Idea Transcript
Chapters 22, 23, and 24 Natural Selection and Mechanisms of Speciation
A. Bergeron AP Biology PCHS
What do you think these data reveal about the state of science education in the United States?
Do you think that this is an appropriate question to ask in a poll?
The Prevailing World View (Before Darwin and his Contemporaries) 1. The Earth is young (approximately 6,000 years) ! -Promoted by Bishop James Ussher 2. Each type of species was “created” ! -Species are “fixed”, meaning that they are immutable ! -The number of species on Earth is not changing 3. Any variations observed in an organism are “imperfections” ! -Adaptations are designed by the creator 4. Observation alone should support the view of immutable species.
Evolution Discussion Topics 1. Worldview before Darwin -Carolus Linneaus, Georges Cuvier, James Hutton, Charles Lyell 2. Jean-Baptiste Lamarck ! -Theory of Acquired Characteristics 3. Charles Darwin ! -Theory of Evolution by Natural Selection ! -Artificial Selection ! -Examples of Natural Selection in the “Real World” 4. Evidence for Evolution ! -Extinction ! -Comparative anatomy, vestigial organs ! -Molecular/biochemical evidence ! -Biogeography ! -Fossil record - Transitional forms
Georges Cuvier Promoted “catastrophism” or the idea that catastrophes (i.e. floods, earthquakes, volcanic eruptions) altered the distribution of organisms in a particular geographic area Rejected the idea of evolutionary change James Hutton ! Promoted gradualism or the idea that ! large changes can take place through ! the accumulation of small changes over ! time Charles Lyell ! Promoted uniformitarianism or the idea that ! the geological factors that affect the Earth ! today are the same that affected the Earth ! long ago
Progression of Evolutionary Theory through Time
Lamarck - Theory of Acquired Characteristics One of the first scientists to offer an explanation for how organisms changed over time Promoted the idea of use and disuse Parts of the body that are used become larger and stronger; those that are disused become smaller and weaker Suggested that organisms pass on acquired characteristics to their offspring There is little evidence that acquired characteristics can be inherited by offspring Evidence for Lamarckism?
Descent with Modification??? Jean-Baptiste Lamarck proposed the Theory of Acquired Characteristics Lamarck proposed that by selective use or disuse of organs, organisms acquired or lost certain traits during their lifetime. These traits could then be passed on to their offspring. This process led to change in a population/species over time.
Larmarck or Darwin? Given your understanding of both Lamarck’s and Darwin’s concept of evolution, determine whether the following statements are more Larmarckian or Darwinian. If the statement is Lamarckian, change it to make it Darwinian.
OR
The widespread use of DDT (an insecticide) in the mid-1900’s placed a selective pressure on insects to evolve resistance to DDT. As a result, large populations of insects today are resistant to DDT.
Larmarck or Darwin?
OR
Many of the bacterial strains that infect humans today are resistant to a wide range of antibiotics. These resistant strains were not so numerous or common prior to the use of antibiotics. These strains must have appeared or evolved in response to the use of the antibiotics
Larmarck or Darwin?
OR
The children of bodybuilders tend to be much more athletic, on average, than other children because the characteristics and abilities gained by their parents during their lives have been passed on to their children.
Larmarck or Darwin?
OR
According to one theory, the dinosaurs became extinct because they couldn’t evolve fast enough to deal with climatic change that affected their food and water supplies.
Larmarck or Darwin?
OR
Life arose in an aquatic environment and later invaded land. Once animals came on to land, they had to evolve effective methods of support against gravity and locomotion in order to survive.
Larmarck or Darwin?
OR
A given phenotypic trait (e.g. height, speed, tooth structure) and the genes that determine it may have positive survival or selective value, negative survival or selective value, or neutral survival or selective value. Which of these it possesses depends on the environmental conditions the organism encounters.
Reading Quiz - Chapter 22 1. Charles Lyell promoted a principle of ____________ which stated that geologic processes have not changed throughout Earth’s history. 2. A contemporary of Darwin presented research that was quite similar to Darwin’s theory of natural selection. Who was this individual? 3. Structures that are similar in structure but may be different in function are known as _________. 4. According to the principle of biogeography, organisms living on an island are (more likely/less likely) to resemble organisms living on the mainland. 5. If Darwin was correct and populations have changed over time, ______________ should be found as part of the fossil record.
Do Now - Evolution by Natural Selection 1. How did Darwin define fitness? How did Darwin define evolution? 2. What is the unit of natural selection (i.e. what is selected for or against?) What is the unit of evolution? 3. In a population of mice, some individuals have brown fur and some have black fur. At present, both phenotypes are equally fit. What could happen to change the relative fitness of the two phenotypes in the population?
Evolution by Natural Selection Theory proposed by Charles Darwin in 1859 as part of his book On the Origin of Species by Means of Natural Selection Evolution: A change in a population of organisms over time Darwin’s Theory of Evolution by Natural Selection proposed that individuals better adapted to their environment will: ! -Be more likely to survive and therefore reproduce ! -Pass on beneficial adaptations to their ! offspring (i.e. differential reproductive success) ! -Generate new populations of organisms that are ! better adapted to their environment *Over time, the original population would have changed and the new population/s could become a new species *
Charles Darwin and the Theory of Evolution by Natural Selection Darwin asked, “If evolutionary change has occurred, then HOW do organisms evolve?” The Keys to Natural Selection 1. Struggle for Existence
- Nature produces more organisms than can survive
-Litters of cats, dogs, mice, hamsters, etc.
