DARWIN AND EVOLUTION Chapter 15
Learning Goals Quiz #9 You will be able to answer questions about Darwin s historical voyage on the HMS Beagle You will be able to explain Darwin s original theory of evolution and Natural Selection Quiz #10 You will be able to explain how Darwin s theory has evolved using Hardy-Weinberg equilibrium You will be able to describe genetic variation, speciation, and reading a cladogram
Descent with Modifications In the early 1800 s some extraordinary fossil records were being uncovered. In 1815, the first reference of a dinosaur being an ancient species of an animal was published. Dinosaur bones had been collected since the medieval times, but were mostly attributed to biblical giants or mythical dragons Animals such as the mastodon and apatosaur seemed to have similarities to modern day animals, such as elephants and giraffes Could these animals have been descended from the ancient dinosaurs, with slight modifications?
The HMS Beagle Charles Darwin was 22 years old when he joined the HMS Beagle as the ship s naturalist. The role of the naturalist is Search for valuable plants, spices, minerals, etc. Collect specimens for research Serve as a backup to the ship s doctor, navigator The Beagle spent five years sailing along the southern hemisphere, visiting Australia, Africa, South America and the West Indies Everywhere they went, Darwin had the chance to explore
The HMS Beagle In attempting to build a new collection of specimens, Darwin made some observations He found fossils of sea animals in the middle of continents He found similar animals on totally different continents Darwin began to subscribe to an Old-Earth theory, which says the earth must be billions of years old instead of the traditional thousands of years. He hypothesized that the earth s oceans and continents must have had different boundaries in the past An older earth would have enough time to make changes to the geography, which would explain the fossil locations.
The HMS Beagle Were the similarities between animals on different continents due to random chance? The environments? Common ancestors? Migrations? Darwin s most famous and substantial investigations would occur when the Beagle made a stop on the Galapagos Islands. The Galapagos were an archipelago (a small group of islands.) Some were flat, some mountainous; some sandy, some rocky; and all 500 miles from South America (which meant migration would be difficult). To Darwin, the Galapagos were like a set of miniature continents and could serve as a model for the earth.
The Galapagos Animals Tortoises Each of the islands on the Galapagos had their own species of tortoises Darwin noticed on the barren islands, all the tortoises had longer necks which helped in picking fruit off the cacti On the lush islands, the tortoises all had short necks. The food was easier to reach. Did the islands cause the sizes of the necks in the tortoises to change? Or did the tortoises live on the islands that were best suited for their necks?
Harriet the Tortoise: b. 1835? d. 2006
The Galapagos Animals Finches Today there are only 13 species of finches on the islands, but Darwin found many more. He noticed that although the finch s appearances differed greatly, the one similarity was the relationship between food and beak size. Finches with large beaks broke seeds or nuts and used a long tongue to retrieve food inside Finches with pointed beaks acted like woodpeckers. Once they created a hole they used tools to force insects out. Did food choices cause beak sizes? Or did beak sizes dictate food choices?
Return to England After Darwin s return to England in 1836, he waited 20 years before publishing his findings Partly because his ideas hadn t formed yet, partly because he wanted to experiment, and partly because it was such a radical idea. Darwin s basic theory was that if organisms could change slightly from generation to generation, why couldn t they change dramatically over time? The main problem to tackle was what was the motivation for change? Changes obviously were occurring. But why?
Return to England Darwin finally published his works when, after reading an article by a fellow scientist with similar ideas, he became worried that he d finish in second place. On November 24, 1859, in the middle of the industrial revolution of Europe, Darwin published his manuscript titled On the Origin of Species. Europe had fallen in love with a mechanized economy. Darwin s opinions about a mechanized system of life fit perfectly with the times. In the manuscript, Darwin promoted descent with modifications, a common ancestor, and natural selection
Natural Selection Natural Selection means that organisms change because the environment forces them to or they will die. 1) Organisms in a population have inheritable variations Darwin believed and we have since proven correct that variations are random. Variations are just as likely to be harmful as helpful (Keep in mind, Darwin is publishing the same time Mendel is experimenting. We still don t know about genes or even what nucleic acids are.) 2) Eventually, more individuals are produced in a population than the environment can support Births are usually much higher than deaths in a population too
Natural Selection 3) Some individuals have characteristics that enable them to survive AND reproduce better than other individuals Fitness is described as reproductive success relative to other members of a population. The goal is to be the fittest. If you re able to survive, you re more likely to reproduce and, thus, more likely to pass on the genes that enabled you to survive. If you re less likely to reproduce, your unhelpful genes die with you before you have a chance to pass them on. 4) As generations progress, a higher percentage of individuals will have these traits than previous generations Adaptations are the genes that help organisms to survive in their environments 5) The result are populations built for their local environment
Natural Selection Tortoises In the past all Galapagos islands probably had tortoises with both long and short necks On drier islands, longer necks helped tortoises to eat. They were beneficial. On lush islands, longer necks would have entangled the tortoises more easily. Shorter necks were safer. The tortoises with the best traits were the most likely to reproduce and pass on longer-necked genes Eventually, only these tortoises inhabited these islands
Natural Selection Finches Similarly, the finch s beaks were more or less beneficial based on the food on their island. If you had a big beak on a nut-filled island or a narrow beak on a vegetation-filled island you had an easier time eating. Since you didn t waste time with food, you had more time to focus on mating. Since you mated more often, you passed your traits on more often.
