Popular in Life: Continuity and Change
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This 7 page Class Notes was uploaded by Crystal Florman on Tuesday April 26, 2016. The Class Notes belongs to BIOL 1014 at University of Northern Iowa taught by Dr. Kurt Pontasch in Spring 2016. Since its upload, it has received 19 views. For similar materials see Life: Continuity and Change in Biology at University of Northern Iowa.
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Date Created: 04/26/16
Evolution Variation and Selection A. Introduction Two concepts are fundamental to the theory of evolution Characteristics of living things change with time That change is direct by natural selection The change in characteristics is not in an individual over time- it’s in a population over time Individuals can’t evolve Evolution- the change in the genetic makeup of a population over time Each individual in a population have a unique genotype Genetic recombination is the total of all gene mixing that occurs during sexual reproduction Sexual reproducing organisms tend to produce more offspring than necessary to maintain the population Replace the parents Geese live 10 years- raise 8 young per year If everyone survived and reproduced there would be 19.5 million geese Dramatic changes in the population because the death rate and the birth rate are equal Although total numbness remain relatively constant- the genetic makeup of the population changes to be more adaptive to changes Adaptation- occurs because death is not random- less fit organisms have higher death rates The survivors, for the most part, are better suited to the environment they reproduce more and transmit more good genes to the next generation Acquired characteristics- gained during a lifetime- learned behaviors, phenotypic changes due to the environment- not passed B. Theory of Natural Selection It states that individuals with genes that make them better adaptive to their surroundings are more likely to have a higher survival rates and produce more offspring First proposed by Charles Darwin in 1859 Wallace thought of it too “On the Origin of Species by Means of Natural Selection or the Presentation or Favored Races in the Struggle of Life” The struggle of life not only includes open conflict, but more often competition for resources that are in short supply Natural Selection works by means of differential reproduction Differential reproduction- organisms with genes that make them more fit to produce more offspring, therefore the favorable genes will become increasingly common in the population Natural Selection gives Evolution a direction C. Gene Frequency Studies Measure how often a certain gene turns up in the gametes of a certain population Gene frequencies only change when there is a reason for change and then it is usually slow Assume that alleles for earlobe type have the following frequencies in the human gene pool FF- free earlobes- 60% ff- attached earlobes- 40% o the frequencies will remain the same through each generation Hardy- Weinberg’s Law Gene frequencies remain the same if only 4 assumptions are met o Population is large enough so that chance alone or accidental deaths will not change gene frequencies The population would have to be infinitely large to mathematically rule out chance In reality if there is greater than 10,000 individuals no random change is likely- less than 100, random changes are likely Chance can cause evolutionary change in small populations Genetic Drifts- no direction- since it is not influenced by the relative adaptiveness in the change in allele frequency it results in what we can indeterminate evolution o Mutations must not occur This condition is never met in any population, but they are not a major factor in changing gene frequencies Since mutations are random the trend is often in a direction other than that which other factors are cousing the population to evolve lack of progress Mutations provide “the raw material” for genetic variability and therefore evolution, but do not determine direction o There cannot be any immigration or emigration Most populations experience at least some gene migration between populations- gene flow Increase genetic variability in the population However this condition for genetic equilibrium is sometimes met in nature- no gene flow o Reproduction must be totally random more than just the mating process also includes the many factors that contribute to the reproductive continuity of the population ex: selection of a mate, physical efficiency of the mating process, frequency of mating process, fertility, number of offspring produced by each mating, survival of the offspring, offspring’s reproductive efficiency for reproduction to be totally random all those factors must be random- seldom if ever happens factors listed are almost always related to genotype nonrandom reproduction is the universal rule Nonrandom reproduction is the same thing as differential reproduction and natural selection In all populations there are selection pressures acting to disturb the Hardy- Weinberg equilibrium o Selection pressures give direction to evolution o Complete equilibrium in a gene pool is not expected, therefore evolutionary change is a fundamental characteristic of all populations including humans Speciation A. Geographic distribution of a species is known as its range An organism can move to a new geographic region by: Traveling under its own power Be carried by storms and winds Attach to other organisms or human transported objects If the new area is suitable the species will establish a new colony and its range will expand Many plants and animals we consider pests are what we call exotic species Exotic Species- species introduced because of human activity Become pests because there are no natural enemies or competitors in their new home B. How Species Originate Sometimes a portion of the gene pool becomes separated from the rest of the gene pool by some geographic change Geographic isolation Geographic features hat cause geographic isolation are called geographic barriers Following geographic isolation the group of isolated organisms will often experience different environmental conditions than the main population These environmental differences act as natural selecting agents and causes differences in color, height, enzyme production, etc. Over a long period of time these genetic differences result in regional populations known as subspecies Speciation is the process of generating new species- it only occurs when gene flow between the 2 isolated populations does not occur even after the geographic barriers