The Evolution of Populations and the Origin of Species

Chapters 23-24

• Evolution happens to populations/ species over time.

• Lamarckian evolution - evolution result of change in individual in response to environment (i.e. giraffe stretching its neck to eat) - incorrect hypothesis.

http://necsi.org/projects/evolution/lamarck/lamarck/giraffes.jpg

• Population composed of many different genotypes, phenotypes because of alleles carried in population.

• Sum total of all alleles in population - gene pool; variation in gene pool - variations in individual phenotypes.

http://www.brooklyn.cuny.edu/bc/ahp/LAD/C21/graphics/C21_GenePool_2.GIF

• Measure of genetic variation in population - allele frequency of gene.

• # of copies of allele divided by total # of copies of gene in population.

http://anthro.palomar.edu/vary/images/map_of_A_blood_allele.gif

http://www.uni-kiel.de/medinfo/mitarbeiter/krawczak/folien/vorlesung2/img002.jpg

• If population does not change - Hardy-Weinberg equilibrium – no evolution.

• Frequencies change - evolution occurring.

http://www.southtexascollege.edu/crj/human%20evolution.jpg

• Conditions for population to stay at Hardy-Weinberg equilibrium:

• 1Random mating occurs.• 2Population large enough to

avoid random statistical fluctuations in frequencies.

• 3No mutation.

http://w3.dwm.ks.edu.tw/bio/activelearner/18/images/ch18c2.jpg

• 4No migration into/out of population.

• 5No natural selection.• Under conditions - free flow of

genes between members of same species.

• Alleles shuffled up from 1 generation to next.

http://www.artgame.com/images3/migration.jpg

• In wildflower population of 500, 80% (0.8) of flower color alleles are R and 20% (0.2) are r.

• Each gamete - 1 allele for flower color; gamete drawn from gene pool at random has 0.8 chance of bearing R allele, 0.2 chance of bearing r allele.

http://k43.pbase.com/u29/gaocus/upload/17965482.wildflower.jpg

• Rule of multiplication - frequencies of 3 possible genotypes in next generation.

•RR genotype - probability of picking 2 R alleles is 0.64 (0.8 x 0.8 = 0.64 or 64%).

• rr genotype - probability of picking 2 r alleles is 0.04 (0.2 x 0.2 = 0.04 or 4%).

• Heterozygous individuals - either Rr or rR - R allele from sperm or egg.

• Probability of ending up with both alleles is 0.32 (0.8 x 0.2 = 0.16 for Rr, 0.2 x 0.8 = 0.16 for rR, and 0.16 + 0.16 = 0.32 or 32% for Rr + rR).

• p = gene frequency of dominant allele, q = frequency of recessive allele; p + q = 1.

• Equation for Hardy-Weinberg principle is:

• p2 + 2pq + q2 = 1• 2pq - # heterozygotes in

population.

http://bill.srnr.arizona.edu/classes/182/GeneFreqs/HardyWeinberg-lg.jpeg

• Hardy-Weinberg equilibrium, frequency of dominant homozygous curly hair (CC) is 64%. Percentage with curly hair?

• p = frequency dominant allele (C) q frequency recessive allele (c).

• CC frequency 64% so p2 = .64; p = .8.

• (p + q = 1) q = 1 - .8 = 2.• Individual with curly hair - either CC

or Cc.• Percentage of population with curly

hair is p2 + 2pq = .64 + 2(.8*.2) = .96 or 96% of population.

Instabilities in populations

• Conditions can change Hardy-Weinberg equilibrium.

• Mutations cannot happen in equilibrium; occurs in real world.

• Errors in DNA replication accumulate over time as well as mutagenic factors in environment.

• Mutations can lead to new alleles not previously in gene pool.

http://www.sciencemuseum.org.uk/exhibitions/genes/images/1-3-5-1-2-2-2-2-2-0-0.jpg

• Mutations either neutral or harmful on survival of individual.

• New phenotypes in population raw material that natural selection acts on to drive evolution; mutations only source of new alleles.

• Migration affects equilibrium.• Different populations have

different allelic frequencies in gene pools.

http://www.geo.arizona.edu/Antevs/nats104/00lect26humigrout.jpg

• 1 population breeds with another population, frequencies of alleles change (gene flow).

• Small population more likely to have random event than large population.

http://web.pdx.edu/~mfish/image019.jpg

• Genetic drift - changes in allele frequencies in small population caused by random events.

• Even in large population if small # of individuals pass on traits can decrease diversity.

• Individuals that do not pass traits on may have harmful alleles - alters gene pool of next generation (2 ways)

• 1Bottleneck - large population reduced to small # by disease, natural disaster, over-hunting/fishing.

• Individuals left eventually reproduce, generations not representative of original gene pool.

• Inbreeding usually follows bottleneck; individuals with same recessive genes have more chance of passing harmful gene on.

• Population more susceptible to disease/infections that may not have occurred with more diversity in population.

http://www.brooklyn.cuny.edu/bc/ahp/LAD/C21/graphics/C21_Bottleneck_2.GIF

• 2Founder effect - small # of individuals of species migrate into new habitat.

