Evolution I & II (DIY)

• Variation

  • The differences between individuals of the same species (due to the presence of different alleles)
  • Includes morphological, physiological, biochemical and genetic differences
  • Arises spontaneously and not dictated by the need of organisms to survive better in the environment
  • Mutations introduce new alleles and enrich the gene pool - existence of new phenotypes and increased variation
• Types of Natural Selection - 1

  1. Directional Selection

  • Phenotype at one extreme is repeatedly selected for
  • Favours what are initially relatively rare individuals
  • Once the new mean phenotype coincides with the new optimum environmental conditions, stabilising selection takes over
  • E.g. Melanic form of the peppered moth has selective advantage in polluted areas
• Types of Natural Selection - 2

  1. Disruptive selection

  • Intermediate phenotypes are selected against
  • Favours individuals on both extremes of a phenotypic range (2 extremes)
  • Possible to result in polymorphism where 2 or more forms are found in one species
  • E.g.: Snails with pale shells are selected for in dry grasslands whereas those with dark broad bands are favoured in areas with leaf litter
• Types of Natural Selection

  1. Stabilising selection

  • Extreme phenotypes are selected against
  • Favours the more common intermediate variants in a population
  • Does not promote evolutionary change but maintain phenotypic stability with a population over time
  • E.g. Babies who are heavier than and lighter than optimum birth weight are at a selective disadvantage → slightly increased mortality works against extremes of birth weight in humans
• Natural Selection

  • Results in the survival of individuals with the most favourable adaptations to a particular environment
  • Results in differential reproductive success in a population, and over many generations, can change the basic attributes of the population
  • Requires variation within a population - influenced by the organism’s interaction with the environment
  • One of the most powerful drivers of evolutionary change
• Principles of Natural Selection - 1

  1. Overproduction of offspring (O)

  • All organisms produce large numbers of offspring
  • Can lead to an exponential increase in size of any population if all offspring survive
• Principles of Natural Selection - 2

  1. Constancy of numbers (C)

  • Despite tendency towards overpopulation, most populations remain relatively stable in size
  • Only a small fraction of the offspring population survives, matures and produces - majority will die before reproductive age
• Principles of Natural Selection - 3

  1. Struggle for survival (S)

  • Individuals of a species experience competition for finite resources such as food, soil nutrients, water, mates, habitats and other factors such as disease and predators impose a limit on their number
  • Such selection pressure are factors that reduce the reproductive success
• Principles of Natural Selection - 4

  1. Variation within a population (V)

  • The differences between individuals of the same species (due to the presence of different alleles)
  • No variation = No selection as there is no differences between individuals of a population
  • Arises spontaneously and not dictated by the need of organisms to survive better in the environment
  • Meiosis and sexual reproduction is a significant source of variation
  • Mutations introduce new alleles and enrich the gene pool - existence of new phenotypes and increased variation (helps prevent extinction)
• Principles of Natural Selection - 5

  1. Survival of the fittest by natural selection (F)

  • When environmental changes the variation allows some individuals with selective advantage (those with favourable alleles that confer an advantage) to survive and reproduce more successfully than others
  • Survivors have characteristics that are selected/ favoured by the environment
  • Organisms which survive get a chance to produce viable and fertile offspring
  • Any factor that reduces the reproductive success in a proportion of a population is a selection pressure
• Principles of Natural Selection - 6

  1. Like produces like (L)

  • Those which survive breed to produce offspring similar to themselves
  • Advantageous characteristics (favourable alleles) are more likely to be passed on to offsprings
  • For evolution to occur, characteristics involved must be heritable
• Principles of Natural Selection - 7 (Part 1)

  1. Formation of a new species over a long period of time (N)

  • Unequal ability of individuals survive and reproduce will lead to a gradual change in a population
  • With each succeeding generation, the proportion of individuals having the advantageous characteristics (favourable alleles) increases while those lacking the advantageous characteristics decreases
• Principles of Natural Selection - 7 (Part 2)

  • Favourable characteristics, and hence favourable genotype accumulate over time, changing allele frequency
  • Over hundreds and thousands of generations, a new species may form
  • In organisms with a very short generation time, new species can be formed relatively faster
• Disruption of Gene Flow (D)

  • Gene flow is the transfer of alleles from one population to another
  • Achieved through the movement of fertile individuals or their gametes
  • Involves isolating populations through various isolating mechanisms
  • Tends to reduce differences in allele frequencies between neighbouring populations
  • Populations that mix their gene pools frequently tend to have similar allele frequencies
  • Disruption to gene flow can result in differences in allele frequencies over time as it allows isolated populations to evolve independently
  • Required for speciation to occur
• Genetic Drift (G)

