Module 2 Flashcards
Evolution (12 cards)
Describe microevolution and the agents of change that drive evolution, natural selection, genetic drift, mutation, reproduction, and gene flow.
Evolution:
- The cumulative change in a population or species overtime.
Macroevolution:
- Changes that occur among large taxonomic groups over long periods of time
Microevolution (5 agents of change - influence gene pool):
- Subtle changes in the frequency of alleles within a species that occur in shorter periods of time
1. Natural selection: survival and reproduction of the fittest
2. Mutation: the ultimate source of variation
3. Sexual reproduction: recombination of genes, mate choice
4. Genetic drift: change to allele frequencies based on chance
5. Gene flow: migration, movement and hybridisation
Discuss the impact size, distribution and structure has on a population while also looking into two of the agents of change; mutation and reproduction.
Mutation:
- Alter DNA and can introduce variation to a population
- Induced – DNA mutation via chemicals and radiation
- Spontaneous – DNA mutation via replication errors
* Germline mutations: affects gametes (meiosis), mutation transmitted via sexual reproduction, mutations in the germline create new variation (alleles) and can be heritable
* Somatic mutations: affects all the daughter cells of a single cell (mitosis), not heritable (can be passed down in plants through vegetative reproduction)
- Scale of mutations:
* Smaller changes: single (base) substitution, DNA insertion or deletion into the middle of an existing sequence (Indel), frameshift mutation (if indel isn’t multiple of three)
* Larger changes: gene duplication, gene invasion, chromosomes joined together/gained/lost (aneuploidy), entire genomes are duplicated – less common but greater genetic consequence
- Mutations in certain regions can impact gene expression or function:
* Regulatory regions: affects gene expression, increase/decrease mRNA abundance, presence/absence in tissues or cells
* Coding regions: affect protein function, be functionally the same, large or small functional difference
Reproduction:
- Non-mating (asexual): fission, fragmentation, budding, vegetative propagation, clones, no change allelic composition
* Advantages: asexual lineages multiply faster, no risk of sexually transmitted infections, no “search” costs with finding a mate
- Mating (sexual): recombination via meiosis, sperm and egg, novel offspring, change in allelic composition
* Advantages: combining beneficial alleles, generation of novel genotypes, “faster” evolution
- Mating systems:
* Random mating: equal probability that mating will occur between any two individuals in a population
* Non-random mating: probability bias
* Assortative mating (positive assortative): mate with individuals that share alleles – less diversity, inbreeding (extreme case), homozygosity
* Disassortative mating (negative assortative): mate with individuals that don’t share alleles (opposites attract) – more genotypic diversity, more heterozygosity
Interpret how selection can act on a population and consider what fitness means from an evolutionary perspective.
Fitness:
- The success of an organism at surviving and reproducing and thus contributing offspring to future generations
- Relative fitness (w) - describes the success of a genotype at producing new individuals, standardised by the success of other genotypes in the population and ranges from 0-10.
Natural selection:
- A mechanism that can lead to evolution, individuals with phenotypes most suited to the environment (fittest) are more likely to produce offspring.
- Factors contributing to natural selection; competition (mates, food, resources), selection (disease, predation), environment (climate, ecology)
* Three principles of natural selection:
1. Variation: individuals within a population must have variation for selection to act, can differ in appearance, behaviour or physiology
2. Heredity: offspring need to resemble their parents more than unrelated individuals, traits must be heritable
3. Selection: some forms are more successful at reproducing in particular environments; selection acts on dominant alleles faster, selection occurs on traits that increase reproductive success
* Three types of selection:
Directional selection (positive): favours individuals on one end of the distribution of phenotypes
Stabilising selection: favours individuals in the middle of the distribution of phenotypes present in a population
Disruptive selection: favours individuals at either end of the distribution
- Artificial selection: results from human activity, when breeders choose individuals with economically favourable traits to use as breeding stock, they impose strong artificial selection on those traits; favoured dominant alleles usually don’t become fixed in a population as they mask the recessive
- Balancing selection: occurs when selection favours heterozygous individuals over homozygotes
Adaptation:
- Inherited trait of an individual that allows it to outcompete other members of the same population - evolve via natural selection
Selective sweep:
- Rapid increase in the frequency of a favourable allele before recombination disrupts the region of DNA
- Selective sweeps support strong directional (positive) selection of the locus; positive selection removes variation
Linkage disequilibrium:
- New mutation arises, with an adequate advantage, the mutation might be lost by chance (drift) or “sweep” through a population
Analyse gene flow and how genetic material is shared between populations and its impacts on evolution.
