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11.1 KEY CONCEPT A population shares a common gene pool. Genetic variation in a population increases the chance that some individuals will survive. Genetic variation leads to phenotypic variation. Phenotypic variation is necessary for natural selection.
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Genetic variation in a population increases the chance that some individuals will survive. • Genetic variation leads to phenotypic variation. • Phenotypic variation is necessary for natural selection. • Genetic variation is stored in a population’s gene pool. • made up of all alleles in a population • allele combinations form when organisms have offspring
measures how common allele is in population • can be calculated for each allele in gene pool • Allele frequencies measure genetic variation. Calculate the allele frequency for G(Green frogs) in the population Calculate the allele frequency for g (brown frogs) in the population
Genetic variation comes from several sources. Can you think of some? • Mutation is a random change in the DNA of a gene. • can form new allele • How can mutations be passed on to offspring? • Recombination forms new combinations of alleles. • usually occurs during meiosis, What is the process called? • parents’ alleles arranged in new ways in gametes
Genetic variation comes from several sources. • Hybridization is the crossing of two different species. • occurs when individuals can’t find mate of own species • Much more successful/common in plants • topic of current scientific research
11.2 Natural selection acts on distributions of traits. • A normal distribution graphs as a bell-shaped curve. • highest frequency near mean value • frequencies decrease toward each extreme value • Why is this curve called “normal”? • Traits not undergoing natural selection have a normal distribution. Why?
Natural selection can change the distribution of a trait in one of three ways. • Microevolution is evolution within a population. • observable change in the allele frequencies • can result from natural selection • How does natural selection cause a change in allele frequencies?
Natural selection can take one of three paths. • Directional selection favors phenotypes at one extreme. • Bacteria, Greyhounds
Stabilizing selection favors the intermediate phenotype. • Gall flies, Siberian Huskies • What happens to the allele frequency in this distribution? • Natural selection can take one of three paths.
Natural selection can take one of three paths. • Disruptive selection favors both extreme phenotypes. • What happens if the middle cuts off completely?
A Match each graph with the correct type of Natural Selection: Directional Disruptive Stabilizing B C
Plant Height • A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage.
Finch Beaks • Throughout the wet seasons, small seeds are more common and there are such a large supply of the small seeds that the finches only eat small seeds. The smaller, smoother beaks make picking up the small seeds easier and become more common in the population.
Greyhounds • Early breeders were interested in dogs with the greatest speed. They carefully selected from a group of hounds those who ran the fastest. From their offspring, the greyhound breeders again selected those dogs who ran the fastest. By continuing this selection for those dogs who ran faster than most of the hound dog population, they gradually produced a dog who could run up to 64km/h (40mph).
African Butterflies • In a species of African butterfly the colorations range from a reddish yellow to blue. In both cases, these extremes of color, from different ends of the spectrum, mimic other species of butterflies that are not normally the prey of the local predator group of birds and other insects. Those butterflies that are moderate in coloration are eaten in far greater numbers that those at the extremes of the color spectrum.
Human Birth Weight • Babies of low weight lose heat more quickly and get ill from infectious disease more easily, whereas babies of large body weight are more difficult to deliver through the pelvis. Infants of a more medium weight survive much more often. For the larger or smaller babies, the baby mortality rate is much higher.
Rabbit Fur Color • Suppose there is a population of rabbits. The color of the rabbits is determined by incomplete dominance, with a black coloring, white coloring, and gray heterozygous coloring. If this population of rabbits were put into an area that had very dark black rocks as well as very white colored stone, the rabbits with black fur would be able to hide from predators amongst the black rocks and the white furred rabbits would be able to hide in the white rocks, but the grey furred rabbits would stand out in both of the habitats and would suffer greater predation.
Work with your neighbor • Think of an example for each of the types of distributions for natural selection
Genetic Drift Quick Lab • Use a deck of cards to represent a population of island birds. The four suits represent different alleles for tail shape. The allele frequencies in the original population are 25% spade, 25% heart, 25% club, and 25% diamond tail shapes. • Turn to page 337 in the books. With your groups, follow the procedure for the lab. Just do the activity, don’t worry about the questions right now.
bald eagle migration 11.3 Gene flow is the movement of alleles between populations. • Gene flow occurs when individuals join new populations and reproduce. • Gene flow keeps neighboring populations similar. • Low gene flow increases the chance that two populations will evolve into different species. • What is a species? • Why would low gene flow create new species?
