17.1 What Is It That Evolves? Microevolution. Microevolution. Ch. 17 Microevolution. Genes. Population

Transcription

1 Ch. 17 Microevolution 17.1 What Is It That Evolves? Microevolution Population Defined as all the members of a single species living in a defined geographical area at a given time A sexually reproducing unit The smallest unit that evolves Microevolution Genes Genotype dictates phenotype Come in different forms called alleles One organism may possess only two alleles for a given gene One from mother, one from father A population may contain many more than two alleles for a gene Evolution, through natural selection and other mechanisms will work upon all of the alleles in a population This is the gene pool 1

2 Microevolution original coloration (a) Original environment 17.2 Evolution as a Change in the Frequency of Alleles (b) Altered environment lighter coloration population B expanse of barren terrain darker coloration population A Figure 17.1 Genetic Basis of Evolution Genetic Basis of Evolution maternal chromosome 3 paternal chromosome 3 maternal chromosome 3 paternal chromosome 3 Evolution is a change in the gene pool As a change in allele frequency The relative amounts of different alleles in a population Ie. 60% brown eyes, 30% blue eyes, 10% green eyes alleles alleles dark coloration light coloration Figure

3 Genetic Basis of Evolution As allele frequencies change from one generation to the next, the phenotoypic ratios will follow Ie. the allele frequency for green eyes goes up, the frequency of green eyes in the population will increase As one allele frequency increases, the frequency of another allele of the same gene must decrease Frequency of all alleles for a given gene must equal 100% Genetic Basis of Evolution When the allele frequency change in a population, evolution has occurred Microevolution a change of allele frequencies within a population over a relatively short period of time THIS IS THE MOST BASIC MANNER IN WHICH ALL EVOLUTION OCCURRS Two populations experiencing this phenomena in differing ways are said to have Diverged Genetic Basis of Evolution Evolution and Genetics Macroevolution A product of microevolution, is evolution on a larger scale Both geographically and temporally Evolution that results in the formation of new species or other large groupings of living things PLAY 3

4 Five Agents of Microevolution 17.3 Five Agents of Microevolution Five evolutionary forces can result in changes in allele frequencies within a population. These agents of microevolution are: mutation gene flow genetic drift sexual selection natural selection Five Agents of Microevolution Mutations A mutation is any permanent alteration in an organism s DNA, and some mutations are heritable, meaning they can be passed on from one generation to the next. Table

5 Mutations Mutations (a) Normal Point mutation (b) Normal Deletion Mutation happens fairly infrequently, and most mutations either have no effect or are harmful. Yet rare adaptive mutations are vital to evolution in that they are the only means by which entirely new genetic information comes into being. correct nucleotide sequence incorrect nucleotide sequence complete chromosome 5 incomplete chromosome 5 Figure 17.3 Gene Flow Gene Flow (a) Hawaiian silversword (b) Tarweeds in California Gene flow The movement of genes from one population to another population Occurs through migration movement of individuals from one population into the territory of another population North America Hawaiian Islands Pacific Ocean Figure

6 Genetic Drift Genetic Drift Genetic drift The chance alteration of allele frequencies in a population Has its greatest effects on small populations The larger a population, the more resistant it will be to random effects (a) Large population = 10,000 (allele carriers in red) allele frequency = 1,000 10,000 = 10% 50% of population survives, including 450 allele carriers (b) Small population = 10 (allele carriers in red) allele frequency = 1 = 10% 10 50% of population survives, with no allele carrier among them allele frequency = 450 = 9% 5,000 allele frequency = 0 = 0% 5 little change in allele frequency (no alleles lost) dramatic change in allele frequency (potential to lose one allele) Figure 17.5 Genetic Drift Genetic drift can have large effects on small populations through two common scenarios: the Bottleneck Effect the Founder Effect Genetic Drift: Bottleneck Effect Bottleneck effect defined as a change in allele frequencies due to chance during a sharp reduction in a population s size Such as the loss of a large proportion of the population Only a small representation of the original allele frequencies would remain May now have an entirely different set of allele frequencies Some alleles may have been completely lost 6

