CH 17 Variation

Variation

• Every organism on Earth is unique due to genetic and environmental factors.
• Phenotypic variation within a species can be quantified.
• Continuous and discontinuous variation are the two main types of variation.

Continuous Variation

• Characteristics have a graded effect with phenotypes not falling into distinct classes.
• Height and mass in humans are examples of continuous variation.
• Polygenes control such characteristics.
• Polygenes have an additive effect on the overall phenotype.
• Environmental factors greatly influence where on the continuum an organism lies.
• Variation is a product of polygenes and the environment.
• The frequency of heights in a sample of humans produces a bell-shaped curve known as a normal distribution curve when plotted on a graph.

Discontinuous Variation

• Characteristics fit into distinct forms with no intermediate types.
• The ABO blood grouping system is an example of discontinuous variation.
• Few genes or a single gene control this type of variation.
• Discontinuous variation can be represented on bar charts or pie graphs.
• Environmental factors have little influence on discontinuous variation.
• Many human diseases resulting from gene mutations show discontinuous variation.
• Albinism, sickle cell anaemia, Huntington's disease, and haemophilia are examples of discontinuous variation.

Causes of Variation - Genetic Differences

• Genetic variation results from the genotype of each individual, including genes and alleles.
• Genetic makeup varies from generation to generation through:
  • Mutations: changes in the DNA base sequence.
  • Crossing over: non-sister chromatids exchange sections, separating alleles of linked genes.
• Random assortment of pairs of homologous chromosomes: daughter cells are genetically different due to random alignment at the spindle equator during metaphase I of meiosis. The greater the haploid number, the greater the number of possible combinations.
• Random mating between individuals within a species is largely a matter of chance, although there may be an element of choice involved.
• Random fusion of gametes at fertilisation.

Causes of Variation - Environmental Influences

• Environmental influences affect the expression of a genotype, resulting in different phenotypes.
• Variation is affected by climatic factors (temperature, water, sunlight), pH, and nutrients.
• The environment may influence genetic variation by affecting the mutation rate or switching genes on and off.
• Genetic variation may be studied with identical organisms.
• Monozygotic (identical) twin studies can be used to study environmental influences in humans.

Environment and Phenotype

• Phenotype is the result of genotype and environmental effects.
• Organisms of identical genotype show variety when exposed to different environmental influences.
• Temperature and light intensity are environmental influences, largely responsible for continuous variation.
• Alleles provide a blueprint that determines the limits within which an organism will develop.

The Environment's Effect on Phenotype

• The degree to which an allele is expressed depends on the environment
  • The recessive cˢ allele in Siamese cats and the equivalent cʰ allele in Himalayan rabbits code for a heat sensitive form of the enzyme tyrosinase involved in melanin production.
• Over much of the body surface of Siamese cats and Himalayan rabbits the temperature is above 33 °C and so the enzyme is inactive and no melanin is produced during development.
• The fur in these regions is therefore light in colour.
• Over those regions of the body the temperature is usually below 33°C and so melanin is produced which results in colour.
• A small Californian plant, Potentilla glandulosa, has a number of genetic forms, each adapted to growing at different altitudes.
• These forms can be experimented on with cuttings from each location being grown at each altitude. Results differ depending on the environment in which they live.
• Arctic foxes have alleles to make fur pigments and so produce dark coats, but in warmer temperatures.
• The cold temperatures of winter approach and the surface hairs are slowly replaced by white ones ensuring the Arctic fox is camouflaged.
• The height of humans is determined by the range of alleles inherited from parents, but our diet will influence whether we reach maximum height.
• A set of plants grown in soil deficient in nitrogen will develop far less biomass than another genetically identical set grown in soils with a plentiful supply of nitrogen.
• The environment may induce a mutation which affects the phenotype. UV radiation can disrupt DNA replication and lead to melanomas on the skin.
• The genotype determines the range of possible phenotypes, but the environment often determines where within that range the actual appearance of an organism lies.

The t-test

• The t-test determines if the difference between the mean of two sets of continuous data is significant or due to chance.
• The t-test is used if:
  • The data collected are continuous.
  • The data are from a normally distributed population.
  • The standard deviations are approximately the same.
  • Each of the two samples has less than 30 values.
• Where
  • x = mean value
  • s = standard deviation
  • n = sample size (number of observations)
  • v = degrees of freedom
• t-test makes use of the mean and standard deviation.

Mean and Standard Deviation

• A normal distribution curve has the same basic shape but differs with the maximum height and its width.
  • The mean is the measurement at the maximum height of the curve. The mean of a sample of data provides an average value.
  • The standard deviation (s) is a measure of the width of the curve. It gives an indication of the range of values either side of the mean.

