What is evolution? - Helena Curtis and N. Sue Barnes, Biology, 5th ed Worth Publishers, p.974

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3 What is evolution? Evolution is a process that results in heritable changes in a population spread over many generations. Evolution can be precisely defined as any change in the frequency of alleles within a gene pool from one generation to the next." - Helena Curtis and N. Sue Barnes, Biology, 5th ed Worth Publishers, p.974

4 How does Evolution Occur? Jean Baptiste Lamarck was one of the first to propose that organisms change through time He proposed that a change in the environment produced a need to change in the animals Thus if an animal uses one part of its body frequently, that part will become stronger Conversely, if they don t use a part, it will become smaller and disappear. He assumed these acquired traits could be passed on to offspring. animation

5 Influences: Charles Darwin geologist Charles Lyell Wrote Principles ofgeology and claimed that geologic forces produced changes on Earth in the past and those same forces will continue to produce changes in the future. Economist Thomas Malthus Wrote in Essay on the Principle of Populationthat population growth would always overpower food supply growth, creating perpetual states of hunger, disease, and struggle

6 If evolution was a car, the theory of natural selection would be the engine The basic ideas of evolution were discussed long before there was any scientific research done to support them. The evolutionary concept was never able to gain any real steam because it lacked a mechanism. Scientists wanted to believe that species evolved from one form to another, but had no plausible process to make it happen. The theory of natural selection provides that reasonable method of evolution. animation

7 Darwin s Theory of Natural Selection 1. Overpopulation- All species have the potential to overpopulate the earth, but usually remain stable over time. 2. Competitiona large # of the population must die at an early age. 3. Variation- Due to variation, different individuals among populations have different traits

8 Darwin s Idea Cont. 4. Survival of the Fittestmembers of a population whose trait makes them better adapted to their environment are more likely to survive to reproductive age and produce more offspring. 5. Reproductionmembers of a population whose trait makes them better adapted are more likely to reproduce. 6. Speciationgiven time, the process of evolution by natural selection can account for the formation of new species and thus for the diversity of life on earth.

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10 Vocabulary to understand: Population a group of individuals living in the same geographical area and sharing a common gene pool Gene Pool the sum of all genetic information carried by the members of a population Mutation any heritable change in DNA (gene-level and the chromosome-level) Gene Flow The transfer of alleles between populations through interbreeding Natural Selection Differential survival and reproduction of organisms with a result of increase in frequency of best adapted traits. Nonrandom Mating This stud artificial bull is selection the product of generations of selective breeding that has resulted in a double muscled freak that can hardly walk

11 Populations and Gene Pools What s the difference between microevolution and macroevolution? Microevolution is change within species which can occur over dozens or hundreds of generations. Macroevolution usually involves much longer periods of time and includes the origin of new species.

12 What is a gene pool? The collection of alleles available among reproductive members of a population

13 Evolution occurs on the population level Remember, a population is a group of individuals of the same species in a given area whose members can interbreed. Because the individuals of a population can interbreed, they share a common group of genes (gene pool). Each gene pool contains all the alleles for all the traits of all the population. For evolution to occur in real populations, some of the gene frequencies must change with time.

14 Gene Frequencies and H-W Recall, the gene frequency of an allele is the number of times an allele for a particular trait occurs compared to the total number of alleles for that trait. Gene frequency = the number of a specific type of allele / the total number of alleles in the gene pool An important way of discovering why real populations change with time is to construct a model of a population that does not change. This is just what Hardy and Weinberg did. Their principle describes a hypothetical situation in which there is no change in the gene pool (frequencies of alleles), hence no evolution.

15 Recall: The Hardy-Weinberg Law This law states an equilibrium of allele frequencies in a gene pool (using a formula p 2 + 2pq + q 2 ) remains in effect in each succeeding generation of a sexually reproducing population if five conditions are met: 1. Large population- The population must be large to minimize random sampling errors. 2. Random mating- There is no mating preference. For example an AA male does not prefer an aa female. 3. No mutation The alleles must not change. 4. No migration Exchange of genes between the population and another population must not occur. 5. No natural selection- Natural selection must not favor any particular individual.

16 In nature, the conditions of the Hardy-Weinberg law are rarely met Allele frequencies in the gene pool of a population do change from one generation to the next thus evolution happens. The H-W equation provides a baseline by which to judge whether evolution has occurred. Remember that the effect of natural selection on gene frequencies can be quantified. 36% homozygous dominants (AA) 48% heterozygotes (Aa) and 16% homozygous recessives (aa) If a change in allele frequencies occurs over time, you can assume evolution is happening.

17 A hypothetical gene pool Find the Frequencies of A anda and the genotypic frequencies of AA, Aa and aa. As long as the conditions of Hardy-Weinberg are met, the population can increase in size and the gene frequencies of A and a will remain the same. Solution: Thus, the gene pool does not change. f(a) = 12/30 = 0.4 = 40% f(a) = 18/30 = 0.6 = 60% Then, p + q = = 1 and p 2 + 2pq + q 2 = AA + Aa+ aa = = 1

18 Now, suppose more 'swimmers' dive in as shown in What will the gene and genotypic frequencies be? Solution: f(a) = 12/34 =.35 = 35 % f(a) = 22/34 =.65 = 65% f(aa) =.12, f(aa) =.23 and f (aa) =.42

19 The results show that H-W Equilibrium was not maintained. The migration of swimmers (genes) into the pool (population) resulted in a change in the population's gene frequencies. If the migration were to stop and the other agents of evolution (i.e., mutation, natural selection and nonrandom mating) did not occur, then the population would maintain the new gene frequencies generation after generation. It is important to note that a fifth factor affecting gene frequencies is population size. The larger a population is, the number of changes that occur by chance alone becomes insignificant. In the previous analogy, a small population was used to simplify the explanation.

