The evolution of populations

Transcription

1 The evolution of populations Chs Name one biochemical difference between the makeup of DNA and RNA. 2. In Eukaryotes, the process by which DNA is made into RNA is called and happens in the. 3. A gene operon consists of transcribed genes, a promoter, and an. 4. Who is the author of on the origin of species? 5. Give two examples of traits that would be desirable for a plant to have. 1

2 Chestnut blight was caused by a fungus that was introduced from China or Japan. Chestnut trees in China and Japan are resistant to the disease. Could American Chestnut trees have been saved? What if Review NS What requirements must be met in order for a trait to be acted upon by natural selection? The trait must in the population The trait must affect The trait must be There must be differential and. 2

3 Populations are the units of evolution Population: a group of individuals of the same species living in the same place at the same time Can measure evolution as a change in the prevalence of certain heritable traits in a population over generations Ex: the evolution of pesticide-resistant insects Populations are the units of evolution Different populations of the same species may be isolated from each other Little genetic exchange between them Gene pool: the total collection of genes in a population at any one time Consists of all alleles in all the individuals in the population Microevolution: When the relative frequencies of alleles in a population change over time 3

4 What produces genetic variation? Mutation The ultimate source of variation New alleles arise by mutation But most occur in somatic cells and are lost when individual dies Only mutations in gametes affect a population s genetic variability A random mutation is unlikely to improve the adaptation of an individual and enhance its reproductive success More likely if mutations that were once disadvantageous become favorable Mutation rates in plants and animals low prevent most mutations from significantly affecting genetic variation between generations What produces genetic variation? Sexual reproduction Three random components 1. Independent orientation 2. Random fertilization 3. Crossing over 4

5 Hardy-Weinberg Equation Sexual reproduction alone does not lead to evolutionary change in a population Although alleles are shuffled, the frequency of alleles and genotypes in the population does not change Similarly, if you shuffle a pack of cards, you ll deal out different hands, but the cards and suits in the deck do not change H-W can be used to test whether a population is evolving Copyright 2009 Pearson Education, Inc. Hardy-Weinberg Principle States that allele and genotype frequencies within a sexually reproducing, diploid population will remain in equilibrium unless outside forces act to change those frequencies 5

6 Hardy-Weinberg Equation Imagine that there are two alleles in a bluefooted booby population: W and w W is a dominant allele for a nonwebbed booby foot w is a recessive allele for a webbed booby foot Copyright 2009 Pearson Education, Inc. The Hardy-Weinberg Equation Consider the gene pool of a population of 500 boobies 320 (64%) are homozygous dominant (WW) 160 (32%) are heterozygous (Ww) 20 (4%) are homozygous recessive (ww) From this information, what can you tell me about q (the frequency of the recessive allele?) Copyright 2009 Pearson Education, Inc. 6

7 Phenotypes Genotypes Number of animals (total = 500) Genotype frequencies WW Ww ww = = = 0.04 Number of alleles in gene pool (total = 1,000) 640 W 160 W w 40 w Allele frequencies 800 1,000 = 0.8 W 200 1,000 = 0.2 w Hardy Weinberg Frequency of dominant allele (W) = 80% = p 80% of alleles in the booby population are W Frequency of recessive allele (w) = 20% = q 20% of alleles in the booby population are w Frequency of all three genotypes must be 100% or 1.0 p 2 + 2pq + q 2 = 100% = 1.0 homozygous dominant + heterozygous + homozygous recessive = 100% Copyright 2009 Pearson Education, Inc. 7

8 Hardy Weinberg What about the next generation of boobies? Probability that a booby sperm or egg carries W = 0.8 or 80% Probability that a sperm or egg carries w = 0.2 or 20% If a population is in Hardy-Weinberg equilibrium, allele and genotype frequencies will not change unless something acts to change the gene pool Copyright 2009 Pearson Education, Inc. Suppose that webbed footed boobies chose to mate only with other webbed footed bobbies, and vice versa. How do you think the genotypic frequencies of the next generation might look? 8

9 GENERATION 3 Phenotypes Genotypes Number of animals (total = 500) Genotype frequencies Number of alleles in gene pool (total = 1,000) WW Ww ww /500 = 500 = % 640 W /500 = 24% 500 = = W w 80/500 = 16% 40 w 600 W 120 W 120 w 160 w Allele frequencies /1000 = 1,000 = 0.8 W /1000 1,000 = 0.2 = w 72% 28% Compare to original parents 80% W and 20% w What about generation 4? 9

