15.3 Shaping Evolutionary 15.3 Shaping Evolutionary Theory Theory Main idea: The theory of evolution Main idea: The theory of evolution continues to be refined as scientists continues to be refined as scientists learn new information. learn new information. Objectives: Objectives: Discuss patterns observed in evolution. Discuss patterns observed in evolution. Describe factors that influence speciation. Describe factors that influence speciation. Compare gradualism with punctuated Compare gradualism with punctuated equilibrium. equilibrium. Review Vocabulary Review Vocabulary Allele: alternative forms of a character Allele: alternative forms of a character trait that can be inherited. trait that can be inherited.
Transcript
15.3 Shaping Evolutionary Theory15.3 Shaping Evolutionary Theory
Main idea: The theory of evolution continues to Main idea: The theory of evolution continues to be refined as scientists learn new information.be refined as scientists learn new information.
Objectives:Objectives: Discuss patterns observed in evolution.Discuss patterns observed in evolution. Describe factors that influence speciation.Describe factors that influence speciation. Compare gradualism with punctuated equilibrium.Compare gradualism with punctuated equilibrium.
Review VocabularyReview Vocabulary Allele: alternative forms of a character trait that can Allele: alternative forms of a character trait that can
Evolution occurs at the population level, with genes as Evolution occurs at the population level, with genes as the raw material.the raw material.Hardy-Weinberg Principle Hardy-Weinberg Principle states that when allelic frequencies remain constant, a population is in genetic equilibrium. (Even though the number of owls doubled, the ratio of gray to red owls remained the same).
Hardy-Weinberg PrincipleHardy-Weinberg Principle
pp2 2 + 2pq + q+ 2pq + q2 2 = 1= 1
This equation allows us to determine the This equation allows us to determine the equilibrium frequency of each genotype in equilibrium frequency of each genotype in the population: homozygous dominant the population: homozygous dominant (p(p22), heterozygous (2pq), and homozygous ), heterozygous (2pq), and homozygous recessive (qrecessive (q22). Note the sum of these ). Note the sum of these frequencies equals one.frequencies equals one.
Hardy-Weinberg PrincipleHardy-Weinberg Principle
A population in genetic equilibrium must meet five A population in genetic equilibrium must meet five conditions – there must be no genetic drift, no gene flow, conditions – there must be no genetic drift, no gene flow, mating must be random, mutation, and there must be no mating must be random, mutation, and there must be no natural selection.natural selection.
Genetic DriftGenetic Drift
A change in the allelic frequencies in a population that is due to chance and random mating.
In smaller populations, the effects of genetic drift become more pronounced, and the chance of losing an allele becomes greater.
Examples: Founder Effect & Bottleneck
Evolution
Genetic Drift A change in the allelic frequencies in a
population that is due to chance and random mating
In smaller populations, the effects of genetic drift become more pronounced, and the chance of losing an allele becomes greater.
Marble Example
15.3 Shaping Evolutionary Theory
Chapter 15
Founder EffectFounder Effect
Occurs when a small sample of a population settles in a location separated from the rest of the population
Alleles that were uncommon in the original population might be common in the new population.
Examples: Amish & French Canadians
Evolution
Founder Effect
The loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population
15.3 Shaping Evolutionary Theory
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Mainland
Island 3
Island 2
Island 1
BottleneckBottleneck
Occurs when a population declines to a very low number and then rebounds
Evolution
Bottleneck a significant percentage of a population or
species is killed or otherwise prevented from reproducing and can rebound later
15.3 Shaping Evolutionary Theory
Often caused by a natural disaster
Gene FlowGene Flow
A population in genetic equilibrium A population in genetic equilibrium experiences no gene flow, however, few experiences no gene flow, however, few populations are isolatedpopulations are isolated
Random movement of individuals between Random movement of individuals between populations, or migration ipopulations, or migration increases genetic variation within a population and reduces differences between populations
Gene FlowGene FlowGenes entering or leaving a population through Genes entering or leaving a population through migrationmigration
EmigrationEmigration Genes LEAVING a population Genes LEAVING a population ImmigrationImmigration INCOMING genes in a population INCOMING genes in a population
Nonrandom matingNonrandom mating
Rarely is mating completely random in a Rarely is mating completely random in a population.population.
Usually individuals mate with individuals in Usually individuals mate with individuals in close proximity.close proximity.
This promotes inbreeding and could lead to a change in allelic proportions favoring individuals that are homozygous for particular traits
MutationsMutations
A random change in genetic material.A random change in genetic material.
