Mechanisms of Evolution • What caused the world to evolve into what we see now? • 1. Mutations • 2. Gene Flow • 3. Genetic Drift • 4. Sexual Selection • 5. Natural Selection are all mechanisms fueling the evolution of life on our planet.
Darwin….. • Set sail in 1831 on the HMS Beagle • Voyage lasted 5 years • He recorded (detailed diaries) and collected specimens in South America His most famous specimens were the finches. • Reached the Galapagos Islands and noted that the flora and fauna although similar to South America, also had unique traits • Darwin came to the conclusion that perhaps a single ancestral species transported from a single nearby island might give rise to a number of similar but distinct new species
How did one species of finch become so many species? http://www.youtube.com/watch?v=l25MBq8T77w&feature=related
Darwin’s finches had differences in their beaks. • Beaks are associated with eating different foods. • Birds from different geographic location were different species of finches. • By definition, a species is unable to reproduce with any other species. • Darwin’s conclusions: • Small populations of original South American finches land on islands. The variations in the beaks allowed individuals to gather food successfully in different environments. • Over generations, the population of finches changed both anatomically and behavorially and a….New Species emerged. • Time and Space had resulted in their evolution.
The most recognized symbol of the Galapagos is the tortoise. When Charles Darwin visited the Galapagos Islands, the vice-governor of the Islands told him that he could identify what island the tortoise was from simply by looking at him.
1. Mutations • Mutations often get a bad rap, but who doesn't want to mutate from a sweet little pond turtle to a massive crime-fighting ninja reptile? Are mutations good or bad? Eh…it depends. The details make a difference when it comes to mutations.
Organisms, including turtles, are constantly having their DNA mutated by the harmful UV rays in sunlight (if they forget their SPF 50, that is) or by treading through chemicals in a sewer. Occasionally, these mutations can help a member of the population survive. Or fight Shredder. Mutations can also be harmful, such as those that cause cancer.
A mutation is simply a change in the original DNA sequence of an organism. This can be a big change or a small one. Examples of mutations include: • 1. A single base pair change • A base pair is two chemical bases bonded to one another forming a "rung of the DNA ladder." The DNA molecule consists of two strands that wind around each other like a twisted ladder. Attached to each is one of four bases--adenine (A), cytosine (C), guanine (G), or thymine (T). Adenine forming a base pair with thymine, and cytosine forming a base pair with guanine. • 2. An addition of a base pair or larger piece of DNA • 3. A deletion of a base pair or larger piece of DNA • 4. A rearrangement of DNA sequences
Most mutations are small, happen randomly, and don't cause organisms to become mean green fighting machines. In fact, most mutations do nothing at all. One human cell contains over 3 billion base pairs. However, in that ridiculously large amount of DNA, there are only 20,000 or 25,000 protein-coding genes. That means the rest of the DNA could be regulatory regions, other types of RNAs, or it could be simply "junk." In fact, current estimates are that there is way more "junk" DNA than protein-coding DNA. This is actually an extremely good thing for a cell, and for you. It means that a random mutation is more likely to hit a region that is unimportant to you and your cells' functions. • When a mutation does occur within a protein-coding region of a gene, one of the following might happen: • 1. The mutation might not change the protein sequence • 2. The mutation might change the protein, but it has no consequence • 3. The mutation might change the protein, and change its function (positively or negatively) • If a mutation has a negative outcome, the organism probably won't do much reproducing and the population will remain largely unaffected. It's true that most random mutations are deleterious, and are largely selected against in a population. Conversely, an adaptive mutation gives an organism a selective advantage. It drives evolution, albeit like a tortoise. That's okay. Slow and steady wins the race.
Take a pretty little species of butterflies with buttercup colored wings. A female butterfly gets shot with a ray of sunshine that mutates one copy of a gene that codes for a protein involved in wing color. The resulting wings have a greenish hue. These new greener wings help this butterfly blend in with its favorite leafy treat, making it more difficult for predating birds to detect it. This mutated allele is passed down to the butterfly's offspring. Because this new phenotype is advantageous to the organism, the members of the population that carry the mutation live longer and are able to produce more offspring. The allele frequency of the adaptive mutation continues to increase, and microevolution is a result. Before you know it, the world's full of green butterflies.
It's simpler to see how adaptive mutations cause evolution in bacteria. Because of their small size, they replicate quickly, allowing us to see evolution in a shorter amount of time.