- Organisms must COMPETE with one another for limited resources 2. Organisms in a Population Can Be Different
- Variations in physical traits exist within any population
- Variations are passed on from one generation to the next 3. Organisms that Can Adapt to their Environment Can Survive
- Organisms that have inherited favorable traits which allow them
to cope with their environment and out-compete other organisms
are more likely to SURVIVE and REPRODUCE
Details of Darwin’s Theory of Natural Selection ! ! ! ! ! ! ! ! ! ! ! ! ! ! !
1. Variations exist within populations of organisms 2. Variations can be passed from parents to offspring ! -Traits are heritable ! 3. Organisms tend to produce lots of offspring 4. Not all of the offspring that are produced will be ! able to survive due to the limited amount of resources in the environment 5. The organisms that do survive are more likely to REPRODUCE and pass on beneficial traits to their offspring making it easier for them to SURVIVE and reproduce
What Conclusions Can We Draw from the Theory of Evolution? Remember, these are conclusions that are drawn from data. The theory CANNOT be proven (and doesn’t need to be…) 1. Organisms (i.e. living things) have changed over time -Organisms alive millions of years ago are not alive today ! -Many of these organisms are extinct but others have ! “evolved” into the organisms that are alive today 2. All species which exist today descended from (i.e. originated) a common ancestor ! -Modern organisms should have similar characteristics to those organisms that were alive long ago
Descent with Modification
If a Gene Pool Can Change, What is the Origin of this Variation? Sources of Genetic Variation No two organisms are 100% identical at the genetic level (except for identical twins, of course) 1. Mutation ! -Produces new genes (alleles) not ! previously seen in the gene pool !
! ! ! !
-Caused by mistakes made during DNA replication, chemical agents (i.e. mutagens), or the environment (e.g. UV radiation)
-The mutation could potentially alter the ! structure of the protein and potentially ! change its function as well
Sources of Genetic Variation 2. Independent assortment of chromosomes during meiosis
Independent assortment produces gametes that (potentially) have a different distribution of alleles than either parent
Sources of Genetic Variation 3. Genetic recombination - “Crossing over” Crossing over creates new varieties of gametes (sex cells) and new combinations of alleles not previously seen in the population (in parental generation)
Discussion Questions 1. How did knowledge of mechanisms of artificial selection (which was used in developing various strains of domesticated plants and animals) help Darwin understand how evolution could occur? 2. Complete the table Observation
All species populations have the reproduction potential to increase exponentially over time. The number of individuals in natural populations tends to remain stable over time. Environmental resources are limited. Individuals in a population vary in their characteristics. Much of this variation is heritable
How did Darwin make this observation? What did he read or observe that led to his understanding?
Discussion Questions 3. Complete the table Inference Production of more individuals that the environment can support leads to a struggle for existence such that only a fraction of the offspring survive each generation. Survival for existence is not random. Those individuals whose inherited traits best fit them to the environment are likely to leave more offspring than less fit individuals. The unequal ability of individuals to survive and reproduce leads to a gradual change in the population, with favorable characteristics accumulating over many generations.
Observations that led to the inference
The Prevailing World View (After Darwin and his Contemporaries) 1. The Earth is old ! ! -Current estimates place the age of the Earth at ! ! approximately 4.5 billion years 2. Species are related through common descent 3. Adaptation to the environment is accomplished through the interplay of random variation and natural selection 4. The process of evolution can be observed and tested in the laboratory and the field
Fish Species Shark Salmon Trout Catfish Crab Shrimp Lobster Clams Oysters Octopus Cod Swordfish Snapper Sea Bass Grouper Sole Smelt
Contributor
Bio-Rad Kaleidoscope Prestained Protein Standard Myosin B-galactosidase Bovine serum albumin Carbonic anhydrase Soybean trypsin inhibitor Lysozyme Aprotinin
Blue Magenta Green Violet Orange Red Blue
198kD 126kD 85kD 37kD 32kD 17kD 7kD
Distance Migrated (mm)
Protein Molecular Mass (from standard curve)
Fish Species #1
Fish Species #2
Fish Species #3
Fish Species #4
Fish Species #5
Count/ Tally of Proteins
Average Mass of an Amino Acid = 110 daltons Average Molecular Weight of Protein = # Amino Acids x 110 daltons #bp that Encode Protein = (3 bp/amino acid) x 110 daltons
Species #1 Species #2 Species #3 Species #4 Species #5 Species #1 Species #2 Species #3 Species #4 Species #5
Reading Quiz - Chapter 23 1. From an evolutionary perspective, do individual organisms or populations change over time? 2. In H-W equilibrium, p represents the frequency of the __________ allele while q represents the frequency of the __________ allele. 3. Name two potential causes of microevolution within a population. 4. A natural disaster that drastically reduces the proportion of a particular allele in the gene pool is referred to as a __________. 5. In ________ selection, the intermediate phenotype is favored over extreme phenotypes.
Is Natural Selection Occurring within a Population? 1. Is there variation within the population?
