Biological Evolution.

Biological Evolution

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

Voyage of the Beagle,

Succession of Types The changes imply ancestry!
Why should extinct armadillo-like species and armadillos be found on the same continent? The changes imply ancestry! When Darwin visited South America, he saw many similarities between living species and fossils of older, extinct species. Here is a modern-day armadillo and its ancestor the (giant) Glyptodon

“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

Basic Tenets of the Theory of Evolution:

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 Think of the thousands of acorns that one mature oak tree produces every year. A female salmon produces about 28,000,000 eggs when spawning. One oyster can produce 114,000,000 eggs in a single spawning. Darwin calculated that in elephants, which are among the slowest breeding land mammals, if all of the potential young of a single female survived and reproduced at the same rate, after 750 years the descendants of this single mother could number 19,000,000! Clearly, if all of these seeds, eggs, and young survived to become adults who also reproduced, the world would soon be overrun with oak trees, salmon, oysters, and elephants.

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 Play Natural Selection Clip from Howard Hughes DVD

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. Already stated at the beginning, but a good spot to remind them.

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 (Sturnella neglecta) 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.

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.

Artificial Speciation
There are many types of speciation, but allopatric speciation is where a geographical barrier prevents interbreeding. Artificial Speciation

Allopatric speciation of squirrels in the Grand Canyon
Harris’s antelope squirrel (south rim) White-tailed antelope squirrel (north rim)

Ex. Many of the finches in the Galapagos islands.

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.

There have been 5 major extinction events during the history of life on Earth.
Some 99.9% of all species that have ever lived on Earth are now extinct. 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.

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:

#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.

#2: Selective breeding transformed teosinte’s few fruitcases into modern corn’s rows of kernels. The domestication of maize is of particular interest to researchers — archaeologists, geneticists, ethnobotanists, geographers, etc. The process is thought by some to have started 7,500 to 12,000 years ago. Research from the 1950s to 1970s originally focused on the hypothesis that maize domestication occurred in the highlands between Oaxaca and Jalisco

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.

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.

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.

Artificial Selection can also be used as evidence of evolution. We will discuss more forms of evidence next.

Scientific Evidence for Evolution
“Those who cavalierly reject the Theory of Evolution, as not adequately supported by facts, seem quite to forget that their own theory is supported by no facts at all” -Herbert Spencer, Essay Scientific, Political and Speculative, 1891.

Fossils! Fossils do show intermediate stages, despite their rarity.
And geological strata consistently reveal the same sequence of fossils! A quick and simple way to debunk the theory of evolution would be to find a fossil horse in the same stratum as a trilobite.

Fossils! For example, there are now at least eight intermediate fossil stages identified in the evolution of whales.

Vestigial Structures An organ present in the organism but either reduced in size or has no use. Ex. Femur in some whales Dewclaw in dogs Eyes in blind mole rats Fake sex in virgin Whiptail lizard Wings on flightless birds

Human Vestigial Organs
The Appendix

Wisdom Teeth

Human Coccyx (Tailbone)

Body Hair and Erector Pili (Goosebumps)

Ear Muscles

Third Eyelid

Scientific Evidence Homologous Structures
Similar structures in very different organisms is evidence of a common ancestor. An example of this is the similarity of the skeleton between all mammals. Every bone in a bat has is its own identifiable counterpart in a human. Identifiable, because of the order in which they join up. Only the proportions are different.

from Ernst Haeckel's 1904 work Kunstformen der Natur
Just as with the mammalian skeleton, you can match each of the parts of the crustacean exoskeleton with parts from another crustacean species. “Every bit of the exoskeleton is joined to the ‘same’ bits, but the shapes of the bits themselves are very different…. I would say the only sensible interpretation is that all these crustaceans have inherited the plan of their skeleton from a common ancestor. They have molded the individual components into a rich variety of shapes. But the plan itself remains, exactly as inherited from an ancestor.” -Richard Dawkins, “The Greatest Show On Earth” from Ernst Haeckel's 1904 work Kunstformen der Natur

Scientific Evidence Embryonic Development
Physical similarities between embryos of different species at different stages. In 1866, Ernst Haeckel proposed his theory that the embryonic development of an individual organism followed the same path as the evolutionary history of its species. Haeckel formulated his theory as "Ontogeny recapitulates phylogeny". The notion later became simply known as the recapitulation theory. Ontogeny is the growth (size change) and development (shape change) of an individual organism; phylogeny is the evolutionary history of a species. Haeckel's recapitulation theory claims that the development of advanced species passes through stages represented by adult organisms of more primitive species. Embryonic pharyngeal slits, formed when the thin branchial plates separating pharyngeal pouches and ectodermal grooves perforate, open the pharynx to the outside. Pharyngeal pouches appear in all tetrapod animal embryos: in mammals, the first pharyngeal pouch develops into the lower jaw (Meckel's cartilage), the malleus and the stapes. At a later stage, all pharyngeal slits close, only the ear remaining open.[8] But these embryonic pharyngeal arches, pouches, and slits could not at any stage carry out the same function as the gills of an adult fish.

Scientific Evidence Haeckel’s theory has largely been rejected today.
Though humans share common ancestors with other animals, stages of human embryonic development are not functionally equivalent to the adults of these shared ancestors. In other words, just because we develop a tail and gill slits embryonically does not mean that they are functional like they are in fish. Generally, if a structure pre-dates another structure in evolutionary terms, then it also appears earlier than the other in the embryo. Species which have an evolutionary relationship typically share the early stages of embryonical development and differ in later stages.

