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Transcript
Unit 5 Mechanisms of Speciation History of Life on Earth Mrs. Petrov What is a Species? One or more populations of individuals that can reproduce under natural conditions and produce fertile offspring. Different species may arise from _________________________________ __________________ between populations 1 Asexual Vs. Sexual Populations The biological species definition does not apply to ____________________ populations. In those cases, the ability to ________________________ is not a good definition. Instead, we rely on _______________ & ________________________similarities. Genetic Divergence Genetic divergence is the process whereby local individuals of a population become ______________________ _________________________from other individuals and thus experience changes in gene frequencies between the groups If the environments are different between isolated units then natural selection, mutations, and genetic drift will work independently on each. This will speed up genetic divergence and speciation. 2 Isolating mechanisms Mechanisms that lead to, or cause genetic _____________________ by preventing ___________________________between two groups. As long as two groups do not interbreed their gene pools will continue to drift further and further apart. The longer two species are genetically isolated from each other the more different they become from each other. Isolating mechanisms Isolating mechanisms can be divided into two different types. Isolating mechanisms that occur before fertilization are called __________________________________ Isolating mechanisms that occur after fertilization are called __________________________________ 3 Types of Prezygotic Isolating mechanisms _________________________________________: Potential mates meet but cannot figure out what to do about it because patterns of courtship may be altered to the extent that sexual union is not achieved ________________________________________: (Time) Different groups overlap in range but may not be reproductively mature in the same season. ________________________________________: Potential mates attempt engagement but sperm cannot be successfully transferred . This may be due to differences in reproductive organs. Types of Prezygotic Isolating mechanisms cont. _________________________________: Sperm is transferred but sperm and egg are incompatible. __________________________________: potential mates never meet because they live in different habitats 4 Types of Postzygotic isolating mechanisms ______________________________: Egg is fertilized but zygote does not develop properly, dies before birth because parents are genetically incompatible. ______________________________Hybrid very weak and can’t live outside the uterus. _____________________________: Hybrid is sterile. Speciation Speciation occurs when a species gives rise to one or more different species. There are three main speciation patterns ___________________________________ ___________________________________ ___________________________________ 5 Allopatric Allopatric (Allo= different, Patric=homeland) Populations are separated due to _____________________________________________ Rivers, earthquakes, continental drift, glaciation, archipelagos cause allopatric speciation. Examples: Cave fish, Darwin’s finches Antelope squirrels of the Grand Canyon Isthmus of Panama Sympatric speciation Sympatric (Sym = same, Patric=homeland) speciation occurs in the same geographical region _____________ ____________________________________________ A new species can arise in a single generation if a genetic change produces a reproductive barrier between mutants and the parent population. Example: Accidents during cell division that result in extra sets of chromosomes (Polyploidy). Selffertilization can give rise to new individuals that are unable to mate and form fertile off-spring with the parent species. 6 Parapatric Speciation Parapatric (Para = near, Patric=homeland) neighboring populations become distinct species while maintaining contact through Hybrid zones If hybrid zone is removed through increased natural selection, natural disasters or some other means the extremes of a population fail to mate. Examples 1 2 Toad population along the north rim of the Grand Canyon. ___________________ Branching and unbranched evolution ____________________ B Branched evolution. Occurs as populations split and become reproductively isolated from each other. A C __________________ A B C Unbranched evolution. Occurs as changes in allele frequency and morphology accumulate over long periods of time. New species do not live within the same time period 7 Transitional Forms Species which are __________________ in body form between two groups of organisms. Species that are intermediate in time in the fossil record. Archaeopteryx Archaeopteryx Intermediate between birds and dinosaurse Archaeopteryx lived after the development of dinosaurs but before the advent of birds. Archaeopteryx contains features that are both characteristic of reptilian dinosaurs as well as modern birds. Some features are half way inbetween. 8 Feature Birds Archaeopteryx Coelurosaurs Body covering Feathers Scales (?) Metatarsals Fused Not fused Bones Hollow and pneumatic Not hollow or pneumatic Clavicles Fused (wishbone) Not fused Sternum Large, keel Small, no keel Abdominal ribs Absent present The tempo of speciation ______________________________________ New species develop slowly and gradually as an entire species changes over time. This idea was asserted by Darwin. ____________________________________ This theory states that new species arise suddenly and rapidly as small subpopulations of a species split from the populations of which they were a part. 9 What Leads to Speciation? _________________________________ is the major mechanism of evolution. Other mechanisms? Of Course! __________________________ Random chance that some more successful Non-Random Mating: __________ preferences Migrations: movement in and out of population Mutations: change genetics (_______) randomly Modeling Evolution The Hardy-Weinberg Model This is a ___________ model: if assumptions are met, it models that NO evolution is occurring. If a change from 1 generation to the next IS observed, this demonstrates that evolution IS occurring. 