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Biological Evolution. One of the most respected evolutionary biologists has defined biological evolution as follows:
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One of the most respected evolutionary biologists has defined biological evolution as follows: • "In the broadest sense, evolution is merely change, and so is all-pervasive; galaxies, languages, and political systems all evolve. Biological evolution ... is change in the properties of populations of organisms that transcend the lifetime of a single individual. The ontogeny of an individual is not considered evolution; individual organisms do not evolve. The changes in populations that are considered evolutionary are those that are inheritable via the genetic material from one generation to the next. Biological evolution may be slight or substantial; it embraces everything from slight changes in the proportion of different alleles within a population (such as those determining blood types) to the successive alterations that led from the earliest protoorganism to snails, bees, giraffes, and dandelions." - Douglas J. Futuyma in Evolutionary Biology, Sinauer Associates 1986
Evolution and Populations • It is important to note that biological evolution refers to populations and not to individuals and that the changes must be passed on to the next generation. In practice this means that evolution is a process that results in heritable changes in a population spread over many generations.
Competing Theories Lamarck’s Theory of Evolution Proposed in 1809, Lamarck observed that characteristics of living things could change over time. For example, if muscles are used, they grow stronger. These are called “acquired” characteristics.
Competing Theories Lamarck’s Theory of Evolution • Lamark proposed that organisms can pass on these acquired characteristics to their offspring. • For example: ancestors of Giraffes had short necks. Lamark’s theory goes that since they had to stretch to get food, their necks became longer and they could pass this trait to their offspring.
Competing Theories • Darwin and Wallace’s theory of Natural Selection would have explained the giraffe’s long necks in the following way: • An original group exhibited variation in neck length. • Longer necks were more successful in the given environment. • Longer necked giraffes were then able to reproduce more often than shorter necked giraffes and thus pass on their long neck genes to the next generation. • After many generations, the population has a general increase in neck length.
Competing Theories Even though the giraffe's neck is extremely long, it has only seven neck vertebrae, the same number that people and most other mammals have.
Definition of Evolution: The accumulation of changes in the heritable characteristics of a population. Evolution describes the changes in the various genes of a species over time. • These changes are the result of: • Mutations • Natural selection • Chance
Succession of Types Why should extinct armadillo-like species and armadillos be found on the same continent? The changes imply ancestry!
“This wonderful relationship in the same continent between the dead and the living will, I do not doubt, throw more light on the appearance of organic beings on our earth, and their disappearance from it, than any other class of facts.” --The Voyage of the Beagle, ch. 8
1. Populations tend to produce more offspring than the environment can support. • This leads to a struggle for existence in which some individuals survive and some die. Carrying Capacity
2. Natural Selection • Definition: • The over-production of offspring leads to competition and survival of the individuals best suited to that particular environment. • The better adapted individuals tend to survive and reproduce more than the less well-adapted individuals.
3. The variation in individuals in a population is controlled by their genes and is therefore inheritable. • The better adapted individuals pass on their characteristics to more offspring than the less well adapted. • The results of natural selection therefore accumulate. • As one generation follows another, the characteristics of the species gradually change.
How Does Variation Arise? In the DNA: -Sexual Reproduction -Mutations In the Environment: -Genetic Drift
Sexual Reproduction • Role: • Promotes variation in a species through meiosis and fertilization which is essential to natural selection.
Meiosis: • Crossing over and random separation of the homologous pairs of chromosomes during Meiosis. resulting in a gamete that is a mixture of paternal and maternal chromosomes. Crossing Over
Fertilization: • Two gametes from different individuals fuse to create a new organism, leading to further variation
Mutations: Mutations: Changes in the base pair sequence of DNA. Most are neutral or harmful to the organism, but some have positive effects. Note: In an organism that sexually reproduces, the mutation must occur in a gamete (sex cell) if it is to be passed on to offspring, and thus contribute to the evolution of that organism. A mutation has caused the garden moss rose to produce a flower of a different color.
Variation is non-directional (random) • The selection process is directional: It is dictated by the environment (not random). This leads to differential survival: • The result is that the individuals best adapted to a specific environment will survive to reproduce. • They will get the most food • find the best shelter • find a mate • reproduce and care for their offspring • not be eaten by other species.
Differential Survival: • If environments change (either gradually or suddenly), the “best adapted” may also change. • This process can lead to changes in the species
Genetic Drift: • Sometimes the frequency of a gene in a population is determined by chance and not by whether it gives the individual an advantage in the environment. • This is known as genetic drift.
Populations: • It is important to know at this point that evolution doesn’t work on the level of individuals or species- it works on the level of populations. • In other words: individuals do not evolve, populations do.