10 Gametes reflect allele frequencies of parental gene pool W egg p = 0.72 W sperm p = 0.72 WW p 2 = 0.52 Sperm w sperm q = 0.28 Ww pq = 0.20 Eggs w egg q = 0.28 ww qp = 0.20 ww q 2 = 0.07 Next generation: Genotype frequencies Allele frequencies 0.52 WW 0.40 Ww 0.08 ww 0.7 W 0.3 w Question The recessive allele of a gene causes cystic fibrosis. For this gene among Caucasians, p = If a Caucasian population is in Hardy-Weinberg equilibrium with respect to this gene, what proportion of babies is born homozygous recessive, and therefore suffers cystic fibrosis? 10

11 Hardy-Weinberg Equilibrium For a population to remain in Hardy- Weinberg equilibrium for a specific trait, it must satisfy five conditions: 1. Very large population 2. No gene flow between populations 3. No mutations 4. Random mating 5. No natural selection -Rarely met: We do not expect a natural population to be in H-W equilibrium Copyright 2009 Pearson Education, Inc. Hardy-Weinberg Equilibrium Very large population Smaller the population, greater chance alleles will fluctuate by chance No gene flow between populations When individuals move, gene pool altered No mutations Altering or deleting alleles can influence gene pool Random mating If mate preferentially, random mixing does not occur No natural selection Differential survival and reproductive success can alter allele frequencies Copyright 2009 Pearson Education, Inc. 11

12 Hardy-Weinberg Equilibrium Very large population Smaller the population, greater chance alleles will fluctuate by chance (genetic drift) No gene flow between populations When individuals move, gene pool altered No mutations Altering or deleting alleles can influence gene pool Random mating If mate preferentially, random mixing does not occur No natural selection Differential survival and reproductive success can alter allele frequencies Copyright 2009 Pearson Education, Inc. Microevolution Deviations from the five conditions can cause change in gene pools = microevolution The three main causes of microevolution are genetic drift, gene flow, and natural selection 12

13 Genetic Drift The smaller the sample size, the greater chance of a deviation from an idealized result GD: the change in the gene pool of a population due to chance Two main mechanisms of genetic drift Bottleneck effect: chance events reduce population to really small number (oil spill, fire, hunting, etc) remnant population is less likely to have alleles from larger population (they are lost) Ex: African Cheetahs: all extremely similar 13

14 Two main mechanisms of genetic drift Founder effect: Few individuals colonize new habitat, less likely gene pool is actually representative. The Amish Dutch settlers in South Africa and Huntington s disease Other examples Microevolution Deviations from the five conditions can cause change in gene pools = microevolution The three main causes of microevolution are genetic drift, gene flow, and natural selection 14

15 Gene Flow Population may gain or lose alleles when fertile individuals or gametes/spores move between populations Reduces differences between populations Microevolution Deviations from the five conditions can cause change in gene pools = microevolution The three main causes of microevolution are genetic drift, gene flow, and natural selection 15

16 Microevolution Genetic drift, gene flow and mutation are chance events that can cause microevolution Evolution by natural selection is a blend of chance and sorting Only NS consistently leads to adaptive evolution Results in a better fit between organisms and their environment What is fitness? Moths Coloration may hide them from predators Wildflowers Flower characteristics may attract certain pollinators Fitness: The contribution an individual makes to the gene pool of the next generation relative to the contribution of other individuals sisu.typepad.com usps.com 16

17 Natural selection can alter variation in a population in three ways Frequency of individuals Original population Phenotypes (fur color) Stabilizing selection Favors intermediate phenotypes Happens in relatively stable environments Ex: human baby birth weight 17

18 Directional selection Acts against individuals at one of the phenotypic extremes Common during periods of environmental change, or when a population migrates to a new and different habitat Ex: Moth coloration or pesticide resistant insects Disruptive selection Favors individuals at both extremes of the phenotypic range May occur in patchy habitats Ex: beak size 18

19 Natural selection can alter variation in a population in three ways Sexual selection can lead to phenotypic differences between males and females Sexual dimorphism 19

20 Sexual selection Intrasexual selection: Secondary sex structures may be used to compete for mates Intersexual selection: mate choice Intrasexual competition 20

