Chapter 5Evolution & Gene

Frequencies

Populations & Gene Pools

• Evolution-def-described as any change in the frequency of alleles, & resulting phenotypes, in a population.

• Population-def-consists of the animals in a particular place that could interbreed

• Populations evolve as gene frequencies change over time

• Change in frequency of alleles in gene pool indicates the presence of evolutionary change

Hardy-Weinberg Theorem

• Hardy-Weinberg theorem-states that when certain assumptions are met, the frequency of alleles in a population will not change over time = no evolution

• 4 assumptions of Hardy-Weinberg Theorem:

1. Population size must be large to prevent the change in allelic frequencies by chance alone

2. Mating must be random3. Migration must not occur, as it would

add/delete alleles from gene pool4. Mutation must not occur, or mutational

equilibrium must exist

Hardy-Weinberg Theorem

• The assumptions of the Hardy-Weinberg theorem are not typically met in natural populations most populations are evolving

• Neutral Changes-Some of the features that might be changing which have no advantage to the organisms

Evolutionary Mechanisms-Population size & Genetic Drift & Neutral Evolution

• Evolution can result in some individual surviving & being more effective at reproducing than others in a population

• The smaller a pop. size the more significant chances of change to occur

• Genetic drift-def-chance events influencing the frequencies of genes in a population

• Neutral Evolution-def-gene frequencies change independent of natural selection & b/c of this genetic drift is neutral evolution

Evolutionary Mechanisms-Population size & Genetic Drift & Neutral Evolution

• Genetic drift is like flipping a coin:– Lrg sample = closer to 50:50 ration– Sm Sample can =:

• Unusual proportions of alleles due to randomness• Inbreeding can be common genetic drift & inbreeding will likely reduce genetic

variation w/in pop.

• Mutations & genetic drift– If a mutation of an allele gets introduced into a pop. & it

doesn’t make the allele more or less adaptive then the new allele could be:

• Established in the pop.• Or it could be lost in the pop. due to genetic drift

• If genetic drift can occur in sm. pop. Then Hardy-Weinberg equilibrium can’t happen

Evolutionary Mechanisms-Population size & Genetic Drift & Neutral Evolution

• Special Cases of Genetic Drift:– Founder Effect

• Founder effect-def-new pop. emerges from founding individual(s) are more likely to have a distinct genetic make-up w/ less variation in the pop. than a lrg’er pop.

• Founder effect is seen when a sm. Subpop. Fragments from the main pop. & colonizes new habitat

• Often seen on islands & previous uninhabited habitats• Ex/ the Afrikaner population Huntington’s disease• Ex/Amish Community polydactyly • Ex/Pingelan island community total colorblindness

Evolutionary Mechanisms-Population size & Genetic Drift &

Neutral Evolution• Special Cases of Genetic Drift:– Bottleneck Effect-def-pg.69-changes in gene frequency

that result when numbers in a population are drastically reduced as a result of the population being built up again from relatively few surviving individuals

• Ex/Cheetah populations in South & East Africa• Ex/elephant seal in late 1800s

– Increase numbers now however low genetic variability

• Ex/ Human intentions are to revive endangered populations of organisms. Where can you see this becoming a problem?

Evolutionary Mechanisms-Gene Flow &

Mutations• Gene flow-def-pg71- changes in relative allelelic frequencies from migration of inidividuals– Individuals will immigrate into a population– Individuals will emigrate out of a population Hardy-Weinberg theorem assumptions don’t apply &

populations are evolving• Gene Flow Effects can be different:

– Increase in Gene flow between 2 populations =s change in a population

• Ex/island & continental population can affect the genetic make-up of both populations eventually leading to genetic make-up becoming similar

– Lack of Gene flow between 2 populations will make changes in a population be less likely

• Ex/ African elephants-tropical forest elephants vs. savannah elephants

Evolutionary Mechanisms-Gene Flow &

Mutations• Mutations– Source of variation that can prove adaptative for organisms– Counters loss of genetic material from genetic drift & natural

selection– Increase probability that variations will be present to allow future

generations to survive shocks to the environment– Mutations make extinction less likely– Mutations are random events& aren’t affect by mutations’

usefulness– Organism’s can filter out good mutations from bad ones– Most mutations are deleterious– Depending on the environment can be harmful/neutral– Mutational equilibrium-know this concept

• It rarely happens

– Mutation pressure-a measure of the tendency for gene frequencies to change through mutations

Natural Selection Reexamined-Mode of

Selection• Selection pressure-tendency for natural

selection to occur & upset the Hardy-Weinberg Equilibrium

• Modes of Selection:– Many phenotypes are spread out over bell

shaped curve– Natural selection can affect a range of

phenotypes in (3) ways:• Directional selection• Stabilizing selection• Disruptive selection

Natural Selection Reexamined-Mode of

Selection• Modes of Selection:

– Directional Selection- occurs when individuals at one phenotypic extreme are at a disadvantage compared to all other individuals in the population• Deleterious genes decrease in frequency & all

other genes increase in frequency• Can happen when

– mutation gives rise to new gene– Environment changes to select against a phenotype

• Ex/ Industrial Melanism

Natural Selection Reexamined-Mode of

Selection• Modes of Selection:

– Disruptive Selection- circumstances selecting against individual of an intermediate phenotype• Produces distinct subpopulations• Ex/snails of (2) colors in tidepools

– Stabilizing Selection-when both phenotypic extremes are deleterious this leads to narrowing of the phenotypic range• Ex/ horseshoe crab- found on the Atlantic Coast

Balanced Polymorphism

• Polymorphism-occurs in a population when 2 or more distinct forms exist w/o a range of phenotypes between them.

