Populations and evolution Flashcards
Define gene pool
The gene pool is all of the alleles of all the genes of all the individuals in a population at a given time
Define allelic frequency
The number of times an allele occurs within the gene pool.
What is the Hardy-Weinberg principle
- 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.
- The principle makes the assumption that the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next.
What five conditions must be met for the Hardy-Weinberg principle to work
- No mutations arise
- The population is isolated: there is no flow of alleles into or out of the population.
- There is no selection: all alleles are equally likely to be passed onto the next generation.
- The population is large
- Mating within the population is random.
What are the two equations that make up the Hardy-Weinberg principle
1) p+q=1
2) p^2 + 2pq + q^2= 1
What do each of the variables in the Hardy Weinberg equations refer to
- q= frequency of recessive allele
- q^2 = frequency of recessive phenotype (homozygous recessive)
- p= the frequency of the dominant allele in a population
- p^2 = the frequency of homozygous dominant genotype in the population
- 2pq= frequency of heterozygous genotype in the population.
What is the primary source of genetic variation
Mutation
List the three things that genetic variation arises as a result of
1) Mutations
2) Meiosis
3) Random fertilisation of gametes
Explain how mutations lead to genetic variation
Mutations are random changes to the DNA sequence which cause a change to the gene and therefore the phenotype it codes for
Explain how meiosis causes genetic variation
- Meiosis produces new combinations of alleles before they are passed into the gametes, all of which are different.
- The randomness of meiosis and processes such as independent segregation and crossing over during meiosis also increase genetic variation.
Describe how random fertilisation of gametes causes genetic variation
Which gamete fuses with which at fertilisation is a random process which further adds to the variety of offspring two parents can produce.
What are selection pressures
The environmental factors that limit the population of a species
Name three key selection pressures
- Predation
- Disease
- Competition
List three key factors that lead to the process of evolution by natural selection
- Organisms produce more offspring than can be supported by the available resources (supply of food/light/space)
- There is genetic variety within the populations of all species.
- There is a variety of phenotypes that selection operates against
What is intraspecific competition
Competition between individuals of the same species
Explain the role of over-production of offspring in natural selection
- All species have the potential to increase their numbers exponentially
- In reality this never happens which means that the death rate of every species must be high.
- High reproductive rates have evolved in many species to ensure a sufficiently large population survives to breed and produce the next generation.
- This means that organisms produce too many offspring for the available resources.
- This leads to natural selection because there is intraspecific competition between individuals for these resources.
- The individuals with phenotypes best suited to the environment will be more likely to survive.
- (Hence reproduce, pass their genetics on).
Why is genetic variation important in a population (why do organisms not evolve to be exactly the same)
Because genetic variation increases the chance that if an environmental change were to occur, some organisms would have the phenotypes to survive.
List the three main types of selection
1) Stabilising selection
2) Directional selection
3) Disruptive selection
What is stabilising selection
Stabilising selection preserves the average phenotypes by selecting against extreme phenotypes and selecting towards the mean phenotype.
What is directional selection
Directional selection changes the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population- it selects for one extreme and against the mean and the other extreme
What is disruptive selection
Disruptive selection favours individuals with extreme phenotypes at both sides of the mean and selects against those individuals with the mean phenotype.
What does stabilising selection do (in terms of phenotypes) and when does it occur
- Stabilising selection tends to eliminate the extremes of the phenotype range within a population and with it the capacity for evolutionary change.
- It tends to occur where the environmental conditions are constant over long periods of change
Explain what disruptive selection is
- Disruptive selection is the opposite of stabilising selection.
- Disruptive selection favours extreme phenotypes at the expense of the intermediate phenotypes.
- Although it is the least common form of selection, it is the most important in bringing about evolutionary change.
- Disruptive section occurs when an environmental factor takes two or more distinct forms.
What is speciation
The evolution of a new species from existing ones
Define species
A group of organisms that are capable of breeding to produce living, fertile offspring.
How do members of two different species differ
Members of different species are reproductively separated from each other
What are the two forms of speciation
Allopatric speciation and sympatric speciation.
Summarise the way in which new species are formed
Through reproductive separation followed by genetic change due to natural selection
What is adaptive radiation
- Adaptive radiation refers to the adaptation (via genetic mutation) of an organism which enables it to successfully spread, or radiate, into other environments.
- Adaptive radiation leads to speciation and is only used to describe living organisms.
- Adaptive radiation can be opportunistic or forced through changes to natural habitats.
Describe how speciation occurs most commonly
- A population becomes separated from other populations and undergoes different mutations causing it to be genetically different.
- Each of the populations will experience different selection pressures because the environment of each will be slightly different.
- Natural selection will then lead to changes in the allelic frequency of each population.
- The different phenotypes each combination of alleles produces will be subject to selection pressure that will lead to each population becoming adapted to its local environment (adaptive radiation).
- As a result of these genetic differences the populations would be unable to interbreed successfully- so speciation has occurred.
Explain how genetic drift leads to speciation
- Small populations have a smaller variety of alleles in their gene pool: they have a lower genetic diversity.
- As these few individuals breed, the genetic diversity of the population is restricted to those few alleles in the original population.
- As there are only a small number of alleles, there is not an equal chance of each being passed on.
- Those that are passed on will quickly affect the whole population as their frequency is high.
- Any mutation to one of these alleles that is selectively favoured will also more quickly affect the whole population because it’s frequency will be high.
- The effects of genetic drift will be greater and the population will change relatively rapidly, making it more likely to develop into a separate species.
Why does genetic drift not have a big affect in large populations
In large populations the effects of genetic drift are likely to be less, and development into a new species is likely to be slower.
What is allopatric speciation
Allopatric speciation describes the form of speciation where two populations become geographically separated.
What is sympatric speciation
Sympatric speciation describes the form of speciation that results in individuals within a population in the same area becoming reproductively separated.
What are the seven forms of isolating mechanisms that may separate populations
1) Geographical
2) Ecological
3) Temporal
4) Behavioural
5) Mechanical
6) Gametic
7) Hybrid sterility
What does a population being geographically isolated mean
Populations are isolated by physical barriers such as oceans etc
What does populations being ecologically separated mean
Populations inhabit different habitats within the same area and so individuals rarely meet.
What does populations being temporally separated mean
The breeding seasons of each population do not coincide and so they do not interbreed.
What does populations being behaviourally separated mean
- Mating is often preceded by courtship, which is stimulated by the colour or marking of the opposite sex, the call or particular actions of a mate.
- Any mutations which cause variations in these patterns may prevent mating.
What does populations being mechanically separated mean
Anatomical differences are present that prevent mating
Why does populations being gametically separated mean
The gametes may be prevented from meeting due to genetic or biochemical incompatibility.
What is hybrid sterility
Hybrids formed from the fusion of gametes from different species are often sterile because they cannot produce viable gametes.