21 Intersexual selection But these traits that are adaptive can also pose risks! Then why be choosy? Prefer traits that are correlated with good genes Offspring from female frogs that mated with long calling male frogs grew bigger, faster, and survived better 21

22 Question Why doesn t natural selection act to eliminate genetic variation in populations, retaining only the most favorable alleles? Diploidy preserves variation by hiding recessive alleles A recessive allele is only subject to natural selection when it influences the phenotype in homozygous recessive individuals Balancing selection and diploidy Balancing selection maintains stable frequencies of two or more phenotypes in a population One type of balancing selection is heterozygote advantage, where heterozygotes have greater reproductive success than homozygous For example, sickle-cell anemia Copyright 2009 Pearson Education, Inc. 22

23 Balancing selection and diploidy In frequency-dependent selection, two different phenotypes are maintained in a population Selection acts against either phenotype if it becomes too common in the population For example, Indonesian silverside fishes Scale eating fish that attack other fish from behind Copyright 2009 Pearson Education, Inc. Right-mouthed 1.0 Frequency of left-mouthed individuals 0.5 Left-mouthed Sample year 23

24 Balancing selection and diploidy Some variations may be neutral, providing no apparent advantage or disadvantages For example, human variation in fingerprints Natural selection cannot fashion perfect organisms 1. Selection can only act on existing variation Natural selection cannot conjure up new beneficial alleles 2. Evolution is limited by historical constraints 3. Adaptations are often compromises 4. Chance, natural selection and the environment interact Copyright 2009 Pearson Education, Inc. 24

25 Ch 14 Speciation: The bridge between microevolution and macroevolution The need for order Cat? Humans have an innate desire to classify things Taxonomy: the branch of biology that names and classifies species and groups them into a hierarchical framework Linnaeus developed binomial nomenclature Linnaeus, the father of taxonomy 25

26 Taxonomy and Systematics Kingdom Animalia Animalia Animalia Phylum Chordata Chordata Arthropoda Class Mammalia Mammalia Diplopoda Order Carnivora Primate Chordeumatida Family Felidae Homidae Cleigonidae Genus Felis Homo Pseudotremia Species catus sapiens schneiderae Felix catus Linnaeus, 1758 What is a species? Depends on who you ask. 26

27 What is a species? Biological species concept (Mayr 1942) Generally accepted definition Species: a group of populations whose members have the potential to interbreed in nature and produce fertile offspring Thus, different species are reproductively isolated from each other; which keeps them as distinct species from each other Other species concepts are useful in some situations (fossils, prokaryotes, etc.) What is a species? Morphological species concept: Classification based on physical traits that are observable and measurable Ecological species concept: Identifies species based on their ecological niches Phylogenetic species concept: Species: the smallest group of individuals that shares a common ancestor and forms one branch on the TOL 27

28 What keeps species separate? Reproductive barriers! Two kinds Prezygotic Postzygotic Prezygotic Barriers 28

29 Prezygotic Barriers Temporal Mating or flowering occurs at different seasons or times of the day Habitat Barriers occur when two species are confined to live in different habitats Prezygotic Barriers 29

30 Prezygotic Barriers Behavioral Barriers occur when there is little or no sexual attraction between species, due to specific behaviors Video: Blue-footed Boobies Courtship Ritual Video: Albatross Courtship Ritual Video: Giraffe Courtship Ritual Prezygotic Barriers Mechanical: Barriers occur when female and male sex organs are not compatible These snail shells spiral in opposite directions and their genital openings cannot be aligned 30

31 Prezygotic Barriers Gametic: Barriers occur when a male and female from two different species can copulate, but the gametes do not unite to form a zygote Gametes of the red and purple urchins are unable to fuse because proteins on the surface of the egg and sperm can not unite. Prezygotic Barriers 31

32 Postzygotic barriers Occurs after hybrid zygotes are formed 3 different barriers that prevent the development of fertile adults Postzygotic Barriers 32

33 Postzygotic barriers Reduced hybrid viability Most offspring do not survive Californiaherps.com Certain salamanders may hybridize, but most do not survive and those that do are very frail Postzygotic barriers Reduced hybrid fertility Hybrid offspring are viable, but sterile 33