are removed Speciation has 3 steps Geographic isolation of part of the population o Allopatric populations don’t share the same range Differences in selecting agents cause genetic differences Genetic differences become so great that reproduction is impossible between the 2 groups o Even if they once again share the same range o Sympatric population- share the same range C. Maintaining Genetic Isolation Once a new species is formed, the species must prevent mating across species lies Mating between 2 different species are not successful because: o No offspring o Offspring would be infertile Mechanisms that prevent interspecies mating are called: Reproductive Isolation o Eco geographic isolation Occurs when 2 populations become so specialized for different environmental conditions they can’t survive where the other lives o Habitat Isolation Occurs when 2 sympatric species occupy different habitats within their common range, genetically determined preference for different habitats that isolates gene pools o Seasonal Isolation Occurs when two sympatric species breeding during different seasons o Behavior Isolation Occurs when behavioral genes are associated with successful courtship and mating ex: dancing fish and birds o Mechanical Isolation Occurs when structural differences between two closely related species prevents mating Genetic Isolating Mechanisms o Gamete Isolation Occurs when two different species are able to mate, but the egg and the sperm will not form a zygote- no fertilization o Developmental Isolation Occurs when mating and fertilization between two species are successful but the embryo doesn’t develop properly o Hybrid In Viability Occurs when a hybrid resolving from interbreeding dies before reproducing or so malformed that it can’t reproduce o Hybrid Sterility Occurs when a hybrid is healthy, but sterile o Selective Hybrid Elimination Occurs when a hybrid is capable of reproducing, but they and their offspring are less fit and are eliminated from the population within a few generations, but if they are just as fit and sympatric they will soon form a new species Usually several of these factors are in play at one time- most organisms rely on the first five They prevent the mating rather than waste gametes with those that prevent birth or produce hybrids D. Polyploidy So far we have only looked at speciation because of geographic isolation over many years Plants can form new species by another method called Polyploidy The number of chromosomes is increased The increase of chromosome number is the result of abnormal cell division in which chromosomes do not separate properly either during mitosis or meiosis Mitosis- is a plant increases its somatic cell chromosome number it may still be able to form normal gametes and reproduce with the parent population BUT If it forms gametes with too many chromosomes it usually cannot reproduce with the parent population The plant may produce asexually and produce a population of polyploidy plants- those plants can then reproduce sexually among themselves- new gene pool is formed E. Development of Evolutionary Thought For centuries people believed that the various species of various plants and animals remained unchanged since their creation Evolution occurs so slowly you usually can’t see it in your lifetime Early 1700’s- people in France and England became interested in fossils and began to realize that many species were now extinct Mid 1700’s- Buffon- proposed that animals might change over time- but didn’t come up with how they changed 1809-Lamarck- suggested a process by which evolution could occur- acquired characteristics could be passed on to offspring Ex: giraffes started out with short necks- they constantly stretched their necks to get at food so they became slightly longer a lifetime, slightly longer necks were passed on to the offspring- necks got longer with each generation He was wrong because acquired characteristics cannot be passed on 1858- Darwin and Wallace Natural Selection by which evolution could occur Made the following 5 assumptions o All organisms produce more offspring than survive o No two organisms are exactly alike o Among organisms there is a constant struggle for survival o Individuals with favorable characteristics are more likely to survive and produce more offspring o Favorable characteristics become more common in the species and unfavorable ones are lost 1900’s- Mendel’s work was rediscovered and explained how traits could be passed from generation from generation to generation F. Evolution above the Species Level Tracing the evolutionary ancestry of an organism is difficult because many of the ancestors do not exist Some organisms you can trace the fossil record, but that doesn’t tell us anything about the behavior, physiology or ecology of the organism Have to use bits and pieces of evidence to trace the series of evolutionary steps leading to present day organisms The incomplete fossil records is what creationists use to dispute the theory of evolution, but if evolution is defined as a change in the genetic makeup of a population overtime there is no doubt that it exists With the fossil evidence, evolutionary trees are created which trace evolutionary changes over time Most species do not survive into the present At least 99% of all species that have ever existed are extinct This basic pattern of evolution is called divergent evolution Speciation events cause branches in the evolution of a group of organisms There are other patterns as well *** Adaptive radiation- characterized by the rapid increase in the number and kinds of closely related species o Occurs like an explosion of new species form one common ancestor o 2 ways it can work An organism invades a previously unexplored environment An organism evolves a new set of characteristics that allows it to displace organisms that previously occupied an environmental niche (habitat) *** Convergent Evolution- occurs when organisms with widely different backgrounds develop similar characteristics G. Rate of Evolution Can vary greatly- usually evolutionary time is measured in thousands of millions of years Since natural selection is driven by the environment, rapid changes in the environment causes relatively rapid changes in the organisms present ***Microevolution- the type described by Darwin- a slow progressive change ***Macroevolution- whole groups of characteristics change at the same time- many new species appear and many old species become extinct Much of the fossil records support this type ***Punctuated Equilibrium- pattern of slow changes for millions of years followed by rapid evolution