BIOLOGY 101. CHAPTER 23: Evolution of Populations: Alleles Change

Transcription

1 BIOLOGY 101 CHAPTER 23: Evlutin f Ppulatins:

2 Evlutin f Ppulatins: CONCEPTS: 23.1 Genetic Variatin Makes Evlutin Pssible 23.3 Frces that can alter allele frequencies in a ppulatin 23.4 Natural selectin is the nly mechanism that cnsistently drives adaptive evlutin

3 Evlutin f Ppulatins: 23.1 Genetic Variatin Makes Evlutin Pssible What is it that evlves? A species is the minimal living unit that can evlve A species is a grup f interbreeding rganisms that are reprductively islated frm ther similar rganisms

4 Evlutin f Ppulatins: 23.1 Genetic Variatin Makes Evlutin Pssible Cmmn Descent With Mdificatin states that ppulatins change ver time, and that these changes are inherited

5 Evlutin f Ppulatins: 23.1 Genetic Variatin Makes Evlutin Pssible Cmmn Descent With Mdificatin states that ppulatins change ver time, and that these changes are inherited and smetimes lead t the frmatin f new species Micrevlutin is defined as a change in allele frequencies in a ppulatin ver time. Macrevlutin is when these changes lead t frmatin f a new species Natural Selectin is the frce that acts upn these changes (variatin) and favrs traits that give a reprductive advantage

6 Evlutin f Ppulatins: 23.1 Genetic Variatin Makes Evlutin Pssible There is nly ne frce that can change alleles Mutatin is a permanent change t the nucletide sequence f DNA This can ccur due t chemical agents, irradiatin r errrs in replicatin / repair f DNA In multicellular rganisms, nly mutatins in cell lines that frm gametes can be passed n t ffspring Mutatin can ultimately have a large effect n allele frequencies when it prduces new alleles that strngly influence fitness in a psitive r negative way What is it that causes allele frequencies t change in ppulatins?

7 Evlutin f Ppulatins: 23.1 Genetic Variatin Makes Evlutin Pssible A pint mutatin is a change f a single base in a gene. Pint mutatins can have a significant impact n phentype, as in the case f sickle-cell disease. Mst pint mutatins are harmless: - why? Much f the DNA in eukarytic genmes des nt cde fr prtein prducts. Because the genetic cde is redundant, sme pint mutatins in genes that cde fr prteins may nt alter the prtein s amin acid cmpsitin. (aka Silent Mutatin) Even if there is a change in an amin acid as a result f a pint mutatin, it may nt affect the prtein s shape and functin. On rare ccasins, a mutant allele may actually make its bearer better suited t the envirnment, increasing its reprductive success.

8 Evlutin f Ppulatins: 23.1 Genetic Variatin Makes Evlutin Pssible Alteratins in gene number r psitin Chrmsmal mutatins that delete, disrupt, r rearrange many lci at nce are usually harmful. In rare cases, chrmsmal rearrangements may be beneficial. Fr example, the translcatin f part f ne chrmsme t a different chrmsme culd link genes that act tgether fr a psitive effect.

9 Evlutin f Ppulatins: 23.1 Genetic Variatin Makes Evlutin Pssible Gene duplicatin is an imprtant surce f new genetic variatin. Duplicatin may ccur due t errrs in meisis, slippage during DNA replicatin, r the activities f transpsable elements. Duplicatins f large chrmsme segments are ften harmful, but the duplicatin f small pieces f DNA may nt be. Example: mammalian ancestrs carried a single gene fr detecting drs that has been duplicated many times. Mdern humans have abut 1,000 lfactry receptr genes and mice have 1,300. Dramatic increases in the number f lfactry genes benefited early mammals, enabling them t detect faint drs and distinguish amng smells. Because f mutatins, 60% f these genes have been inactivated in humans. Mice, which rely mre n their sense f smell, have lst nly 20% f their lfactry receptr genes

10 Evlutin f Ppulatins: 23.1 Genetic Variatin Makes Evlutin Pssible HOX Gene Mutatins can have a dramatic affect n bdy plans and rganizatin Hx prteins cntrl the type f segment structures (e.g. legs, antennae, and wings; r the different types f vertebrae in humans) that will frm n a given segment. They cntrl segmental identity, but d nt frm the actual segments themselves Hx genes prduce Transcriptin Factrs and can enhance the transcriptin f sme genes and repress thers. They regulate genes that cntrl large netwrks f ther genes that ultimately frm the tissues, structures, and rgans f each segment. Fr example, variatins in Hx gene expressin are respnsible fr the frmatin f different frelimbs in vertebrates. Hx Gene mutatins can explain the transfrmatin f ne limb type t anther

