sign in sign up
<<
Home , Population genetics

/

Through the efforts of in the mid- 1800‘s the theory of was developed œ He believed that through competitive advantage for limited resources that some bred and continued into the next generation, while those less well adapted did not œ The characteristics that provided for this advantage were passed on to the next generation, natural selection has taken place œ the theory of natural selection has been summarized as: Population Genetics/Natural Selection

1.Organisms beget like organisms

2.Chance variations exist amongst individuals of the same species and some of these variations are heritable

3.More offspring are produced each generation than can be supported by the environment

4.Some individuals, because of their physical or behavioral traits, have a higher chance of surviving and reproducing than other individuals in the same population Population Genetics/Natural Selection

Darwin developed the mechanism but it was not until Mendel that the actual process was described œ Mendel found that the inheritable characteristics were actually the result of Population Genetics/Natural Selection

• Variation Within - Populations include genetic and pheontypic variation among individuals, which are significant aspects of population structure œ Phenotypic and in Potentilla glanulosa Population Genetics/Natural Selection

• Clausen, Keck, Hiesey(1940) œ looked at clones of different ecotypes of the —sticky cinquefiol“ œ they grew them at different altitudes and found

œ Plants showed both environmental and genetic effects

œ Plants either died or seeds were not viable in many plants that were grown at environmental extremes Population Genetics/Natural Selection

• Combining Molecular and Morphological Information œ Hansen, Elven, and Brochmann (2000) looked at Potentialla growing in Norway » Looked at two different species groups (P. nivea and P. pulchella) » On the basis of believed that there were three species for P. nivea and P. pulchella » DNA evidence (READ APPLICATION AND TOOLS SECTION PAGES 218 œ 222) showed that in fact three separate species did exist for P. nivea but the differences observed in P. pulchella were the results of morphological changes based upon plasticity Population Genetics/Natural Selection

œ Variation in Animal Populations œ Case (1976) examined variation in size of Sauromalus obesus with respect to environmental differences œ lives in hot dry climates with large amount of variation: Population Genetics/Natural Selection

• Average Summer Temp œ 23.8o to 35o C

• Average Annual Rainfall œ 25 - 194 mm

• Large variation attitudinally 4 œ 1,166 m

• Average winter rainfall correlated with elevation

• Winter rainfall important for growth of feed plants for lizards Population Genetics/Natural Selection

The variation in rainfall in these areas translates to variation in the chuckwalla‘s food source, which needs winter moisture to grow œ What do you think he found? Population Genetics/Natural Selection

Was this variation genetically based? œ How do you think he tested this?

Raising individuals from different altitudes under the same conditions and found: Population Genetics/Natural Selection

• Genetic Variation in Alpine Fish Populations œ cold headwaters of Alpine rivers provide refuge for cold loving organisms —leftover“ from the last ice age œ isolated populations have given rise to reduced

œ Douglas and Brunner (2002) studied whitefish, Coregonus, (related to trout and salmon) that live in these cold waters Population Genetics/Natural Selection

» Studied both natural populations and introduced populations for the food industry

» Looked at anatomical features number of dorsal, anal, pelvic, and pectoral fin rays fin pigmentation number of gill rakers on 1st gill arch

» Genetically looked a specific DNA segments at six different loci

» Using both genetics and morphology they were able to assign specimens to their appropriate ancestral group 79% Population Genetics/Natural Selection

• The Hardy-Weinberg equilibrium model helps identify evolutionary forces that can change gene frequencies in populations

œ Calculating Gene Frequencies œ studies were performed by Dobzhansky(1937) and Tan (1946) using Asian lady beetles, Harmonia axyridis, saw that color patterns were the affect of different alleles Population Genetics/Natural Selection

If we can use a that can tell us the genotype mating experiments can easily be performed Population Genetics/Natural Selection

After collecting a number of beetles from an Asian forest we find the following genotypic frequencies:

SS = 0.81 (81%) SA = 0.18 (18%) AA = 0.01 (1%)

Using the above information, what is the frequency of the S ?

S = 0.9 A = 0.1

Hardy-Weinberg Principle œ in a population mating at random in the absence of evolutionary forces, will remain constant Population Genetics/Natural Selection

S (.9) A (.1)

S (.9) .81 .09

.09 .01 A (.1) Population Genetics/Natural Selection

An algebraic equation was written to express these matings with p = the frequency of one allele and q representing the other (p+q)2 = (p+q) x (p+q) = p2 + 2pq + q2 = 1.0

Using our numbers

(.9)2 + 2(.9x.1) + (.1)2 = 1

(.81) + (.18) + (.01) = 1

We can also use our numbers to calculate the genotypes in our population

SS = p2 = .81 SA = 2pq = .18 AA = q2 = .01 Population Genetics/Natural Selection

The previous allele frequencies were said to remain constant when no evolutionary forces were at work and mating was random œ the conditions for allele frequency to remain constant are:

1. Random mating

(does this occur in nature?)