2. Are characteristics within the population heritable? 3. Not all members of the population that are born will survive and reproduce
4. Certain members of the population are more likely to survive and reproduce than others
Let’s Analyze Several Populations to See if These Four Tenets of Natural Selection Are Applicable…
Natural Selection - Nylon-eating Bacteria The nylon polymer (synthetic, human-made) was invented by DuPont in 1934 Nylon was invented to replace the silk and/or hemp-based products that were used to make parachutes Nylon requires a great deal of energy to be broken down. Incineration produces hydrogen cyanide Strains of Pseudomonas bacteria grown in an environment that contains a molecular precursor for nylon synthesis (only source of carbon and nitrogen in the media) evolve enzymes that break down nylon Experiment has been repeated with similar results each time
Natural Selection - Antibiotic Resistance Imagine a scenario where a population of Mycobacterium tuberculosis (the cause of TB) infect lung tissue What selective pressure/s could act on the bacterial population? Could this population change over time?
Infection
Mycobacterium tuberculosis
Selective Pressure = Addition of an Antibiotic 1. Will there be variation within the bacterial population? 2. Are characteristics within the population heritable? 3. Will all members of the population survive and reproduce? 4. Will certain members of the population be more likely to survive and reproduce than others? If so, which ones?
Drug-resistant Tuberculosis
Approximately 50% of antibiotics target prokaryotic ribosomes, rendering them unable to synthesize bacterial proteins Natural selection favors those bacterial variants that possess ribosomes that are unaffected by antibiotic Over time bacterial populations have the potential to become resistant to multiple types of antibiotics (e.g. extensively drug resistant strains)
The Case of the Alpine Skypilot
What conclusions can we draw regarding the flower size of alpine skypilots growing in each environment? Why is this important?
Hint: Is there variation in the population?
Plants growing above tree line were propagated and transplanted to locations below the tree line Flower size of offspring was compared to the flower size of the original parent (i.e. the mother) What does the best fit line tell us about the heritability of flower size?
Is flower size heritable?
Is there limited reproductive success in the population? (Are more individuals “born” than can survive and reproduce?)
Only certain transplanted individuals originating from the tundra environment were able to survive in the forest environment
Certain individuals in a population are more likely to survive and reproduce than others. Hypothesis: Transplanted flowers that are pollinated by bumblebees are more likely to survive and reproduce than flowers that are pollinated by flies. How could this hypothesis be tested?
What can we conclude regarding a bumblebee’s preference for flower size in the transplanted skypilot population?
What can we conclude regarding the relative fitness of a transplated skypilot and the number of bee visits that it received? Biological fitness refers to an organism’s ability to produce fertile (fecund) offspring Plants with the largest number of bee visits (and thus the largest flowers) are most likely to survive, reproduce, and pass on characteristics to the next generation of plants
Evidence for Evolution 1. Fossil record •Extinction •Transitional fossils 2. Comparative anatomy • Homologous structures • Embryological origins • Vestigial organs 3. Molecular and biochemical evidence 4. Biogeography
Fossils Depict Evolutionary Change Organisms that lived in the past are very different from organisms alive today
Transitional Forms Fossil evidence supports the view that land-dwelling mammals are the ancestors of modern-day whales All fossils have distinctive types of ear bones that only organisms in this lineage share Fossils were found in rocks that could only be formed from ocean deposits Legs became smaller as the organism “switched” to a new habitat
Evolution of the Horse Evolution is NOT goal-oriented Examining selected fossils from the descendants of the modern horse might lead one to believe that large, one-toed horses were the eventual goal Examination of the entire horse fossil record reveals multiple branch points Although one type of horse (Equus) was ultimately favored other others, this result was not pre-determined
Homologous Structures Structures in different organisms that are similar due to common ancestry
Embryological Origins Strong resemblance of embyros from a variety of species Suggests descent from a common ancestor
Vestigial Organs Reduced or incompletely developed structures that have no (or reduced) function in an organism Show similarities to functioning organs or structures in other species - Suggested descent from a common ancestral form 1. Human coccyx Reduced form of the tailbone found in primates 2. Human appendix Reduced version of the cecum, aids in the digestion of cellulose 3. Goosebumps Original purpose allowed hair shaft to stand erect, signaling anger or fear
Transitional Forms Fossil evidence supports the view that land-dwelling mammals are the ancestors of modern-day whales All fossils have distinctive types of ear bones that only organisms in this lineage share Fossils were found in geological formations that could only be formed from ocean deposits Legs became smaller as the organism “switched” to a new habitat
Biochemical Evidence Comparing nucleotide sequences of genes found in all organisms suggests common descent Number of differences has allowed for the formation of “genetic clocks” Clocks are supported by position of fossils within the fossil record
Tracking Evolutionary Divergence Biochemical evidence has allowed for the construction of phylogenetic trees which demonstrate the similarities between organisms at the molecular level
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Development of Plants & Animals from an Embryo
Genetic Equivalence Hypothesis Hypothesis of differential gene content not supported experimentally M H
K
M H
K
fertilized egg
fertilized egg
M
H
K
M H
K
M H
K
M H
K
All cells have the same genes but different genes are expressed in different cells Differential expression of genes leads to the development of different cell types
Testing Genetic Equivalence
How Is Cell Fate Determined?
