Unformatted text preview: Biology in the News [see folder on BB] •
• • • • • ants have evolved social behavior ("eusociality") mul=ple =mes independently a recent eﬀort has begun to sequence the genomes of several ant species to try to uncover the evolu=onary gene=c basis of the evolu=on of eusociality some of the most important features of the eusociality of ants, bees, and termites are diﬀeren=a=on of roles (castes) within the hive (queen, drone, worker, etc.) as well as the ability to communicate informa=on between individual members (e.g. when bees do a dance to indicate to other bees the direc=on of nectar sources) red harvester ants (Pogonomyrmex barbatus) ‐ have more genes associated with smell (olfactory genes) than other insect genomes; consistent with ants using chemical communica=on genes encoding components of the immune system are very diﬀerent from other insects ‐ possibly rela=ng to the need for crowded hives to ﬁght disease outbreaks it has also been found that genes regula=ng the development of wings and ovaries in ants show diﬀerences from other insects possibly rela=ng to their need to be expressed diﬀerently between the diﬀerent castes hHp://www.nsf.gov/news/news_summ.jsp?
Pogonomyrmex_barbatus,_worker,I_ANT123.jpg varia=on at the protein and DNA sequence level • in the 1960s‐1980s it became apparent that molecular varia=on within popula=ons and species was very, very common • The idea that natural selec=on might have liHle or no eﬀect on this varia=on came from the above observa=on – this varia=on could mostly be “adap=vely neutral” • many amino acid subs=tu=ons might be func=onally equivalent – don’t change the charge or shape of a protein domain • very many DNA nucleo=de subs=tu=ons might be selec=vely neutral – noncoding regions – synonymous subs=tu=ons in coding regions Evolu=on of neutral muta=ons (correc&on in green boldface) •
• • • Motoo Kimura in 1968 proposed that most molecular varia=on in natural popula=ons is neutral: Neutral Theory of Molecular Evolu&on Selec=on has no eﬀect on this varia=on; evolu=on is by gene&c dri: ‐ Recall “boHlenecks” and other examples from previous lecture Muta=on rate: µ In diploids: # gene copies =2N (N= popula=on size) Average number of new muta=ons each genera=on: =µ•2N Probability that each new neutral muta=on will increase to 100% = its frequency =1/2N Overall frequency of new muta=ons increasing to 100% =µ•2N * 1/2N = µ , the muta=on rate (popula=on size doesn’t maHer) • Larger popula=ons have more muta=ons than smaller popula=ons • Smaller popula=ons have fewer muta=ons but each one is more likely to go to 100% than in a larger popula=on Constancy of neutral muta=ons going to 100% (ﬁxa=on) leads to the idea of a molecular clock – Can use molecular data to es=mate the =me two species have been diverging Selec=on at the molecular level • Recall two types of selec=on from previous lecture – Posi=ve (direc=onal) and stabilizing • Neutral evolu=on – Rate of synonymous subs=tu=on approx. = rate of nonsynonymous subs=tu=on • Stabilizing selec=on (selec=on for no change) – Rate of synonymous subs=tu=on > rate of nonsynonymous subs=tu=on • Posi=ve (direc=onal) selec=on (selec=on promotes change in amino acid sequence) – Rate of synonymous subs=tu=on << rate of nonsynonymous subs=tu=on Evolu=on of foregut fermenta=on • Posterior esophagous or stomach modiﬁed to hold bacteria, which digest plant maHer (cellulose) via fermenta=on • Evolved independently in primates (langurs) and ruminants (caHle) • Also evolved independently in one lineage of birds (hoatzin) Molecular evolu=on of lysozyme in foregut fermenters •
• Most animals, defensive enzyme (digests bacterial cell walls) In foregut fermenters, modiﬁca=ons of lysozyme have evolved – Enables some foregut bacteria to release nutrients, which are absorbed as food • 5 amino acid changes have evolved independently in both langurs and caHle • Some of these have also evolved in hoatzins Varia=on in gene number among species • Why? – Complexity – Other reasons? – What about coding vs. noncoding DNA? Varia=on in the amount of noncoding DNA • Why? – Diﬀering rates of loss of nonfunc=onal DNA – Rela=onship between DNA content and reproduc=ve rate genomes have a lot of genes • where do they come from? • new genes are only very rarely “invented” – could be from purng together pieces of previously exis=ng genes • many genes arise from duplica=on and diversiﬁca=on of pre‐exis=ng genes – gene families hHp://www.pnas.org/content/98/19/10763/F1.large.jpg Gene duplica=on 1 specia=on 1 1 Gene duplica=on SPECIES 1 Ancestral copy 2 1a 2 SPECIES 2 • Genes 1 and 1a are paralogs of each other (paralogous) • Genes 1 and 2 are orthologs (orthologous) • Genes 1a and 2 are orthologs (orthologous) • All 3 genes are homologous to each other (less speciﬁc term) Evolu=on of gene families (#s are es=mated # of DNA sequence changes) What is a phylogeny? • Also called a phylogene&c tree • Illustrates the hypothesized rela=onships among a set of taxa (singular = taxon) More recent !me ancient ...
View Full Document
- Spring '08
- Biology, synonymous subs=tu=on, neutral muta=ons, new neutral muta=on, previously exis=ng genes, Muta=on rate