Evolution_of_Populations[1]

The Evolution of Populations : 

The Evolution of Populations BIOLOGY

What Causes Variations in a Population? : 

What Causes Variations in a Population? Gene mutations random source of new alleles beneficial, harmful, or neutral hidden until an environmental change makes it beneficial Chromosome mutations may be caused by an alteration in chromosome numbers may be caused by an alternation in the arrangement of alleles (inversion, deletion, translocation) Recombination of alleles and chromosomes -- crossing-over & independent assortment during meiosis -- caused by random gamete union during fertilization -- new combination of alleles may have great selective value

Evolution in a Genetic Context : 

Evolution in a Genetic Context In population genetics, the various alleles at all the gene loci in all individuals make up the gene pool of the population. To determine the frequency of each allele, calculate its percentage from the total number of alleles in the population. For example, 36% of Drosophila in a population are homozygous dominant for long wings, 48% are heterozygous, and 16% are homozygous recessive. In a population of 100 fruit flies, there are 36 LL, 48 Ll and 16 ll. Number of L alleles: LL (2 L x 36) = 72 Ll (1 L x 48) = 48 ll (0L) = 00 Total = 120 120/200 total L alleles = .6 (&l = .4)

Slide 4: 

An equilibrium state was independently recognized by G.H. Hardy and W. Weinberg in 1908. They used the binomial expression p2 + 2pq + q2 to calculate the genotypic and allele frequencies of a population. p + q = 1 (there are only 2 alleles) p2 + 2 pq + q2 = 1 (these are the only genotypes possible) p2 = % of homozygous dominant individuals (AA) p = frequency of the dominant allele (A) q2 = % of the homozygous recessive individual (aa) q = frequency of the recessive allele (a) 2pq = % of heterozygous individuals (Aa) Hardy-Weinberg Law

Figure 23.3a The Hardy-Weinberg theorem : 

Figure 23.3a The Hardy-Weinberg theorem

Figure 23.3b The Hardy-Weinberg theorem : 

Figure 23.3b The Hardy-Weinberg theorem

Sample Hardy-Weinberg Problem : 

Sample Hardy-Weinberg Problem One in 10,000 babies in the United States is born with PKU. Calculate the frequency of carriers in the population. What do we know? 1/10,000 = 0.0001 q2 = 0.0001 (Individuals with PKU = aa) Therefore, q = 0.01 (square root) p = 1-q Therefore p = 0.99 Carriers = 2pq = 2 X .99 X .01 = .0198 or about 2% of the population

Conditions of the Hardy-Weinberg Equilibrium: : 

Conditions of the Hardy-Weinberg Equilibrium: No mutations: there are either no changes in alleles or changes in one direction are balanced by changes in the opposite direction. No gene flow: migration of alleles into or out of the population does not occur. Random mating: individuals pair by chance and not according to specific genotypes or phenotypes. Large population: changes in allele frequencies due to chance alone are insignificant (no genetic drift). No selection: no selective force favors one genotype over another. Note: Any changes in the gene pool of a population signifies that microevolution has occurred.

What Causes Microevolution? : 

What Causes Microevolution? Genes mutate --mutated alleles may be more adapative than others Genes flow --variation in a population brought in from another population Mating is not always random --assortative mating individuals mate with those with the same phenotype; e.g. tall humans mating with tall humans --sexual selection—females choose males with a particular phenotype or males compete for the right to reproduce Genetic Drift --Founder Effect—by chance, founding individuals do not contain the full genetic diversity of the original gene pool. --Bottleneck Effect—a natural disaster subjects a population to near extinction and chance alone determines survivors

Gene Flow : 

Gene Flow

Figure 23.16x1 Sexual selection and the evolution of male appearance : 

Figure 23.16x1 Sexual selection and the evolution of male appearance

Genetic Drift—Bottleneck Effect : 

Genetic Drift—Bottleneck Effect

Figure 23.5x Cheetahs, the bottleneck effect : 

Figure 23.5x Cheetahs, the bottleneck effect

Natural Selection requires: : 

Natural Selection requires: Variation Inheritance (some differences are environmental) Differential adaptiveness—some differences affect how well an organism is adapted to its environment Differential reproduction—better adapted individuals are more likely to reproduce and their offspring will be a greater proportion of the next generation.

