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Transcript
16-2 Evolution as Genetic Change Natural selection NEVER acts directly on genes An entire organism, survives, reproduces Natural selection determines which organisms survive to reproduce Organisms dies before reproduction it does not pass its alleles onto subsequent generations Organisms reproduce – alleles pass on to subsequent generations EVOLUTION = change over time in the relative frequencies of alleles in a POPULATION Populations evolve; not organisms Natural Selection on Single Gene Traits Natural selection on single gene traits can lead to changes in allele frequency which leads to evolution Example: brown lizards with red and black forms(due to mutations) Red lizards more visible to predation – die allele for red less common Black lizards – more radiation – faster, avoid predation better than brown – allele becomes more frequent If allele has no effect on “fitness” allele is not under pressure from natural selection Natural Selection on polygenic traits Natural selection can affect the distributions of phenotypes in any of three ways: directional selection, stabilizing selection, disruptive selection Directional selection: individuals on one end of curve have higher fitness than opposite end or middle of curve – big beak birds able to crack large seeds – if food shortage causes only large size seeds to be available – big beak birds survive – population favors big beak Stabilizing selection: individuals in center of curve have higher fitness – change of curve doesn’t change much – human infants – small babies, low survival, big babies low survival (difficulty being born) average size babies – higher fitness Disruptive selection – opposite ends of curve favor selection – results in 2 sub group populations Genetic drift In small populations the expected results of genetic crosses is not what is expected – random change in allele frequency = genetic drift In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just BY CHANCE. Over time these CHANCE occurances cause an allele to become more common Founder effect: small group of individuals from original population colonize a new habitat. – these carry different alleles from original population – create new populations of organism – members of population of different habitat have allele frequencies different from parent population Evolution versus genetic equilibrium To understand how evolutionary change operates look at what happens when no change takes place Identify conditions underwhich NO EVOLUTION will occur Hardy-Weinberg principle – allele frequencies in a population will remain constant unless one or more factors cause the frequencies to change Genetic equilibrium – when no change occurs in the allele frequencies 5 conditions to maintain genetic equilibrium o Must be random mating o Population must be very large o no immigration or emigration o no mutations o no natural selection Hardy Weinberg equation o p2 + 2pq + q2 = 1 o p = frequency of dominant allele o q = frequency of recessive allele o pq = hybrid o Process of speciation: o how does natural selection or change in allele frequencies result in speciation? o Species – a group of organisms that breed with one another and produce fertile offspring – share a common gene pool o Gene pools of two populations must become separated for them to become new species o REPRODUCTIVE ISOLATION – as new species evolve, populations become reproductively isolated – cannot interbreed successfully REPRODUCTIVE ISOLATION – due to: 1. behavioral isolation (different mating songs) 2. geographic location – break up of Pangea (ostrich, rhea, emu) 3. temporal isolation – reproduction is at different times – different species of orchids release pollen only on a single day – thus the pollen cannot pollinate another species Speciation in Darwin’s Finches o founders arrive (from mainland – Ecuador) o geographic isolation – members of founders move to another island o changes in gene pool – each island has specific environmental pressurs – favoring certain characteristics – changes in allele frequencies o reproductive isolation – beak “fetishes” among finches o ecological competition – competition for available resources “selects” traits o continued evolution – over many generations until 13 different species established themselves Understanding evolution is important because evolution drives changes in the world of drug resistance in bacteria and viruses, and pesticide resistance in plants Evolution of disease resistant bacteria A gene on bacteria mutated – causing bacteria to 1) destroy antibiotic or 2) hinder effectiveness of antibiotic o result – a class of antibiotic resistant bacteria – named “R plasmids” R = resistant o should use of antibiotics be restricted???