Homeobox Genes Suggest that Embryos Have Similar Developmental Programs
Homeobox genes - Encode proteins that can bind to DNA; involved in regulating the transcription of genes as part of a developmental pathway
Hox Genes Hox genes are involved in pattern formation during embryonic development Hox (short for homeobox) are master control genes transcripton factor proteins bind to DNA ➔ regulate transcription of dev. genes Hox genes are often found in clusters on a chromosome Position-specific along body axis Genes are expressed in the same general regions of the developing embryo suggesting a structure/function relationship
Hox Genes - Deep Homology The expression of Hox genes provides the basis for anteriorposterior axis specification throughout the animal kingdom The expression of human HOXB4 gene can mimic the function of its Drosophila homologue, deformed (dfd), when introduced into Dfd-deficient Drosophila embryos (i.e. gene homology)
•Changes in the Hox protein-responsive elements (DNA) of downstream genes •Changes in Hox gene transcription patterns (mRNA, protein) within a portion of the body •Changes in the number of Hox genes (duplications, deletions, etc.)
Homeotic Mutations Affect Embryonic Development and Formation of Adult Structures
Homeotic mutation - Mutations to either the homeotic gene or the homeobox target to which the gene product (i.e. protein) can bind
Homeotic Mutations Affect Embryonic Development and Formation of Adult Structures
Hox Genes - Evidence for Evolution (Deep Homology)
Hox Genes - Snakes Have Legs! (Or at Least Primordial Femurs...?)
In most vertebrates, forelimbs develop anterior to the expression of HoxC-6 HoxC-6 and HoxC-8 expression specify the development of thoracic vertebrae
Effect of Paralogous Hox Gene Mutations on Vertebral Development
Hox Genes - Deep Homology
Paralogous genes: Partner genes that have a similar function in the absence of a partner
How Would You Evaluate These Explanations of Darwin’s Ideas? This question was included as part of a Biology I exam at the University of _________ (wherever you choose to attend). In two or three sentences describe Darwin’s theory of descent with modification and the mechanism, natural selection, that he proposed to explain how this change occurs. Two students’ answers to the questions are given on the next slide. Evaluate and grade each response on how well it represents Darwin’s ideas. Indicate why you would or would not have awarded full credit.
Answer #1: Darwin saw that populations increased faster than the ability of the land to support them could increase, so that individuals must struggle for limited resources. He proposed that individuals with some inborn advantage over others would have a better chance of surviving and reproducing offspring and so be naturally selected. As time passes, these advantageous characteristics accumulate and change the existing species into a new species. Answer #2: Darwin’s theory of evolution explains how new species arise from already existing ones. In his mechanism of natural selection, organisms with favorable traits tend to survive and reproduce more successfully, while those that lack traits do not. Beneficial traits are passed on to future generations in this manner, and a new species will be created in the end!
The allele for black coat is recessive to the allele for white (pink) coat. Can you determine the number of recessive alleles in this population of pigs?
What is Microevolution? Microevolution is a change in the genetic composition of a population
Populations and Gene Pools Gene pool: All of the genes (alleles) that are present in a population of organisms The allele frequencies in a population always equal 100% If the composition of genes (alleles) in the gene pool changes, there is a good chance that the population of organisms is changing or evolving Microevolution - Online Activity 14.4 - Alter a Gene Pool
Populations and Gene Pools (continued) Sample Population
48% heterozygous black
Frequency of Alleles allele for brown fur
allele for black fur
16% homozygous black
36% homozygous brown
Suppose two genes (alleles) control fur color in mice. The black allele is dominant to the brown allele The collection of both alleles make up the gene pool for this particular population of mice
Calculating Allele Frequencies In a population of ten (10) flowers, you can assume that there are twenty (20) total alleles (diploid organisms) If the genotypes of the organisms in the population are known, you can calculate the frequency of each allele in the gene pool
Calculating Allele Frequencies
If a Gene Pool Can Change, What is the Origin of this Variation? Sources of Genetic Variation No two organisms are 100% identical at the genetic level (except for identical twins, of course) 1. Mutation ! -Produces new genes (alleles) not ! previously seen in the gene pool !
! ! ! !
-Caused by mistakes made during DNA replication, chemical agents (i.e. mutagens), or the environment (e.g. UV radiation)
-The mutation could potentially alter the ! structure of the protein and potentially ! change its function as well
Sources of Genetic Variation 2. Independent assortment of chromosomes during meiosis
Independent assortment produces gametes that (potentially) have a different distribution of genes than either parent
Sources of Genetic Variation 3. Genetic recombination - “Crossing over” Crossing over creates new varieties of gametes (sex cells) and new combinations of genes not previously seen in the population (in parental generation)
Introduction to Hardy-Weinberg Equilibrium In order to determine if a population is evolving, we must have some sort of baseline for comparison Population: A localized group of individuals belonging to the same species Species: A group of populations whose individuals have the potential to interbreed and produce fertile offspring A non-evolving population should have a gene pool that is in equilibrium Gene pool = All of the alleles present in a population The relative frequency of dominant and recessive alleles should remain constant from one generation to the next Any changes to the allelic frequencies in a gene pool suggest that the population is changing or evolving
Hardy-Weinberg Equilibrium Practice Problems 1. Which five (5) conditions must be met in order for HardyWeinberg equilibrium to exist within a population? 2. Researchers examining a particular gene in a fruit fly population discovered that there are two alleles for the gene, designated A1 and A2. Gene pool analysis revealed that 70% of the gametes produced in the population contained the A1 allele. If the population is at H-W equilibrium, what proportion of the flies carries both A1 and A2? 3. In a certain population of 1000 fruit flies, 640 have red eyes while the remainder have sepia eyes. The sepia eye trait is recessive to red eyes. How many individuals would you expect to be homozygous for red eye color?