Genetic drift is a change in allele frequencies due to chance. • Genetic drift causes a loss of genetic diversity. • It is most common in small populations. Why? • A population bottleneck can lead to genetic drift. • It occurs when an eventdrastically reducespopulation size. • Example? • The bottleneck effect isgenetic drift that occursafter a bottleneck event.
It occurs when a few individuals start a new population. • Can you think of an example of when this might occur? • The founder effect is genetic drift that occurs after start of new population. • The founding of a small population can lead to genetic drift.
less likely to have some individuals that can adapt • Why does that occur more in small population? • Penny Activity: • Flip the penny 3 times, record how many head and how many tails you flipped. • Flip the penny 7 more times, record how many heads and tails you flipped. • Which results were closer to the 1:1 ratio you expect to get? • harmful alleles can become more common due to chance • Genetic drift has negative effects on a population.
Sexual selection occurs when certain traits increase mating success. • Sexual selection occurs due to higher cost of reproduction for females. • males produce many sperm continuously • females are more limited in potential offspring each cycle • Result: Females are picky!
There are two types of sexual selection. • intrasexual selection: competition among males • Example: • intersexual selection: males display certain traits to females • Example: These birds have huge red air sacs which make them easier for predators to spot. Why would they have evolved these, then? Male Irish elks, now extinct, had 12-foot antlers. Describe how sexual selection could have caused such an exaggerated trait to evolve.
11.5 The isolation of populations can lead to speciation. • Populations become isolated when there is no gene flow. • Isolated populations adapt to their own environments. • Genetic differences can add up over generations. Two small, isolated populations of dolphins in Tin Can Bay and the Great Sandy Strait are at risk of extinction, and are the focus of a study by Southern Cross University researcher Daniele Cagnazzi. MrCagnazzi has been studying dolphins along the Queensland coast for the last three years. ~ABC News
Reproductive isolation can occur between isolated populations. • members of different populations cannot mate successfully • Prezygotic: temporal isolation, behavioral isolation • Postzygotic: hybrid sterility • final step to becoming separate species • Speciation is the rise of two or more species from one existing species.
Populations can become isolated in several ways. • Behavioral barriers can cause isolation. • called behavioral isolation • includes differences in courtship or mating behaviors
Geographic barriers can cause isolation. • called geographic isolation • physical barriers divide population • Temporal barriers can cause isolation. • called temporal isolation • timing of reproductive periods prevents mating
11.6 Evolution through natural selection is not random. • Natural selection can have direction. • The effects of natural selection add up over time.
Convergent evolution describes evolution toward similar traits in unrelated species What examples can you think of of convergent evolution? Wings of insects and birds; fins/tails of sharks and fish
red fox kit fox ancestor Other examples of divergent evolution? • Divergent evolution describes evolution toward different traits in closely related species.
Species can shape each other over time. • Two or more species can evolve together through coevolution. • evolutionary paths become connected • species evolve in response to changes in each other
THINK! Why do the stinging ants live in the holes of the thorns that are supposed to protect the plant from herbivores that would eat the leaves? How does the plant benefit? How does the ant benefit? • Coevolution can occur in beneficial relationships.
Coevolution can occur in competitive relationships, sometimes called evolutionary.
Species can become extinct. • Extinction is the elimination of a species from Earth. • Background extinctions occur continuously at a very low rate. • Same rate as speciation • Few species in small area • caused by local changes in environment
Mass extinctions are rare but much more intense. • destroy many species at global level • catastrophic events • 5 in last 600 million years
Speciation often occurs in patterns. • A pattern of punctuated equilibrium exists in the fossil record. • theory proposed by Eldredge and Gould in 1972 • Episodes of speciation occur suddenly followed by periods of little change • revised Darwin’s idea
Many species evolve from one species during adaptive radiation. • ancestral species diversifies into many descendent species • descendent speciesadapt to different environments