Evidence The fossil record The fossil record is the history of life told through remains from the past (plant, animal, bacterial, geologic) The fossil record shows plants and animals from the past that are currently extinct but similar to today s plants and animals Example: The ancient mesohippus and the modern-day horse Bones of animals millions of years old and 5-6 times the size of modern day animals are remarkably similar Some even show hybrids, like the archaeopteryx, a reptilelike creature with avian-like feathers
Evidence Biogeography Biogeography is the study of the distribution of plants and animals throughout the world Why does South America have no rabbits? No cacti in northern Africa? The Galapagos tortoises and finches only in the Galapagos? These species would thrive if they were found there. Darwin concluded that these species could not have inhabited these lands because they had no ancestors who lived in these lands. Organisms do not randomly emerge. Someone has to migrate there
Evidence Common Anatomy An enormous number of species all show common homologous structures (anatomically similar in different species) Forelimbs of humans, horse, whale, cat, bird This should not be confused with analogous structures, or structures which serve a similar purpose but are not ancestrally related Wings of a bird vs an insect
Evidence Common Anatomy cont Vestigial structures are anatomical features that are functional in some species but not in others Wings of an ostrich Eyes of cave-dwelling salamanders Spending time building structures that serve no purpose seems like a waste. Most likely they either USED to serve a purpose, or eventually WILL serve a purpose.
Evidence Biochemical evidence The final piece of evidence is biochemical evidence and was formed years after Darwin The system of chemistry that codes for organisms is universal All life is built on DNA, which contains the same 4 nucleotides All life is built on proteins containing the same 20 amino acids The same codons code for the same amino acids The same basic proteins exist in single and multicellular organisms
The Modern Theory of Evolution Darwin provided compelling evidence that species and populations change. What he didn t know (and neither did anyone else at the time) was the how What are the mechanisms for passing on traits? How are they passed on? How does this turn into a trait? Now we will talk about how the parts of the theory of evolution changed, was altered, thrown out, or confirmed after the discovery of genes
Microevolution In a population, all of the alleles for a specific gene within the population are called a gene pool Gene pools are described using allele frequencies If we count a population of 1000 people for the earlobe trait, each person has two alleles for a total of 2000 alleles in the population If 500 of those alleles are for unattached earlobes, then the allele frequency is 500/2000=25%. Today, the study of evolution is the study of tracking the allele frequencies in a population
Microevolution Some common misconceptions about gene frequencies (in other words, these are incorrect): The majority of the gene pool is always the dominant allele Gene frequencies over generations will tend to favor the dominant allele The dominant allele is the best allele. Which allele is dominant in a gene pool has nothing to do with whether or not an organism wants the allele Polydactylism, Huntington s Disease
Allele Frequency Evolutions cannot occur unless the allele frequency of a population changes. In order for the frequency to not change, five characteristics about a population must be true for each gene. 1. No mutations. Either mutations don t occur or two mutations cancel each other out 2. No gene flow, either immigration or emigration Immigration: into your population Emigration: out of your population
Allele Frequency 3. Random mating. Individuals pair by chance, not according to genotypes or phenotype 4. Large population size The population is large enough that chance alone does not affect frequencies 5. No natural selection. No selective agent in the environment favors one genotype over another. If just one of these five is not met, an evolution can occur
Genetic Mutations Without mutations, there could be no new variations among members of a population Many mutations are not immediately detected because they do not affect phenotypes Once a mutation has occurred, the right combinations of genes and/or environment factors may make a mutated genotype more advantageous than the wild genotype In this case, the favorable genotype will be passed on over generations more than the unfavorable genotype
Gene Flow (or Gene Migration) Gene flow is the movement of alleles between populations by migration of the individuals Large migrations of individuals, into or out of a population, will carry their alleles with them This inevitably affects the frequencies of the population It is possible that one allele s arrival or departure may be so dramatic that the result is only one allele existing in the population in future generations
Nonrandom Mating While we d like to think everyone s equal, almost all species on the planet are picky about their mates If an organism has features that are undesirable or unhealthy, their alleles have a smaller chance of being passed along Therefore, their allele frequencies will decrease in future generations Assortative mating: individuals mate with those with specific phenotypes Sexual selection: males compete over the right to mate, but the winner gets as many mates as he chooses
