Topic 7: Populations and Evolution Flashcards
Define population
A group of organisms of the same species in the same space at the same time that can interbreed to produce fertile offspring
Define gene pool
All the alleles of all genes of all 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?
A mathematical equation used to calculate allelic frequencies.
p = frequency of dominant allele
q = frequency of recessive allele
Allele frequency:
p + q = 1
Genotype frequency:
p^2 + 2pq + q^2 = 1
What are the conditions for the Hardy-Weinberg principle to work?
Only works if the evolution of dominant and recessive alleles of any gene in a population remains constant from one generation to the next so there are some assumptions:
- No mutations arise
- Population is isolated (no flow of alleles in or out)
- No selection pressures (all alleles equally likely to be passed on)
- Population is large
- Mating within the population is random
How does phenotypic variation arise?
Occurs within a population due to a combination of genetic and environmental factors.
How does genetic variation arise?
Within a population all individuals have the same genes, but different alleles.
3 main sources:
- Mutations: main source, sudden changes to genes and chromosomes may or may not be inherited.
- Meiosis: produces new allele combinations before they are passed onto gametes, all of which are different
- Random fertilisation of gametes: produces new combinations of alleles where the parents can produce a variety of offspring
What happens to the range of phenotypes of organisms when variation is largely due to genotype?
Individuals fall into a number of distinct classes / categories with few intermediate forms, as environmental factors have very little influence
What happens to the range of phenotypes of organisms when variation is largely due to environmental influence?
Environment often causes variation around the distinct classes determined by genotype. This causes classes to grade into one another, forming a continuum = a normal distribution curve
What are polygenes?
When multiple genes determine a single characteristic
What are selection pressures? Give some examples
Environmental factors that limit the population of a species and determine allele frequencies in a gene pool.
e.g predation, disease, competition
What does the process of evolution by natural selection depend on?
- Variety of phenotypes in the population
- An environmental factor resulting in differential survival and reproduction of individuals
- Over-production of offspring
- Genetic variation
How does over-production of offspring enable natural selection?
Organisms produce more offspring than can be supported by the environment in order to ensure a large enough population survives and breeds for the next generation.
Compensates for high death rates, so some species have lower reproductive rates with more parental care instead.
Over-production increases intraspecific competition for the limited resources, so the better-suited are more likely to survive, leading to natural selection
How does genetic variation enable natural selection?
Variation within a population means there will almost always be some individuals suited to any new conditions, making the environment less population less vulnerable to change (e.g climate / new diseases) as they have the capacity to evolve
What are the three forms of selection?
- Stabilising
- Directional
- Disruptive
What is stabilising selection?
Preserves the average phenotype (individuals existing around the mean).
Eliminates phenotypic extremes and the capacity for evolutionary change (preserves characteristics).
Tends to occur when conditions are constant over a long period of time.
e.g human birth weights
How does stabilising selection show on a graph?
Normal distribution curve becomes taller at the mean and thinner.
What is directional selection?
Favours phenotypes that vary in one direction from the mean.
If environmental conditions change, so does the optimum phenotype for survival. There is then a selection pressure favouring one or extreme, so the mean moves left or right of its original position (natural selection).
e.g antibiotic resistance
How does directional selection show on a graph?
Normal distribution curve stays the same shape but moves left or right of its original position
What is disruptive selection?
Favours extreme phenotypes over those around the mean.
The most important form of selection for bringing about evolutionary change. Occurs when an environmental factor takes 2 or more distinct forms.
e.g if temperature alternates drastically between seasons, it could lead to 2 separate species - one with long fur and active in the winter, one with short fur and active in the summer
What is speciation?
The evolution of a new species from existing ones.
How does speciation occur?
- The individuals of one species can freely interbreed as they share the same area.
- Populations of species become isolated from each other, and each population now exists in different biotic and abiotic conditions, so have different selection pressures.
- Within each population there is variation caused by mutations (cause production of different polypeptides that lead to different phenotypes).
- The phenotypes that are better able to survive are selected for.
- As these organisms survive, they have greater reproductive success.
- In each of the isolated populations the frequency of the allele for the advantageous phenotype increases as the allele is passed on.
- No interbreeding can occur between the populations so there is no gene flow between gene pools.
- Over time, the differences between the gene pools become so great that the populations are unable to breed and produce fertile offspring = different species.
This is also known as adaptive radiation.
What is allopatric speciation?
2 populations become geographically isolated where conditions vary on either side of the barrier. Natural selection influences the 2 populations differently, leading to local adaptation.
Genetic drift also causes differences in the gene pools, leading to reproductive isolation and new species.
What is sympatric speciation?
Where 2 populations exist in the same area, but become reproductively isolated by behavioural changes that prevent them from interbreeding.
Mutations and genetic drift cause differences in the populations, and over time they may become so different that they cannot breed to produce fertile offspring.
What are some different isolating mechanisms that can lead to speciation?
- Geographical: populations isolated by physical barriers
- Ecological: populations inhabit different habitats in the same area so individuals rarely meet
- Temporal - their breeding seasons don’t coincide so they don’t breed
- Behavioural: mutations causing changes to courtship behaviour may prevent mating
- Mechanical: anatomical differences may prevent reproduction
- Gametic: gametes may be prevented from meeting due to genetic or biochemical incompatibility
- Hybrid sterility: hybrids formed from the fusion of gametes from different species are often sterile they cannot produce viable gametes (e.g they have an odd number of chromosomes)
What is genetic drift?
The change in allele frequencies due to chance events.
What type of populations does genetic drift have the greatest effect in?
Occurs in small populations because chance plays a bigger role in determining which alleles are passed on, so there can be significant fluctuations in allele frequency.
In small populations, the few members possess a smaller variety of alleles than large ones = lower genetic diversity. Means that any mutations causing advantageous alleles will more quickly affect the whole population as their frequency is high, so a new species is more likely to develop.
What is a genetic bottleneck?
When there is a sharp reduction in the size of the population. Some alleles may be lost, so only a random subset survive. Allele frequencies change.
What is the founder effect?
When a population is descended from a small number of colonising ancestors. Alleles of all descendants are inherited from a small initial subset = low genetic variation.
