18: Populations and evolution Flashcards
Define gene pool
All alleles of all genes of all the individuals in a population at a given time
Define allelic frequency
Number of times an allele occurs within the gene pool
What does the Hardy-Weinberg principle assume?
Proportion of dominant and recessive alleles of every gene remains constant between generations This means: No mutations arise Isolated population No natural selection Large population Mating within the population is random
What are the Hardy-Weinberg equations?
Frequency of allele A = p
Frequency of allele a = q
p + q = 1
p^2 + 2pq + q^2 = 1
What is variation in phenotypes usually caused by?
Both genetic and environmental factor
How does genetic variation occur?
Mutations - sudden changes to genes which could be passed on. Main source of variation
Meiosis - produces new form of alleles before passed into gametes (independent assortment and crossing over)
Random fertilisation - random which gamete fuses with another meaning there is an increased variety
How does genetic and environmental influences affect the phenotype?
Genetics set the limits but the environment determines where within those limits the final phenotype is found
How does the environment affect genes?
Environment influences the way the organism’s genes are expressed
What does polygenes when describing a phenotype describe?
One phenotype that is determined by many genes
Define a phenotype
The physical expression of genes which results from both the environment and the genotype of the organism
Define selection pressures
Environmental factors that limit the population of a species
Determine the frequency of all alleles within the gene pool
E.g. Predation, disease and competition
What factors does natural selection depend upon?
Organisms produce more offspring than can be supported by available supplies (food,light,space)
Genetic variety within the populations of all species
Variety of phenotypes that selection operates against
Why is variation important in natural selection?
Greater variety raises chance one or more individuals will have the combination of alleles which lead to a beneficial phenotype
Accelerates natural selection
Define stabilising selection
Preserves phenotypes around the mean and selects against the extreme phenotype
Define directional selection
Changes the phenotypes of the population by favouring phenotypes in one direction, selects one extreme phenotype
Define disruptive selection
Favours individuals with both sides of the extreme phenotype rather than the mean
When does stabilising selection occur?
When the environment conditions are constant over very long periods of time
When does directional selection occur?
When the environment conditions change in one direction
When does disruptive selection occur?
Occurs when an environmental factor takes two distinct forms, or due to a barrier causing two environments
Define speciation
The evolution of new species from existing ones
Define a species
A group of individuals that have a common ancestry and can breed to produce fertile offspring
What is adaptive radiation?
Different phenotypes due to different selection pressures in different environments
What is genetic drift?
Allele frequencies of a population change over generations due to chance
More prominent in small populations as lower genetic diversity
What are the types of speciation?
Allopatric speciation
Sympatric speciation
Define allopatric speciation
Speciation which forms due to members of a species becoming geographically separated
Different locations have different conditions causing natural selection
Define sympatric speciation
Speciation which occurs within a population in the same area
What are the isolating mechanisms?
Geographical Ecological Temporal Behavioural Mechanical Gametic Hybrid sterility
What occurs in geographical isolation?
Populations are isolated by physical barriers such as oceans, mountains etc
What occurs in ecological isolation?
Populations inhabit different habitats within the same area so individuals rarely meet
What occurs in temporal isolation?
Breeding seasons of each population do not coincide and cannot interbreed
What occurs in behavioural isolation?
Courtship or other mating prerequisites are changed by variation meaning mating does not occur
What occurs in mechanical isolation?
Anatomical differences may prevent mating
E.g. May be physically possible for the penis to enter the females vagina
What occurs in gametic isolation?
Gametes may be prevented from meeting due to genetic or biochemical incompatibility
What occurs in hybrid sterility
Hybrids are the offspring of the parents of two different species often are sterile as cannot produce viable gametes
Define genotype
The genetic constitution of an organism
Why can protein structure be used to investigate evolutionary relationships between different species?
Closer the amino acid sequence the closer the relationship
Protein structure related to the DNA base sequence
Why does comparing the base sequence of genes provides more evolutionary information than
comparing the structure of proteins?
More bases in the base sequence than amino acids in amino acid sequence
Due to introns being present