• Balanced Polymorphism-occurs when different phenotypes are maintained at relatively stable frequencies in the population & may resemble a population in which disruptive selection operates

Heterozygote Superiority

• What is heterozygote superiority?– When the heterozygote is more fit than

the either homozygous organism to survive in the given environment.

– This can lead to balanced polymorphism which can lead to speciation

– Ex/Sickle Cell anemia

Species & Speciation

• Fundamental unit of classification= species• Taxonomists classify species based on:

– Similarities– differences

• Species-a group of population in which genes are actually & potentially exchanged through interbreeding– This definition causes taxonomists problems:

• Morphological characteristics • Reproductive criterion must be assumed based on

morphological & ecological information • Fossil material

Species & Speciation• Taxonomists generally incorporate the following into their

categorization:– Morphology Criterion– Physiology Criterion– Embryology Criterion– Behavioral Criterion– Molecular Criterion– Ecological Criterion

• What is speciation?– The formation of a new species

• Only happens when a subpopulation can’t interbreed• when gene flow doesn’t happen between population &

subpopulation

Species & Speciation

• How can speciation happen?– Reproductive isolation-def-when a

populations are reproductively isolated, natural selection & genetic drift can result in evolution taking a different course in each subpopulation.

– Types of Reproductive Isolation:• Premating Isolation:

1. Impenetrable barriers2. Different mating behavior3. Different breeding periods4. Different habitats

Species & Speciation– Types of Reproductive Isolation:

• Post mating Isolation- prevents successful fertilization & development even though mating can occur:

1. Hybrids-usually sterile2. Mismatched chromosomes3. Developmental failures of fertilized egg & embyro

-Types of speciation:• Allopatric speciation-def-occurs when subpopulation

become geographically isolated from one another– Most common type of speciation– Ex/ Galapagos Finches

» Combined forces of natural selection, mutation, isolation

Species & Speciation– Types of Speciation

• Parapatric speciation-def-pg75-occurs in small, local population called demes

• Demes-areas that are not completely isolated from each other– Members w/in demes experience different selection

pressures speciation can occur– i.e. tidepools, ponds,etc.– This is theoretical & has not been observed

no known examples

• Individuals w/in demes more likely to reproduce with each other than those outside of demes

Species & Speciation

• Types of Speciation:– Sympatric speciation- speciation that

occurs w/in a single population in which a new species develops when members of a population develop a genetic difference that prevents then from reproducing w/members of original species

– Happens most often in plant species

Rates of Evolution

• Phyletic Evolution-def-pg75-the idea that evolutionary changes occur at a slow, constant pace over millions of years– Periods of stasis = equilibrium which result in stabilizing

selection

• Periods of stasis (a.k.a.equilibrium) can be interrupted by geological/climate/habitat change– These changes can cause some evolutionary

changes to happen rapidly• These cause disruptive & directional selection to occur

– These rapid changes “punctuates” the equilibrium results in the Punctuated equilibrium model

Rates of Evolution

• Punctuated Equilibrium Model-def-pg76-long periods of stasis interrupted by brief periods of change– Rapid evolutionary changes have been

observed in sm populations• Ex/pest acquiring resistance to pesticides• Ex/ bacteria acquiring resistance to antibiotics

– The punctuated equilibrium model is can be used to explain the gaps in the fossil records between organisms that may not have a transitional stage

Rates of Evolution

• Molecular evolution & Gene duplication– Molecular evolution-def-involves all

evolutionary changes which results from changes in the base sequence in DNA and/or the amino acids sequence in proteins

– Scientists study the base sequences & protein sequences of organisms to see if they are highly conserved (closer evolutionary relationship) vs. not highly conserved (further evolutionary relationship)

• But scientists compare many proteins or genes

• Ex/ Cytochrome c

Rates of Evolution

• Molecular Evolution & Gene Duplication– Gene duplication-def- the accidental

duplication of a gene on a chromosome– So how does gene duplication fit in with

molecular evolution?• As long as there is a good copy of the gene it

should work in the organism this can lead to extra genetic material which can cause an organism to evolution at a molecular further along the evolutionary timeline

Rates of Evolution

• Gene Duplication– Ex/ hemoglobin vs. myoglobin

• Mosaic Evolution-def-a change in a portion of an organism while the basic form of the organism is retained– Ex/Birds

• basic body type-highly conserved• Particular parts of birds are rapidly changing

– beaks, wing modification, legs