Natural Selection

• Natural selection is the process by which better adapted organisms survive and reproduce.
• Adapted organisms pass on favourable alleles to their next generation.
• Charles Darwin and Alfred Wallace developed the theory of evolution by natural selection based on these principles:
  • Organisms produce more offspring than can be supported.
  • Most populations remain constant in size.
  • Competition among members of a species to survive (intraspecific competition).
  • Genetic variations within a population occur in all species.
  • Some individuals possess alleles that make them better adapted to survive (fitter) and breed.
  • Surviving and breeding individuals pass on their alleles.
  • Favourable alleles are therefore passed on to the next generation giving them the advantage.
  • Beneficial alleles increase in number at the expense of less favourable alleles overtime.
  • The frequency of favourable alleles in the population increases over time.

Examples of Specific Natural Selection

• Antibiotic resistance in bacteria:
  • Bacteria acquire resistance through chance mutations.
  • Mutation allows bacteria to produce penicillinase which breaks down antibiotic penicillin.
  • Only susceptible bacteria are killed as a result of exposure to antibiotics leading to increased resistance bacteria.
  • Allele for antibiotic resistance is carried on plasmids which can be transferred from cell to cell by natural and artificial means increasing risk of resistant strains being over ones that are susceptible to the antibiotic.
• Industrial melanism:
  • Species have distinct forms called morphs.
  • Polymorphism = ('poly' = many; 'morph' = form).
  • The peppered moth originally existed only in its natural light form until a black melanic form occurred as a result of a mutation.
  • Light forms were easily seen against the light background of lichen-covered trees and eaten by bird predators reducing its numbers.
  • After industrial development, buildings, walls and trees were blackened, meaning bird predators ate the light form more than the black melanic form.

Molecular Evidence for Similarities Between Organisms

• Molecular evidence, such as mitochondrial DNA comparisons, reveals relationships.
• Mitochondrial DNA is made of relatively few genes and only inherited along the female line. Differences can be revealed using mutations which change the patterns of nucleotide bases overtime.
• The sequence of amino acids in proteins reflects evolutionary relationships since determined by DNA.
• Similarity in amino acid sequences of the same protein in two species reflects how closely related the two species are.

The Roles of Over-Production and Variation in Natural Selection

• The process of evolution depends upon:
  • Organisms produce more offspring than can be supported.
  • Genetic variety exists within all species.

Over-Production of Offspring

• Species have the potential to increase their numbers exponentially; however they cannot control:
  • The climate, rate of predation, availability of food.
• Species ensure survival from generation to generation by producing vast numbers of offspring. This compensates for rates of predation, lack of food and water, extremes of temperature, natural disasters and disease.
• Examples of how organisms over-produce depends: Bacteria - binary fission Fungi - spores Higher Plant - vegetative propagation Flowering Plants - pollen from Their anthers, many millions of seeds Animals - vast numbers of sperm and sometimes large numbers of eggs also, blue tits and rabbit’s multiply every year
• The importance of over-production to natural selection = competition= intraspecific competition, with the limited resources available
• The individuals best suited have adaptations that increase ability to: Hide from/escape predators Obtain light/catch prey resist disease
• Better suited individuals will be more likely to breed and pass on their alleles to the next generation.

Variation and Natural Selection

• If an organism can survive, you may wonder why it doesn't produce offspring that are identical to itself instead of undergoing variation.
• A populations wide range of different individuals are less vulnerable to climate changes, allowing: Changes resulting from allele frequencies within other species. Adaptations Mutations
• In conclusion, the larger a population, the more variety is in the organisms allowing for advantageous alleles over time to give individuals an advantage in the struggle for survival.

Why Organisms Become Extinct

• Climate change
• Habitat loss
• Competition
• Hunting and fishing

Environmental Factors as Forces of Natural Selection

• Environmental factors help contribute to variation within a population.
• Environmental factors may be an agent for constancy or change according to the selection pressure they exert.

Selection Pressure

• Every organism faces a process of selection based upon their suitability for survival:
• Competition for food
• Competition for a space in which to live, breed and rear young.
• Need for light, water, oxygen, etc.
• Climate changes e.g temperature, rainfall, wind/water currents
• Predation.
• Disease. The extent and direction of selection pressures varies from time to time and place to place.
• Selection pressures determine the frequency of an allele within the gene pool
• A gene pool is the total of all the alleles of all the genes of all individuals within a particular population at a given time.
• There are three main types of selection:
• selection that preserves the characteristics of a population by favouring average individuals (those at or near the mean of the population) = stabilising selection.
• selection that changes the characteristics of a population by favouring individuals that vary in one direction from the mean of the population = directional selection.
• selection that changes the characteristics of a population by favouring individuals at the extremes rather than those around the mean of the population = disruptive selection.

Directional Selection

• There will be a range of individuals in respect of any one characteristic.
• Continuous variation amongst these individuals forms a normal distribution curve which has a mean that represents the optimum value for the characteristic under the existing conditions.
• If the environmental conditions change, so will the optimum value needed for survival. Some individuals, either to the left or the right of the mean, will possess a phenotype with the new optimum for the characteristic. A selection pressure moving the mean to either the left or the right results in directional selection.

Stabilising Selection

• Stabilising selection eliminates the extremes of the phenotype range within a population leaving no opportunity for evolutionary change.
• Arises where the environmental conditions are constant and tends to eliminate variation.
• It leads to the same mean, but there will be fewer individuals at either extreme.