Hardy-Weinberg Equilibrium Practice Problems 4. The frequency of a recessive allele in a population of antelopes is determined to be 0.27. What is the frequency of the dominant allele? 5. An investigator has determined by inspection that 16% of a human population has a recessive trait. What are the genotype and allele frequencies for the population, assuming that the conditions for H-W equilibrium have been/are met? 6. In a certain population, 21% are homozygous dominant, 49% are heterozygous, and 30% are homozygous recessive. what percentage of the next generation is predicted to be homozygous dominant, assuming a H-W equilibrium? 7. The genotype frequencies of a population are determined to be 0.6 BB, 0.0 Bb, and 0.4 bb. Is it likely that this population meets all the conditions required for genetic equilibrium?
Hardy-Weinberg Equilibrium Practice Problems 8. After graduation, you and 19 of your closest friends (lets say 10 males and 10 females) charter a plane to go on an around-the-world tour. Unfortunately, you all crash land (safely) on a deserted island. No one finds you and you start a new population totally isolated from the rest of the world. Two of your friends carry (i.e. are heterozygous for) the recessive cystic fibrosis allele (c). Assuming that the frequency of this allele does not change as the population grows, what will be the incidence of cystic fibrosis on your island? 9. In a given population, only the "A" and "B" alleles are present in the ABO system; there are no individuals with type "O" blood or with O alleles in this particular population. If 200 people have type A blood, 75 have type AB blood, and 25 have type B blood, what are the allelic frequencies of this population (i.e. What are p and q)?
Hardy-Weinberg Equilibrium Practice Problems 10. The allele for right-pointing mouth in cichlids (A) is dominant over that for left-pointing mouth (a). Use the Hardy-Weinberg principle to calculate the allele frequencies that would correspond to equal frequencies of the two phenotypes. Which allele has the higher frequency? How are these scale-eating fish an example of a balanced polymorphism?
Requirements for Hardy-Weinberg Equilibrium In an ideal breeding population: 1. Population size is very large 2. Mating is random. Mating pairs show no preference for one phenotype over another. There is no sexual selection. 3. There is no mutation. 4. Immigration and emigration do NOT occur. No gene flow. 5. There is no natural selection Hardy Weinberg Equilibrium Hardy-Weinberg Equilibrium - LabBench
Hardy-Weinberg Equilibrium Requirements (assumptions) for Hardy -Weinberg equilibrium to be “true” 1. Population size is very large 2. No mutation is occurring 3. Organisms in the population reproduce sexually and randomly. 4. Natural selection is not occurring 5. No emigration (i.e. out of) the population or immigration (i.e. into) the population If all of these conditions are met, the allele frequencies in the population are not changing and the population is NOT evolving Hardy-Weinberg Animation
Hardy-Weinberg Equilibrium A simple mathematical model of the interplay between alleles that are present in a particular gene pool At H-W equilibrium, the frequency of the dominant allele (p) and the frequency of the recessive allele (q) always equal 1 p+q=1 The frequency of the homozygous dominant genotype in the population is expressed as p2 The frequency of the homozygous recessive genotype in the population is expressed as q2 The frequency of the heterozygous genotype in the population is expressed as 2pq Thus at H-W equilibrium... p2 + 2pq + q2 = 1
Hardy-Weinberg Equilibrium Frequency of dominant allele = p Frequency of recessive allele = q
p+q=1
p2 + 2pq + q2 = 1 p2 = frequency of homozygous dominant genotype q2 = frequency of homozygous recessive genotype 2pq = frequency of heterozygous genotype
AP Biology Lab #8 - Population Genetics
Exploring H-W Equilibrium - Activity 1 Dominant: Taster - AA or Aa Recessive: Non-taster - aa Tasters
Non-tasters
Tasters + Non-Tasters
Allele Frequency A (p)
A (q)
Class Totals
What is the frequency of heterozygous tasters in the class? Does the class meet any/all of the requirements for H-W equilibrium? Would you predict the allelic frequencies for PTC tasting to remain constant for the next 200 years?
Exploring H-W Equilibrium - Activity 2 Effect of a small population size on allelic frequency in a gene pool Initial Class Frequencies
p= 0.5
q= 0.5
Initial Genotype = A/a Class Totals
My Genotype Generation 1
A/A
A/a
a/a Generation 5 frequencies
Generation 2 Generation 3
p=
Generation 4
q=
Generation 5
Exploring H-W Equilibrium - Activity 3 Applying Selective Pressure Initial Class Frequencies
p=
q=
Initial Genotype = A/a Class Totals
My Genotype Generation 1
A/A
A/a Generation 5 frequencies
Generation 2 Generation 3
p=
Generation 4
q=
Generation 5
Do Now - Evolutionary Change? Genome sequencing projects may dramatically affect how biologists analyze evolutionary changes in quantitative traits. For example, suppose that the genomes of many living humans are sequenced and that genomes could be sequenced from individuals who lived several millennia in the past (You may assume preserved tissue is readily available.) If 20 genes have been shown to influence height, how could you use the sequence data from these genes to test the hypothesis that human height has evolved in response to natural selection?
