Genetic Drift and Gene Flow Illustration

Transcription

1 Gntic Drift and Gn Flow Illustration This is a mor dtaild dscription of Activity Ida 4, Chaptr 3, If Not Rac, How do W Explain Biological Diffrncs? in: How Ral is Rac? A Sourcbook on Rac, Cultur, and Biology. Carol C. Mukhopadhyay, Rosmary Hnz, and Yolanda T. Moss. 2 nd Edition AltaMira Prss. Ths activitis wr originally dvlopd by or adaptd for us hr in 2006 by Scott Smith, thn a graduat studnt, Dpartmnt of Anthropology, Univrsity of California, Rivrsid. Scott Smith Gntic Drift Illustration Objctiv: To illustrat th ffcts of gntic drift on alll frquncis in a population. Concptual Ovrviw Gntic drift is on of th four forcs of volution and is dfind as th fluctuation of alll frquncis du to random factors. Gntic drift is strongly tid to population siz and bcoms influntial in situations whr a small sgmnt of an original population splits off or bcoms isolatd from th parnt population. Somtims this occurs whn a small group founds a nw population in gographic isolation from th parnt population. This is rfrrd to as foundr ffct. Othr tims this procss occurs whn a parnt population undrgos a dvastating population rduction rfrrd to as a population bottlnck. Ths procsss both produc a situation whr a non-rprsntativ sampl of allls is carrid ovr to th nw population. For xampl, assum a parnt population of

2 1000 individuals and at on particular locus ach individual has on of two forms of a gn, B or b. In th original parnt population 700 individuals hav th B alll and 300 hav th b alll. Th frquncy thn of th B alll is.7 and th frquncy of th b alll is.3. Say 200 individuals bcom angry with th parnt population and dcid to lav and start a nw sttlmnt far away from th original parnt population. Just by chanc 180 of ths individuals hav th B alll and only 20 hav th b alll. Th nw frquncy of th B alll in this population is thn.9 and th nw frquncy of th b alll is.1. In this cas bcaus a random sampl of allls was carrid ovr to a nw foundr population that is much smallr than th original parnt population, alll frquncis wr altrd. Sinc volution is dfind as a chang in alll frquncis ovr tim w s how gntic drift can b an important volutionary forc. Matrials (pr group of studnts) Dry bans of two diffrnt colors (thy should b as clos to th sam siz as possibl) (othr matrials such two diffrnt color marbls may b usd altrnativly) A st of masuring cups A larg bowl A caftria tray or a pan of som sort on which to sort bans (somthing with dgs is usful so th bans don t go shooting around th room) Instructions 1) Divid studnts into groups 2) Explain that th bans rprsnt two diffrnt allls (g. B and b) at on particular locus on a chromosom. You might say that ths bans rprsnt th

3 gn for tongu-rolling ability (which is a discrt trait). On typ of ban rprsnts th tongu-rollrs and th othr th non-tongu-rollrs. You can us any discrt trait hr though. 3) Hav studnts tak a cup of ach typ of ban and mix thm in th larg bowl. Explain to th studnts that sinc th allls ar about th sam siz and th sam volum was takn of ach thr should b th sam numbr of ach alll in th bowl (if you want to tak th tim you can hav ach group count out vn numbrs of bans but ach group should us about a cup of ach typ of ban). Th bans now in ach group s bowl ar th original parnt population of allls and th frquncy of ach typ of ban (alll) is.5. 4) In ordr to simulat th ffcts of gntic drift studnts will bgin taking sampls of bans (allls) and rcording th counts and frquncis of ach typ of ban on a data tabl (s xampl tabl blow). Explain to studnts that ach sampl thy tak rprsnts a group of individuals who hav bcom angry with th original parnt population and hav dcidd to lav and sttl somwhr ls. 5) Groups will bgin by taking a ½ cup sampl (25%) of th original parnt population and counting th numbr of ach typ of ban (alll). Studnts should rcord this numbr in thir data tabl and thn calculat th frquncy of ach typ of ban by dividing th numbr of ach typ by th total numbr of bans in th ½ cup sampl. Explain that th two frquncis should add up to 1.0 (100% of th ½ cup sampl). Th frquncis of ach typ of ban (alll) in this sampl will probably b clos to that of th original parnt population of bans (1 cup of ach).

4 6) Studnts should continu by taking a ¼ cup sampl and rcording ban counts and frquncis. Following this a 50 ban sampl and thn thr 20 ban sampls should b takn. All sampls should b takn without looking at th bans so as to assur that th sampl is as random as possibl. 7) Aftr all groups hav finishd counting and rcording thir sampls ask studnts to discuss thir rsults among thmslvs and rport back to th class. Studnts should find th ban (alll) frquncis vary in gnral mor dramatically from th original parnt population frquncis whn a smallr sampl is takn. Sinc volution at th population lvl is dfind as a chang in alll frquncy ovr tim w can s how gntic drift can b a strong volutionary forc in crtain situations. Explain to studnts that in th nw foundr populations (th 20 ban sampls) alll frquncis can fluctuat mor dramatically and it is mor likly that a locus will bcom fixd. That is, it is mor likly that on particular alll will vanish from th population. In othr words, if thr ar 1000 bans and w switch 1 ban from on typ to anothr (simulating for xampl th ffcts of rcombination), it rprsnts a.1% chang in th composition of th population. Howvr, if th population is only 10 bans as opposd to 1000, switching 1 ban from on typ to anothr rprsnts a 10% chang in th composition of th population. Hr w s that a smallr population siz can account for a much mor dramatic shift in th composition of th population.

5 Activity Extnsion Gn Flow This can b xtndd to illustrat anothr volutionary forc, gn flow. Rcall that gn flow is th xchang of gns btwn populations. Unlss th two populations hav xactly th sam frquncis of a particular gn th ovrall composition of th rsulting population will b altrd. Rmmbring that th volution is dfind as a chang in gn frquncis ovr tim w s how gn flow can b an volutionary forc. W can xtnd th gntic drift activity to dmonstrat th ffcts of gn flow on gn frquncis. 1) Ask ach group of studnts to sav thir last 20 ban (alll) sampl. 2) Each group of studnts from th gntic drift activity should combin with anothr group to form a largr group. Each largr group should now hav two distinct 20 ban sampls and a rcord of th ban (alll) frquncis for thir sampl. 3) Explain that ths sampls ar gntically distinct populations. It is important to mphasiz that ths gntically distinct populations do not rprsnt diffrnt spcis and that th two distinct thortical populations can intrbrd. 4) Explain that our angry populations from th gntic drift activity which split away from th original parnt population in that activity hav all ncountrd anothr split off population and dcidd to liv togthr. In ach larg group of studnts th two 20 ban sampls rprsnt th angry populations who split off from th original parnt populations. Sinc now, for ach group of studnts, th two populations bcom ffctivly on brding population, tll studnts to combin

6 both 20 ban sampls into on bowl which maks on population of 40 bans. Studnts should count th numbr of ach typ of ban (alll) for this nw population and dtrmin th frquncis of ach. Studnts should thn compar ths nw ban (alll) frquncis with thos of th two original 20 ban populations and not how thy hav changd. 5) Ask studnts to discuss ths nw rsults among thmslvs and thn rport back to th class. Exampl of Data Tabl Ban Typ 1 (alll B) Count Original Parnt Population ½ Cup ¼ Cup 50 Ban 20 Ban 20 Ban 20 Ban Frquncy Ban Typ 2 (alll b) Count Frquncy