10. Changes in the genetic makeup of populations Flashcards
What is a population?
A group of organisms of one species that interbreed and live in the same area at the same time.
What is monomorphism?
Populations that show one form of a characteristic are monomorphic, all members of a population are identical with respect to a particular trait. No variation exists in the population.
What is polymorphism?
Polymorphic populations show 2 or more variants for a particular characteristic. Polymorphic traits show continuous or discontinuous variation.
What are monogenic traits?
Variation that occurs due to a single gene. The variation is due to the action of different alleles of a single gene.
Discontinuous variation?
When members of a population can be classified into a few distinct and non overlapping classes. Monogenic traits show discontinuous variation.
Polygenic variation
Variation that is caused by the action of two or more genes at different loci. The genes involved are called polygenes and their expression is also often influenced by environmental factors.
Continuous variation
When members of a population cannot be classified into a few distinct groups, they show a variety of phenotypes. Polygenic traits show continuous variation.
What does diploid mean?
When just the two matched sets of a chromosome are present an organism and its cells are diploid.
What is polyploidy?
A condition in which an organism has more that two matched sets of chromosomes.
Autopolyploidy
In some cases a polyploid individual will have additional complete sets of chromosomes from its own species (autopolyploidy) produces plants with larger cells and organs
Allopolypoid
The additional chromosome sets in a polyploidy may comes from another species allopolyploid.
What is hybrid vigour
Many allopolyploids show hybrid vigour indication that they are more robust and display superior qualities as compared to the parents from which they were derived.
What is aneuploidy?
A condition in which the chromosomal number is more or less that the normal diploid or haploid cell.
How does aneuploidy come about?
During meiosis the normal separation of homologous chromosomes does not occur during meiosis 1 or sister chromatids fail to separate during meiosis 2
-Gametes can end up with more or less than the normal chromosomes
Difference between asexually and sexually reproducing organisms in terms of how offspring are produced
A: organisms are produced by mitosis and are genetically identical to the parent
S: Organisms are produced as a result of the fusion of egg and sperm from two individuals that develops into an organism that is genetically different to either parent.
Difference in terms of variation
A: mutations and effects of environments
S: Different allele combinations, recombination (meiosis), independent assortments (meiosis) random fusion of egg and sperm, mutations and effect of environment
Difference in terms of advantage
A: No mate needs to be found, large number of offspring can be produced in a short time
S: In a changing environment a variant pollution ensures that at least some offspring with suitable characteristics will survive
Difference in terms of disadvantage
A: In a changing environment entire population may be wiped out because all the offspring are genetically identical to the parent
S: A mate needs to be found but the signal sent may also signal their location to predators. Also it takes time
What is a gene pool?
A gene pool is the sum total of the genetic information present in a population at any one time
What is the hardy weinberg principle?
-No matt3er how many times alleles are segregated by meiosis and recombined by fertilisation the allele frequency remains constant generation after generation, the alleles are just shuffled about within the breeding population
What conditions must apply in a population if allele frequencies are to remain constant?
Large population
Random mating, with all mating equally fertile producing equal numbers of viable offspring, no migration, no selection pressure or mutation
What is microevolution?
A permanent change in the allele frequencies on a population is termed microevolution
What are agents that can cause allele frequenting to change over time?
- Selection (natural an artificial)
- Gene flow (immigration and emigration)
- CHance events (genetic drift, bottlenecks and founder effect)
What is fitness value?
The measure of the genetic contribution to the next generation of one phenotype relative to other phenotype for the same gene locus
Selective advantage
Describes a phenotype that has a higher fitness value
Selective agent
an environmental factor that acts on a particular phenotype, causing it to have a reduced fitness with respect to the other phenotype
Complete selection
against a phenotype occurs when any organism with a given phenotype cannot reproduce because of death before reproductive age is reached or because of sterility
Partial selection
against a phenotype occurs when matings involving that phenotype produce of average fewer viable and fertile offspring relative to other matings
Natural selection
Process in which agents of selection act differentially and with no human intervention on various phenotypes in members of a population
Differential reproduction
Occurs when one inherited variety (phenotype) in a population produces more viable offspring than other varieties and so make a greater contribution of alleles to the gene pool in the next generation
Step 1 natural selection
VARIAtioN IN A POPULATION
Within a population of (organisms) there was variation in the characteristics of (diggle) which was genetically determined and can be passed on to their offspring.
