Variation & Evolution

Variation & Evolution

1. Variation

   • Due to:
     • Different genotypes
     • Epigenetic changes on same genotype
     • Different environments
   • The differences between organisms of the same species
   • Heritable
     • Change that can be inherited in genes
       • Bought over by:
         • Meiosis (Only in sexual reproduction)
           • Mixing parental genotypes
           • Random Assortment
           • Crossing over
         • Mutations (Asexual/sexual reproduction)
           • Create long-lasting variation
           • Occurence is rare
   • Non-Heritable
     • Change brought about by environment (no epigenetic change)
   • Continuous variation
     • Controlled by multiple genes (Polygenic)
       • Can result in a range of characteristics
         • Influenced by the environment
   • Discontinuous variation
     • Controlled by single genes (Monogenic)
       • Code for discrete characteristics
         • No intermediates

2. Competition for breeding success & survival

   • When no environmental resistance organisms over produce
     • Finite resources bring about limited reproduction
   • Types of competition
     • Inter-specific
       • Between organisms of different species
     • Intra-specific
       • Between organisms of the same species

3. Selective Pressure

   • An environmental factor that can alter the frequency of alleles in a population
     • Example of natural selection
     • Factors = Selective agents
       • Examples
         • Availability of nesting sites
         • Day lengths affect reproductive behaviour
         • Overcrowding allows diseases to spread
         • Predation & mimic species
         • Temp/Human impact
   • Value of allele is dependant on environment
     • Phenotypes more suited to the environment
       • Are more likely to be passed on
         • Thus becoming more abundant in the environment

4. Population genetics

   • Gene pool
     • All the alleles present in a population at a given time
   • Allele frequency
     • The % of alleles for a gene present in a gene pool
   • Genetic Drift
     • Random changes in allele frequencies in a population
       • Particularly prevalent & an important evolutionary mechanism in small populations
         • Not due to natural selection
   • Hardy-Weinberg Principle
     • In ideal conditions
       • Allele & Genotype are constant generation to generation
       • Ideal Conditions
         • The organisms:
           • Are diploid
           • Allele frequencies are equal in both sexes
           • Reproduce sexually and randomly
           • Generations don't overlap
         • The population
           • Large population
           • No migration, mutation, selection
     • Hardy-Weinberg equilibrium equation
       • p = Frequency of dominant allele (AA)
       • q = Frequency of recessive allele (aa)
       • 2pq = Frequency of heterozygous genotype (Aa)

5. Evolution

   • In general, ideal Hardy-Weinberg conditions don't exist
     • Allele frequency/phenotypes change over time, Leading to evolution
       • Evolution = Change in the average phenotype of a population
         • If a big change in phenotype occurs, the organisms can't reproduce w/initial population
           • Leading to Speciation: Formation of new species
       • Changes brought by: Mutation, Genetic Drift, Gene Flow, Natural Selection
   • Founder Effect
     • Loss of genetic variation in a new population
       • Established by a small No. of individuals
         • That have become isolated from the rest of the population
     • Whilst the founder population stays small
       • It may undergo genetic drift, making them even more different
         • As many genes are subject to change
           • Average phenotype can change quickly
             • The smaller the population, the more significant the change
   • Natural selection
     • Stabilising Selection
       • Characteristic w/continuous variation has a range of values
         • In a certain environment, average phenotype might be better than the extremes
           • Meaning next gen will have smaller deviation & higher peak
       • E.g Human birth weight
     • Directional Selection
       • Selection when an extreme phenotype proves more advantageous
       • E.g Cichlid fish lake victoria, bigger fins to escape predators
     • Disruptive selection
       • Average phenotypes have no advantage, extremes favoured
         • Graph becomes bimodal (2 peaks)
       • E.g Coho Salmon mating, small sneak, big fight, middle neither > fail