• If only a few individuals colonize new area, new population reflect only their gene pool not larger gene pool where they came from.

http://www.answersingenesis.org/creation/images/v18/i3/p13_step3.JPG

• Nonrandom mating - equilibrium cannot occur.

• Individuals must choose mate randomly without respect to phenotype.

• If phenotype influences selection, genotypes and phenotypes of population will be changed.

http://bioweb.wku.edu/courses/Biol430/wsquirrelHR.jpg

• Self-fertilization in plants has this effect.

• Reduces # of heterozygotes in population; increases # of homozygotes.

• Many species exhibit sexual selection (form of nonrandom mating)

http://www015.upp.so-net.ne.jp/shuri/shuri_study/what_is_si.GIF

• Natural selection - differential production of offspring based on inherited traits.

• Individuals with more favorable phenotypes may survive, reproduce; alters population frequencies.

• Fitness - key description of natural selection.

http://www.telomere.org/images/Sloth.jpg

• Fitness - organism’s ability to contribute alleles, traits to future generations.

• Factors involved - ability to survive to reproductive age, mate and produce offspring, raise offspring to maturity.

• Other factors - ability to escape predation, gather food, attract mates, or care provided to offspring.

http://www.agpix.com/catalog/AGPix_CoTo11/large/AGPix_CoTo11_0092_Lg.jpg

• Individual with long life but few offspring - poor fitness if other individuals have more offspring.

• Animals that take care of offspring - greater fitness than those that do not.

• Balanced by having more offspring that receive little care/fewer offspring that receive more care.

http://www.facstaff.bucknell.edu/ddearbor/BFAL_feeds.jpg

• Three types of selective pressures that affect natural selection over time.

• Any given population, trait distribution bell-shaped.

http://www.csulb.edu/~kmacd/346NotesI_files/normalCurve.gif

• 1Stabilizing selection - not change average, tends to sharpen curve.

• Newborns can have problems if too large or too small at birth – stabilizing selection pushed average to 8 pounds; perfect size for newborn infant.

http://img.sparknotes.com/figures/A/a3aa6bb95c7d70781cc0089d17f9160f/stable.gif

• 2Disruptive selection - either side of bell curve favored - leads to 2 different peaks in distribution of population.

• 3Directional selection - change in average for trait in population occurs.

• Giraffes live in area where vegetation high up - favor those with longer necks - drives evolution of longer necks.

• Kin selection - individuals with many of same alleles live with other members with same alleles.

• Lions live with many female relatives, help raise young, even if it is not his.

• Increases fitness of mother, even if it doesn’t increase fitness of male.

http://www.anth.ucsb.edu/faculty/gurven/images/orangatan.jpg

• Species - group of organisms able to interbreed productively with rest of group, not with other organisms.

• Reproductive isolation – 2 populations of birds live in different areas never mix - not necessarily separate species.

http://www.lakelandwildlife.co.uk/images/species2.jpg

• If they breed and produce fertile offspring when placed together - same species.

• Separation of species can promote evolution into 2 separate species - no gene flow possible between 2 species.

• Result of separation of gene pools over period of time.

• Cladogenesis - formation of 2 species from 1 ancestor species.

• Occurs when populations occupy same area or when separated geographically from each other.

• 1Allopatric speciation - populations separated by geographic barrier followed by reproductive isolation.

http://taxonomy.zoology.gla.ac.uk/~rdmp1c/teaching/L1/Evolution/l6/grandcanyon.gif

• Evolution of many diversely adapted species from common ancestor - adaptive radiation.

• 2Sympatric speciation - speciation by populations that occupy same region.

• New species arise within range of parent populations.

• Can occur through sudden dramatic genetic change - result of polyploidy of genome.

• Can occur within species if individual spontaneously (through mistake in meiosis) produces offspring with 2X normal chromosomal number.

• Also result when cross between 2 related species produces hybrid with chromosomal average of both parents.

http://en.wikipedia.org/wiki/Hybrid_animals

A sheep goat

• Tetraploid cannot produce fertile offspring if it mates with diploid, could fertilize itself through self-pollination if plant.

• Polyploidy more common in plants - has selective advantage over diploid parents.

• Mechanisms of reproductive isolation that cause speciation divided into 2 groups.

• 1Prezygotic barriers prevent 2 species from mating.

• Occurs several ways.

http://www.sci.uidaho.edu/bionet/biol115/t9_species/images/L9_Mating-Barriers.jpg

• AHabitat isolation: species separated because of where they live.

• BBehavioral isolation: species have different mating habits - never interact.

• CTemporal isolation: breed at different times of the year.

• DMechanical isolation: not anatomically correct for each other.

• 2Postzygotic barriers do not prevent mating - prevent formation of fertile offspring.

• Examples of this - reduced hybrid viability (hybrid aborts spontaneously), reduced hybrid fertility (hybrids infertile), hybrid breakdown (1st generation hybrids viable and fertile, next generation feeble or sterile).

http://www.kyhorsepark.com/imh/bw/images/dmconf.jpg

• Rate of speciation and relationship between speciation and evolution still controversy.

• Punctuated equilibrium - evolution occurs rapidly in association with speciation in small isolated populations followed by long periods where species changes little.