Step 2 natural selection
DIFFERENTIAL SURVIVAL AND REPRODUCTION
When the environment changed due to diggle or in a diggle environment the bob ogden trait resulted in the organisms possessing it having a greater chance of survival because… The legdig was a selection pressure or agent so that organisms with the advantageous characteristic had a greater fitness value, more likely to survive and reproduce
Step 3 natural selection
OFFSPRING AND GENETIC INHERITANCE
The organisms that survive to reproduce will produce offspring that are likely to possess the bob ogden trait due to the alleles inherited from their parents. This will also have a greater chance of survival in a diggle environment
Step 4 natural selection
CHANGE IN POPULATION OVER TIME
Over a number of generations the frequency of bob ogden phenotype all increase until the entire population will display to. Thus a change in the species has occurred as a population adapts to the environments provided selection pressure remains the same.
How does natural selection act on the phenotypes in populations?
- Eliminates or reduces the reproductive success of individuals will poorly suited phenotypes, their alleles become less common in the genes pool
- Enhances the survival and reproductive success of individuals with well suited phenotypes, their alleles become more common in the gene pool
Gene flow
Movement of genes into or out of a population, immigration and emigration
How does population size affect gene flow
- Large populations have more stable allele frequencies because they have a greater reservoir of variability and are less affected by changes involving only a few individuals
- Small populations often have a fewer alleles to begin with and so the severity and the speed of change in the allele frequencies are often greater
What is genetic drift?
Hone the direction of change is unpredictable and can vary from one generation to the next when the change in allele frequencies from one generation to the next is random.
Why does genetic drift occur?
Due to random fertilisation of eggs by sperm that carry different alleles so different allele and genotype frequencies can result in the next germination due to chance.
What is the bottleneck effect?
When the size of a population is drastically reduced for at least one generation and the few survivors that reproduce to give the next generation may by chance be an unrepresentative sample of the gene pool of the original population.
What does the bottleneck effect do?
- THe alleles that remain in the gene pool may be present in different frequencies to the original population and some alleles may be lost.
- This can lead to random shift in the allele frequencies among survivors.
- Severely limited genetic variation may be an outcome because the population must rebuild from so few individuals
- Small surviving population affected by inbreeding resulting in very little variation
Founder effect
- Small number of individuals from a population may migrate away or become isolated from the original population.
- Founder population will have small unrepresentative allele sample from parent population’s gene pool and reduced genetic variation
- May evolve differently
How is a species defined?
Members of the same species are able to reproduce viable and fertile offspring under natural condition
Pre-mating isolating mechanisms
- Isolation in time: difference in time of activity
- Isolation in space: difference in habitat
- Isolation by behaviour: one species does not recognise signs of sexual readiness
- Isolation by anatomy: mating unsuccessful due to physical differences
Post mating isolating mechanisms
- Incompatibility of gametes: sperm cannot permeate outer coats surrounding egg
- Zygote mortality: fertilisation occurs, zygote fails to develop
- Inviability of zygote: zygote develops into embryo but not beyond that
- Sterility of hybrid
Phyletic evolution
One ancestral species progressively changes over time to ultimately be recognised as a new species
Branching evolution
A population of one species splits and evolves separately into a new species
Allopatric speciation
Speciation that results when populations become geographically isolated, being subject to different natural selection pressure ultimately reproductive isolation. Ould not be able to produce viable offspring
Steps in allopatric speciation
- Some members of a species become geographically separated
- Gene flow between populations does not occur, no interbreeding
- Members in isolated populations will be exposed to different selective agents due to different environmental conditions, effect of genetic drift, mutations
- Gene pool becomes more different over time
- Different phenotypes will be selected for, different phenotypes increase in frequency over many generations
- Over time members of population become so different that races or sub species results and two sub species become so different they can no longer mate to produce viable offspring.
- Reproductively isolated