7 Genetic Drift: Bottleneck Effect Genetic Drift: Bottleneck Effect Bottleneck effect (cont.) If population re-expands, the gene pool will now consist of only those alleles present in the survivors of the bottle neck event Therefore, bottleneck events reduce diversity in a population bottleneck only allows a few individuals through Original population, original allele frequency. Hunting of seals in late 1800s greatly reduced population size. Surviving population had different allele frequency and little genetic diversity. This different allele frequency is reflected in today's population. Figure 17.6 Genetic Drift: Founder Effect Sexual Selection Founder effect 2nd type of genetic drift when a small subpopulation migrates to a new area to start a new population it is likely to bring with it only a portion of the original population s gene pool Sexual selection A form of natural selection that can affect the frequency of alleles in a gene pool It occurs when differences in reproductive success arise because of differential success in mating 7

8 Sexual Selection Sexual Selection A given male in a population may, for example, sire many more offspring than the average male in the population May be due to mate selection Displays etc. Or simple superior reproductive abilites If so, this male s alleles will increase in frequency in the next generation of the population. Figure 17.7 Natural Selection Natural Selection Natural selection Within a population some individuals will be more successful than others in surviving, and hence reproducing owing to traits that better adapt them to their environment Natural selection The only agent of microevolution that consistently acts to adapt organisms to their environments As such, it is generally regarded as the most powerful force underlying evolution Not entirely accepted as only agent Many scientists believe drift has as great effect Called the neutral theory 8

9 Number of individuals Natural Selection average beak depth, 1976 beak depth average beak depth, Natural Selection and Evolutionary Fitness Beak depth (mm) Shift of average beak depth following drought Figure 17.9 Evolutionary Fitness survival of the fittest misleading because it implies that evolution works to produce generally superior beings who would be successful competitors in any environment Evolutionary Fitness Evolutionary fitness has to do only with the relative reproductive success of individuals in a given environment at a given time 9

10 Evolutionary Fitness One individual is said to be more fit than another to the extent that it has more offspring than another More accurately - offspring that survive to reproductive age Evolutionary Fitness Individuals are not born with invariable levels of fitness Fitness can change in accordance with changes in the surrounding environment Therefore fitness is a relative term Modes of Natural Selection 17.5 Three Modes of Natural Selection Natural selection has three modes: stabilizing selection directional selection disruptive selection 10

11 Three Modes of Natural Selection Stabilizing selection Directional selection Disruptive selection Three Modes of Natural Selection Time (many generations) Stabilizing selection moves a given character in a population toward intermediate forms and hence tends to preserve the status quo. Range of a particular characteristic (in this instance, lightness or darkness of coloration) In stabilizing selection, individuals that possess extreme values of a characteristic here, both the lightest and the darkest colors are selected against and die or fail to reproduce. Over succeeding generations, an increasing proportion of the population becomes average in coloration. In directional selection, one of the extremes of a characteristic is better suited to the environment, meaning that individuals at the other extreme are selected against. Over succeeding generations, the coloration of the population moves in a direction in this case toward darker coloration. In disruptive selection, individuals with average coloration are selected against and die. Over succeeding generations, part of the population becomes lighter, while part becomes darker meaning the range of color variation in the population has increased. Figure Stabilizing Selection Directional Selection Percent of infant deaths infant deaths infant births Infant mortality is lowest among infants of average birth weight Percent of births in population Directional selection moves a given character toward one of its extreme forms Birth weight in pounds Figure

12 Directional Selection Disruptive Selection A. afarensis A. africanus H. habilis Cranial capacity (the volume of the skull) has increased in hominins over time H. ergaster H. erectus H. sapiens Cranial capacity (cubic centimeters) Disruptive selection moves a given character toward two extreme forms Present Earliest fossil record (millions of years ago) Figure Disruptive Selection Three Modes of Natural Selection PLAY 12