Scientific Evidence Darwin's view: that early embryonic stages are similar to the same embryonic stage of related species but not to the adult stages of these species, has been confirmed by modern evolutionary developmental biology Whales, which have evolved from land mammals, don't have legs, but tiny remnant leg bones lie buried deep in their bodies. During embryonal development, leg extremities first occur, then recede. Similarly, whale embryos have hair at one stage (like all mammalian embryos), but lose most of it later.

Scientific Evidence Similarity of Genetic Code (DNA)
All organisms share the same genetic code, based on the series of bases: A, T, G, and C. The more similar two species are the more alike the sequence of bases in their DNA are.

Common Misconceptions

#1: Isn’t evolution just a theory that even scientists can’t agree on?
First we need to look at what the word theory means. The Oxford English Dictionary gives two meanings: Theory, Definition 1: A scheme or system of ideas or statements held as an explanation or account of a group of facts or phenomena; a hypothesis that has been confirmed or established by observation or experiment, and is propounded or accepted as accounting for the known facts; a statement of what are held to be the general laws, principles, or causes of something known or observed. Theory, Definition 2: A hypothesis proposed as an explanation; hence, a mere hypothesis, speculation, conjecture; an idea or set of ideas about something; an individual view or notion.

#1: Isn’t evolution just a theory that even scientists can’t agree on?
Darwin’s Theory, as with all scientific theories follow the first definition. Having your explanation for a group of observations being called a theory is an honor in the scientific field; it means that it is the best explanation we have for why something happens. Does being a scientific law mean you have more evidence to support your explanation than a theory? Not at all… laws and theories are used to describe different things in science. In general, Scientific Laws describe what is happening. Examples include: Newton’s Laws of Gravity, Kepler’s Laws of Planetary Motion, Law of Conservation of Energy, Laws of Thermodynamics…. Scientific Theories explain why something happens. Examples include: The Big Bang Theory, Cell Theory, Theory of Relativity, String Theory… Laws describe what is happening. I can use the Laws of Gravity to describe/predict what will happen will I throw a pencil in the air. Theories explain why something is happening. I can use the theory of relativity to explain why the pencil will fall toward the ground.

#2: Was one of my ancestors really a chimpanzee?
NO! Evolution points to humans sharing a common ancestor with chimpanzees and other great apes around 7-8 million years ago. Chimps have continued to evolved alongside us since then. The ancestor you share between yourself and the chimpanzee at the zoo probably did look something like this though: Sahelanthropus tchadensis

#2: Was one of my ancestors really a chimpanzee?
In fact we have shared the Earth with many other human like species (called hominids). We even shared the Earth at the same time with other hominid species such as Homo Neanderthalensis. And this recent find: Homo Floresiensis, the hobbit people on the island of Flores in Indonesia. Genetic evidence suggests interbreeding took place with Homo sapiens (anatomically modern humans) between roughly 80,000 and 50,000 years ago in the Middle East, resulting in 1–4% of the genome of people from Eurasia having been contributed by Neanderthals.[4][5] Courtesy of Smithsonian, 03/10

#3: How can natural selection lead to extremely complicated structures/organs like the human eye?
Though the eye is incredibly complex, even Darwin back in his day saw that the Theory of Evolution could account for it: “...if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real” -Charles Darwin, Origin of Species

The first step in the evolution of the eye, would be a set of photoreceptor proteins that sense light called photoreceptors. Photoreceptors can sense ambient brightness, and distinguish light from dark. These allowed unicellular organisms to move toward where they sensed light, probably to use for photosynthesis. Example of a photoreceptor in a Euglena

The next step would be indenting the tissue behind the eye to make a cup behind the photoreceptor cells. This enlarges the area of photoreceptor cells as well, now known as the retina. These primitive eyes are called eyespots. Example of an eyespot in a planarian (flat worm) "eyespots", and have evolved independently somewhere between 40 and 65 times. These eyespots permit animals to gain only a very basic sense of the direction and intensity of light – enough to know when they are safely in a cave[citation needed], for example, but not enough to discriminate an object from its surroundings.[16]

The “pinhole camera” eye stage was next. This is where the eyespot hollows out more and reduces the size of the hole where the light comes through. This gave the organism true imaging, allowing for directional and shape sensing. Example of a pinhole eye in a nautilus. It is likely that a key reason eyes specialize in detecting a specific, narrow range of wavelengths on the electromagnetic spectrum—the visible spectrum—is because the earliest species to develop photosensitivity were aquatic, and only two specific wavelength ranges of electromagnetic radiation, blue and green visible light, can travel through water. This same light-filtering property of water also influenced the photosensitivity of plants.[22][23][24]

Next came eyes that were fully enclosed, fluid-filled chambers. A refractive lens has also evolved over the pin-hole. Refraction is when light waves change speed as they enter a new medium. This bends the angle of the light, and in the case of an eye (or camera) directs the light into a single beam on one spot. The refractive lens in most animals is known as the cornea. The iris later evolved, which allows the organism to adjust the “pin-hole”, controlling the amount of light that enters the eye. In the picture, X represents where the pencil really is, the light waves bend as they change velocity and enter a new medium (air), giving the illusion that the pencil is higher.

So if we remember that evolution is an accumulation of heritable changes, the evolution of complex structures like they eye, though remarkable, isn’t all that unrealistic.

“Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone on cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.” -Charles Darwin, On the Origin of Species

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