Causes of Evolutionary Change - Genetic Drift The gradual change in the frequency of alleles within a small population of organisms Changes are due to chance
Causes of Evolutionary Change - Genetic Drift Assume that you have a population of red and green insects at Hardy-Weinberg equilibrium
Causes of Evolutionary Change - Genetic Drift After a windstorm, a large proportion of the population blows away The distribution of alleles has changed dramatically
If only red alleles were passed on to the next generation, what would the population look like after several generations?
Sources of Genetic Drift 1) Bottleneck Effect Typically caused by a natural disaster that eliminates a substantial portion of a population and the corresponding component of the gene pool The genetic diversity of the remaining population is limited
2) Founder Effect Caused by a small part of a population emigrating to a new location Genetic diversity of the new population much smaller than in the original population
Other Causes of Evolutionary Change in a Population 1. Gene flow Caused by emigration or immigration of individuals out of or into the population, respectively 2. Mutation Directly changes one allele into another (or another form) 3. Natural selection -Particular individuals are more likely to survive and reproduce in the population -These individuals will pass on their genetic information to their offspring -Consequently, the population may change (I.e. evolve) over time
Is Natural Selection Occurring within a Population? 1. Is there variation within the population? 2. Are characteristics within the population heritable? 3. Not all members of the population that are born will survive and reproduce 4. Certain members of the population are more likely to survive and reproduce than others
Let’s Look at How Natural Selection Can Alter a Gene Pool and the Population Represented by the Gene Pool…
Do Now - Evolution as Genetic Change The residues ("tailings") of mines often contain such high concentrations of toxic metals (e.g., copper, lead) that most plants are unable to grow on or near them. However, some hardy species (e.g. certain grasses) are able to spread from the surrounding uncontaminated soil onto such waste heaps. These plants develop resistance to the toxic metals while their ability to grow on uncontaminated soil decreases. Because grasses are wind pollinated, breeding between the resistant and nonresistant populations goes on. 1. What type of natural selection is affecting the population? 2. Predict what could happen to this population after many thousands of generations
Disruptive Selection Individuals at both extremes (in terms of phenotype) have the highest fitness (most likely to survive and reproduce)
Disruptive Selection
Ducks with long beaks can find food at the bottom of a river or stream while ducks with short beaks can easily feed at the water’s surface Ducks with intermediate sized beaks are at a disadvantage Over time, the extreme phenotypes will be favored over the intermediate phenotype and the population will change
Stabilizing Selection Individuals closest to the mean or average phenotype have the highest fitness within a population of organisms
Stabilizing Selection
Large lizards are more likely to be seen and caught by predators while small lizards will have difficulty escaping from predators on the ground Intermediate-sized lizards in the population are more likely to survive and reproduce
Effect of Sickle Cell Mutation on Hemoglobin Structure
Normal vs. Sickle-Cell Red Blood Cells
Sickle-Cell Disease and Heterozygote Advantage Heterozygous individuals do not suffer from sickle cell anemia and are less susceptible to the parasite that causes malaria, Plasmodium falciparum When the parasite enters the RBC it induces hypoxia which leads to a sickling of RBCs in individuals who are either heterozygous OR homozygous for the sickle cell allele Individuals who do not have a sickle cell allele have RBCs that do not sickle in the presence of the parasite - These individuals are at the greatest risk of contracting malaria Spleen eliminates sickle-shaped RBCs (which removes the parasite from the body) leading to anemia in persons who are homozygous for the sickle cell allele Heterozygotes do not suffer from anemia because a sufficient supply of normal RBCs remains in the body
Directional Selection Individuals which possess one phenotype (one of the extremes) have the highest level of biological fitness
Directional Selection
During dry years on the Galapagos, hard seeds were more prevalent than soft seeds Birds with deep beaks were better able to crack open hard seeds than birds with narrow beaks Over time, the mean beak depth shifted as the finch population adapted to their environment
Types of Natural Selection - A Summary
How Do New Species Come to Be? Monochromatic P. pundamilia
P. nyererei
Let’s Play Everyone’s Favorite New Game... Name that Reproductive Barrier! Decide whether the following descriptions are prezygotic or postzygotic barriers to reproduction. In addition identify the specific type of reproductive barrier
Different species of bowerbird construct elaborate bowers and decorate them with different colors in order to woo females. The MacGregor’s Bowerbird builds a tall tower of sticks and decorates with bits of charcoal.
Two species of frogs with overlapping ranges breed at different times of the year. Rana aurora (left) breeds January - March while Rana boylii - breeds late March May (right)
Imagine a situation in which a population extends over a broad geographic range, and mating throughout the population is not random. Individuals in the far west would have zero chance of mating with individuals in the far eastern end of the range.