Disruptive Selection

• Disruptive selection favours the two extreme phenotypes at the expense of the intermediate phenotype. Although the least common form of selection, it is the most important in bringing about evolutionary change.
• Occurs when an environmental factor, such as temperature, takes two or more distinct forms. An example is coho salmon, where large males and small males have a selective advantage over intermediate-sized males in passing on their alleles to the next generation

Allelic Frequencies

• A sexually mature individuals are capable of breeding with any other and this means that the alleles of any individual organism may combine with the alleles of any other individual in the population.
• A population is a group of organisms of the same species that occupies a particular space at a particular time and may potentially interbreed. Any species may exist as one or more populations.
• All the alleles of all the genes of all the individuals in a population at any one time is known as the gene pool. The number of times an allele occurs within the gene pool is referred to as the allelic frequency.

The Hardy-Weinberg Principle

• In any population the total number of alleles is taken to be 1.0.
• However, in practice the a population is made up of one is all three varying with the population.
• States that the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next: No mutations arise Population is isolated No selection Population is large Mating is random

The Processes Affecting Allelic Frequencies

• Environmental changes affect the probability of an allele surviving in a population.
• Environmental factors do not affect the probability of a particular mutant allele occurring.

Sickle Cell Anaemia

• Sickle cell anaemia is the result of a gene mutation which causes the wrong amino acid to be incorporated for B-globin polypeptide chain of haemoglobin. The result is red blood cells with a sickle (crescent) shape.
• Single gene for the B-globin polypeptide chain of haemoglobin has two codominant alleles, HbA (normal) and Hbs(sickled). The malarial parasite, Plasmodium, is unable to exist in red blood cells with Hbs. -Homozygous for haemoglobin-S (HbHbs) - individuals with sickle cell anaemia and are considerably disadvantaged without medical attention. -Homozygous for haemoglobin-A (HbAHbA) - individuals lead normal healthy lives, but are susceptible to malaria.
• Heterozygous for haemoglobin (HbAHbs) - individuals are said to have sickle cell trait, have protection against malaria, but not badly affected except when the oxygen concentration of their blood is low.

The Founder Effect

• Occurs when a population colonizes a new region: These few individuals will carry only a small fraction of the alleles in the species as a whole. This means that the gene pool of the founder population may not be representative of the parent gene pool, and the new population will show reduced diversity.

Isolation Mechanisms in the Evolution of New Species

• Speciation is the evolution of new species from existing ones.
• A species is a group of individuals that have a common ancestor. They are able to breed with one another to produce fertile offspring meaning they are reproductively isolated.

How New Species Are Forms

• Cross fertilisation between individuals of two different species that leads to the formation of a hybrid.
• Reproductive isolation followed by genetic change resulting from natural selection: -Allopatric speciation - where two populations become geographically isolated. -Sympatric speciation - population can no longer interbreed successfully, meaning groups are separate.

Types of Isolation Mechanisims

• Geographical: population is isolated by physical barriers, mountains, rivers oceans.
• Ecological: population inhabits different habitats within the populations doesn’t mix. Temporal: Each populations breeding seasons do not coincide, don’t Interbred.
• Behavioural: Variations in these patterns may prevent mating e.g can stimulate certain colours.
• Mechanical: prevent mating.
• Gametic: Sperm are destroyed due to chemicals.
• Fertility: Offspring do not have the ability to bear offspring
• Inviablity: Does not properly generate.
• Breakdown: 1st hybrids fine, but 2 generation sterilitey.

Selective Breeding

• Selective breeding, also known as artificial selection, involves identifying individuals with the desired characteristics and using them to parent the next generation.
• Offspring that do not exhibit (show) the desired characteristic are killed, or at least prevented from breeding.
• Selective breeding leads to animals with the desired qualities.

Differences Between Natural and Selective Breeding

• Selection pressure - humans
• Diversity is lowered.
• Doesn't lead to new species. Inbreeding is common.
• Genetic isolation mechanisms don't operate.

Or

• Selection pressure - natural Diversity is high.
• Can lead to new species.
• Proportions are high
• Outbreeding is common.
• Operate frequently.

Methods of Selective Breeding

• Inbreeding: Used to keep a desirable characteristic that has arisen by chance mutation. However can weaken population
• Outbreeding: Breeding of unrelated individuals to try and find combination of two different characteristics which produces the tougher individuals, with better yield known as ''hybrid vigour''

Selective Breeding in Cattle

• Volume of milk produced each day.
• Protein and fat content of milk.
• Type of udder.
• Quality and type of feed.
• Disease resistance and temperament.
• Pedigree records
• Progeny testing
• Artificial instemination

Crop Improvement by Selective Breeding

• To supply the food required to satisfy the increasing demands of an expanding world population, crops need to be constantly improved so they yield more resulting in ''artificial or selective breeding''
• To decide which characteristics is desirable
• Select the parents possessing these
• Choose the offspring possessing a certain characteristic and reproduce it. Repeat process to make plant stronger