✅18 - Populations And Evolution Flashcards
What is a population?
A group of organisms of the same species that occupy a habitat at a particular time and that can interbreed.
What is a gene pool?
All the alleles of all the genes of all the individuals in a population at a given time
What is allelic frequency?
The number of times an allele occurs within the gene pool
How does the number of organisms compare to the number of alleles in a gene pool?
The number of alleles is twice the number of organisms as every organism has two copies in each of their cells
What is the Hardy-Weinberg principle?
Provides a mathematical equation that can be used to calculate the frequencies of the alleles of a particular gene in a population
What assumptions does the Hardy-Weinberg principle make?
No mutations arise
The population is isolated, so no flow of alleles in and out
No selection, so all alleles equally as likely to be passed on
Population is large
Mating within population is random
What is the equation for Hardy-Weinberg?
p2 + 2pq + q2 = 1.0
P + Q = 1
What is the difference between the hardy weinberg equations
P2 + 2PQ + Q2 = 1 = phenotype
P + Q = 1 = allele frequency (as a fraction of total population)
What are the factors which can cause variation?
Mutation
Meiosis
Random fertilisation of gametes
How can mutation cause variation?
The sudden changes to genes and chromosomes may or may not be passed on to the next generation. Mutations are the main source of variation
How can meiosis cause variation?
Produces new combinations of alleles before they are passed into gametes, so all are different
How can random fertilisation of gametes cause variation?
In sexual reproduction this produces new combinations of alleles and the offspring are therefore different from parents. Which gamete fuses with which at fertilisation is random.
How can environmental factors affect an organism?
Factors such as height can be influenced by environmental factors, and they influence where on the normal distribution curve the organism lies.
What are selection pressures?
The environmental factors that limit the population of a species
What factors does natural selection require to happen?
Organisms produce more offspring than can be supported by the available supply of food, light, space etc
There is genetic variety within the populations of all species
A variety of phenotypes that selection operates against
Why do many species produce more offspring than they can support?
To ensure a sufficiently large population survives to breed and produce the next generation.
What do high reproduction rates compensate for?
High death rates, predation, competition, natural disaster, extremes of temperature etc
What is intraspecific competition?
Competition between organisms of the same species
What does the selection process depend on?
Individuals being genetically different
Why is having a wide range of genetically different organisms beneficial to a population?
Because it means that there will likely be an individual who can survive in almost any new set of circumstances
What is stabilising selection?
Preserves the average phenotype around the mean of a population by favouring average individuals. Selects against extreme phenotypes
What is directional selection?
Changes the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population. Selection for one extreme phenotype
What is disruptive selection?
Favours individuals with extreme phenotypes rather then those with phenotypes around the mean
What is an example of stabilising selection?
Fur length in mammals where temperature is constant
What is an example of directional selection?
Antibiotic resistance in bacteria
What is an example of disruptive selection?
Coho salmon - small males can sneak up on females in spawning grounds and large males are fierce competitors, intermediate sized ones die out
What is an example of evolutionary changes in recent years?
The peppered moth
What type of selection has the peppered moth undergone?
Directional
What is evolution by natural selection?
A change in the allelic frequencies within a population
What is speciation?
2 populations reproductively isolated
Mutation occurs
Conditions different for 2 populations
Different features are selected for to survive and breed
Populations become genetically different and there is a change in allele frequency
What is a species?
A group of individuals that have a common ancestry and so share the same genes but different alleles and are capable of breeding with one another to produce fertile offspring
How are new species formed?
Through reproductive separation followed by genetic change due to natural selection
What is genetic drift?
Takes place in small populations, less genetic diversity, alleles passed on quickly affects whole population and frequency becomes high
What is allopatric speciation?
Formation of a new species from population through geographical isolation
What is sympatric speciation?
Formation of a new species that results within a population in the same area leading to them becoming reproductively separated
What is an example of allopatric speciation?
The Galapagos finch - populations spread over different islands and each evolved according to environment
What is an example of sympatric speciation?
Apple maggot fly - originally laid eggs in hawthorns, but apples introduced later. Females lay eggs on type of fruit which they developed in, same for males they mate with, so flies raised in apples lay eggs on apples, and vice versa for hawthorns
What are the isolating mechanisms?
Geographical Ecological Temporal Behavioural Mechanical Gametic Hybrid sterility
What is geographical isolation?
Populations isolated by physical barriers eg oceans, mountains rivers
What is ecological isolation?
Populations inhabit different habitats within same areas, so individuals rarely meet
What is temporal isolation?
Breeding seasons of each population do not coincide and they do not interbreed
What is behavioural isolation?
Mating preceded by courtship, which is stimulated by colour or markings of opposite sex, or actions of mate. Any mutations which cause variation in these prevent mating
What is mechanical isolation?
Anatomical differences may prevent mating occurring
What is gametic isolation?
Gametes may be prevented from meeting due to genetic or biochemical incompatibility, eg sperm destroyed by chemicals in female
What is hybrid sterility isolation?
Hybrids formed from the fusion of gametes from different species are often sterile because they cannot produce viable gametes