Genetic Drift due to Small populations Genetic drift is changes to allele frequencies due to chance issues in the population Larger populations are less likely to have a natural disaster, disease, or other issue to dramatically affect the frequencies Drift is random, so a change in one population may not have any effect whatsoever on a neighboring population A drift that occurs in elm trees in northern California will probably not be seen in a similar population in northern Idaho
Genetic Drift due to Small Populations Disasters can sometimes cause a near-extinction called a bottleneck effect, where most members of a species die. Only a few will make it through the bottleneck The few remaining survivors alleles then make up the new allele frequencies Cheetahs are an example of this. All cheetahs, no matter their locations, have a dramatically similar genome It is believed sometime between 4,000 and 10,000 years ago they suffered a near-extinction, the result being low variations in genotypes and alleles
Genetic Drift due to Small Populations The founder effect is when a rare allele or combination of alleles is unusually high in a population of a species that is isolated from the other populations The founding members of this population happen to carry these rare alleles. Thus, all their offspring over the generations contain the same rare combination Since the population doesn t interbreed with other populations, the frequency is much higher than usual. Example: the dwarfism fingers in the Amish population of Pennsylvania
Natural Selection Natural Selection is when the environment favors one allele and/or genotype instead of another. Darwin hypothesized a form of natural selection. The modern version of natural selection is as follows: At least two phenotypes exist, & they are controlled by genes. Genes are able to be passed from parents to offspring One allele is better suited for an environment than the other Organisms with the better allele will have offspring; organisms with the less favored allele will not. Offspring of parents with the better allele will also have the better allele. The less favored allele has died off.
Natural Selection In short, if an individual has an allele that allows them to survive better in an environment than a different allele, they will have a better chance of reproducing If they have a better chance of reproducing, the next generation will have a higher percentage of offspring with the alleles for the favorable trait Eventually, the alleles for the favorable trait will be the only alleles in the population
Generation 1
Generation 2
Generation 3
Generation 4
Generation 5
Generation 10?
Generation 1 Generation 10
Speciation Speciation is the splitting of one species into two or more species, or the transformation of one species into a new species over time. A species is a group of organisms all capable of interbreeding and producing fertile offspring It s important to identify and separate species from each other because genes only flow between populations of similar species, not different ones. It s also important to note when species separate themselves to the point where a new species emerges
Speciation For two species to separate, they must be reproductively isolated. There are many ways this occurs 1) Habitat isolation When two species occupy different habitats, even within the same geographic range, they are less likely to meet and attempt reproduction. Red Maples of the East Coast vs Bigleaf Maples of the West Coast 2) Temporal Isolation Two species live in the same area but reproduce at different times of the year The leopard frog, which mates in April and the bull frog, which mates in July
Speciation 3) Behavioral Isolation Two species have different courtship patterns that allow males and females to recognize one another. Fireflies of different species have different flashing patterns to attract mates Gypsy moths give off pheromones only detectable by members of their own species 4) Mechanical Isolation When two species genitalia or structures are incompatible, reproduction cannot occur Male dragonflies anatomy are so uniquely shaped they only fit with other members of their own species
Speciation 5) Gamete Isolation Two species may be able to mate, but their gametes may be incompatible with each other The Gray Fox has 66 chromosomes; the Red Fox has 34 6) Zygote Mortality Fertilization occurs, but the zygote does not survive Many zygotes of coral hybrids do not survive development 7) Hybrid Sterility The zygote of a hybrid is able to develop and be born, but the offspring are sterile When a male donkey and a female horse reproduce, the result is a mule. Mules live healthy lives, but they are sterile
Evolution Extra Credit QUIZ 9: The warty hammer orchid is a plant whose flower has developed to both look and smell like a like a female wasp s body during ovulation (ready to be impregnated) The male wasp lands on the flower attempted to mate with it. In the process, the wasp collects pollen on its body which it then carries to the next WH orchid plant in another attempt to mate. Without a brain or an ability to plan, reason, observe, or infer, explain in no more than a 3-5 sentence paragraph how a flower develops into the perfect shape and scent to trick a wasp into mating with it. QUIZ 10: On average any random 50 chimpanzees contain more genetic variations than any random 50 humans. What does this tell you about human genetic history vs chimps?