Chapter 21The Evolution of Populations Following the Big

Chapter 21~The Evolution of Populations

Following the Big Ideas �Evolution- Microevolution, or evolution within populations, is measured as a change in allele frequencies over generations! Genetic diversity allows individuals in a population to respond differently to the same changes in environmental conditions Continuity of homeostatic mechanisms reflect a common ancestry, while changes may occur in response to different environmental conditions.

Overview: The Smallest Unit of Evolution �One common misconception is that organisms evolve during their lifetimes �Natural selection acts on individuals, but only populations evolve �Consider, for example, a population of medium ground finches on Daphne Major Island �During a drought, large-beaked birds were more likely to crack large seeds and survive �The finch population evolved by natural selection

Figure 21. 1 Is this finch evolving?

Concept 21. 1: Genetic variation makes evolution possible �Sources of variation gene mutations- 1/10, 000 gametes may have a change in a gene, usually negative. Chromosomal mutations- new combinations of genes on chromosomes in organism that can be inherited together Errors in mitosis and meiosis result in changes in phenotype. Sexual Reproduction- Recombination- crossing over. Immigration or emigration- brings new genes into the population or removes genes from the population Migration- tends to have the greatest affect on small populations

Populations evolve �Natural selection acts on individuals differential survival � “survival of the fittest” differential reproductive success � who bears more offspring �Populations evolve genetic makeup of population changes over time favorable traits (greater fitness) become more common Mummichog

Modern Synthesis: Comprehensive Theory of Evolution Integrating Many Ideas �Population genetics- study of changes in genetic make-up of populations population- group of interbreeding organisms of the same species living together individuals do not evolve, populations do species- a group of similar organisms that can breed to produce fertile offspring

Allele frequencies �each organism has a set of individual alleles, but individuals can carry some of the same alleles �in any population there a certain number of alleles of each kind �some alleles are more common than others �gene pool- total of all of the alleles in a population �at any given time, each allele occurs in the gene pool with a certain frequency �Evolution- gradual change in the allele frequencies in a population

Differential Reproduction (Natural Selection) �Mutations lead to new alleles and are the primary source of variation that can be acted on by natural selection. �DNA mutations can be positive, negative or neutral based on their effect or lack of effect they have on the resulting nucleic acid or protein and the phenotypes that are conferred by the protein. �Alterations in the DNA sequence can lead to changes in the type or amount of protein produced and consequent phenotype. (Phenotypes are determined by protein activity!) �Whether a mutation is positive or negative (or neutral) depends on the environmental context. If environment changes, screening effect of natural selection will change.

�Differential reproduction will cause certain allele frequencies to increase and others to decline �Alleles that benefit organism are passed on and alleles that don’t benefit organism are not passed on due to early death �The frequency of alleles that gives individuals an advantage under new conditions will increase in the population. The frequencies of alleles that are disadvantageous will decrease

Concept 21. 2: The Hardy-Weinberg equation can be used to test whether a population is evolving � 1908 - Hardy and Weinberg showed that the segregation and recombination of genes in sexual reproduction could not by itself change allele frequencies �if the frequency of allele p is 90% and allele q is 10%, ordinary random mating would produce a new generation with 90% p and 10% q �law is used to figure out allele frequencies at equilibrium (when there is no evolution) by mathematical equation. Any deviation from the reference point establishes evolution

Hardy- Weinberg Theorem �The alleles are for instance, B and b, the genotypes that could be made are: � the equation for phenotypes � black white p + q = 1 �the equation for alleles p 2 + 2 pq + q 2 = 1 BB (p 2) (homo dom. ) Bb (2 pq) (heteroz. ) bb (q 2) (homo rec. ) B b B BB Bb bb

Hardy-Weinberg Theorem � Serves as a model for the genetic structure of a non-evolving population (equilibrium) � Evolution = change in allele frequencies in a population hypothetical: what conditions not would cause allele frequencies to change? non-evolving population REMOVE all agents of evolutionary change 1. very large population size (no genetic drift) 2. no migration (no gene flow in or out) 3. no mutation (no genetic change) 4. random mating (no sexual selection) 5. no natural selection (everyone is equally fit) The first 2 conditions listed could occur, but the rest never do. Mutations always occur and reproduction is seldom random or nonselective due to differential reproduction

Hardy-Weinberg Equation � p=frequency of one allele (A); other allele (a); p+q=1. 0 q=frequency of the (p=1 -q & q=1 -p) � p 2=frequency of AA genotype; 2 pq=frequency of Aa genotype; q 2=frequency of aa genotype; � frequencies of all individuals must add to 1 (100%), so: p 2 + 2 pq + q 2 = 1 G. H. Hardy mathematician W. Weinberg physician

Using Hardy-Weinberg equation population: 100 cats 84 black, 16 white How many of each genotype? p 2=. 36 BB q 2 (bb): 16/100 =. 16 q (b): √. 16 = 0. 4 p (B): 1 - 0. 4 = 0. 6 2 pq=. 48 Bb q 2=. 16 bb Must. What assume arepopulation the genotype is in frequencies? H-W equilibrium!

