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
