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Unformatted text preview: Chapter 18 Processes of Evolut 2 Processes of Evoluti Outline Microevolution Hardy-Weinberg Causes of Microevolution Natural Selection Types of Selection Macroevolution 3 Processes of Evoluti Hardy-Weinberg The Hardy-Weinberg principle: Allele frequencies in a population will remain constant assuming: No Mutations No Gene Flow Random Mating No Genetic Drift No Selection Calculating Gene Pool Frequencies Using the Hardy-Weinberg Equation 4 Industrial Melanism and Microevolution 5 6 Processes of Evoluti Hardy-Weinberg Required conditions are rarely (if ever) met Changes in gene pool frequencies are likely When gene pool frequencies change, microevolution has occurred Deviations from a Hardy-Weinberg equilibrium indicate that evolution has taken place 7 Processes of Evoluti Causes of Microevolution Genetic Mutations The raw material for evolutionary change Provides new combinations of alleles Some might be more adaptive than others 8 Processes of Evoluti Causes of Microevolution Gene Flow Movement of alleles between populations when: Gametes or seeds (in plants) are carried into another population Breeding individuals migrate into or out of population Continual gene flow reduces genetic divergence between populations Gene Flow 9 10 Processes of Evoluti Causes of Microevolution Nonrandom Mating When individuals do not choose mates randomly Assortative mating: Individuals select mates with their phenotype Individuals reject mates with differing phenotype Sexual selection: Males compete for the right to reproduce Females choose with males possessing a particular phenotype Both of these cause an increase in homozygotes 11 Processes of Evoluti Causes of Microevolution Genetic Drift Occurs by disproportionate random sampling from population Can cause the gene pools of two isolated populations to become dissimilar Some alleles are lost and others become fixed (unopposed) Likely to occur: After a bottleneck When severe inbreeding occurs, or When founders start a new population Stronger effect in small populations Genetic Drift 12 13 Processes of Evoluti Genetic Drift Bottleneck Effect A random event prevents a majority of individuals from entering the next generation Next generation composed of alleles that just happened to make it 14 Processes of Evoluti Genetic Drift Founder Effect When a new population is started from just a few individuals The alleles carried by population founders are dictated by chance Formerly rare alleles will either: Occur at a higher frequency in the new population, or Be absent in new population Founder Effect 15 16 Processes of Evoluti Natural Selection Adaptation of a population to the biotic and abiotic environment Requires: Variation - The members of a population differ from one another Inheritance - Many differences are heritable genetic differences Differential Adaptiveness - Some differences affect survivability Differential Reproduction Some differences affect likelihood of successful reproduction 17 Processes of Evoluti Natural Selection Results in: A change in allele frequencies the gene pool Improved fitness of the population Major cause of microevolution 18 Processes of Evoluti Types of Selection Most traits are polygenic - variations in the trait result in a bell-shaped curve Three types of selection occur: (1) Directional Selection The curve shifts in one direction Ex - when bacteria become resistant to antibiotics Directional Selection 19 20 Processes of Evoluti Types of Selection Three types of selection occur (cont): (2) Stabilizing Selection The peak of the curve increases and tails decrease Ex - when human babies with low or high birth weight are less likely to survive (3) Disruptive The curve has two peaks Ex When Cepaea snails vary because a wide geographic range causes selection to vary Stabilizing Selection 21 Disruptive Selection 22 23 Processes of Evoluti Maintenance of Variations Genetic variability Populations with limited variation may not be able to adapt to new conditions Maintenance of variability is advantageous to population Only exposed alleles are subject to natural selection 24 Processes of Evoluti Maintenance of Variations Recessive alleles: Heterozygotes shelter recessive alleles from selection Allows even lethal alleles to remain in population at low frequencies virtually forever Lethal recessive alleles may confer advantage to heterozygotes Sickle cell anemia is detrimental in homozygote However, heterozygotes more likely to survive malaria Sickle cell allele occurs at higher than expected frequency in malaria prone areas Sickle-cell Disease 25 26 Processes of Evoluti Species Definitions Species Definitions Morphological Can be distinguished anatomically Specialist decides what criteria probably represent reproductively isolated populations Most species described this way 27 Processes of Evoluti Species Definitions Species Definitions Biological Populations of the same species breed only among themselves Are reproductively isolated from other such populations Very few actually tested for reproductive isolation Biological Species Definition 28 29 Processes of Evoluti Species Definitions Species Definitions Phylogenetic Can be shown to have genetic differences Usually based on DNA sequence analysis Very few species determined this way, but growing in use 30 Processes of Evoluti Reproductive Isolating Mechanisms Reproductive isolating mechanisms inhibit gene flow between species Two general types: (1) Prezygotic Mechanisms - Discourage attempts to mate Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gamete Isolation Temporal Isolation 31 32 Processes of Evoluti Reproductive Isolating Mechanisms Two general types: (2) Postzygotic Mechanisms - Prevent hybrid offspring from developing or breeding Zygote Mortality Hybrid Sterility Reduced F2 Fitness 33 Processes of Evoluti Modes of Speciation Speciation: The splitting of one species into two, or The transformation of one species into a new species over time Two modes: (1) Allopatric Speciation Two geographically isolated populations of one species Become different species over time Can be due to differing selection pressures in differing environments Allopatric Speciation 34 35 Processes of Evoluti Modes of Speciation Two modes: (2) Sympatric Speciation One population develops into two or more reproductively isolated groups No prior geographic isolation Tetraploid hybridization in plants Results in self fertile species isolated from either parental Reproductively species 36 Processes of Evoluti Adaptive Radiation Adaptive Radiation When members of a species invade several new geographically separate environments The populations become adapted to the different environments Many new species evolve from the single ancestral species This is an example of allopatric speciation 37 Processes of Evoluti Review Microevolution Hardy-Weinberg Causes of Microevolution Natural Selection Types of Selection Macroevolution Ending Slide Chapter 18 Processes of Evolu ...
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