Hardy-Weinberg Theorem �How is theorem useful? demonstrates that evolution in a pop. does occur theorem tells us that under certain conditions allele frequencies would remain constant and no evolution would occur that allele frequency changes tells us external factors are causing them to change failure of theorem- shows that evolution occurs and the extent of variation from theorem, shows the extent of evolution

Concept 21. 3: Microevolution �Definition small scale evolutionary change represented by a generational shift in a population’s relative allelic frequencies and genotypes there are 5 possible agents for microevolution, but only natural selection generally leads to an accumulation of favorable adaptations in a population.

5 Agents of evolutionary change Gene Flow Genetic Drift Mutation Non-random mating Selection

Microevolution I- Genetic Drift � A change in the gene pool of a population over a succession of generations � 1 - Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability)

Microevolution I: type of genetic drift �The Bottleneck Effect: type of genetic drift resulting from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population

Cheetahs �All cheetahs share a small number of alleles less than 1% diversity as if all cheetahs are identical twins � 2 bottlenecks 10, 000 years ago � Ice Age last 100 years � poaching & loss of habitat

Conservation issues Peregrine Falcon �Bottlenecking is an important concept in conservation biology of endangered species loss of alleles from gene pool reduces variation reduces adaptability Breeding programs must consciously outcross Golden Lion Tamarin

Microevolution I: type of genetic drift �Founder Effect: a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population just by chance some rare alleles may be at high frequency; others may be missing skew the gene pool of new population � human populations that started from small group of colonists � example: colonization of New World

Microevolution II- Gene Flow � 2 - Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations) • seed & pollen distribution by wind & insect • migration of animals

Microevolution III- Mutations � Mutations: Mutation creates variation a change in an organism’s DNA (gametes; many generations); original source of genetic variation (raw material for natural selection) most are harmful, even lethal. Some are neutral, very few are beneficial. Always random

Microevolution IV- Nonrandom Mating � Nonrandom mating: (selective breeding)� inbreeding- mating with close neighbors- increases frequency of homozygous recessive allelic combinations � assortive mating- individuals mate with others with the same phenotype (for a specific trait) � sexual selection- males compete to mate (collect a harem), or females choose a mate based on some physical characteristic (male bird plumage) ) Those most “fit” are chosen first!

Sexual selection �Sexual dimorphism: secondary sex characteristic distinction �Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism

Concept 21. 4: Natural selection is the only mechanism that consistently causes adaptive evolution Microevolution VNatural Selection � differential success in reproduction; � climate change � food source availability � predators, parasites, diseases � toxins � only form of microevolution that adapts a population to its environment • combinations of alleles that provide “fitness” increase in the population

Natural Selection �Selection acts on any trait that affects survival or reproduction predation selection physiological selection sexual selection

� Natural Selection- based on relative fitness- fitness of one phenotype (allele at one loci) compared to another. Like all genetics, the phenotype may change depending on the selection pressures at the time. Heterozygotes can be a depository for recessive genes. Operates in three ways: directional- the extreme phenotype is favored so allele frequencies shift in that one direction. Ex- peppered moth stabilizing- the intermediate phenotype is selected (~7 lb. birth weight) disruptive- favors 2 or more extreme phenotypes over the intermediates (specialization gives advantage over the generalists). Often each extreme has adapted to a specific habitat, predator, food, etc. (takes two directions)

Effects of Selection �Changes in the average trait of a population DIRECTIONAL SELECTION giraffe neck horse size STABILIZING SELECTION human birth weight DISRUPTIVE SELECTION rock pocket mice

Other interesting notes! �Genetic diversity allows individuals in a population to respond differently to the same changes in environmental conditions. EX. Not all animals in a population stampede. EX. Not all individuals in a population in a disease outbreak are equally affected; some may not show any symptoms, some show mild symptoms and others show severe symptoms.