  • A change in allele frequency due to chance events
  • When alleles end up lost from the original gene pool, it is an indiscriminate, random event
  • ** Different from natural selection - alleles in this case are lost from random individuals instead of from individuals with disadvantageous phenotypes
  • Tends to reduce genetic variation in populations through such losses of alleles
  • The smaller the original population is, the greater the impact of genetic drift
• Factor of Genetic Drift - 1

  Founder Effect

  • A few, random individuals from a larger population become pioneers of a newly isolated population
  • Not likely to carry all the alleles present in the original population
  • New population is usually small and reproductively isolated
  • If there is continuous breeding within this pioneer population, rare alleles may become more common
• Factors of Genetic Drift - 2

  Bottleneck Effect

  • A catastrophic event leading to a drastic reduction in population results in the reduction of allele frequencies and even elimination of alleles in a random nature
  • E.g. Disease Epidemic/ natural disaster or any unfavourable environmental change
  • The few, random surviving individuals constitute a random genetic sample of the original pre-catastrophe population
  • Certain alleles may be over-represented/ under-represented among the survivors
•  Non-Random Mating (N)

  • Takes into account mate selection → influences evolution
  • In natural populations, individuals preferentially choose mates
  • Random mixing of gametes in natural populations therefore does not occur
  • E.g. Peacock spiders, blue-footed booby from galapagos islands (mate selection is based on courtship display)
  • Sexual selection: the presence of one or more inherited phenotypes improve the chances of successful mating
  • The allele may then be more prominent in the next generation
  • Artificial selection: where humans select the breeding pair (Non-random mating)
• Mutation (M)

  • A mutation is a random change in an organism’s DNA and are a source of new alleles
  • Must occur in gametes to have an impact on evolution
  • If mutations occur in somatic cells (non-gametic), it will not be passed down to the next generation → not considered a source of new alleles → not responsible for evolution
  • Only agent of evolutionary change that results in the formation of new alleles
  • Does not act directly on allele frequencies
  • Once a mutation generates a new allele, evolutionary forces like natural selection can take over and change the allele frequencies
• Populations Evolve, Not individuals

  • A population is a group of interbreeding individuals belonging to a particular species and sharing a common geographic area
  • Natural selection occurs through interactions between individual organisms and their environment
  • But the collective genetic response of the population is what determines survival of the species and the formation of new species
• Mutations - 1

  • Gene mutation
  • Substitution, deletion or addition of a nucleotide that changes the triplet code, and hence the amino acid
  • In non-coding regions such as the promoter and enhancer, can result in phenotypic variation as well (polymorphisms)
• Mutations - 2

  • Chromosomal mutations
  • Polyploidy: when more than 2 homologous sets of chromosomes are present
  • Aneuploidy: when a particular chromosome is over-represented or under-represented
  • Deletion: when a segment of a chromosome is missing
  • Duplication: when an extra segment of a chromosome is present
  • Inversion: when a chromosome segment is detached, flipped around 180 degrees and reattached to the rest of the chromosome
  • Translocation: when a segment from one chromosome is detached and reattached to a different chromosome (alters linkage relationships)
• Mutations - 3

  • MEIOSIS
  • Independent assortment
  • Separation of homologous chromosomes during metaphase I and anaphase I respectively
  • Separation of sister chromatids during metaphase II and anaphase II respectively
  • Will result in gametes with numerous Como
  • Binational of maternal and paternal chromosomes
  • Crossing over between non-sister chromatids of homologous chromosomes 
  • results in more alleles combinations
• Mutations - 4

  • SEXUAL REPRODUCTION
  • Random fusion of gametes
  • Adds to the variety of genotypes
  • Different genotypes will result in different phenotypes and these act as raw materials for natural selection to act on
• DIPLOIDY/HETEROZYGOTE PROTECTION

  • most eukaryotes are diploid, and a considerable amount of genetic variation is hidden from selection as recessive alleles
  • Dominant allele masks the effect of the recessive allele
  • Natural selection only acts on phenotypes
  • Recessive alleles are exposed to natural selection only when the individual Carrie’s 2 copies of this allele
  • Heterozygote protection maintains a huge pool of alleles that might not be favoured under the present conditions
  • Some of which could bring new benefits when the environment changes
• BALANCING SELECTION

  • Occurs when natural selection maintains 2 or more alleles at a locus
  • Can arise by the heterozygotes having a selective advantage (sickle cell anaemia)
  • Can also arise through frequency- dependent selection, where fitness depends on how common an allele is
• Factor of Balancing Selection - 1