2. No

(does this occur in nature?)

3. Large

Is this always the case in nature? Changes in allele frequency do to chance events in small populations is called

4. No

(does this occur in nature?)

5. All genotypes have equal

(is this the case in nature?) Population Genetics/Natural Selection

Change Due to Chance

• Random processes such as genetic drift can change gene frequencies in populations, especially in small populations

This is of particular interest today for two reasons: 1.populations of organisms are becoming smaller as these organisms are being exploited, and becoming endangered 2. are becoming fragment by development and exploitation Population Genetics/Natural Selection

• The Process of Natural Selection œ natural selection: œ is the result of differences in survival and among œ Can act against different segments of the population under different circumstances and produce vastly different results Population Genetics/Natural Selection Population Genetics/Natural Selection

œ Evolution by Natural Selection and Genetic Variance œ the production of evolutionary change depends upon natural selection and (the passing on of traits from one generation to the next) œ can be expressed algebraically:

2 h = VG/VP

VG = genetic variance

VP = phenotypic variance

Simplification

VP = VG + VE

VE = environmental variance

2 h = VG/VG + VE Population Genetics/Natural Selection Natural Selection

• Natural selection, which changes genotypic and phenotypic frequencies in populations, can result in organisms adapting to the environment

œ Adaptive Change in Colonizing Lizards œ Losos, Warhelt, and Schoener (1997) œ looked at the brown anole because

1.There is a great deal of variation in this genus with respect to size

2.Length of hind limb reflected vegetation use and a tradeoffs between the two

a.Longer limb length leads to increased speed

b.Shorter limbs aids movement on narrow surfaces Population Genetics/Natural Selection

They performed transplant experiments where they took lizards from a parent population and introduced onto islands where anoles did not exist œ they hypothesized that:

1.The extent to which the colonizing populations change morphologically from the source population will correlate with the amount of difference in vegetative structure on the experimental islands, and the parent island

2.The Anolis populations on the experimental island and parent island should show a significant correlation between relative hind limb length within populations and average perch diameter used on the islands

They found: Population Genetics/Natural Selection

These studies did not rule out the possibility that environment affected the morphology of the lizards Population Genetics/Natural Selection

œ Rapid by Soapberry Bugs to New Host Plants œ there exists a very intimate relationship between herbivores and the plants they eat with respect to defenses produced by the plants and developed by the herbivores œ Carroll and Boyd (1992), studied the soapberry bug in relation to this œ this bug feeds on seeds by piercing the walls of the fruits (therefore the beak must be long enough to reach the seed)

there was a natural shift in bugs from their natural host to several introduced hosts that occurred in the southern US œ using herbarium sheets and museum collections they found: Population Genetics/Natural Selection Population Genetics/Natural Selection

œ Evidence of Genetic Drift in Chicuahua Spruce œ ideal to study because small, fragmented populations of this spruce live only at high altitudes in glacial refugia œ Lediget. al. (1997) examined the amongst these populations by studying the genes responsible for alloenzymes of 16 enzyme systems in populations ranging in size from 17 œ 2,441 individuals œ they suspected that genetic variation should be greatest in the larger populations, and found: Population Genetics/Natural Selection Population Genetics/Natural Selection

œ Genetic Variation in Island Populations œ Frankham (1996) realized that rates of were higher on islands than adjacent mainland populations œ he hypothesized that this was the result of a decreased genetic diversity in the island populations œ his studies were designed to examine:

• Do mainland populations of sexually reproducing species have lower genetic variation than comparable mainland populations?

• Do endemic island populations, which have lived in isolation on islands long enough to diverge substantially from mainland populations, have lower genetic variation than mainland populations?

His studies showed: Population Genetics/Natural Selection Population Genetics/Natural Selection

œ Genetic Diversity and Butterfly œ Frankham‘s studies did not answer the question as to if a decreased genetic diversity did in fact lead to extinction œ Hanski, Kuussaari, and Nieminen (1994), using small populations of the Glanville fritillary butterfly found that they did become more readily extinct

It was theorized that it was that led to genetic a decreased variation in small populations which in turn translated to an increased risk of extinction Population Genetics/Natural Selection

Saccheri et. al. (1998) showed that this indeed was the case œ they studied 1,600 meadows and found: # of Fields with Year Butterlies 524 1993

401 1994

384 1995

320 1996

They observed 200 extinctions and 114 colonizations œ they also performed isoenzyme studies and found low levels of heterozygositywhich resulted in:

smaller larvae with lower survival rates pupae remained in this stage longer, resulting in increased parasitism 24 œ 46% lower egg hatching rate Population Genetics/Natural Selection

Be sure to read and study Applications & Tools pgs. 218 - 222