calculating allele freq formula in cheat sheet
Gene flow:
- Transfer of genetic information from one population to another and can alter allele frequencies
- Tends to homogenise more connected populations
- Lack of gene flow promotes interpopulation differentiation
- For gene flow to occur, individuals must be able to disperse, interbreed and produce viable offspring
* Migration: between distinct populations
* Movement: between sub-populations
* Barriers: influence connectivity between populations and the extent of gene flow between populations
- The impact of gene flow on the gene pool depends on:
* The level of migration, movement or hybridisation (m)
* The genetic difference between populations
* Gene flow has a large impact on the gene pool of a population when:
The allele frequencies in residents (p) and migrants (x) differ
Migration rate (m) is high
Evaluate genetic drift is an agent of change, influencing microevolution through changing allele frequencies by chance.
Describe genetic drift, bottlenecks, selection and the founder effect.
Genetic drift:
- Involves random changes in allele frequencies
- There is always randomness in determining which alleles are passed on
- Buri (1956):
* Allele frequencies change with each successive generation
* One allele can reach a frequency of 1
* Cannot predict which allele will be fixed/lost
* Unlike selection, doesn’t favour any allele
* Effect of genetic drift is most pronounced in smaller populations – outcomes are more predictable, probability of large changes is greater
* Large populations – buffer genetic drift making it a less significant agent of change, random sampling doesn’t impact allele frequencies significantly
- Genetic bottleneck: caused by events that reduces the size and genetic diversity of a population significantly (reduce n.o individuals or separate a population)
* Bottleneck event: population goes through a severe reduction where only a few members survive
* Founder event: smaller group from a larger population creates a new distinct population
* Implications:
Conservation – fragmented populations will continue to lose genetic diversity via genetic drift
Speciation – populations that stop exchanging alleles and continue to differentiate due to genetic drift/other agents may eventually become a different species
Explore some of the reproductive barriers that drive speciation and how hybridisation can make this species boundary semi-permeable.
- Speciation:
- The evolutionary process by which new species arise through reproductive isolation, speciation causes one evolutionary lineage to split into two or more lineages
- Reproductive barriers prevent gene flow and enable speciation:
- Pre-mating reproductive isolation (geographic and behavioural):
Isolating barriers that impede gene flow before sperm or pollen can be transferred to the other species
Geographical isolation prevents reproduction and can enable the agents of change to drive speciation (allopatric speciation)
Behavioural isolation due to courtship calling; e.g. three related species with different call types
Selection for different mating signals creates reproductive isolation in the same (sympatric) population - Pre-zygotic reproductive isolation (timing)
Isolating barriers that impede gene flow before fertilisation of the zygote
Genetic, behavioural, physiological or ecological aspect preventing the sperm from one species from fertilising eggs of another species
Timing: spawning times of two coral species do not overlap - Post-zygotic isolation (inviable offspring)
Isolating barriers that act after a zygote begins to develop
A post-zygotic reproductive barrier is an aspect of the genetics, behaviour, physiology or ecology of a species that prevents hybrid zygotes from successfully developing and reproducing themselves - Reproductive barriers:
Allopatric speciation: speciation in different geographic locations
Sympatric speciation: speciation in the same location - Hybridisation:
- Interbreeding of individuals from genetically distinct populations or closely related species to produce viable offspring
- The offspring displays traits and characteristics of both parents but may be sterile
- Outcomes of hybridisation:
Adaptive introgression – inheritance of beneficial variation from related species that accelerate adaptation to, and survival in new environments
Define the biological species concept
Species are groups of potentially interbreeding natural populations that are reproductively isolated from other such groups
Define population
- A group of organisms that interact and share genetic information
- Can vary in size, distribution and structure
- Can be identical - organisms that reproduce asexually
Define gene pool
- The genetic information carried by a population and is dynamic
Define genetic variation
- Differences that exist between individuals in a population
- A larger gene pool leads to greater genetic diversity
- N = total n.o individuals
- Ne = effective population size
- Natural selection and genetic drift affects smaller populations to a greater extent
Define polygenic vs monogenic and know how to use punnet squares
Polygenic:
- most traits are complex and involve the cumulative action of many genes
Monogenic:
- a single gene produces a trait
Punnet squares:
- explain expected genotype frequencies of Mendelian traits in genetic crosses
Contrast between allele, genotype and phenotype
Allele:
- Alternative form of the same gene, e.g. B & b
Genotype:
- Allelic composition of an individual or cell, e.g. BB or Bb
Phenotype:
- Physical characteristic of an individual