11 Evlutin f Ppulatins: 23.1 Genetic Variatin Makes Evlutin Pssible Rearrangement and Distributin f Existing Alleles Sexual Reprductin: In rganisms that reprduce sexually, mst f the genetic variatin in a ppulatin results frm the unique cmbinatin f alleles that each individual receives frm its parents. Three mechanisms cntribute t this shuffling: 1. crssing ver, 2. independent assrtment f chrmsmes, and 3. Fertilizatin (unin f 2 gametes)

12 Evlutin f Ppulatins: 23.3 Frces that can alter allele frequencies in a ppulatin Frces that Cause Allele Frequencies t Change in ppulatins Gene Drift is when a catastrphe leads t the separatin f a large ppulatin int smaller fragments (generally resulting in a dramatic reductin in the number f alleles within a ppulatin) Gene Flw is when ppulatins merge (generally intrducing new alleles t a ppulatin) Gene flw can als reduce certain alleles in sme cases Natural Selectin acts upn different (existing) alleles, imprving the match between rganisms and their envirnment. Natural selectin is nt a direct change t alleles r allele frequencies, but acts upn changes t alleles r allele frequencies!

13 Evlutin f Ppulatins: 23.3 Frces that can alter allele frequencies in a ppulatin Frces that Cause Allele Frequencies t Change Genetic drift, resulting in smaller fragmented ppulatins may ccur as a result f tw situatins: the funder effect r the bttleneck effect The funder effect ccurs when a new ppulatin is started by a small number f individuals wh d nt represent the gene pl f the larger surce ppulatin. At an extreme, a ppulatin culd be started by a single pregnant female r a single seed with nly a tiny fractin f the genetic variatin f the surce ppulatin. The funder effect is due t a separatin frm a larger ppulatin

14 Evlutin f Ppulatins: 23.3 Frces that can alter allele frequencies in a ppulatin Frces that Cause Allele Frequencies t Change Genetic drift, resulting in smaller fragmented ppulatins may ccur as a result f tw situatins: the funder effect r the bttleneck effect The funder effect ccurs when a new ppulatin is started by a small number f individuals wh d nt represent the gene pl f the larger surce ppulatin. At an extreme, a ppulatin culd be started by a single pregnant female r a single seed with nly a tiny fractin f the genetic variatin f the surce ppulatin. The funder effect is due t a separatin frm a larger ppulatin this ften results in inbreeding and the appearance f adverse recessive alleles

15 Evlutin f Ppulatins: 23.3 Frces that can alter allele frequencies in a ppulatin Frces that Cause Allele Frequencies t Change Genetic drift, resulting in smaller fragmented ppulatins may ccur as a result f tw situatins: the funder effect r the bttleneck effect The bttleneck effect ccurs when the numbers f individuals in a large ppulatin are drastically reduced by a disaster (death) By chance, sme alleles may be verrepresented and thers underrepresented amng the survivrs. Sme alleles may be eliminated altgether. The bttleneck effect and funder effect bth act t reduce the number f cmmn alleles in a ppulatin

16 Evlutin f Ppulatins: 23.3 Frces that can alter allele frequencies in a ppulatin Frces that Cause Allele Frequencies t Change Gene flw is the transfer f alleles amng ppulatins due t the migratin f fertile individuals r gametes. (new alleles are generally intrduced) Fr example, if a nearby wildflwer ppulatin cnsisted entirely f white flwers, its pllen culd be carried int ur hypthetical ppulatin. The result wuld be an increase the frequency f new alleles in the hypthetical ppulatin in the next generatin. Gene flw tends t reduce differences between ppulatins. If extensive enugh, gene flw can cmbine neighbring ppulatins int a single ppulatin with a cmmn gene pl.

17 Evlutin f Ppulatins: 23.3 Frces that can alter allele frequencies in a ppulatin Frces that Cause Allele Frequencies t Change Natural Selectin is based n differential survival and reprductive success Individuals in a ppulatin vary in their heritable traits. Individuals with variatins better suited t the envirnment tend t prduce mre ffspring As a result f selectin, alleles are passed n t the next generatin in frequencies different frm thse in the present ppulatin.