The formation of a river separates the insect population into two distinct groups; one on either side of the river
The morphology of the damselfly penis (Yes, insects have sex!) is evidence for a specific form of reproductive isolation. What is it?
Imagine a species of louse living on a species of gopher. When the gophers get together to mate, the lice get an opportunity to switch gophers and perhaps mate with lice on another gopher. Gopher-switching allows genes to flow through the louse species.
That’s Bananas! The modern Cavendish banana has 11 pairs of chromosomes (2n=22). The Cavendish is an autotriploid species with a complex evolutionary history. Illustrate a possible sequence of events that could have led to the appearance of the Cavendish banana as a result of an autopolyploid event. (Remember an autopolyploid speciation event is fundamentally different from an allopolyploid event)
What is a Species? Biological species concept: A population or group of populations whose members have the potential to interbreed and produce viable, fertile offspring (genetic exchange is possible and that is genetically isolated from other populations)
General Forms of Speciation Anagenesis
Accumulation of heritable changes resulting in a new species (Lamarckian)
Cladogenesis
“Branching evolution;” New species arises from a population that branches from an original parent species; Parent species remains (Darwinian)
Process of Speciation Reproductive Isolation results from Isolating mechanisms which include Behavioral isolation
Geographic isolation
Temporal isolation
produced by
produced by
produced by
Behavioral differences
Physical separation
Different mating times
which result in Independently evolving populations which result in Formation of new species
Prezygotic Isolating Mechanisms Prezygotic barriers: Impede mating between species or prevent the fertilization of the ova Geographical Isolation Population is distributed in a way that prevents or inhibits efficient reproduction/mating Barriers that prevent organisms from encountering one another could lead to the production of new species Smaller organisms with limited ranges are affected greatly by geographic barriers Larger organisms that can travel large distances or plants that use air currents to distribute pollen may not be affected
Prezygotic Isolating Mechanisms Prezygotic barriers: Impede mating between species or prevent the fertilization of the ova Geographical Isolation Population is distributed in a way that prevents efficient reproduction/mating Behavioral/Sexual Isolation -Albatross mating ritual -Blue-footed boobies courtship ritual Temporal Isolation Mating/reproductive periods are separated among members of a population ! Two fruit fly species have overlapping ranges but Drosophila ! !
psedoobscura is sexually active in the afternoon while Drosophila persimilis is active only in the morning
Do Now - Exploring Speciation Drosophila “Speciation” via Nutrient Modification Original WT fly population
Some flies raised on starch medium
Some flies raised on maltose medium
Mating experiments between both fly populations
After several generations of living on each type of specialized medium, flies adapted to digest starch (or maltose) more efficiently The populations had “evolved”
Drosophila “Speciation” via Nutrient Modification At your tables, predict the results of matings between “starch” flies and “maltose” flies At your tables, predict the results of matings between flies taken from two different “starch” populations or two different “maltose” populations Female
9
8
20
Mating frequencies; Experimental group
Different
Same
Starch
22
Same
18
15
Different
Maltose
Male
Starch
Maltose
Male
Female
12
15
Mating frequencies; Control group
Assortative (Sexual Selection) Mating in Cichlids Male and female fish were placed in two aquarium tanks; one with natural light and one with monochromatic (orange) light. Under natural light, the colorations of the two different fish species are readily apparent However, under monochromatic light, the colorations appear almost identical Females mated only with males of their own species under normal light Natural Monochromatic Under monochromatic light, females mated indiscriminately
P. pundamilia
P. nyererei
Examples of Postzygotic Isolating Mechanisms Postzygotic barriers: Fertilization occurs, but the hybrid zygote does not develop into a viable, fertile adult Reduced hybrid viability Hybrid embyro fails to develop normally Reduced hybrid fertility Hybrid organism survives but is infertile Example - Horse ♥ Donkey --> Mule (infertile) “Backbreeding” of a mule to a horse or donkey fails Hybrid breakdown Offspring of hybrid are infertile Example - Cotton; 2nd generation hybrids are sterile
Allopatric vs. Sympatric Speciation
Allopatric (“Other Country”) Speciation Production of new species through the interruption of a population’s gene flow due (primarily) to a change in the local geography
Sympatric (“Same Country”) Speciation Production of a new species within members of the population that are not geographically isolated from one another Reproductively isolated subpopulation in the midst of its parent population (results in change in genome) Common in plants but less so in animals
Speciation Practice Problem In Chapter 24, you read that bread wheat (Triticum aestivum) is an allohexaploid, containing two sets of chromosomes from each of three different parent species. Genetic analysis suggests that the three species pictured below each contributed chromosome sets to T. aestivum. (The capital letters shown on the right represent sets of chromosomes rather than individual alleles.) Evidence also indicates that the first polyploid event was a spontaneous hybridization of the early cultivated wheat species T. monococcum and a wild grass species. Based on this information, draw a diagram of one possible chain of events that could have produced the allohexaploid T. aestivum.