Bacteria reproduce much quicker than eukaryotes; some can duplicate themselves by the time you're out of the shower. For that reason, it's much easier to see adaptive mutations causing evolution in bacteria. In a real world example, an insertion mutation in a species of bacteria gave a few individuals a new way to take up delicious copper cobalt. Having this adaptive mutation meant better growth in environments with large amounts of metal. Over multiple generations, these mutants were selected for their new adaptation, becoming a bigger part of the population. • A bacterium isolated from rust samples of the RMS Titanic appears to be accelerating the wreck's disintegration. • The bacteria are eating the wreck's metal and leaving behind "rusticles," or icicle-like deposits of rust.
(Mad) Scientists often try to mutate something and see what happens. In one experiment, the British biologist C. Waddington put a couple fruit fly embryos under heat stress. This mutated their DNA and caused a few of them to grow funky looking wings. Through multiple generations, he was able to see these wings get passed on to their offspring. Scientists sometimes try to create their own teenage mutant flies.
Are inheritable changes in the genotype. • Provide the variation that can be acted upon by natural selection. • Mutations provide the raw material on which natural selection can act. • Only source of additional genetic material and new alleles. • Can be neutral, harmful or beneficial( give an individual • a better chance for survival). • Antibiotic resistance in bacteria is one form.
Some strains of Staphylococcus (large) have acquired antibiotic resistance, a scary trait that can be passed from one population of bacteria to another. Gene flow can happen quickly in bacteria. The CDC (along with the general population) is nervous about the rise in antibiotic resistant bacteria. Because they can multiply at ridiculously fast clips, some bacteria are evolving antibiotic resistance at a rate that medicine can't keep up with. Strains of bacteria with antibiotic resistant genes can pass on their genes to other bacteria when they hang out together.
Causes of Mutations • DNA fails to copy accurately. • External influences can create mutations. • Mutations can also be caused by exposure to specific chemicals or radiation.
Gene Flow and Genetic Drift • Remember Gilligan's Island? Gilligan's crew is part of a very small human populations separated from the rest of the human species. If they never got off the Island, their gene pool would be considered to be shallow. • Although it happens that some populations of the same species are geographically isolated, this is not usually the case. One population of squirrels could live near another population of squirrels. All they'd need to do is hitch a ride in the back of a pickup truck to end up in the middle of the first population's territory. Once there, the alien squirrels might reproduce with a member of the first population. When genes are exchanged due to the mixing of populations, the result is gene flow. Picture genes literally flowing from one population to the next
2. Gene Flow: • Is the movement of alleles into or out of a population (immigration or emigration). • Gene flow can introduce new alleles into a gene pool or can change allele frequencies. • The overall effect of gene flow is to counteract natural selection by creating less differences between populations. • Example: • Plant pollen being blown into a new area
Gene flow also often happens to populations that migrate, or move from one location to another location at a specific time year after year. Imagine that several populations of geese migrate to the same sunny Florida getaway every winter. While vacationing in the Keys, this species has plenty of opportunity to interact with other populations and acquire some genetic diversity.
Gene flow is what happens when two or more populations interbreed. This generally increases genetic diversity. Imagine two populations of squirrels on opposite sides of a river. The squirrels on the west side have bushier tails than those on the east side as a result of three different genes that code for tail bushiness. If a tree falls over the river and the squirrels are able to scamper across it to mate with the other population, gene flow occurs. The next generation of squirrels on the east side may have more bushy tails than those in the previous generation, and west side squirrels might have fewer bushy tails.
Gene Flow Some individuals from a population of brown beetles might have joined a population of green beetles. That would make the genes for brown beetles more frequent in the green beetle population.
3. Genetic Drift • The change in allele frequencies as a result of chance processes within their own populations. • These changes are much more pronounced in small populations. • Directly related to the population numbers. • Smaller population sizes are more susceptible to genetic drift than larger populations because there is a greater chance that a rare allele will be lost.
Imagine that in one generation, two brown beetles happened to have four offspring survive to reproduce. Several green beetles were killed when someone stepped on them and had no offspring. The next generation would have a few more brown beetles than the previous generation—but just by chance. These chance changes from generation to generation are known as genetic drift.
In a population of 100 bears, suppose there are two alleles for fur color: • A1 (black) and A2 (brown). • A1 has a frequency of .9, • A2 a frequency of .1 (1.0 = 100%). • The number of individuals carrying A2 is very small compared to the number of individuals carrying A1, and if only fifty percent of the population survives to breed that year, there's a good chance that the A2s will be wiped out.
