1. The Evolution of Populations

2. Overview: The Smallest Unit of Evolution Natural selection acts on individuals But only populations evolve Genetic variations in populations contribute to evolution An individual organism cannot change during its lifetime Microevolution Evolution on its smallest scale Change in allele frequencies in a population over generations.

3. Variations are the raw material for evolutionary change Genetic variations arise through mutation and sexual reproduction Darwin could not explain how organisms passed heritable traits to their offspring Mendel wrote a paper shortly after The Origin of Species that proposed a mode of inheritance Darwin never learned about genes, but these genetic differences is what evolution is based on Two processes that produce genetic differences is mutation and sexual reproduction

4. Genetic Variation Individual variation occurs in all species Some variation we can see in the phenotype Some occur at the molecular level, for example blood type Some phenotypic variation is not heritable Phenotype is the product of an inherited genotype and many environmental influences Only the genetic part of variation can have evolutionary consequences Bodybuilders cant pass on muscles Caterpillars have different appearances to chemicals in their diets, not their genotypes

5. Genetic Variation within a population Characters can be one of two Discrete characters- classified on an either-or basis. Like Mendels purple or white flowers These are determined by a single gene locus with different alleles that produce the phenotype Quantitative characters- vary along a continuum. Determined from the influence of two or more genes on a single phenotypic character Scientists will consider both the gene variability and the nucleotide variability (molecular level of DNA) within a population Gene variability tends to be greater within a population because a gene can consist of thousands of nucleotides. A difference at only one of these nucleotides can be sufficient to make two alleles of that gene different and thereby increase gene variability.

6. Genetic Variation between populations Species also exhibit geographic variation – differences in the genetic composition of separate populations This usually occurs because of some kind of separation, for example mountains, usually by chance – not natural selection Another example is as a cline – a graded change in a character along a geographic axis. These result from natural selection.

7. Variation between populations Cline Change in a population due to altitude These variations are due mutation

8. Genetic Variation - Mutation Ultimate source of new alleles is muation – change in the nucleotide sequence of an organisms DNA Only mutations in gametes can be passed to offspring In plants and fungi not as limiting, many different cell lines can produce gametes Examples Point Mutations Deletions Duplications Translocation These can be harmful or sometimes have no effect, but some mutations can result in positive effects

9. Genetic Variation – Mutation Rates Mutation rates are low in animals and plants Long generation times Diploid genomes The average is about one mutation in every 100,000 genes per generation Mutations are more rapid in microorganisms Have short generation spans Haploid genomes

10. Genetic Variation – Sexual Reproduction In sexually reproducing organisms most genetic variation in a population results from the unique combination of alleles that each individual receives Differences at the nucleotide level have originated from past mutations Sexual reproduction shuffles existing alleles through Crossing over Independent assortment Fertilization All provide genetic variation

11. The Hardy-Weinberg equation can test whether a population is evolving Gene Pools and Allele Frequencies Population is a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring. A population can be characterized by describing its gene pool Gene pool – all of the alleles for all the loci in all individuals of the population Each allele has a frequency in the population Alleles can be homozygous, or heterozygous

12. Hardy-Weinberg The Hardy-Weinberg theorem describes a population that is not evolving Under such conditions, the Hardy-Weinberg equilibrium states that allele frequencies don’t change and predicts what the frequency of genotypes should be in a population. Among all of the individuals in the population, there are only two different alleles for a gene at the same loci. So, if you know the frequency of the alleles in a population, you can figure out the frequency of the genotypes in the next generation If mating is random If no evolutionary forces are changing the allele frequencies in the next generation.

13. Hardy-Weinberg The five conditions for non-evolving populations rarely met in nature Extremely large population size No gene flow No mutations Random mating No natural selection

14. The Big Idea Hardy-Weinberg equilibrium helps scientists determine when natural selection or genetic drift is at work. If the results deviate from the prediction, you know that one of the aforementioned has occurred. Instead of solving Punnett squares Hardy-Weinberg considers the combination of alleles in all of the genetic crosses in a population All alleles are mixed together in a large bin (representing the gene pool) Reproduction occurs by selecting alleles at random Like pollen blown by the wind Thus mating is random

15. The Equation If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then p 2 + 2pq + q 2 = 1 And p 2 and q 2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype p = the allele frequency of the dominant trait q =the allele frequency of the recessive trait p + q will always equal 1 Youtube: Bozemanbiology solving hardy-wienberg problems If talking about allele frequencies then it is referring to the p value or the q value If talking about individual people, organisms or phenotypes (a population is people) then it is referring to p 2 or 2pq or q 2 Almost always they will have to give you the homozygous recessive

16. Natural selection, genetic drift, and gene flow can alter a population’s genetic composition Three major factors alter allele frequencies and bring about most evolutionary change: Natural selection Genetic drift Gene flow

17. Mechanisms that cause most evolutionary change Natural Selection Genetic Drift Gene flow Natural Selection (review) Differential success in survival and reproduction Individuals in a population exhibit variations Those with traits that are better suited to their environment tend to produce more offspring Selection results in alleles being passed to the next generation in proportions different from their proportions in the present generation.