How Can Sympatric Speciation Account for New Species Arising by Genetic Variation? Polyploidy in Certain Species of Plants -Autopolyploidy - Multiple sets of chromosomes from the same species
How Can Sympatric Speciation Account for New Species Arising by Genetic Variation? Polyploidy in Certain Species of Plants -Allopolyploidy - Multiple sets of chromosomes from two or more different species
Possibility #1
Possibility #2
Processes of Evolutionary Change Species that are
Unrelated
form
Related
in
under
under
in
in
Inter-relationships
Similar environments
Intense environmental pressure
Small populations
Different environments
can undergo
can undergo
can undergo
can undergo
can undergo
Co-evolution
Convergent evolution
Extinction
Punctuated equilibrium
Adaptive radiation
Do Now - Exploring Speciation in Drosophila Using the pictures below, write a short story that explains how the original Drosophila popluation could split into two new species. 1.
2.
3.
4.
Adaptive Radiation
Some organisms manage to be the first to colonize new islands The new environment allows for this original population to undergo evolutionary change Members of this population may adapt to the environment or may move to a neighboring island (if it exists) The gradual accumulations of beneficial adaptations may lead to the generation of new species from the original population
Adaptive Radiation At least 13 separate species of finch inhabit the Galapagos islands All of these species diverged from a single ancestral, ground-dwelling, seed eating finch species Each species has become adapted for different diets: seeds, insects, flowers, leaves, and the blood of seabirds
Convergent Evolution Different organisms living in different places experience similar environmental influences Consequently, these organisms may develop analagous structures, structures similar in structure and function but with different evolutionary histories (i.e. not derived from common descent) While homologous structures indicate common ancestry, analagous structures do not
Convergent Evolution (cont.) Some ocean-dwelling fish live in the waters surrounding Antarctica where the water is very cold These fish have adapted to their frigid environment by synthesizing “antifreeze proteins” that prevent their blood from freezing at low temperatures Natural selection would favor any fish that had this beneficial mutation/adaptation Fish that live in the Arctic also have these proteins but the gene that encodes the Arctic protein is very different from the gene that encodes the Antarctic protein The antifreeze gene evolved at least twice in each of these separate populations of fish
Coevolution (The Biological Arms Race!) Different organisms can evolve together if they depend on each other for survival and/or reproductive success
Red squirrels and crossbilled birds both eat pine (cone) seeds In some places, there are no squirrels and the birds are free to consume as many seeds as they can Unfortunately for the birds, the pines have developed a defense mechanism which makes it harder for the birds to eat their seeds
Coevolution (cont.) Pinecones with large, thick scales are harder for birds to eat But the birds have developed a counterattack against the pine trees! Birds with deeper, shorter, less curved beaks are better able to eat seeds from large, thick pine cones Hypothesis Testing 1. Predict what type of pinecone you would expect to observe in an area without birds (but with squirrels) or an area without squirrels (but with birds). 2. Predict the beak phenotype of birds living in an area where pinecones have thick scales. Do the same for birds living in an area where pinecones have thin scales.
Punctuated Equilibrium Punctuated equilibrium refers to the tempo of speciation Some species appear in “bursts” due to the rapid accumulation of modifications Often occurs in small populations that are in a “challenging” new environment This rapid speciation may not leave fossil evidence and the surviving species may appear to have been “created” without any ancestral species Punctuated equilibrium helped explain the non-gradual appearance of some species in the fossil record
Punctuated Equilibrium Mollusk (i.e. snail) population at stasis (i.e. genetic equilibrium)
A drop in sea level isolates one part of the population from the other
The isolated population changes in an “attempt” to adapt to the new (terrestrial?) environment
Punctuated Equilibrium 1.
3.
2.
4.
5.
Evolution and Speciation Review Questions 1. Natural selection can lead to the evolution of prezygotic isolating mechanisms but not postzygotic mechanisms. Explain how this is possible. 2. Directional selection can lead to the fixation of favored alleles. When this occurs, genetic variation is zero and evolution stops. Explain why this rarely occurs. 3. Explain why genetic drift is much more important in small populations than in large populations. 4. Conservation managers frequently use gene flow, in the form of transporting individuals or releasing captive-bred young, to counteract the effects of drift on small, endangered populations. Explain how gene flow can also mitigate the effects of inbreeding.
Evolution and Speciation Review Questions 5. In 1789 a small band of mutineers led by Fletcher Christian took over the British warship HMS Bounty. They fled first to Tahiti and then to tiny Pitcairn Island in the South Pacific, along with a small population of native Tahitians. Some of the sailors married Polynesians there and raised families. All contemporary Pitcairn islanders can trace at least part of the family tree back to the colonization event. What evolutionary forces were at work in this small, isolated population during and after the arrival of the mutineers and Polynesians?
Instructions for Performing an Entrez/pBLAST alignment 1. Enter www.Pubmed.gov into a web browser 2. Click Protein at top of page 3. Perform a search for common gene sequences Examples: Elongation factor 1 alpha, alcohol dehydrogenase, cytochrome C, catalase, carbonic anhydrase,or you choose one of your own. 4. Locate the amino acid sequence of your selected gene 5. Highlight, copy and paste the sequence to pBLAST
Initial Class Frequencies
p=
q=
Initial Genotype = A/a Class Totals
My Genotype Generation 1
A/A
A/a Generation 5 frequencies
Generation 2 Generation 3
p=
Generation 4
q=
Generation 5 Generation 6 Generation 7 Generation 8 Generation 9 Generation 10