Natural Selection

D4.1.1: Natural Selection as the mechanism driving evolutionary change 


Natural Selection 

  • Evolution : the change in the heritable characteristics of a population over time 

  • Natural Selection - the process where organisms better adapted to their environment tend to survive and produce more offspring


Nature of Science: Paradigm 

  • A paradigm is a distinct set of concepts and understandings including theories 

  • IN Darwin’s time, evolution was understood but mechanism was not clear

  • Darwin’s theory of natural selection provided a convincing mechanism

  • Darwin’s and Wallace’s theory of natural selection is an example of paradigm from Lamarck’s theory of acquired inheritance


D4.1.2: Roles of mutation and sexual reproduction in generating the variation on which natural selection acts


Variation and Natural Selection 

  • NS only occurs if there is genetic variation within a population of a species 

  • Variation is the difference in traits within a population 


Genetic Variation 


  • Mutations generate alleles in a population 

  • Meiosis produces gametes with variation because of crossing over of alleles, independent assortment of homologous chromosomes during metaphase l

  • Sexual reproduction involves the fusion of gametes. Each parent contributes half of their genetic material to generate new combinations of alleles



D4.1.3: Overproduction of offspring and competition for resources as factors that promote natural selection 


Overproduction of Offspring 

  • Populations of species produce more offspring than can survive in an ecosystem 

  • Populations increase until they reach carrying capacity 


Competition for Resources 

  • Individuals of the same species compete with each other for the following resources 

  • food 

  • Water 

  • Shelter

  • Territory 

  • Avoiding predators 

  • The overproduction of offspring and competition promote natural selection where the better adapted individuals are more likely to survive and reproduce . 



D4.1.4: Abiotic factors as selection pressures 


Selection Pressures 

  • Selection pressures cause a particular phenotype to be more favorable in certain environmental conditions 

  • Phenotype is the observable traits of an organism, resulting from genotype and environmental factors 

  • Both biotic and abiotic factors often act as selection pressure s


Biotic and Abiotic Factors 

  • Biotic factors - living factors that affect the survival of other organisms 

  • Abiotic factors - nonliving parts of an ecosystem that can affect the survival of organism 


Density Independent factors 

  • Density independent factors limit population growth but are not dependent on population density 

  1. Usually abiotic 

  2. May affect the survival of individuals in an ecosystem 

  • The individuals which are best adapted to the abiotic factors in an ecosystem are more likely to survive and reproduce 


D4.1.5: Differences between individuals in adaptation, survival and reproduction as the basis for natural selection 


Natural Selection 

  • Natural selection occurs because of interspecific competition for resources 

  1. Better adapted individuals tend to survive and produce more offspring

  • Less well-adapted tend to die or produce fewer offspring

  1. Individuals with beneficial genotypes are more likely to survive and reproduce than other members of a population 


D4.1.6: Requirement that traits are heritable for evolutionary change to occur 


Heritable traits

  • Heritable traits encoded in the nucleotide base sequence of genes in organisms are required for natural selection 

  1. Organisms with favorable genes that increase chance of survival/reproduction in an environment are more likely to pass their genes to next generation 



Natural Selection of Peppered Moths 

  • Selection pressure: the dark colored trees and predators 

  • Genetic variation w/in population: Some moths have an allele for being light and others have the allele for being dark. Sources of genetic variation are mutations, meiosis and sexual reproduction

  • Intraspecific competition: Moths produce more offspring than can survive. Moths compete for resources and avoid predators.

  • Favourable adaptations: The dark moths were better adapted to the environment

  • are better camouflaged on dark trees, reducing predation.

  • Survive and Reproduce: The dark moths are better adapted and are more likely to survive and reproduce than light moths.

  • Inheritance of Genes: The allele for the favourable trait (being dark) in peppered moths is more likely to be passed to the next generation of moths.

  • Natural selection: The frequency of the allele for being dark increases in the moth population over time. 

  •  example of evolution by natural selection.