6. Isolation & Speciation

   • Species = Groups of organisms that interbreed to make fertile offspring
     • New species may arise from
       • Abruptly, polyploidy = Chromosome No. doubles by endomitosis, but rare
       • Gradually, by isolating groups of individuals
       • Population = Inter-breeding group of 1 species
         • Sub-groups in this population may breed more often w/each other than the rest of the population
           • These are called demes
             • If a deme gets isolated, it can't breed w/other demes
               • So gene flow in/out of this deme is prevented
                 • This is an example of reproductive isolation
                   • Reproductive Isolation = Prevention of reproduction, and therefore the gene flow between the demes of a species
                     • If demes are isolated for many gens, mutations/changes in allele frequency occur
                       • Making them unable to inter-breed, hence speciation occurs
   • Types of Reproduction Isolation
     • Pre-Zygotic
       • Geographical isolation
         • Population split by physical barrier into separate demes
           • When demes are in different areas, speciation is allopatric
             • Allopatric Speciation: Evolution of new species from demes isolated in different geographical locations
       • Gametes are prevented from fusing, thus zygote never formed
       • Behavioural Isolation
         • Species have specific mating rituals & courtship displays
           • But if Male A doesn't sings, Male B sings, Female B may mate w/Male A
             • The 2 groups will diverge across generations that they become different species
               • Called Sympatric Speciation = Evolution of new species from demes sharing a geographical location
       • Morphological Isolation
         • Individuals of species have certain body shapes in order to reproduce (Mechanical isolation)/pollinate
       • Gametic Isolation
         • Gametes in open environments (water) meet gametes of other species
           • Can often make viable offspring (Coral reefs)
             • Barriers prevent fusion of gametes of different species
               • Leading to isolated populations
                 • Gamete membrane has molecules that prevent inter-species fusion
                 • Female secretes chemo-attractants that are only recognised by males of same species
                   • Gametic isolation lead to sympatric isolation
       • Seasonal (Temporal) Isolation
         • Reproductive organs of different demes mature at different times of year
           • Thus unable to hybridise & are genetically isolated
             • Grow in the same area, so this leads to sympatric isolation
     • Post-Zygotic
       • Gametes fuse, zygote forms, organism is infertile so genes are separate & species don't merge
       • Hybrid inviability
         • Fertilisation occurs,
           • Incompatibility means embryo fails to develop
             • Leading to hybrid = Offspring of a cross between members of different species
       • Hybrid Sterility
         • Some cases an embryo develops
           • But if chromosomes aren't similar enough, the don't pair at prophase 1
             • Meaning no gametes = Hybrid is sterile
               • Sterility prevents gene flow between the parental species
                 • Meaning the species remains distinct
         • Plants only
           • If infertile F1 offspring undergoes endomitosis
             • & Doesn't undergo cytokinesis
               • Chromosome No. doubles >
                 • 2 copies of each chromosome are present
                   • Pairing occurs > Offspring fertile
       • Hybrid Breakdown
         • Some F1 are fertile, Some F2 infertile
           • More common in plants (Cotton, Rice)
             • Due to Incompatibility between nuclear genes & mitochondria/chloroplast genes
   • Speciation occurs as a result of the founder effect, genetic drift & natural selection
     • It is enhanced by Reproductive isolation

7. Darwinian Evolution

   • Darwin went to galapagos islands
     • Found 14 species of finch, found nowhere else on earth
       • Sub-species had adapted to environmental conditions of their habitat
         • Filling vacant niches free from competition from other flinches that failed to reach the island
           • Sub species differed (beak shape & size) reflecting habitat
             • Darwins observations & deductions, in modern terms
               • 1) Competition for limited resources
                 • 2) Individuals w/alleles making better-adapted phenotypes reproduce more successfully
                   • 3) Passing alleles of phenotype to offspring, continues over generations
                     • 4) Those alleles start to appear more frequently in the gene pool of a population
                       • 5) Causing the average phenotype to change (This change = Evolution)
                         • 6) If changes are significant enough, individual can't interbreed w/member of original population
                           • 7) Forming a new species
                             • 8) Natural selection maintains phenotypes in stable environment, Changes them in a changing environment
   • 2 types of evolutionary product
     • Adaptations are features used in original form
     • Exaptations are structures that appear to have their original use modified
     • Natural Selection, founder effect, genetic drift, only modify phenotype by acting on alleles already present
       • Therefore evolution only changes what is already there
         • & only does so in response to environmental changes
           • Evolution isn't always direct or follows a linear path
             • Some structures don't appear ideally suited to their function

8. Niche: Particular area in a population

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