10 H-W Assumptions _______________________________________ _______________________________________ _______________________________________ _______________________________________ _______________________________________ All conditions seldom met! H-W Model Equations p2 + 2pq + q2 = 1 p+q=1 p2= decimal of homozygous dominant individuals 2pq = decimal of heterozygous individuals q2 = decimal of homozygous recessive individuals p = decimal of dominant alleles q = decimal of recessive alleles 11 Practice Problem In a population of 100 trees, 30 individuals are homozygous recessive for small leaves. If the population is in HW equilibrium (no evolution occurring), how many dominant alleles should be in the population next generation? How many heterozygous individuals should there be? Natural Selection Vs. Genetic Drift Differential success in ____________________________ results in certain alleles being passed to the next generation in greater proportions For example, an allele that confers resistance to DDT increased in frequency after DDT was used widely in agriculture 12 Genetic Drift The smaller a sample, the greater the chance of deviation from a predicted result Genetic drift describes how allele frequencies fluctuate _________________________________ from one generation to the next Genetic drift tends to _______________genetic variation through ____________ of alleles Animation: Causes of Evolutionary Change Right-click slide / select “Play” 13 Case Study: Impact of Genetic Drift on the Greater Prairie Chicken Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched Figure 23.11a Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) Greater prairie chicken (a) Range of greater prairie chicken 14 Figure 23.11b Location Illinois 1930–1960s 1993 Population size Number Percentage of alleles of eggs per locus hatched 1,000–25,000 <50 5.2 3.7 93 <50 Kansas, 1998 (no bottleneck) 750,000 5.8 99 Nebraska, 1998 (no bottleneck) 75,000– 200,000 5.8 96 (b) Effects of Genetic Drift: A Summary 1. Genetic drift is significant in _____________ populations 2. Genetic drift causes allele frequencies to change at __________________ 3. Genetic drift can lead to a loss of genetic variation within populations 4. Genetic drift can cause __________________ alleles to become _______________ 15 A Closer Look at Natural Selection Natural selection brings about ___________________ evolution by acting on an organism’s ______________________ Relative Fitness The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals Reproductive success is generally more subtle and depends on many factors 16 ________________________fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals Selection favors certain genotypes by acting on the phenotypes of certain organisms Directional, Disruptive, and Stabilizing Selection Three modes of selection: Directional selection favors individuals at one end of the phenotypic range Disruptive selection favors individuals at both extremes of the phenotypic range Stabilizing selection favors intermediate variants and acts against extreme phenotypes 17 Frequency of individuals Figure 23.13 Original population Evolved population (a) Directional selection Original population Phenotypes (fur color) (b) Disruptive selection (c) Stabilizing selection The Key Role of Natural Selection in Adaptive Evolution Striking adaptations have arisen by natural selection For example, cuttlefish can change color rapidly for camouflage For example, the jaws of snakes allow them to swallow prey larger than their heads 18 Figure 23.14 Bones shown in green are movable. Ligament Natural selection increases the frequencies of alleles that enhance survival and reproduction Adaptive evolution occurs as the ____________ between an organism and its environment increases Because the environment can change, adaptive evolution is a ______________________________ process 19 Genetic drift does not consistently lead to adaptive evolution as it can increase OR decrease the match between an organism and its environment Sexual Selection Sexual selection is natural selection for _______________ success It can result in sexual _________________________ : Distinct differences between the sexes. 20 Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his _________________________________________ Figure 23.15 21 How do female preferences evolve? The “_________ ________________” hypothesis suggests that if a trait is related to male health, both the male trait and female preference for that trait should increase in frequency Balancing Selection Balancing selection occurs when natural selection maintains stable frequencies of ___________________ ____________________________________________ Balancing selection includes Heterozygote advantage Frequency-dependent selection 22 Heterozygote Advantage Heterozygote advantage occurs when heterozygotes have a higher __________________than do both homozygotes Natural selection will tend to maintain two or more alleles at that locus The sickle-cell allele causes mutations in hemoglobin but also confers _____________________________ __________________________________________ Figure 23.17 Key Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote) 5.0–7.5% 7.5–10.0% 10.0–12.5% >12.5% 23 Frequency-Dependent Selection In frequency-dependent selection, the fitness of a phenotype _________________________ if it becomes too _______________________in the population Selection can favor whichever phenotype is __________ common in a population For example, frequency-dependent selection selects for approximately equal numbers of “right-mouthed” and “left-mouthed” scale-eating fish Figure 23.18 “Left-mouthed” P. microlepis Frequency of “left-mouthed” individuals 1.0 “Right-mouthed” P. microlepis 0.5 0 1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90 Sample year 24