20 How much variation does a large population of sexually reproducing organisms have in their gene pool? Typically about 0.5% of the DNA bases are different In fruit flies there are 165 million base pairs, so ~1 million nucleotides sites differ. In humans with 3 billion base pairs, this means there are about 15 million different/variable nucleotides).

21 Mutations and Peppered Moths In 1850, f(l)=.95 and f(l) =.05 light colored moths on light colored trunks were camouflaged In 1900, f(l) =.05 and f(l) = generations later: light lichen was killed by pollution So the dark tree trunk was exposed Therefore, light colored moths became easy prey for birds

22 Positive, Negative, or Neutral: Mutations are random and unpredictable Some are lethal and kill individuals before they are born. Some are harmful, but masked by a dominant allele each of us carry 7-8 lethal recessive genes Some are neutral and have little to no affect on an organisms long term survival. Some are beneficial and help the organism survive to reproductive age and reproduce

23 Gene Flow the exchange of alleles Alleles move through populations when organisms interbreed or migrate Gene flow can increase variation within a population by introducing new alleles

24 Genetic Drift random changes in the gene pool Genetic drift causes gene pools of two isolated populations to become dissimilar some alleles get lost and other get fixed This causes a reduction in the frequency of the heterozygotes over time. The smaller the population size the faster the effect is seen

25 Why does it affect small populations more? In a small population, allele frequencies are likely to be atypical just by chance. If you were to toss a coin 1000 times and get heads 750 times you would be very surprised. If you tossed a coin 4 times and got 3 heads, you would not be so surprised! Remember, chance has a greater influence upon gene frequencies in a small population

26 When is genetic drift likely to occur in nature? Most populations are large enough so genetic drift does not occur Some populations crash in numbers due to natural disasters or over-harvesting by humans( bottleneck effect ) Another example is when a new habitat (e.g. an island) is colonized by just a few dispersing individuals ( founder effect ) The suddenly small population may, purely by chance, contain a different frequency of different genotypes that the original large population.

27 Founder Effect Genetic drift can also occur when a small number of individuals from a large population emigrate to a new area. The small number of emigrants are likely to have an genetic structure that differs, purely by chance, from the main population

28 An Example of Founder s Effect: In the 1700 s 200 German Amish immigrated to Pennsylvania to start a community. These people carried a high concentration of a mutation which causes a form of polydactylism. Individuals in this group tended to marry within so there's a greater likelihood that the recessive genes of the founders will come together in the cells that produce offspring.

29 Inbreeding Breeding between close relatives. This causes the gradual increase in homozygosity (or loss of heterozygotes. Example: In most species, related individuals share about 80% of the same genes. With cheetahs, this figure rises to approximately 99%. The genetic inbreeding in cheetahs has led to low survivorship (a large number of animals dying), poor sperm quality, and greater susceptibility to disease.

30 The Bottleneck Effect From an original large population, only few survive to repopulate the habitat. The few survivors are likely to have a genetic structure (a set of genotype frequencies) that is unrepresentative of the ancestral large population

31 Example of a genetic 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 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

32 Comparing the 2 Effects: Bottleneck Effect is sampling error as a result of only a few individuals surviving a population crash. Founder Effect is sampling error as a result of movement of a few individuals away from the main population

33 What s inbreeding depression? The fertility and survival are reduced compared with populations that are not inbred. Caused by an increase in homogous individuals. Ex. Outbreeding enhancement/hybrid vigor is when they manage a species and bring back heterogeneity. In the adder case, introducing snakes from other populations a population of 40 adders experienced inbreeding depression when they were isolated them from other adder populations. As a result, a higher proportions of stillborn and deformed offspring were born in the isolated population than in the larger populations

34 Artificial Selection A change in plant or animal population by selective breeding Much of what Darwin learned about Natural Selection, came from his observations from selectively bred crops and domestic animals. This showed that continued selection was powerful enough to bring about largescale changes within a species.

35 Fitness The suitability of an organism to a given environment This is often measured by the number of offspring that an individual has However, the offspring must survive to contribute to the following generation Consider two zebras: Both live in the savannah and must escape predation by lions while also finding food & water. As babies, they were protected by the herd Zebra 1- had a mutation with deformed its rear leg Zebra 2- is fast and strong

36 Soon after they both became juveniles, a lion pride attacked the heard. Zebra one, not being able to run fast, was caught by the lion pride and eaten. Zebra 2 escaped this, and many more attacks. Lived to have many offspring who were also fast and strong runners. So Zebra 2 was more fit than Zebra 1, by surviving.

37 Adaptations that affect Fitness Camouflage- color is not necessarily relevant Pattern and shades matter Pattern breaks up an organism s outline Protective Coloration- Color of animal blends in with environment However, this can allow the organism s shadow to reveal the outline

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