VARIATION AND EVOLUTION

VARIATION AND EVOLUTION

1. VARIATION

   • CONTINUOUS
     • quantitative differences
     • varies within a range of values
     • normal distribution curve
     • environnment has a large effect
     • eg height and mass in humans
     • polygenic
     • each allele has a small additive effect on the phenotype
   • DISCONTINUOUS
     • qualitative differences
     • monogenic
     • environment has a little effect
     • bar graph
     • distinct categories, no intermediates
     • each allele has a large different effect on the phenotype
     • eg blood group, gender, height in peaplants
   • intraspecific: variation within a species
     • interspecific: variation between species

2. NATURAL SELECTION

   • Stabilising selection:
     • genetic variation is basis of evolution by natural selection
       • a variety of genetic combinations of alleles allows the best adapted variant to be selected for
         • when there is little change in the environment the same alleles are likely to be selected for
           • there will be little changes in characteristics of the population over time
             • allele frequency stays the same (this is stabilising selection)
   • selection pressure: thing that makes an organism likely to survive eg predation, disease and competition. Things that create a struggle for survival
   • Model answer about NS:
     • There is a random genetic mutation/ genetic variation...
       • The selection pressure is... predators/ competition for food
         • Species X has the selective advantage because...
           • Therefore species X is more likely to survive, reproduce and pass on the advantageous allele.
             • There is therefore an increase in allele frequency of the advantageous allele over many generations.
               • This leads to evolution.
   • selection in a stable environment: stabilising selction, selection in a changing environment: directional selection

3. GENETIC DRIFT

   • Change of allele frequency which leads to some phenotypes becoming rarer and some more common
   • In evolution by NS the environemtn effects which individuals survive and breed
     • In genetic drift evolution, it is a chance effect that occurs which alters the frequency of which alleles are passed on.
       • The characteristic has no effect in the chance of survival.
         • It has biggest effect when populations are small
           • Founder effect: more of one phenotype in original popualation by chance
   • HOW IT WORKS
     • individuals within a population show variation in their genotypes (eg A and B)
       • by chance the allele for one genotype (B) is passes onto offspring more often than others therefore number of individuals with ths allele increases
         • can lead to evolution if by chance same allele is passed on more often again as the allele frequency increases in the population
           • greater effect in small populations as chance has greater influence

4. HARDY WEINBERG PRINCIPLE

   • predicts allele frquency in a population wont change from one genration to the next but only under these two condtions
     • 1) it has to be a large population with no immigration, emmigration, mutations or natural selection
     • 2) there needs to be random mating - all possible genotypes can breed with all others
   • allele frequency
     • p + q = 1
     • total frequency of all possible alleles for a characteristic in a certain population is 1
     • frequency of dominant and recessive alleles must add up to 1
     • p= frquency of dominant allele
     • q= frequency of recessive allele
   • genotype frquency
     • total frequency of all possible genotypes for a characteristic in a certain population is 1
     • p2= frquency of homozygous dominant genotype
     • q2= frequency of homozygous recessive genotype
     • 2pq= frequency of heterozygous genotype
     • p2+2pq+q2= 1

5. ARTIFICIAL SELECTION

   • ARTIFICIAL VS NATURAL SELECTION
     • SIMILARITIES
       • make use of random genetic mutations when they occur
       • leads to increase in allele frequency of desirable alleles
     • DIFFERENCES
       • AS: humans apply selection pressure, NS: environmental factors apply selction pressure
       • AS: end results are predetermined, NS: end results are unpredictable
       • AS: makes the species more useful to man, NS: makes species better adapted to environment
       • AS: may not benefit species (could harm animals eg udders too large so uncomfy), NS: benefits the species
       • AS: faster, NS: slower
   • MODEL ANSWER
     • Humans apply selection pressure
       • Select oragnisms with advantageous alleles (eg cow with highest milk yield)
         • Breed with bull who produces cow with highest milk yield
           • Select offspring with highest milk yield and repeat (breed together)
             • Over several generations
               • There is an increase in the allele frequency of the advantageous allele/ high milk yield
     • DAIRY COW: high milk quality, long lactation period, large udders, resistance to mastitis and dsieases, calm tmeperament
   • AS of wheat
     • to produce varieties that: increase yield, are resistant to infections, have higher protein content, have strong/stiff stem, resistance to wind and rain (tolerance to cold)
   • Hybrids are sterile as no homologous pair thing BUT if random genetic mutation occurs where chromosomes double then hybrid can reproduce

6. SPECIATION

   • The formation of a new species
     • Members of a population become reproductively isolated from eachother.
       • Can be due to:
         • Geographical barriers: moutains, islands, separated by man (fence), flood, earthquale etc
           • experience different conditions so NS diffeent characteristics are more common
             • different advantageous alleles, muattions either side of barrier, changes in genotype and phenotype
         • Behavioural barriers: different courtship behaviours
         • Reproductive barriers: mechanical problems with mating, gamete incompatbility, hybrid produced is sterile (meiosis cannot take place and so gametes are not formed)
         • Temporal barriers: reproduce at different times of the year, one species active at night the other in the day
   • SPECIES CONCEPTS
     • different species cant mate to form fertile offspring as there'll be no homolgous pairs of chromosomes so meiosis cant happen thus no gametes form hence infertility
     • BIOLOGICAL SPECIES CONCEPT
       • members of same species can reproduce to give fertile offspring
       • seprates species into Kingdom , Phylum, Class, Order, Family, Genus, Species
         • This also reflects evolutionary histories of species
       • This concept is useful in explaining how new species can evolve
         • However can only be used for species that reproduce sexually, problems arise if species reproduce asexually or are extinct
     • PHYLOGENTIC SPECIES CONCEPT
       • focuses on the phylogenetic (evolutionary) relationships between organisms
       • species that are closely related in evolutionary terms are grouped together
       • relies on molecular analysis of DNA and RNA for gentic divergence (differences between base pairs analysed)
       • If 2 organisms have very similar DNA/RNA they would be considered to be in the same species
       • wolves and dogs have same ancestor and similar DNA so would be in a monophyletic group together
       • less clear cut that biological concept
       • Does allow a decision to be made
       • Ignores whether or not they can reproduce to produce fertile offspring

7. KEY WORDS

   • Genotype: combination of alleles possessed by an individual
   • Phenotyple: physical characteristic expressed
   • Alleles: different forms of the same gene