Examples of Genetic Drift • A) The Founder Effect: A founder effect occurs when a new colony is started by a few members of original population. • Small population that branches off from a larger one may or may not be genetically representative of the larger population from which it was derived. • Only a fraction of the total genetic diversity of the original gene pool is represented in these few individuals.
In the founder effect, genetic drift happens because a few individuals broke off and started their own population. Both gene flow and genetic drift alter allele frequencies. Both rely on the Hardy-Weinberg principal.
For example, the Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists. Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington’s disease, because those original Dutch colonists just happened to carry that gene with unusually high frequency. This effect is easy to recognize in genetic diseases, but of course, the frequencies of all sorts of genes are affected by founder events.
Examples of Genetic Drift • B) Population Bottleneck: • Occurs when a population undergoes an event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing. • The event may eliminate alleles entirely or also cause other alleles to be over-represented in a gene pool. EX. Cheetahs http://www.nytimes.com/1985/09/17/science/loss-of-gene-diversity-is-threat-to-cheetahs.html
Bottleneck = any kind of event that reduces the population significantly..... earthquake....flood.....disease.....etc.…
An example of a bottleneck: Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000 but their genes still carry the marks of this bottleneck. They have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.
Sexual Selection- both Natural and Artificial • We know we aren't supposed to judge a book by its cover. No one told the peacocks, though. Peacocks can be totally judgmental. Peacocks are interested in other peacocks—especially the fancy-pants ones. Do they judge another bird by its looks? You bet your plumage they do. Although this is the antithesis of what we're taught as little kids, it's actually a pretty common concept in nature. It's the basis of sexual selection.
Evolution is really all about the ability of a species to reproduce. What's the purpose of life? In nature, the answer is quiet easy. It is to pass on your genes. Sexual selection describes when an organism chooses a mate based on some phenotype that makes it more attractive. It throws random mating right out the window. • An organism can't choose a mate based on its genotype. Not even the best X-ray vision allows us to know someone's DNA just by looking. Phenotypes, however, are the physical consequence of genotypes, and these are fair game. When it comes to choosing a mate based on looks, a female peacock is subconsciously picking a mate with better genes. A male peacock with brighter, fuller feathers might be healthier, strong, and disease-free. A peacock with the most bling for plumage will attract the most mates and pass on its genes to a greater majority of the population. Its alleles will therefore increase in frequency.
There are two types of sexual selection. • intrasexual selection: competition among males • intersexual selection: males display certain traits to females
sexual selection -- includes any trait that allows you to reproduce more ( like attracting mates more or rearing healthy young more).
Artificial Selection • Domesticated breeds have not always been in their current form. This change has been achieved by repeatedly selecting for breeding the individuals most suited to human uses. This shows that selection can cause evolution. Breeders have been making these changes for centuries.
Dog diversity illustrates artificial selection • A comparison of (a) the ancestral dog (the gray wolf, Canis lupus) and (b) various breeds of modern dogs. Artificial selection by humans has caused a great divergence in size and shape of dogs in only a few thousand years.
Humans have been improving domesticated plants and animals species for thousand of years. • Offspring are selected with desirable traits as breeding stock of the next generation • Selective breeding techniques have led to these genetically superior beef cattle that have incredible muscles and strength known as “Belgian Blue”. Modern breeds of Belgian Blue cattle are the creation of genetic engineering, with the specific goal of expanding the muscular content of the animals as much as possible. • http://www.youtube.com/watch?v=Nmkj5gq1cQU
Useful Selective Breeding Artificial selection has been used in modern farming to get: • The best beef cattle – taste and texture • The best milking cows – yield and disease resistance • Wheat – better yields and disease resistance • Flowers – bigger and more colourful • Chickens – egg size and number
Now the evidence continues • Lamarck – each species gradually became more complex and that new simple species were created by spontaneous generation (He did not believe that a single species could give rise to additional species (no common ancestor)). • He believed in the inheritance of acquired traits (changes in an individual resulting from interaction with the environment) Giraffe’s with longer necks (acquired trait inherited by the offspring) • -now know that acquired traits cannot be inherited • -he does deserve credit for his recognition of the role of the environment in driving evolutionary change
“Use and Disuse” – CHARACTERISTICS that were in constant use developed(were ACQUIRED) and those that were not used were LOST.
A Compromise Between Opposing Pressures • (a) A male giraffe with a long neck is at a definite advantage in combat to establish dominance. • (b) But a giraffe's long neck forces it to assume an extremely awkward and vulnerable position when drinking. Thus, drinking and male–male contests place opposing evolutionary pressures on neck length.