Chapter 5 Evolution & Gene Frequencies

1. Chapter 5Evolution & Gene Frequencies

2. 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

3. 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: • Population size must be large to prevent the change in allelic frequencies by chance alone • Mating must be random • Migration must not occur, as it would add/delete alleles from gene pool • Mutation must not occur, or mutational equilibrium must exist

4. 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

5. 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

6. 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

7. 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

8. 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?

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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: • Impenetrable barriers • Different mating behavior • Different breeding periods • Different habitats

19. Species & Speciation • Types of Reproductive Isolation: • Post mating Isolation- prevents successful fertilization & development even though mating can occur: • Hybrids-usually sterile • Mismatched chromosomes • 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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