Occasionally mutations provide an Occasionally mutations provide an advantage to organisms and become advantage to organisms and become more common in subsequent generations.more common in subsequent generations.
Basis upon which natural selection works.Basis upon which natural selection works.
What is a gene pool?What is a gene pool?Gene poolGene pool – combined genetic – combined genetic info of all members of a info of all members of a populationpopulation Contains two of more alleles Contains two of more alleles
(genes) for the same trait(genes) for the same trait
Allele frequency – number of Allele frequency – number of times an allele occurs in a times an allele occurs in a gene pool compared to the gene pool compared to the number of times another allele number of times another allele occurs (expressed in percents)occurs (expressed in percents)
Sample Population
48% heterozygous
black
36% homozygous
brown
16% homozygous
black
Frequency of Alleles
allele for brown fur
allele for black fur
Relative Frequencies of AllelesRelative Frequencies of AllelesSection 16-1
Natural SelectionNatural SelectionActs to select the individuals that are best adapted for survival and reproductionStabilizing selection operates to eliminate extreme expressions of a trait when the average expression leads to higher fitness. (Birth Weights)Directional selection makes an organism more fit. (Peppered Moths)Disruptive selection is a process that splits a population into two groups. (African Finches)
Evolution
Stabilizing selection operates to eliminate extreme expressions of a trait when the average expression leads to higher fitness.
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Ex. Siberian Husky
Evolution
Directional selection makes an organism more fit.
15.3 Shaping Evolutionary Theory
Chapter 15
Favors the extremes Ex. Greyhound Dog
Sexual SelectionSexual Selection
Sexual selection operates in populations where males and females differ significantly in appearance. (Males are largest and most colorful in the group).Qualities of sexual attractiveness appear to be the opposite of qualities that might enhance survival. (More likely target for predators yet bodies enhance reproductive success).Examples: Peacocks
Reproductive IsolationReproductive Isolation
Two types of reproductive isolating Two types of reproductive isolating mechanism prevent gene flow among mechanism prevent gene flow among populations:populations: Prezygotic isolating mechanisms – operate Prezygotic isolating mechanisms – operate
after fertilization has occurred to ensure that after fertilization has occurred to ensure that the resulting hybrid remains infertile.the resulting hybrid remains infertile.
Prezygotic IsolationPrezygotic Isolation
prevents reproduction by making fertilization unlikely.prevents genotypes from entering a population’s gene pool through geographic, ecological, behavioral, or other differencesEastern and Western meadowlarks similar in appearance but their songs separate them behaviorally.
Postzygotic IsolationPostzygotic Isolation
Occurs when Occurs when fertilization fertilization has occurred but a hybrid offspring cannot develop or reproduce.Prevents offspring survival or reproduction.A Liger is the offspring of a male lion and a female tiger. Ligers are sterile.
SpeciationSpeciationA population must diverge and then be A population must diverge and then be reproductively isolated for speciation to occur.reproductively isolated for speciation to occur.Two types of speciation:Two types of speciation: Allopatric speciation - Allopatric speciation - A physical barrier
divides one population into two or more populations. (The Grand Canyon separates the Abert and Kaibab squirrels).
Sympatric speciation - Sympatric speciation - A species evolves into a new species without a physical barrier. The ancestor species and the new species live side by side during the speciation process. (Occurs frequently in plants due to polyploidy).
Patterns of EvolutionPatterns of Evolution
Adaptive Radiation - Adaptive Radiation - Can occur in a relatively short time when one species gives rise to many different species in response to the creation of new habitat or some other ecological opportunity.Follows large-scale extinction events
CoevolutionCoevolutionThe relationship between two species might be so close that the evolution of one species affects the evolution of the other species.Mutualism – occurs when two species benefit each other. (Moth and the comet orchid it pollinates exist in a mutualistic relationship).Coevolutionary arms race – one species can evolve a parasitic dependency on another species.
Example: A plant and an insect pathogen that is dependent on the plant for food. The plant evolves a chemical defense; the insect evolves a biochemistry to resist the defense; response continues to escalate.
Convergent EvolutionConvergent Evolution
Unrelated species evolve similar traits even though they live in different parts of the world.
Occurs in environments that are geographically far apart but have similar ecology and climate.
Rate of SpeciationRate of Speciation
Evolution proceeds in small, gradual steps according to a theory called gradualism.Punctuated equilibrium explains rapid spurts of genetic change causing species to diverge quickly.