Natural Selection of Antibiotic Resistance in Bacteria 


  • Selection pressures: An antibiotic, such as penicillin

  • Genetic variation: There is genetic variation within the bacteria population. 

  • Some bacteria have an allele for antibiotic resistance, and others do not. 

  • Sources of genetic variation are mutations.

  • Intraspecific competition: Bacteria produce more offspring than can survive. Bacteria compete to survive in the presence of antibiotics.

  • Favourable adaptations: antibiotic-resistant bacteria are better adapted to their environments, as they can survive in the presence of the antibiotic.

  • Survive and Reproduce: The antibiotic-resistant bacteria are more likely to survive and reproduce than other bacteria..

  • Genetic Inheritance : The allele for the favourable trait of being resistant to the antibiotic in bacteria is more likely to be passed to the next generation of bacteria.

  • Natural selection: The frequency of the allele for antibiotic resistance increases in the bacteria population over time. This is an example of evolution by natural selection.



D4.1.7: Sexual selection as a selection pressure in animal species 


  • Sexual selection is the natural selection arising through preference by one sex for certain physical or behavioural traits in individuals of the other sex






D4.1.8: Modelling of sexual and natural selection based on experimental control of selection pressures


Background: Evolution 

  • Colouration of guppies is an inherited trait.

  • Predators are more likely to kill colourful guppies.

  • Female guppies are more likely to mate with colourful male guppies. This is an example of sexual selection.


Endler’s Experiments with Guppies


  • John Endler designed and carried out experiments to investigate the evolution of coloration of guppies in the presence and absence of predators.

  • The guppies in streams with dangerous predators remained dull coloured.

  • Guppies in streams without dangerous predators evolved to be colourful due to sexual selection.


D4.1.9: Concept of the gene pool


Gene pools 

  • A gene pool consists of all of the genes and their different alleles in an interbreeding population.

  • Evolution is a change of gene frequencies in a population’s gene pool over time.

  • Gene pools changing over time results in a change in the traits of organisms



D4.1.10: Allele frequencies of geographically isolated populations


Geographically Isolated Populations

  • Reproductive isolation occurs when there is a barrier which prevents individuals from reproducing.

  1. Geographical isolation(form of reproductive isolation)- occurs when 2 populations of the same species are prevented from reproducing because of geographical features such as rivers, mountains or being on different islands.



D4.1.11: Changes in allele frequency in the gene pool as a consequence of natural selection between individuals according to differences in their heritable traits


Neo-Darwinism 

  • Neo-Darwinism is the fusion of Darwin’s theory of natural selection with the principles of genetics. Neo-Darwinism provides a more comprehensive and detailed explanation of the process of evolution.

  • Darwin developed the theory of evolution by natural selection without an understanding of genetics (He was unaware of Mendel’s work on genetic inheritance)

  • Neo-Darwinism can be traced back to Thomas Hunt Morgan’s work on genetics after the rediscovery of Mendel’s work. 


D4.1.12: Differences between directional, disruptive and stabilizing selection


Natural selection reduces variation within a population.


Stabilizing Selection 

  • Occurs when the extreme phenotypes are selected against and the middle range of phenotypes are selected for:

- most common phenotype increases in frequency 

- extreme phenotypes become less common 


  • Stabilizing selection occurs most of the time for a population adapted to a habitat, and prevents divergence.


Directional selection

  • Occurs when one extreme of a range of phenotypes is advantageous to survival. 

- Selection pressure is on one extreme of a range of phenotypes.

- Organisms with the advantageous phenotype are more likely to survive. 

  • Causes a frequency distribution of the trait towards the advantageous phenotype

  • the genes for this phenotype becoming more common.

  • occurs when the environment changes.


