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Biology 30

Biology 30. Population Studies. Introduction of Some terms. A population consists of all the members of a species that occupy a particular area at the same time The members of a population are more likely to breed with one another than with other populations of the same species

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Biology 30

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  1. Biology 30 Population Studies

  2. Introduction of Some terms • A population • consists of all the members of a species that occupy a particular area at the same time • The members of a population are more likely to breed with one another than with other populations of the same species • Therefore, genes tend to stay in the same population for generation after generation

  3. INTRODUCTION • The total of all the genes in all the members of a population at one time is called the …population's gene pool • Evolution • is the change in the frequency of genes… • in a population's gene pool… • from one generation to the next.

  4. Hardy-Weinberg Law • In order to see how a population evolves, it is helpful to examine the genetics of a population that does not change from generation to generation • The Hardy-Weinberg Law provides a model of an unchanging gene pool • This law states that the frequencies of alleles in a population's gene poolremain constantover generationsif all other factors remain constant

  5. Hardy-Weinberg Law • For a gene pool to be in the Hardy-Weinberg equilibrium, 5 conditions must be met: • The population must be closed. This means that no immigration or emigration can occur. • Random mating takes place. There can be no mating preferences with respect to genotype. • There can be noselection pressure. A specific gene must not affect the survival of the offspring. • No mutation of the particular alleles examined can occur. • The population must be very large. This equilibrium is based on statistical probabilities and random sampling.

  6. Hardy-Weinberg Law • If all these conditions are met, the frequencies of two alleles (A and a) will remain constant in a population forever • or until conditions change • Recall our definition of Evolution • Change of frequency of genes or alleles • The Hardy-Weinberg law points out that sexual reproduction reshuffles genes but does not by itself cause evolution

  7. Hardy-Weinberg Law • The mathematical expression of the Hardy-Weinberg equilibrium is… p + q = 1 wherep= frequency of the dominant allele & q= frequency of the recessive allele

  8. Hardy-Weinberg Law • Example: • suppose a certain allele A has a frequency of 0.6 in a population • since the two alleles must add up to 1… then • p + q = 1 (1 - 0.6 = 0.4) • the frequency of a is 0.4 Let's see what happens during reproduction

  9. A (0.6) a (0.4) A (0.6) a (0.4) Hardy-Weinberg Law • First, let’s arrange the two alleles and their frequencies on a Punnett square

  10. A (0.6) a (0.4) A (0.6) AA (0.36) Aa (0.24) a (0.4) Aa (0.24) aa (0.16) Hardy-Weinberg Law • Then, fill in frequencies for the possible offspring

  11. Hardy-Weinberg Law • Go ahead and add up your values for the allele frequencies. • What do you get? • The mathematical relationship governing the gene frequencies is… p2 + 2pq + q2 = 1 AA + 2Aa + aa = 1 (or 100%) Since p = 0.6 and q = 0.4, then (0.6)2 + 2 (0.4 x 0.6) + (0.4)2 must equal 1 (0.36) + 2(0.24) + (0.16) = 1

  12. Practice

  13. Mutations & Evolutionary Change • Mutations violate the conditions for Hardy-Weinberg equilibrium because one gene changes into another and therefore alters gene frequencies in the population

  14. Mutations & Evolutionary Change • Review: a mutation is any inheritable change in the DNA of an organism • Chromosome mutation • results form non-disjunction, chromosome breakage or translocation • Gene mutation • changes in the nucleotides of a DNA molecule • If a population has a stable gene pool and gene frequencies, it is not evolving.

  15. Mutations • If the population does not demonstrate Hardy-Weinberg equilibrium (i.e. its gene frequencies are not stable) it is in evolutionary change

  16. Evolutionary Change • Micro-evolution • a change in the gene pool of a population over successive generations • Potential causes of micro-evolution are • mutation • genetic drift • gene flow • non-random mating • natural selection

  17. Evolutionary Change • Mutation • A new mutation that is transmitted in gametes immediately changes the gene pool of a population by substituting one allele for another • A mutation by itselfdoes not have much effect on a large population in a single generation • If, however, the mutation gives selective advantage to individuals carrying it, then it will increase in frequency and the population gene pool will change over successive generations

  18. Evolutionary Change • Genetic drift • evolution can occur simply by chance • Random events may bring death or parenthood to some individuals regardless of their genetic makeup • The resulting change in the gene pool is called genetic drift • Genetic drift plays more of a role in small populations than in large ones

  19. Evolutionary Change • Genetic drift • Example • Flipping a coin 1000 times compared with flipping a coin 10 times • Example • a population of plants consists of only 25 individuals, 16 are AA, 8 are Aa and 1 is aa • AA plants are destroyed in a rock slide, which alters the relative gene frequencies for subsequent populations.

  20. Evolutionary Change Founder Effect • Genetic drift that occurs when a small number of individualsseparate form their original population and start a new population • Allele frequencies of the new population will be different than the original population • depend on gene pool of the founding population

  21. Evolutionary Change Bottleneck Effect • A dramatic reduction in population size resulting in genetic drift • The frequency of alleles in the remaining members of the population is very different from the original population.

