Processes of Evolution

Transcription

1 Processes of Evolution Microevolution Processes of Microevolution How Species Arise Macroevolution

2 Microevolution Population: localized group of individuals belonging to the same species with the potential to interbreed Individuals of a population share morphological, physiological, and behavioral traits/ heritable basis/ shared genes Variations within a population arise from different alleles of shared genes Dimorphic: a trait with only two forms Polymorphic: traits with more than two distinct forms

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5 Members of a population share a gene pool: All alleles at all gene loci in all individuals of a population Allele frequency: abundance of a particular allele among members of a population Microevolution: Changes in the allele frequencies of a population

6 How Do We Know When A Population Is Evolving? G. Hardy and W. Weinberg realized that under certain theoretical conditions, the frequencies of alleles and genotypes in a population remain constant over the generations despite of meiosis and sexual reproduction / Hardy-Weinberg genetic equilibrium

7 Five necessary conditions to maintain genetic equilibrium: 1. Population must be very large 2. No gene flow between populations 3. No mutations 4. Random mating 5. No natural selection/ all individuals survive and reproduce the same number of offsrpring

8 Mathematics of Hardy-Weinberg Frequency of dominant allele: p Frequency of recessive allele: q p + q = 1 Frequency of genotypes: Homozygous dominant: p 2 Homozygous recessive: q 2 Heterozygote: 2pq p 2 + 2pq + q 2 = 1

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10 Consider a hypothetical gene that encodes a blue pigment in flowers: Two alleles of this gene, B and b, are codominant A plant homozygous for the B allele (BB) has dark-blue flowers A plant homozygous for the b allele (bb) has white flowers A heterozygous plant (Bb) has medium-blue flowers

11 1,000 plants in a population: 490 BB individuals: 980 B alleles 420 Bb individuals: 420 B and 420 b alleles 90 bb individuals: 180 b alleles The frequency of alleles B and b : B (p) = ( ) 2,000 alleles = 1,400 2,000 = 0.7 B(q) = ( ) 2,000 alleles = 600 2,000 = 0.3

12 0.7 B 0.3 b 0.7 B BB (p 2 ) Bb (pq) b Bb (pq) bb (q 2 ) BB (p 2 ) Bb (2pq) bb (q 2 )

13 The Hardy-Weinberg Equation is Useful in Public Health Science Phenylketonuria (PKU): inability to breakdown the aa phenylalanine Recessive trait The HW equation can be used to estimate the number of carriers (heterozygotes) p 2 + 2pq + q 2 = 1 Which term in the Hardy-Weinberg equation corresponds to the frequency of individuals who have no alleles for PKU?

14 1 PKU occurrence in 10,000 births q 2 = 1/10,000 = q = = 0.01 p = 0.99 Frequency of carriers: 2pq = 2 x x 0.99 =

15 What Drives Microevolution Mutation Natural selection Genetic drift Gene flow

16 Natural selection can alter variation in a population in 3 ways Affects the frequency of heritable traits in a population in different ways Directional selection Stabilizing selection Disruptiveselection

17 Directional selection shifts the range of variation in traits in one direction

18 Stabilizing selection favors intermediate forms of a trait

19 Disruptive selection favors forms at the extremes of a range of variation

20 Natural Selection and Reproduction Sexual dimorphism Sexual selection/ Advantage in securing mate

21 Natural Selection and Maintaining Multiple Alleles Balanced polymorphism: two or more alleles in high frequency in a population Can result from environmental pressures that favor heterozygous individuals

22 What Drives Microevolution Mutation Natural selection Genetic drift Gene flow

23 Genetic Drift Change in gene pool due to chance Reduces genetic variation More pronounced in small populations

24 Genetic Drift Bottleneck event that drastically reduces population size/ earthquakes, fires- small surviving population with reduced genetic variation Founder effect Small group of organisms colonizing an island or a new habitat/ less genetic variation/ high frequency of inherited disorders

25 Founder Effect Elis-van Creveld Syndrome (EVC) Autosomal recessive; shortlimbed dwarfism, extra fingers, heart disease, sparse hair, teeth at birth 1 in 60,000 to 200,000 newborns Much more common in the Old Order Amish population of Lancaster County, Pennsylvania, and in the indigenous (native) population of Western Australia

26 Bottleneck About 10,000 years ago and because of climate changes: cheetahs faced extinction/ genetic variation dramatically reduced With the drastic reduction in their numbers, close relatives were forced to breed: preserving the low genetic variation found within the species

27 What Drives Microevolution Mutation Natural selection Genetic drift Gene flow

28 Migration (or Gene Flow) Populations may gain or lose alleles as individuals move in and out of a population

29 Speciation Process by which new species arise Reproductive isolation (the end of gene flow between populations) is always a part of speciation Reproductive isolation could be prezygotic or postzygotic

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31 Prezygotic Barriers Ecological Isolation These two different species of garter snakes do not mate because one lives in water and one lives on land

32 Prezygotic Barriers Temporal Isolation Different breeding seasons for the eastern spotted skunk and the western spotted skunk

33 Prezygotic Barriers Behavioral Isolation Elaborate courtship ritual specific to species Specific songs, displays

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35 Prezygotic Barriers Mechanical Isolation Male and female sex organs are not compatible/ anatomical incompatibility

36 Prezygotic Barriers Gametic Incompatibility Gametes of the red and purple sea urchins do not fuse because of the incompatibility of their surface proteins Similar mechanism in plants

37 Postzygotic Barriers Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown

38 Modes of Speciation Allopatric, Sympatric and parapatric

39 Allopatric Speciation Speciation by geographic isolation Isolation might occur because of great distance or a physical barrier, such as a desert or river

40 Hawaiian honeycreepers Ancestral species/ housefinch

41 Sympatric Speciation New species arise without any geographic isolation In plants: Polyploidy

42 Polyploid plants Oats Potatoes Bananas Barley Wheat Plums and many more

43 Sexual selection: mate choice Cichlids

44 Parapatric Speciation Populations maintaining contact along a common border evolve into distinct species Hybrid Zones Reproductive isolation/ sterility of hybrids

45 Macroevolution The major events in the history of life on Earth Macroevolution refers to evolutionary changes at or above the species level Origin of new taxonomic groups

46 Patterns of Macroevolution Stasis: a lineage which persists for millions of years with little or no change Exaptation: Adaptation of an existing structure for a completely different purpose Mass extinction: species that has been permanently lost Adaptive radiation: Lineage rapidly diversifies into several new species Coevolution: Joint evolution of two closely interacting species

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48 History of Life Scientific evidence indicates that Earth formed about 4.6 billion years ago but life originated about 3.5 billion years ago

49 History of Life Fossil stromatolites provide the most ancient records of life on Earth Produced by the activity of photosynthetic prokaryotes (Cyanobacteria) The layers were produced as calcium carbonate precipitated over the growing mat of bacterial filaments

50 Major Events in the history of Life

51 Major Events in the history of Life