18. Mechanisms of evolutionary change - Genetic Drift A change in the gene pool of a population due to chance The smaller the population, the more impact genetic drift is likely to have. The frequencies of alleles stay more stable if the population is large Over time genetic drift tends to reduce genetic variation through the losses of alleles Includes Founder effect and Bottleneck effect

19. Genetic Drift Only the five plants in white boxes produce fertile offspring, by chance events like the soil has nutrients that support those plants. In F2 generation a moose steps on the CwCw plants leaving more CR alleles. If the CRCW plants produce few offspring, and by chance every egg and sperm pair carried the CR allele, not the Cw allele, this would account for the change in allele frequencies seen in the F3 generation CRCRCRCR CRCRCRCR CWCWCWCW CRCRCRCR CRCWCRCW CRCRCRCR CRCWCRCW CWCWCWCW CWCWCWCW CRCWCRCW CRCWCRCW CRCRCRCR CRCWCRCW CRCWCRCW CRCRCRCR CRCRCRCR CRCWCRCW CWCWCWCW CRCWCRCW CRCRCRCR Only 5 of 10 plants leave offspring Only 2 of 10 plants leave offspring CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Generation 1 p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3

20. Two situations that can shrink populations down to a small size so that genetic drift can have a large impact Bottleneck effect Founder effect

21. The Founder Effect The founder effect occurs when a few individuals become isolated from a larger population This new group may establish a new population whose gene pool differs from the source population Example: Members are blown by a storm to a new island Can account for the relatively high frequency of certain inherited disorders among isolated human populations In human populations, small groups settle, a recessive allele is present, over time that allele becomes more prevalent in the founders than in the original population from which they came

22. The Bottleneck Effect The bottleneck effect is a sudden change in the environment that drastically reduces population size. Ex. Earthquakes, floods, fires The resulting gene pool may no longer be reflective of the original population’s gene pool Human actions can sometimes create severe bottlenecks.

23. Original population Bottlenecking event Surviving population Certain alleles may be present at higher frequency in the surviving population than in the original population Even if the population recovers in size, it may have low levels of genetic variation for a long period of time due to the genetic drift. Elephant seals hunted and survived by bottleneck.

24. Effects of Genetic Drift: a summary Genetic drift is significant in small populations Genetic drift can cause allele frequencies to change at random Genetic drift can lead to a loss of genetic variation within populations Genetic drift can cause harmful alleles to become fixed.

25. Another source of evolutionary change Gene Flow Gene flow -a population may gain or lose alleles when fertile individuals move into or out of a population Or when gametes (such as plant pollen) are transferred between populations It tends to reduce differences between populations over time Ex. Humans moving all over the world Increased gene flow between populations Gene pools can become homogenized Reducing geographic variation in appearance

26. Gene Flow Summary Gene flow can introduce new alleles into a population Can occur at a higher rate than mutation More likely than mutation to alter allele frequencies directly Once a new allele is introduced, natural selection may then cause the new allele to increase in frequency or decrease in frequency

27. Natural selection is the primary mechanism of adaptive evolution Evolution by natural selection works by chance – creating new genetic variations (by mutation). And sorting – favoring some alleles over others. Only natural selection consistently increases the frequencies of alleles that provide reproductive advantage and thus leads to adaptive evolution.

28. Natural Selection and Relative Fitness Relative Fitness and the different ways that an organisms phenotype is subject to natural selection Relative Fitness and Reproductive Success Relative Fitness – the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals It is not a “survival of the fittest” One organism may be more efficient at collecting food Body color conceals one moth better than others, thus it survives longer and produces more offspring Natural Selection acts directly on the phenotype, indirectly affecting the genotype

29. Natural Selection Directional, Disruptive, and Stabilizing Selection Selection favors certain genotypes by acting on the phenotypes of certain organisms Three modes of selection: Directional Disruptive Stabilizing Selection will always favor individuals whose heritable phenotypic traits provide higher reproductive success than others.

30. Original population Evolved population Phenotypes (fur color) Original population Directional selectionStabilizing selection Frequency of individuals Favors individuals at one end of the phenotypic range. Common when a populations environment changes or members migrate to new habitat Favors individuals at both extremes of a phenotypic range. Example birds with different beak sizes. Small to feed on soft seeds, large in cracking hard seeds. Intermediate are inefficient and have low frequencies Favors intermediate variants. Acts against extremes. Birth weights of humans (6.6- 8.8lbs))those smaller or bigger suffer higher rates of mortality Disruptive selection

31. Sexual Selection Intrasexual selection Selection within the same sex Individuals of one sex compete directly for mates of the opposite sex Occurs often between males Males will patrol and defend his status by defeating smaller weaker or less fierce males in combat. Intersexual selection Individuals of one sex (usually female) are choosy in selecting their male mates Often depends on the showiness of the male’s appearance or behavior Showiness may be a danger to males, but gives them reproductive success. These traits correlate with “good genes” and females pick these males. (whether it be good traits, or good health)

32. Why Natural Selection Cannot Fashion Perfect Organisms Selection can act only on existing variations Ne advantageous alleles do not arise on demand Evolution is limited by historical constraints Evolution operates on the traits an organism already has Even if flight would be a huge advantage, an organism cannot grow wings Adaptations are often compromises Each organism must do many different things Seals spend time on rocks, which legs would be a better adaptation, but then it would not swim nearly as well if it didn’t have flippers Chance, natural selection, and the environment interact If a wind blows insects to a remote island it doesn’t necessarily transport those best suited to the new environment Also environments can change from year to year