18 Evlutin f Ppulatins: 23.3 Frces that can alter allele frequencies in a ppulatin Frces that Cause Allele Frequencies t Change Natural Selectin is based n differential survival and reprductive success By cnsistently favring sme alleles ver thers, natural selectin can cause adaptive evlutin (evlutin that results in a better match between rganisms and their envirnment)

19 Evlutin f Ppulatins: 23.4 Natural selectin is the nly mechanism that cnsistently causes adaptive evlutin Frces that Cause Allele Frequencies t Change Natural selectin is the nly evlutinary mechanism that cnsistently leads t adaptive evlutin. Genetic drift can cause the frequency f a slightly beneficial allele t increase, but it als can cause the frequency f such an allele t decrease. Gene flw may intrduce alleles that are advantageus r nes that are disadvantageus. Mutatin may change alleles in a way that may be either advantageus r disadvantageus

20 Evlutin f Ppulatins: 23.4 Natural selectin is the nly mechanism that cnsistently causes adaptive evlutin Three mdes f Natural Selectin: Natural selectin can alter the frequency distributin f heritable traits in three ways, depending n which phentypes in a ppulatin are favred. Directinal Selectin Disruptive Selectin Stabilizing

21 Evlutin f Ppulatins: 23.4 Natural selectin is the nly mechanism that cnsistently causes adaptive evlutin Three mdes f Natural Selectin: Directinal selectin is mst cmmn during perids f envirnmental change r when members f a ppulatin migrate t a new habitat with different envirnmental cnditins. Directinal selectin shifts the frequency curve fr a phentypic character in ne directin by favring individuals wh deviate frm the average. Fr example, a greater number f dark mice survived in an envirnment with dark rcks

22 Evlutin f Ppulatins: 23.4 Natural selectin is the nly mechanism that cnsistently causes adaptive evlutin Three mdes f Natural Selectin: Disruptive selectin is when envirnmental cnditins favr individuals at bth extremes f the phentypic range ver thse with intermediate phentypes. Fr example, an envirnment with scattered black and white rcks wuld favr bth black and white mice Intermediate (gray) mice wuld nt be favred Gray (intermediate) mice wuld nt be selected

23 Evlutin f Ppulatins: 23.4 Natural selectin is the nly mechanism that cnsistently causes adaptive evlutin Three mdes f Natural Selectin: Stabilizing selectin favrs intermediate variants and acts against extreme phentypes. Stabilizing selectin reduces variatin and maintains the status qu fr a trait. An example wuld be a granite envirnment that is gray and favrs gray mice. Bth White and Black mice wuld stand-ut and wuld nt be selected

24 Speciatin: Reprductive Islatin Reprductive Islatin Leads t Speciatin Cmmn Descent With Mdificatin states that ppulatins change ver time, and that these changes are inherited and smetimes lead t the frmatin f new species Micrevlutin is defined as a change in allele frequencies in a ppulatin ver time. Macrevlutin is when these changes lead t frmatin f a new species Natural Selectin is the frce that acts upn these changes (variatin) and favrs traits that give a reprductive advantage. This can lead t adaptive evlutin

25 Speciatin: Reprductive Islatin Reprductive Islatin Leads t Speciatin Cmmn Descent With Mdificatin states that ppulatins change ver time, and that these changes are inherited and smetimes lead t the frmatin f new species Micrevlutin is defined as a change in allele frequencies in a ppulatin ver time. Macrevlutin is when these changes lead t frmatin f a new species Natural Selectin is the frce that acts upn these changes (variatin) and favrs traits that give a reprductive advantage. This can lead t adaptive evlutin

26 Speciatin: Reprductive Islatin Reprductive Islatin Leads t Speciatin What is it that evlves? A species is the minimal living unit that can evlve A species is a grup f interbreeding rganisms that are reprductively islated frm ther similar rganisms

27 Speciatin: Reprductive Islatin Reprductive Islatin Leads t Speciatin The strngest and mst cmmn starting pint fr speciatin is Allpatric Speciatin Allpatric Speciatin is due t Extrinsic Reprductive Islating Mechanisms Gegraphic Islatin is respnsible fr allpatric speciatin and is the result f physical barriers t mating (muntain ranges, ceans, cntinental separatin, etc.) There is n strnger barrier t mating than physical separatin! Allpatric speciatin ften leads t Sympatric Speciatin Sympatric speciatin is due t Intrinsic Reprductive Islating Mechanisms Intrinsic Mechanisms are thse due t physilgical, anatmical r behaviral difference that prevent successful mating

28 Speciatin: Reprductive Islatin Intrinsic Reprductive Islating Mechanisms Prezygtic Barriers Habitat Islatin: Due t the selectin f habitat Behaviral Islatin: Lw sexual attractin depends n curtship behavirs Tempral Islatin: Due t seasnal differences (mating seasns) Barriers t Mating Mechanical Islatin: Incmpatible genitalia Gametic Islatin: Sperm cannt fertilize the egg Pstzygtic Barriers Hybrid Inviability: Hybrids d nt survive t reprduce Hybrid Infertility: Hybrids frm, but are sterile (Mule)