Speciation • When two groups of a species are in different environments they cannot interbreed. • If the selection pressure is different, they will eventually become different species: aka speciation. • The eastern meadowlark (Sturnella magna) and the western meadowlark (Sturnellaneglecta) have very similar body shapes and coloration. • Their ranges overlap in the middle of the country. • Considered different species: • Songs are different • Behavioral differences that prevent interbreeding.
In the summer of 1995, at least 15 iguanas survived Hurricane Marilyn on a raft of uprooted trees. They rode the high seas for a month before colonizing the Caribbean island, Anguilla. These few individuals were perhaps the first of their species, Iguana iguana, to reach the island.
Speciation: Artificial Speciation
Allopatric speciation of squirrels in the Grand Canyon Harris’s antelope squirrel (south rim) White-tailed antelope squirrel (north rim)
Why are finches different on different islands? • Islands have different kinds of seeds available which select for different types of beaks to eat them. • In time the birds become so genetically/physically different they cannot interbreed.
Environments constantly change and if a species cannot adapt, they may go extinct. Ex. Dinosaurs • Couldn’t adapt to colder climate • Their place was taken by mammals (homeothermic-warm blooded) • Not all dinosaurs went extinct. Some also evolved to become warm-blooded birds.
Some 99.9% of all species that have ever lived on Earth are now extinct. There have been 5 major extinction events during the history of life on Earth. The Cretaceous-Tertiary Extinction (65.5 MYA) is probably the most well known, as it killed off all non-avian dinosaurs. 75% of all species went extinct.
There have been 5 major extinction events during the history of life on Earth. This wasn’t even the largest though. That honor goes to the Permian-Triassic Extinction (251 MYA) which caused the extinction of 96% of all marine species and 70% of all land species.
Examples of Evolution: Archeopteryx: A transition fossil between reptiles and birds.
Example #1: Peppered Moths • A famous example from England that demonstrates natural selection is the Pepper Moth. • Two different species of moths, one light colored and one dark. • Before the 1850’s the dark one was very rare, after the 1850’s the light color became rare. What happened in the 1850’s? INDUSTRIAL REVOLUTION Coal fired plants produced lots of soot that covered the trees and buildings, making everything dark. So where once the birds preferentially picked off and ate the dark moths, they now saw and ate the light ones instead.
Example #2: Resistance of Bacteria to Antibiotics Through Exposure • As the use of antibiotics have become widespread • many disease-causing bacteria have developed resistance against known antibiotics. • Only the resistant bacteria will survive and reproduce
This means that if you become infected with these bacteria, treatment with antibiotics will not cure you and the disease may become fatal.
This is more likely to occur when a small dose of antibiotics is used over a short time. • It will kill some of the bacteria but not all. • Next time antibiotics are used, these bacteria may be less vulnerable and more survive. • Repeated small dosages can produce very resistant strains.
http://www.usatoday.com/story/news/nation/2013/03/05/superbugs-infections-hospitals/1965133/http://www.usatoday.com/story/news/nation/2013/03/05/superbugs-infections-hospitals/1965133/
In the USA, • half the livestock is fed antibiotics to increase the growth of the animals. • This leads to strains of antibiotic resistant bacteria being discovered in the gut of these animals and then in human guts. • The runoff from the waste and feed contains the antibiotic resistant strains that can get out to the rest of the world.
Example #3: Artificial Selection • There are numerous examples of humans performing their own type of selection (hence: artificial) on desired, heritable traits belonging to a variety of organisms. Since we seem to like the #3, we have… 3 examples:
Example #3: Artificial Selection #1: The domestication of the wolf (or domestication of any animal) is a good example: Further selection of favored traits resulted in the varied amount of dog breeds we have today.
Example #3: Artificial Selection #2: Selective breeding transformed teosinte’s few fruitcases into modern corn’s rows of kernels.
Example #3: Artificial Selection #3: By selecting various traits of the common wild mustard plant to breed for, we have created many common vegetables we use today such as broccoli, cauliflower and brussels sprouts.
Example #3: Artificial Selection Artificial selection is also important as it acts as an experiment on natural selection. Experimentation is the ultimate test of a scientific hypothesis, without it you can never be sure that a correlation (i.e. the environment selecting for traits) you observe is significant.
Example #3: Artificial Selection In artificial selection, humans are the manipulators (we are the environment)- we choose which individuals get to reproduce. We would expect to see what is found in nature- that the individuals who reproduce pass on their genes/traits, and that is exactly what we see.
Example #3: Artificial Selection Artificial Selection can also be used as evidence of evolution. We will discuss more forms of evidence next.