Variation & natural selection �Variation is the raw material for natural selection there have to be differences within population some individuals must be more fit than others

Genetic Variation � Variation within populations polygenic characters- vary quantitatively within the population along a continuum- height in humans (genetic polymorphism) polymorphism (discrete characters)- contrasting formspink vs white flowers. Forms are called morphs. A population is called polymorphic if two or more distinct morphs are in noticeable frequency. (phenotypic polymorphism) Both of the above contribute to variation

Variation between populations � geographical variation- environmental factors effect natural selection between two locales. Cline- graded change in some trait along a geographical transect Characteristics of a species often are different in different parts of the range, due to varying selective environmental pressures. Race Circle-adjacent populations in a range can interbreed to produce normal offspring (subspecies), but nonadjacent populations cannot. They are still considered the same species because there is continuous interbreeding among adjacent subspecies throughout the range. If a population varies so much from its neighbors that it can no longer produce normal offspring with them then speciation has occurred.

Allopatry

Variation preservation � Prevention of natural selection’s reduction of variation Diploidy 2 nd set of chromosomes hides variation in the heterozygote � Balanced polymorphism 1 - heterozygote advantage (hybrid vigor; i. e. , malaria/sickle-cell anemia); 2 - frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i. e. , parasite/host)

Adaptations �adaptation- any kind of inherited trait that improves the chance of survival and reproduction of the organism in a given environment. The selecting force is the environment itself. �Types of adaptations Structural adaptations- those that involve the body of the organism- wings, leaves w/ large surface area, fins, webbed feet (morphological) Physiological adaptations- involve the metabolism of the organism- protein web made by spiders, poison venom made by snakes Behavioral adaptations- mating behavior, migration by birds, spawning of fish, hibernation

Coloration- An Important Adaptation! �Camouflage- adaptation for protection (cryptic coloration) counter shading- dark top, light under belly �Warning coloration- (aposematic coloration) colors make animal easier to see, must have a reason; venom, poisonous, bad taste, a really bad guy! Or to protect young. �Mimicry- one organism is protected against enemies by resembling another, unrelated organism- Viceroy butterfly is tasty, but resembles the Monarch butterfly which is bad tasting, thus birds do not attempt to eat it.

Cryptic coloration aposemetic coloration Mimicry

Observed Natural Selection � Industrial Melanism- When industry emits pollutants that change the color of the habitat leading to selective forces that select dark colored forms of insects. Peppered moth B. betularia is found in wooded areas around London on lichen-covered trees 2. Before the industrial revolution(1850) most pepper moths were a light pepper in color, blending well into the habitat. The black form was rare. 3. 1850 -1900 - England became industrialized and heavy soot darkened the tree trunks killing the lichen on them. In 1890’s- 99% of moths were black in color, the light pepper variety was rare. 5. In cleaner areas, the light pepper colored moths still predominated. WHY? 6. After 1950 - England has installed air pollution controls and the pepper moth population has returned to the original color of before 1850.

Industrial Melanism- Peppered moths

�Insect resistance to DDT- Most died; those resistant to DDT live to pass trait on. �Bacterial resistance to antibacterials. �Remember- Phenotypic variations are not directed by the environment but occur through random changes in DNA and through new gene combinations!!!!!

Clones and Natural Selection � Bacterial resistance to antibiotics � clones- single-species population that has descended from one ancestor; all individuals are genetically identical because they have been reproduced asexually. � When random mutations occur, a new trait appears and the mutant will asexually produce a population with the new trait. � Such is the case with penicillin-resistant bacteria that thrive in an environment rich with penicillin. � Penicillin destroys the cell wall of bacteria, preventing them from carrying on normal metabolism. Penicillin-resistant bacteria are not affected by penicillin. � Population of resistant bacteria thrives quickly as susceptible bacteria get killed off. (Ex. Penicillin-resistant Gonorrhea bacteria)

Why Natural Selection Cannot Fashion Perfect Organisms 1. Selection can act only on existing variations 2. Evolution is limited by historical constraints 3. Adaptations are often compromises 4. Chance, natural selection, and the environment interact

Connecting the Concepts with the Big Ideas � Evolution Natural selection acts on trait variation, and trait variation is determined by genes. Whether or not a trait gives an advantage depends on the environment. Thus genes, traits, environment, and natural selection are all involved in microevolution. Microevolution occurs when allele frequencies in a population change over time; Hardy and Weinberg devised a mathematical method by which geneticists can measure that change. If there is no gene flow or genetic drift, random mating, occurrence of mutation, or any type of selection, microevolution should not occur. Small populations are especially vulnerable to genetic drift- chance events that in a small population may remove some alleles completely and may cause others to become more frequent. Some genes, such as those for sickle cell anemia, are maintained in populations living in particular environments due to stabilizing selection. Mutation and genetic variation are the ultimate sources of the raw materials for evolution.