental conditions
Genetic drift
Random changes in allele frequencies, especially in small populations. Though it can reduce diversity, it can also play a role in maintaining unique alleles in isolated populations
Sexual selection
Selection based on traits related to mate choice, which may maintain diversity in traits like coloration or display behaviors
Balancing selection
A type of natural selection that maintains multiple alleles at a locus within a population (e.g., through heterozygote advantage or frequency-dependent selection)
Red Queen Processes definition
Evolutionary processes where species must continuously adapt to survive against co-evolving organisms
These processes maintain genetic diversity by favoring adaptations that keep pace with changing environmental or biotic factors, such as the evolution of pathogens or predators
Balanced Polymorphism definition
The maintenance of two or more alleles at a locus in a population at stable frequencies due to selective forces that maintain the diversity (e.g., heterozygote advantage or frequency-dependent selection)
Environmental Heterogeneity definition
Variability in environmental conditions across space or time. It can maintain genetic diversity by favoring different alleles in different environments
Eg. one allele may be advantageous in one environment, while another allele may be advantageous in a different environment.
Heterozygote Advantage definition
Form of balancing selection where individuals with two different alleles at a particular locus (heterozygotes) have a higher fitness than individuals with two identical alleles (homozygotes)
Can maintain genetic diversity at that locus
Frequency-Dependent Selection definition
A type of selection where the fitness of a phenotype depends on its frequency relative to other phenotypes in the population.
For example, rare phenotypes may be more advantageous because predators or competitors focus on more common phenotypes, which maintains genetic diversity.
Case study analysis - Red Queens
Co-evolution between host and parasite (eg. pathogens)
Case study analysis - environmental heterogeneity
Different alleles are favored in varying environments (eg. Drought vs flood tolerance in plants)
Case study analysis - heterozygous advantage
Sickle cell trait providing malaria resistance
Case study analysis - frequency-dependant selection
Rare traits have higher fitness due to predator avoidance (eg. guppy coloration)