  • Heterozygote Advantage
  • When individuals who are heterozygous at a particular locus have greater fitness than both kinds of homozygotes
  • Seen in sickle cell anaemia
  • 2 beta globin alleles important for the inheritance of sickle cell anaemia
  • Individuals who are heterozygous produce both normal and abnormal haemoglobin - individuals are usually healthy, but may suffer some symptoms under conditions of low blood oxygen (carriers)
  • The sickle cell trait is selected for in regions of endemic malaria
• Factor of Balancing Selection - 2

  • Frequency-dependent selection
  • The fitness of the phenotype depends on how common it is
  • Scale-eating fish in Africa which attacks other fish from behind
  • These fish are either left-mouthed or right-mouthed
  • Will attack the prey’s opposite side
  • Thus, the prey guards itself against attack from whatever phenotype of scale-eating fish is most common in the lake
  • Thus, selection favours whichever mouth phenotype is least common
  • Frequency of left-mouthed and right-mouthed fish oscillates over time
• MODIFICATION

  • Evolution only occurs when there are changes in allele frequency within a population over time
  • Allele frequency is a measure of the chances of finding a particular allele of a gene within the population
• DESCENT

  • Evolution involves passing on these heritable genetic changes to the next generation
• COMMON ANCESTOR

  • Present day species are related through a common prehistoric ancestor that have changed and adapted over time
  • All species come from pre-existing species
• Anatomical Homology

  • Organisms with anatomical homology have morphological structures that are derived from a common ancestor
  • Homologous structures represent variations on a structural theme that was present in their common ancestor
  • Characteristics present in an ancestral organism may show ‘modifications’ in its descendents over time as they face different environmental conditions
  • May serve different functions and may superficially look very different but retains a common underlying similarity
• • How are fossils formed?
  • Sedimentary rocks are a rich source of fossils
  • Aquatic and terrestrial organisms that are swept into the seas and swamps die, settle to the bottom together with sediments from land and when their deposits pile up and compress, they form strata
  • Organic substances of dead organisms decay rapidly but parts rich in minerals remain as fossils
  • Minerals in water also seep into tissues of dead organisms and replace their organic material, subsequently crystallising and forming a cast in the shape of the organism
• • Transitional Fossils
  • Any fossilised remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group
  • Many fossil records are incomplete and this lack of continuous fossils is a major limitation in tracing the descent of biological groups
  • Especially important when the descendent group is sharply differentiated by gross anatomy and mode of living from the ancestral group
• Biogeography

  • The study of the past and present geographic distribution of organisms
  • Supports the evolutionary deductions based on homologies
  • Plant and animal species are discontinuously distributed throughout the world
  • Ecological factors alone cannot account for this discontinuous distribution
  • A pattern in the geographical distribution of species, extinct or extant needs to be observed in order to appreciate how biogeography supports the theory of evolution
  • This pattern links to an evolutionary concept such as descent from a common ancestor
• Molecular Homology

  • All forms of life use the same genetic language of DNA and RNA
  • Genetic code is essentially universal
  • Likely that all species descended from common ancestors that used this code
  • Just like anatomical homologies, an ancestral gene would be modified in terms of nucleotide sequence over many generations
  • The more closely related the species, the fewer differences they have
  • The sequences of closely related species tend to be more similar to each other than they are to distantly related species
• Biological Species concept

  • A group of organisms capable of interbreeding and producing fertile, viable offspring
  • Organisms of the same species are reproductively isolated from those of a different species
  • Have a common gene pool and same chromosome number
  • Usually have similar morphological, physiological and behavioural features
• Biological Species (Benefits & Costs)

  • (+) To determine if organisms are of the same species, they can be studied to see if they can interbreed and produce fertile, viable offspring
  • (-) Cannot be applied to asexually reproducing organisms and extinct species whose breeding behaviour cannot be observed in nature
  • (-) In plants, 2 different species can mate and give rise to viable hybrids offspring that are potentially fertile because they are capable of polyploidy (Rare in animals)
• Advantages of Linnaean Classification

  1. Hierarchical classification is a systematic way of grouping organisms

  • A newly discovered species can be easily categorised and named

  1. The binomial nomenclature provides an accurate identity to each species

  • Far more precise to identify a species using its binomial nomenclature than its common name
• Disadvantages of Linnaean Classification

  1. Based on Linnaean classification, we cannot infer the evolutionary relationships between members of each category
  2. Unable to tell how distantly related one species is to another and the name does not tell us the evolutionary history of that species