Chapter 23 - Lecture notes 23 PDF

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NOTES FOR BIOLOGY 1202 DR. STEVEN POMARICO, INSTRUCTOR CHAPTER 23 The Evolution of Populations >>>Evolution and population Remember that evolution is the change in gene frequencies in the population. ---Population is a localized group of individuals that belong to the same biological species.    

The change in the genetic makeup of a population over time is the measurement of evolution A define area This is an interaction that leads to reproductive actions for the next generation Looking for a change in gene frequencies

Populations are the smallest relevant evolutionary unit -individuals don't evolve -evolution measured by change in population -evolution acts only on heritable traits >>>>>>Populations show variations Some variation is the result of heritable changes (mutation and/or genetic recombination) and some variations is nonheritable (see fig 23.5)    

Crossing over- Meiosis Prophase 1 Independent assortment- Meiosis Metaphase 1 If the offspring survive better than others it is an evolutionary change Nonheritable variation example o Environmental influences o Like animals who change coat/fur color during the changing of seasons o Changing of the phenotype isn’t a heritable trait that can’t be passed on for evolutionary change

2 >>>>>>Variation between populations Population of the same organism can show variations. This population variation can often be geographic. The variation between populations can be discrete or continual.   



We are all the same organism, but we aren’t the same Variation can be geographic and occur over a geographic range o It can be discrete or a continual variation Population can occur in the same place and not be a single population o Same species, 2 separate population  Ex. Porcupine herd and Fortymile range  Never run into one another Is the variation based on heritable differences or something else? o Is it discrete or continual?

Frequently continual variation in a population will exist as a cline --- Cline is a graded variation in a trait that parallels a gradient in the environment

>>>>>>The integration of Darwinism (evolution) and Mendelism (genetics) When the field of evolution (and the study of populations) merged with genetics, the science of population genetics was formed. 

Population genetics is the study of how populations change over time o population is the unit of evolution o natural selection is the primary mechanism for evolutionary change

>>>>>>A population has a genetic structure defined by its gene pool The genes in a population make up the gene pool for that population ---Gene pool is the total aggregate of genes in a population at any one time

-all the alleles at all the loci for all individuals in a population -most alleles in the gene pool will be combined to produce the next generation. If you add up all the alleles for a particular trait, the allele frequency for an allele can be

3 determined. -if only one allele exists for a trait, then that trait is "fixed" in the population The allele frequency is 1.0 (100%) -most traits have multiple alleles and each allele has a frequency of occurrence.

>>>>>>The Hardy-Weinberg Principle In the absence of other factors, the segregation and recombination of alleles during meiosis and fertilization will not alter the overall genetic makeup of a population. -the gene frequencies will remain constant unless nature forces a change. A population of 100 peas The peas are diploid for the pea shape gene and are either round or wrinkled The round allele (R) is dominant over the wrinkled allele (r) In the population 84 plants have round seeds, and 16 have wrinkled seeds. Of the 84 plants with round seeds 36 are homozygous (RR) and 48 are heterozygous (Rr). Since each plant has 2 genes for shape there are 200 pea shape genes in the population. 120 of those shape genes are dominant (R) (36 x 2) homozygous

+ 48 heterozygous

= 120

The frequency for the R allele is 0.6 120/200 The frequencies for both alleles (R and r) must add up to equal 1 0.6 + frequency for r = 1 so the frequency for r must be 0.4

4 You can also get this by the same method we got the frequency for R (16 x 2) homozygous

+

48 heterozygous

= 80

80/200 = 0.4 Assuming that mating in the population is completely random, then the frequencies of R and r will remain constant. The chance of getting two R genes in the F1 generation is 0.6 x 0.6 = 0.36

This means 36% of the F1 generation is RR

The chance of being a heterozygous individual is 2 (0.6 x 0.4) = 0.48

This means 48% of the F1 generation is Rr

The reason you multiply by 2 is there are 2 ways to get a heterozygous individual. You can get R first followed by r or you can get r first followed by R. The chance of getting two r genes in the F1 generation is 0.4 x 0.4 = 0.16

This means 16% of the F1 generation is rr.

NOTICE => These percentages are the same as the P generation If reproduction continued with segregation, recombination and random mating the frequencies would remain the same (i.e., no evolution)

>>>HARDY-WEINBERG EQUILIBRIUM -Genotype frequency stays the same from generation to generation because the frequency of alleles (gamete frequency) stays the same. -The population is not evolving Remember the theorem stated: In the absence of other factors, the segregation and recombination of alleles during meiosis and fertilization will not alter the overall genetic makeup of a population.

5 The conditions needed to maintain the equilibrium: 1. No mutations 2. No gene flow - Population is isolated from other populations 3. Population is very large 4. Random mating (every individual has an equal chance of mating with any other individual) 5. No natural selection ==> these criteria are essentially never met, and the result is evolution. Four important points about evolution (if we don’t meet the criteria above evolution occurs) 1. Natural selection doesn’t cause genetic change in individuals (it selects the variation) 2. Outcome of natural selection is evolution, which affects the population (genetic make-up of the group will shift in favor of whoever is reproducing more) 3. Evolution is spread because of differential reproduction or fitness (how many offspring the organism leaves) 4. Evolutionary changes aren’t absolutely good, but are good in the environmental context

MECHANISMS OF EVOLUTION 1. Mutations: adding a new allele to the gene pool o Mutations are the only source of new alleles o Mutations occur spontaneously o The creation of new allele forms  Some are meaningful, some are not, some you can’t tell. >>>>>>Gene flow can cause evolution by transferring alleles between populations ---Gene flow is genetic additions to or subtractions from a population resulting from the movement of fertile individuals or gametes

6 -The gene pool of one population mixes with the gene pool of another -If the mixing is extensive then the two populations become one.

>>>>>>Genetic drift: evolution by chance. (see fig 23.9) ---Genetic drift

-In a small population chance events have a greater impact. The larger the population the less important genetic drift becomes. Two situations which result in small populations. (A) Bottleneck Effect (see fig. 23.10) -drastic reduction in the population size -the surviving population probably has a different genetic makeup -the population is now more susceptible to genetic drift. (B) Founder Effect -a few individuals colonize a new habitat -the founding population probably has a different genetic makeup -the population is now more susceptible to genetic drift.

>>>>>>Non-random mating can cause evolution by shifting the frequency of genotypes There are two kinds of non-random mating: 1) Inbreeding -individual in a population mate with close neighbors. -results in a shift in genotype frequency, but not allele frequency 2) Assortative mating -individuals mate with similar phenotypes Both usually result in an increase in homozygous individuals Many mammal and bird species exhibit sexual selection in which only one sex is involved in the mate selection process. This can be intrasexual selection or

7 intersexual selection --- intrasexual selection

--- intersexual selection

>>>>>>Natural selection causes evolution via differential reproduction In a sexually reproducing population some individuals produce more offspring than others. =>Note these offspring must survive and reproduce. This difference in reproductive success is natural selection. Natural selection is usually the only adaptive form of evolution The selection is of a phenotype which reflects the underlying genotype. >>>>>>Modes of natural selection There are three modes of natural selection: (see fig 23.13) 1) Directional -shifts the population toward one extreme 2) Disruptive -shifts the population toward both extremes and away from the middle 3) Stabilizing -reduces phenotypic variation >>>The preservation of variation Diploidy -the recessive genes can hide -the heterozygous individuals may have an advantage

8 Heterozygote advantage - heterozygosity confers an advantage over either homozygote...