Disruptive selection

  • Occurs when individuals at the two extremes of the phenotypic range have advantages that allow them to survive better than individuals with intermediate phenotypes 

- Individuals with phenotypes between the two extremes are less likely to survive and intermediate phenotypes become less common

    - Disruptive selection usually occurs when intraspecific competition is high as there is a higher competition for resources



D4.1.13: Hardy-Weinberg equation and calculations of allele or genotype frequencies


Hardy-Winberg Equilibrium 

  • Occurs when allele and genotype frequencies in a population remain constant from generation to generation 


Equation 

  • Model for predicting allele frequencies in populations that are not evolving 

  • p + q = 1 for alleles frequencies 

  • Therefore the equation is p² + 2pq +q² = 1 for genotype frequencies

  • p represents the frequency of the dominant allele in a population 

  • q represents the frequency of the recessive allele in a population.

  • p^2 represents the frequency of homozygous dominant genotype in the population.

  • 2pq represents the frequency of the heterozygous genotype in a population.

  • q^2 represents the frequency of the homozygous recessive genotype in the population.

  • Used to determine the frequency of alleles and genotypes in a population


Steps for solving Hardy-Weinberg problems


Identify the frequency of the homozygous recessive genotype (q^2).

Calculate the frequency of the recessive allele (q) by finding the square root of frequency of the homozygous phenotype.

Calculate the frequency of the dominant allele (p), by rearranging:

 p + q = 1 to p = 1 - q. Solve for p.

Calculate the frequency of the homozygous dominant genotype p, by calculating p^2.

Calculate the frequency of the heterozygous genotype (2pq) by calculating 2pq.


Example problem 

1: Identify the frequency of the homozygous recessive genotype q²

  • White (b) is the recessive allele. 40% of the population are white and must be homozygous recessive

  • 40% is written as 0.4

  • q² = 0.4 = the frequency of the homozygous recessive individuals (bb) in the butterfly population 

2. Calculate the frequency of the recessive allele (q) by finding the square root of frequency of the homozygous phenotype 

  • If q2 = 0.4 then q = square root of 0.4 =0.63 = the frequency of the recessive brown allele in the butterfly population 

3. Calculate the frequency of the dominant allele (p), by rearranging:

 p + q = 1 to p = 1 - q. Solve for p.

  • q = 0.63 has been calculated

  • 1 - q = p therefore 1 - 0.63 = p = 0.37  = frequency of the dominant brown allele (B) in the population of butterflies


4. Calculate the frequency of the homozygous dominant genotype p, by calculating p2.

- If p = 0.37, then p2 = 0.372 = 0.14 = the frequency of homozygous dominant brown butterflies (BB) in the population


5. Calculate the frequency of the heterozygous genotype (2pq) by calculating 2pq.

  • p and q have been calculated. p = 0.37 and q = 0.63

  • 2pq = 2 x 0.37 x 0.63 = 0.47 = the frequency of heterozygous brown butterflies (Bb) in the population.


6. Check answers by inputting the values into the Hardy-Weinberg Equation





D4.1.14: Hardy-Weinberg conditions that must be maintained for a population to be in genetic equilibrium 


Hardy-Weinberg equilibrium

  • The following conditions are assumed from a population in the HW equilibrium

  • The population is large 

  • No immigration or emigration is occurring

  • Mating is random within the population (No sexual selection)

  • No mutations are occuring within the population

  • No natural selection is occurring in the population (Survival rates of phenotypes is the same for all phenotypes)


D4.1.15: Artificial selection by deliberate choice of traits 


Artificial selection

  • Artificial selection , known as, selective breeding, is the carried out in crop plants and domesticated animals by choosing individuals for breeding that have desirable traits

Corn 

  • There Is genetic variation in the size of corn seeds 

  • Human farmers selected the corn with bigger seed and allowed it to reproduce

  • The process was repeated over many generations to produce modern corn from teosinte

  • Gene frequency for the size of corn has changed over the generations

  • Corn has evolved by artificial selection 


Natural Selection or Artificial Selection 

  • Unintended consequences of human actions, such as the evolution of resistance in bacteria when an antibiotic is use are due to natural rather than artificial selection