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Chapter 18 Populations and evolution textbook notes Unit 7 Genetics, populations, evolution and ecosystems Summary AQA Biology, ISBN: 9780198351771

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Chapter 18 Populations and evolution notes from AQA A Level Biology (2nd edition). Authors: Glenn Toole, Susan Toole Publisher: Oxford University Press (including Nelson Thornes) with specification reference and exam questions at the end.

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biology
aqa
a level
specification
chapter 18
science
medicine
textbook notes
unit 7
module 7
genetics
populations
evolution and ecosystems
sixth form
372
373
chapter 18 populations and evolution

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18 Populations and evolution
18.1 Population genetics
 Population: group of organisms of same species that occupies same space at
same time and can potentially interbreed (interact)
o A species can exist as 1/more populations
 Gene pool: all alleles of all genes of all individuals in a population at a given time
 Allelic frequency: no. times an allele occurs w/in gene pool
o An allele of a gene is the same in every cell ∴ only count a pair of alleles/gene/individual in terms of gene pool
(eg 1000 individuals, 2000 alleles in gene pool of that gene)
o If probability of heterozygous is 1.0 then freq of allele X is 0.5, freq of allele x is 0.5
Hardy Weinberg principle
 Proportion of dominant and recessive alleles of any gene in a population remains constant from generation to
generation, under such conditions:
o No mutations
o No migration, ∴ isolation, no flow of alleles
o No selection (natural, artificial, sexual) alleles are equally likely to be passed to next generation
o Large population
o Random mating
p+q=1
p2 + 2pq + q2 = 1

18.2 Variation in phenotype
Variation due to genetic factors
 All members in a population have the same genes but different alleles
o This applies to individuals as well as cross generations
 Genetic variation occurs as result of;
o Mutations: sudden changes to genes and chromosomes which could/not be passed on to next generation
(main source of variation)
o Meiosis: nuclear division producing new combinations of alleles before passed to gametes therefore all
different
o Random fertilisation of gametes: sexual reproduction produces new combinations of
alleles and offspring are different from parents. Random fertilization further adds
variety of offspring
 When variation is largely result of genetic factors, organisms fit into distinct groups and there
are no intermediate types
o ABO blood group system: 4 distinct groups a, b, ab, o controlled by single gene and
environmental factors have little influence
Variation due largely to environment influences
 Environment exerts influence on all organisms affecting way in which organisms genes are
expressed
 Genes set limits but environment determines where organism lies within limits
o Buttercups: a plant may be determined to grow taller than other plants but a seed
germinating in poor light/soil nitrates won't grow properly and will be short
 Environmental features include: climatic conditions (temp, light, rainfall), soil conditions, pH, food availability
 Some characteristics of organisms grade into one another, forming a continuum
o Humans (height and mass): variation controlled by many genes (polygenes)
o Environmental factors play major role in where on continuum organism lies (eg individuals genetically
predetermined to be same height actually grow different height due to variations i environmental factors such
as diet)
 This type of variation is a product of polygenes and environment
 Most cases of variation are due to combined factors therefore it is hard to distinguish btw the 2 and draw conclusions
abuts causes of variation
18.3 Natural selection
 Every organism is subjected to a process of selection based on its suitability for survival under conditions that exist at
that time
 Selection pressures: environmental factors that limit population of a species
o Predation, disease, competition
o Vary from time to time and place to place
o Determine frequency of all alleles within gene pool
 Gene pool: total no. al alleles of all genes of all individuals within a particular population at a given time
 Process of evolution by means of natural selection depends on:
o Organisms produce more offspring than can be supported by available food, light, space

, o Genetic variety within populations of all species
o Variety of phenotypes that selection operates against
Role of over producing of offspring in natural selection
 Charles Darwin: acknowledged all species have potential to increase numbers exponentially but in nature, populations
rarely increase in size at such a rate therefore the death rate must be extremely high
 High reproductive rate has evolved in many species to ensure sufficiently large population survives to breed and
produce next generation
o Compensates for high death rates from predation, competition for food (light in plants), water, extremes of
temp, natural disasters, disease
 Some species have evolved lower reproductive rates with high parental care
 Lower death rates help maintain population size
 Link btw over production and natural selection is that when there are too many offspring for available resources, there
is intraspecific competition (competition amongst individuals) for limited available resources
 Greater numbers, greater competition, more deaths (although not random):
o Individuals in population best suited to prevailing conditions (better able to hide/escape from predators/better
able to obtain light/catch prey/better able to resist disease/find a mate) will more lily survive, breed, and pass
on favourable allele combinations to next generation which will have diff allele frequency to previous
 Depends on individuals of population being genetically different
Role of variation in natural selection
 Conditions change over time therefore having a wide range of genetically different (and ∴ phenotypes) in population
means that some will have combination of genes needed to survive in almost every new circumstance
 Population with very little individual genetic variation are more vulnerable to new diseases and climate changes
 Important that species are able to adapt to changes due to evolution of other species
 The larger the population, the more genetically varied the individuals are in it, the greater chance that individuals will
have a combination of alleles that lead to phenotype advantageous for survival
o These individuals ∴ more likely to bred, pass on their allele combination to next generations
 ∴ variation gives potential for a population to evolve and adapt to new circumstances
18.4 Effects of different forms of selection on evolution

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Chapter 18 Populations and evolution textbook notes Unit 7 Genetics, populations, evolution and ecosystems Summary AQA Biology, ISBN: 9780198351771

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