Types of Selection : 

Types of Selection Directional Selection An extreme phenotype is favored and the distribution curve shifts in that direction. population is adapts to a changing environment. Stabilizing Selection Occurs when an intermediate phenotype is favored. adaptation of the population to constant conditions. Disruptive Selection Two distinctly different phenotypes are found in the population. Leads to polymorphism.

Figure 23.12 Modes of selection : 

Figure 23.12 Modes of selection

Figure 23x2 Polymorphism : 

Figure 23x2 Polymorphism

Maintenance of Variations : 

Maintenance of Variations Populations with limited variation may not be able to adapt. The heterozygote—potential protector of recessive alleles that would otherwise be weeded out of the gene pool. Balanced polymorphism—the relative fitness of the heterozygote versus the two homozygotes determines their percentages in the population. Example: Sickle-cell Disease a. Individuals with sickle-cell disease die young. b. Individuals with no sickle-cell gene often die from malaria. c. Heterozygotes (sickle-cell trait) only have sickle-shaped red blood cells when the oxygen is low and the malaria parasite cannot live in their cells

Figure 23.10 Mapping malaria and the sickle-cell allele : 

Figure 23.10 Mapping malaria and the sickle-cell allele

Speciation : 

Speciation The splitting of one species into two or more species. OR The transformation of one species into a new species over time.

What is a Species? : 

What is a Species? Biological definition of species: a population or group of populations whose members have the potential to interbreed with one another in nature to produce fertile offspring. They have a shared gene pool and are reproductively isolated from other species. Limitations of the Biological Species Concept: Cannot be used for asexually reproducing organisms Often unclear whether two populations are subspecies or separate species Some animals interbreed but remain separate species dogs, wolves and coyotes

Figure 24.9 Ensatina eschscholtzii, a ring species : 

Figure 24.9 Ensatina eschscholtzii, a ring species

Reproductive Isolating Mechanisms: : 

Reproductive Isolating Mechanisms: Premating barriers a. Habitat isolation—species live in different habitats within the same territory and rarely meet. b. Temporal isolation—two species breed at different times of the day or in different seasons. c. Behavioral isolation—little or no sexual attraction exists. d. Mechanical isolation—structural differences in genitalia or flowers prevent copulation or pollen transfer.

Reproductive Isolating Mechanisms: : 

Postmating or postzygotic barriers a. Gamete isolation—sperm cannot reach or survive to reach egg to fertilize it. b. Zygote mortality—fertilization occurs but the zygote does not survive or the organism fails to reach sexual maturity. c. Hybrid sterility—the hybrid survives but is sterile e.g. donkey and horse produce a sterile mule. d. F2 fitness—the hybrid is fertile, but the F2 hybrid has reduced fitness. Reproductive Isolating Mechanisms:

Modes of Speciation : 

Modes of Speciation Allopatric Speciation— a population forms a new species when geographically separated from its parent population. Gene flow stops and variations build up causing first postmating and then premating reproductive isolation to occur.

Modes of Speciation : 

2. Sympatric Speciation—a population develops two or more reproductively isolated groups without geographic isolation. Usually a mutation erects a reproductive barrier between the mutants and the parent population. Modes of Speciation

Figure 24.8 Has speciation occurred during geographic isolation? : 

Figure 24.8 Has speciation occurred during geographic isolation?

Adaptive Radiation : 

Adaptive Radiation Is allopatric speciation. The rapid development from a single ancestral species of many new species, which have spread out and become adapted to various ways of life. --as the parent population increases in size, daughter populations are subjected to the founder effect and the process of natural selection. --Examples: 13 species of Galapagos finches 20+ species of Hawaiian honeycreeper 500 species of Drosophila in Hawaii