DNA, Mutations & Evolution

Random Fact A panda’s thumb is actually an enlarged wrist bone that has evolved to allow the panda to hold onto its favorite food: bamboo

Evolution as Genetic Change Populations: collection of individuals of the same species in a given area whose members can breed with one another. E.g. all of the fishes of a certain species in a pond Because these individuals of the population can interbreed, they and their offspring share a group of genes (gene pool). Each gene contains a number of alleles – forms of a certain gene at a given point on a chromosome – for each heritable trait

Cards as Alleles

Cards as Alleles But shuffling the cards will not change the number of kings and queens, spades or aces in the deck. Evolutionary change involves a change in the relative frequencies of alleles in the gene pool of a population

Shaping Evolutionary Theory Mechanisms of Evolution Population Genetics When the frequency of alleles remain constant, a population is in genetic equilibrium. NO CHANGE IN ALLELE FREQUENCY = Genetic equilibrium A population in genetic equilibrium DOES NOT evolve

Shaping Evolutionary Theory Genetic equilibrium occurs when ALL five conditions are met. If the conditions are NOT met, then the population can change or evolve: The population is large. There is no immigration or emigration. Mating is random. Mutations do not occur Natural selection does not occur. All five conditions rarely exist in nature. Therefore, most populations can evolve.

Bozeman – Microevolution 7:48

Shaping Evolutionary Theory Mechanisms of Evolution Genetic drift, gene flow, nonrandom mating, mutation and natural selection are mechanisms of evolution. (Microevolution)

Shaping Evolutionary Theory How does genetic drift lead to evolution? 1. Genetic Drift Random change in the frequency of alleles in a population In smaller populations the effects of genetic drift are more pronounced, and the chance of losing an allele becomes greater. Due to the random assortment of chromosomes in meiosis. (Change by chance) Genetic drift is another way a population can evolve. **Unlike natural selection, adaptations arising from genetic drift are not always best. Sometimes important adaptations can be lost by genetic drift.

Shaping Evolutionary Theory What is Founder Effect? Founder Effect Occurs when a small sample of a population settles in a location separated from the rest of the population Often occurs on islands where new species can result from a few founders of the original population. Occurs in the US – Amish and Mennonite people rarely marry outside their communities and hence, have many unique genes. Alleles that were uncommon in the original population might be common in the new population. The few individuals present to reproduce carry a random subset of genes from the original population. **Unlike natural selection, traits that result may or may not be the best available for the environment

Shaping Evolutionary Theory What happens in a genetic bottleneck? Genetic Bottleneck Occurs when a population declines to a very low number and then rebounds Bottlenecks reduce the total alleles in a population Genes within the resulting population can be unusually similar

Shaping Evolutionary Theory Cheetahs in Africa – might have gone through a bottleneck a few thousand years ago. Cheetahs are genetically similar and appear to be INBRED. *Inbreeding reduces fertility and can eventually cause extinction!

Shaping Evolutionary Theory What is Gene Flow? 2. Gene Flow If a population is in genetic equilibrium, then NO gene flow can occur. (No new genes would enter or leave the population) Few populations are actually isolated, therefore, gene flow occurs when individuals move among populations. Gene flow increases genetic variation within a population and reduces differences between populations.

Shaping Evolutionary Theory With whom do organisms mate? 3. Nonrandom Mating Mating within a population is usually nonrandom. Promotes inbreeding and could lead to a change in allelic proportions favoring individuals that are homozygous for particular traits NON-random mating that implies every member of the population has an equal chance to mates and have offspring. Mates are chosen on their physical appearance also known as phenotype to ensure offspring have good genes (genotype).

Shaping Evolutionary Theory How do mutations affect genetic equilibrium? 4. Genetic Mutations The cumulative effect of mutations might change the allelic frequencies in a population and disrupt genetic equilibrium Mutations MAY provide an advantage for an organism and be more common in future generations. Mutations form the basis of Natural Selection.

Shaping Evolutionary Theory 5. Natural Selection Acts to select the individuals that are best adapted for survival and reproduction There are three ways that Natural Selection can change populations

Shaping Evolutionary Theory What are the different types of natural selection? A. Stabilizing Selection Removes organisms with extreme forms of a trait Most common form of selection Favors the average value of a trait when the average expression leads to higher fitness

Shaping Evolutionary Theory A. Stabilizing Selection E.g. Human babies born with below-normal or above normal birth weights are less likely to survive than babies born at average weights. Therefore, the average birth weights in humans remains about the same.

Shaping Evolutionary Theory What are the different types of natural selection? B. Directional Selection Shifts the populations toward the beneficial trait. Favors the extreme form of a trait when the extreme form of the trait results in higher fitness.

Shaping Evolutionary Theory B. Directional Selection E.g., Galapagos Islands – years with little water saw a decrease in food supply with remaining food as hard seeds. Birds with small beaks starve because they were unable to crack the seeds, but larger beak sizes could more easily crack seeds to survive. Little water – average beak size increases. More rain – average beak size decreases because more food is available to those with smaller beaks.