  22. Evolutionary Change • Gene flow • The gene pools of most populations of the same species exchange genes. • This violates the Hardy-Weinberg condition that populations must be closed to be in equilibrium • Animals may leave one area and contribute their genes to the pool of a neighbouring population • migration • or a high wind may disperse seeds or pollen far beyond the bounds of the local population • Gene flow between populations may change gene frequencies and therefore may result in evolution

  23. Nonrandom Mating • Mates are chosen based on different characteristics (not just love the one you’re with) • Sexual Selection • Chances of being selected depend on animal’s traits (what makes him more desirable to the female) • Includes Physical and Behavioural Differences between sexes

  24. Male Male Female Female Nonrandom Mating • Sexual Dimorphism • Striking physical differences between males and females

  25. Nonrandom Mating • Natural Selection • Environment selects for particular traits that are more favourable for surviving in that environment • “Survival of the Fittest”

  26. Interactions in Ecological Communities

  27. Population Interactions Definitions • Population – Any group of individuals of the same species who live in the same area at the same time • Eg. Population of humans in the food court • Community– The association of interacting populations that live in a defined area • Eg. Population of the food court, tables, chairs, trays, and pets and wild animals that wander into the food court. • Niche – An organism’s habitat and role within a community • Includes all factors needed to survive and the organism’s interactions with other species

  28. Population Interactions • In any community, individuals of many populations need to live among each other • Some possible scenarios: • Competing for limited resources • One species preying on another • One species relying on another for survival • Competition occurs whenever two or more organisms attempt to exploit a limited resource • Food • Living space

  29. Population Dynamics • The interactions among individuals – either within the same population or from different populations – are the driving force behind population dynamics • The changes that occur in a population over time

  30. Population Dynamics • Individuals are always competing for resources in order to survive • Competition for resources can occur: • Among individuals of the same species • Natural selection • Survival of the fittest • Between individuals of different species • Hence, there are two basic categories of competition

  31. Intraspecific Competition Intraspecificcompetition – Competition for limited resources among members of the same species • SURVIVAL OF THE FITTEST among members of the same species aka NATURAL SELECTION • Eg. Seeds • On the forest floor there are thousands of seeds • Each seed requires water, nutrients, sunlight, space to grow and mature • Only a few seeds will be able to compete successfully to obtain what they need of the limited available resources

  32. Intraspecific Competition • Intra-specific competition is very common since the members of a population have the same requirements • Intra-specific competition occurs when individuals of a species are competing for resources within their niche

  33. Interspecific Competition Interspecific competition – Competition for limited resources between members of different species in the same community • Tree competing with a shrub for light and growing space • Recall a niche is an organism’s habitat and role in a community • Due to interspecific competition, no two organisms can share the exact same ecological niche

  34. Interspecific Competition • If no two species can share the exact same ecological niche – then why is there interspecificcompeition? • Interspecific competition occurs when individuals of two different species are competing for resources within overlapping niches

  35. Competition – Gause’s Principle The Theory of Competitive Exclusion • Two species with very similar niches cannot survive together because they compete so intensely that one species eliminates the other

  36. Competition – Gause’s Principle • Experiment: Gause raised two species of paramecium with similar food requirements in the same culture • One species always eliminated the other (the particular conditions in the culture determined which species survived) • In nature, species can avoid direct competition by • Feeding at different times of the day (e.g. Hawks and owls) • Dividing resources in some other way (e.g. Different organisms hunt for insects in different parts of coniferous trees)

  37. Producer-Consumer Interactions • Not all interspecific interactions in a community are classified as competitive…

  38. Predation • The most obvious population interaction in a community are those in which a predator eats its prey • Predators that specialize in eating only one prey species play an important role in controlling the population size of the prey species • Eg. Canada lynx and snowshoe hare • The terms predator and prey apply not only to animals that eat other animals, but to any type of producer and consumer relationship • Eg. Plants and Herbivores

  39. Predation Plant defense mechanisms against herbivores: • Thorns • Microscopic crystals in their tissues • Spines or hooks on leaves • Distasteful or harmful chemicals • Some well-known poisons and drugs are secondary compounds produced by plants: • Strychnine • Morphine • Nicotine • Mescaline

  40. Predation • Active animal defenses against predation • Fighting • Hiding • Escaping • Four types of passive defense

  41. Predation – Passive Defense Type I Mechanical or chemical defense mechanisms include porcupine quills, the skunk's offensive odour, the bad taste of monarch butterflies

  42. Predation – Passive Defense Type II Camouflage or protective coloration makes it difficult to spot prey

  43. Predation – Passive Defense Type III Deceptive coloration, warning coloration

  44. Predation – Passive Defense Type IV Mimicry, where one species resembles another • Monarch and viceroy butterflies • Coral snake and harmless species • Wasps and non-biting flies

  45. Symbiotic Relationships Symbiosis is a close relationship between members of different species (3 categories) 1. Mutualism- Both species benefit from the association • Coliform bacteria in the human gut, nitrogen-fixing bacteria in nodules of legumes, protists in a termite's gut 2. Commensalism- One species benefits while the other neither benefits nor is harmed • Remora and the shark 3. Parasitism- One species, the parasite, benefits at the expense of the host • The parasite takes nourishment directly from the tissues of its host's body

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