34 Postzygotic barriers Hybrid breakdown the first-generation hybrids are viable and fertile, but the offspring of the hybrids are feeble or sterile Nespal.org Where do new species come from? Biological diversity exists and the environment selects. Species do not deliberately change. Two different mechanisms can lead to speciation Differ based on whether the speciation event happens in the same place ( SYMPATRIC ), or via geographic isolation ( ALLOPATRIC ) 34

35 Allopatric Speciation Populations of the same species become geographically isolated, creating a barrier to gene flow Changes in the allele frequencies of each subpopulation may be caused by natural selection, genetic drift, and mutation, unaffected by gene flow from other subpopulations Ex: subdividing lake, grand canyon South North 35

36 Allopatric Speciation Likelihood of allopatric speciation increases when a population is small and isolated A small population may have a different gene pool due to the founder effect Genetic drift and natural selection may have a greater effect in a small population in a new habitat Sympatric Speciation New species may arise within the same area as the parent population. Gene flow between the populations may be reduced by polyploidy, habitat differentiation, or sexual selection. 36

37 Chromosomes not homologous (cannot pair) 1 2 The hybrid may reproduce asexually. Species A 2n = 4 Gamete n = 2 3 Species B 2n = 6 Gamete n = 3 Sterile hybrid n = 5 Viable, fertile hybrid species 2n = 10 Polyploidy 80% of all living plant species are the descendants of ancestors that formed by polyploid speciation Polyploid food plants include oats, potatoes, bananas, peanuts, barley, plums, apples, sugarcane, coffee, and bread wheat 37

38 AA Triticum monococcum (14 chromosomes) 1 BB Hybridization Wild Triticum (14 chromosomes) AB Sterile hybrid (14 chromosomes) AA BB T. turgidum Emmer wheat (28 chromosomes) 2 Cell division error and self-fertilization 3 DD Hybridization T. tauschii (wild) (14 chromosomes) ABD Sterile hybrid (21 chromosomes) 4 Cell division error and self-fertilization AA BB DD T. aestivum Bread wheat (42 chromosomes) Sympatric Speciation New species may arise within the same area as the parent population. Gene flow between the populations may be reduced by polyploidy, habitat differentiation, or sexual selection. 38

39 Sympatric Speciation Habitat differentiation and sexual selection Ex: cichlid fish in Lake Victoria Explore different food sources Different habitats Different coloration Evolution.berkeley.edu 39

40 How do reproductive barriers arise? Reproductive barriers arise as a byproduct of changes in populations as they adapt to different environments. Reproductive Barriers Dodd s experiments with fruit flies Raise flies on different food sources After many generations, Flies raised on starch digest starch more efficiently Flies raised on maltose digest maltose more efficiently When flies were combined in mating experiment, show preferential mating Control flies mate randomly 40

41 Bumblebees prefer pink Receive more hummingbird visits Typical Mimulus flowers Scientistaltered flowers Hummingbirds prefer red Receive more bee visits A change in a single gene can influence pollinator preference which provides a reproductive barrier! 41

42 What happens if isolated populations reconnect? 42

43 Reinforcement Fusion Stability If hybrids are less fit than parent species, natural selection strengthens reproductive barriers Parent species continue to mate with themselves Weak reproductive barriers between the two species, with considerable gene flow, reverses speciation and two species become one again Pollution in lake making it hard to distinguish between two species Many hybrid zones are stable, continuing to produce hybrids; this allows some gene flow between populations, but each species maintains its own integrity Adaptive Radiation The evolution of many diverse species from a common ancestor Typically occurs when a few organisms colonize new, unexploited areas or when environmental changes cause extinctions and present new opportunities for survivors Adaptive radiations are linked to new opportunities: lack of competitors, varying habitats and food sources, evolution of new structures 43

44 Adaptive Radiation and the finches Completely isolated on the island, the founder population may have significantly changed as it adapted to its novel environment (species 2) Later a few individuals migrate to another island, encounter new conditions, accumulate enough changes to become a new species (species 3) Multiple colonizations and speciations on separate islands probably followed Cactus-seed-eater (cactus finch) Seed-eater (medium ground finch) Tool-using insect-eater (woodpecker finch) 44

45 Speciation can occur rapidly or slowly Punctuated equilibrium Long periods of little change, with abrupt speciation events Gradual patterns Differences gradually evolve in populations as they become adapted to their local environment Lab Finish lab 9 For next week Lecture Review/Office Hours Tuesday evening Exam and lecture 45