Shaping Evolutionary Theory What are the different types of natural selection? C. Disruptive Selection Removes organisms with average forms of a trait Creates two populations with extreme forms of a trait

Shaping Evolutionary Theory C. Disruptive Selection E.g., Water snakes on the shores of Lake Erie: Mainland snakes live in grass habitats and have brown skin. Island snakes live on rocky shores and have grey skin. Colors of both snakes helps them blend in with their habitat. Snakes with intermediate color would be obvious to predators.

Shaping Evolutionary Theory What is Sexual Selection? Example: Peacock Sexual Selection Occurs when certain traits are inherited because they increase the chance of attracting a mate. Found in populations where males and females differ significantly in appearance Qualities of sexual attractiveness appear to be opposite of qualities that might enhance survival Traits selected are not necessarily those that are best for the survival in the environment.

Shaping Evolutionary Theory Speciation Process where some members of a sexually reproducing population change so much they can no longer produce fertile offspring that can mate with the original population. Gene flow can be prevented by two types of reproductive isolating mechanisms: Prezygotic isolation and Postzygotic Isolation

Shaping Evolutionary Theory Reproductive Isolation Prezygotic Isolation Prevents reproduction by making fertilization unlikely Prevents genotypes from entering a population’s gene pool through geographic, ecological, behavioral or other differences. Takes place BEFORE fertilization occurs E.g., Time can be a reproductive barrier – different species of fireflies mate at different times of the night. Therefore, they do not interbreed.

Shaping Evolutionary Theory Prezygotic Isolation E.g., Eastern and Western Meadowlarks: Exhibit a form of behavioral isolation. They have a similar appearance and live in overlapping areas, BUT use different mating songs and do not interbreed.

Shaping Evolutionary Theory Reproductive Isolation Postzygotic Isolation Occurs when fertilization has occurred but a hybrid offspring cannot develop or reproduce. Prevents offspring from surviving or reproducing Lions and tigers are considered separate species but sometimes do mate. The offspring – a liger – is sterile and cannot reproduce.

Shaping Evolutionary Theory Speciation: Occurs when a population reproduces in isolation Two types of speciation are: allopatric speciation and sympatric speciation

Shaping Evolutionary Theory Speciation Allopatric Speciation Most common form of speciation A physical barrier divides one population into two or more populations (e.g., mountain ranges, wide rivers, and lava flows) Grand Canyon is a geographical barrier separating Albert and Kaibab squirrels After a long period of time, the two populations will contain organisms that can no longer successfully breed with one another.

Shaping Evolutionary Theory Speciation Sympatric Speciation Occurs when a species evolves into a new species without a physical barrier Ancestor species and new species live in the same habitat during the speciation process Common in plants – mutation called polyploidy, which increases the number of chromosomes in an organism might be the reason for sympatric speciation in plants. Plants that result from polyploidy no longer are able to interbreed with the main population.

Crash Course – Speciation – Of Ligers and Men 10:38 min

Shaping Evolutionary Theory Patterns of Evolution Many details of speciation are not yet known. Speciation is a long process Observations of speciation are rare Evidence of speciation is visible in most patterns of evolution

Shaping Evolutionary Theory When does adaptive radiation occur? Adaptive Radiation Occurs when one species evolves in a short period of time into a number of new species. Can occur when a species evolves a new, useful trait or when a species arrives in a new habitat. Also called, divergent evolution when occurring on a large scale. E.g., Mass extinction of the Cretaceous period saw mammals become more diverse. This was adaptive radiation on a large scale.

Shaping Evolutionary Theory How do species coevolve Coevolution: Relationship between two species might be so close that the evolution of one species affects the evolution of the other species Mutualism – benefits both species E.g., Flowers have markings to guide bees to nectar. When bees gather nectar, they pollinate the flower. The flowers and bees have coevolved in a way that benefits both species.

Shaping Evolutionary Theory What is convergent evolution? Convergent Evolution: Unrelated species evolve similar traits even though they live in different pars of the world. Similar environments can cause similar organisms to evolve by natural selection E.g., Deserts in North America and Africa are similar Produces organisms with similar morphology, physiology, and behavior, even though the organisms are unrelated.

Shaping Evolutionary Theory What is convergent evolution? Convergent Evolution:

Shaping Evolutionary Theory How quickly do species evolve? Gradualism: The idea that evolution occurs in small steps over millions of years (much evidence to support the theory) Punctuated Equilibrium: Speciation occurs in sudden bursts followed by long periods of stability Stability does not mean an organism is NOT changing – genes might be changing but effects are not seen in the fossils of the organism. Some scientists think the fossil record shows that most change occurs in small bursts. Other scientists believe that evolution occurs in a combination of gradual and punctuated changes.