chapter 18 process of evolution 18.1 microevolution principles of genetics explain how populations...

Chapter 18

Process of Evolution

18.1 Microevolution

Principles of genetics explain how populations vary and change population genetics

Modern Evolutionary Synthesis applies natural selection to genetics

microevolution refers to changes within a population, all the members of one species (that interbreed) in a particular area

18.1 Microevolution population genetics, cont.

gene pool: all alleles at all gene loci in all individuals of a population

variation in gene pool is key to natural selection

a gene pool is to a population as a genotype is to an individual

gene pools can be described in terms of gene frequencies, the percentage occurrence of particular alleles or genotypes


gene pool, cont.example:

suppose that in a Drosophila population 30% of flies are homozygous dominant for grey bodies, 45% are heterozygous, and 25% have black bodies (homozygous recessive)

What are the allele frequencies? What are the frequencies in the

following generation?


gene pool, cont.we find that sexual reproduction alone does not change allele frequencies

Hardy and Weinberg independently realized the possibility of equilibrium of gene pool frequencies in 1908


Hardy-Weinberg principle: equilibrium of allele frequencies in a gene pool will remain in effect in each succeeding generation if

1. no mutations (changes in alleles)2. no gene flow (allele migration)3. random mating (chance pairing)4. no genetic drift (large population, insignificant chance changes)

5. no selection


Hardy-Weinberg principle, cont.these conditions are rarely metthese are the factors that cause evolution, the change in allele frequencies over time

natural selection can be seen as a change in allele frequencies

more frequent alleles: more fit less frequent alleles: less fit (but

rarely removed completely)


microevolution examplepeppered moth in England

pre-Industrial Revolution, white moths more common, rested on white trees to avoid being bird food

during Industrial Revolution, trees covered with soot

black moths survived, white moths became bird food, so black moth frequency (and allele) increased

called industrial melanism

Fig. 18.2a Moths on light tree trunk

Fig. 18.2b Moths on dark tree trunk


Hardy-Weinberg equationallows one to measure allele frequencies

by comparing frequencies over several generations, changes can be detected and measured

if frequencies stay constant over time, the population is in equilibrium


Hardy-Weinberg equation, cont.if there are only 2 alleles,

let p = frequency of dominant allele let q = frequency of recessive allele then p + q = 1 (100%)

individuals have 2 alleles/trait, so p2 = frequency of AA 2pq = frequency of Aa q2 = frequency of aa, and p2 + 2pq + q2 = 1


Hardy-Weinberg equation, cont.using the equation

vestigial wings are recessive in flies given a population of 500 flies and

80 flies with vestigial wings, what are the frequencies of the wild and vestigial wing alleles?


Hardy-Weinberg equation, cont.using the equation

the only observable value is….. q2

q2 = 80/500 = 0.16 q = = 0.4 p + q = 1 p = 1 - q = 1 - 0.4 = 0.6 frequency of dominant allele = 0.6 frequency of recessive allele = 0.4

016.


Hardy-Weinberg equation, cont.using the equation, cont.

if we measured p and q in a few generations and the values were the same, then the population is in equilibrium

if this is true, then all five conditions are being met

However, suppose it is fit for flies to fly?

18.1 Microevolution Causes of Microevolution

Genetic Mutationsresult in multiple alleles

Gene Flowmovement of alleles between populations by migration of breeding individuals

can increase variation in a population, decreases isolation

makes gene pools similarcan prevent speciation

Fig. 18.3 Gene flow

18.1 Microevolution Causes of Microevolution, cont.

Nonrandom Matingassortative mating: individuals mate with others of the same phenotype

intrasexual selection: males fight for the right to mate

example: Bighorn sheepintersexual selection: females exhibit choosiness

example: peahens


Genetic Driftchanges in allele frequencies due to chance

more likely in small populationsbottleneck effect: prevents most genotypes from participating in production of next generation

example: California condors, population dropped to 20 birds, limits variation

Fig. 18.4 Genetic drift


Genetic Drift, cont.founder effect: small, “strange” population breaks off of larger population

example: Amish have more polydactyl dwarves then rest of the world

Natural Selectionbiggest influence on frequenciesresults in adaptation (others don’t)

Fig. 18.5 Founder effect

Causes of microevolution

18.2 Natural Selection

Natural selection results in adaptation to the environment

natural selection is the process that results in adaptation of a population to the biotic and abiotic environments

requires variation inheritance differential adaptiveness differential reproduction

Effect of selection on finch beak size

18.2 Natural Selection Types of Selection

natural selection usually acts on polygenic traits

polygenic traits display a range of phenotypes

directional selection occurs when an extreme phenotype is favored

example: antibiotic resistance in bacteria

Normal distribution

Directional selection

Fig. 18.6 Directional selection

18.2 Natural Selection Types of Selection, cont.

stabilizing selection occurs when an intermediate phenotype is favored

examples: clutch size in Swiss starlings, size of galls made by gall-flies

Stabilizing selection

Fig. 18.7 Stabilizing selection

18.2 Natural Selection Types of Selection, cont.

disruptive selection occurs when extreme phenotypes are favored over the intermediate phenotype

examples: British land snail coloration, male lazuli bunting coloration

Disruptive selection

Fig. 18.8 Disruptive selection

18.2 Natural Selection Maintenance of Variations

genotypic variation is maintained by:mutationrecombination (ex: flowers prevent self-pollination)

gene flow (ex: male wolves out of pack)

disruptive selection

18.2 Natural Selection Maintenance of Variations, cont.

diploidy makes heterozygotes possible

heterozygotes maintain recessive alleles

heterozygotes sometimes have an advantage over homozygotes

example: malaria resistance and sickle cell anemia

Heterozygote advantage

Fig. 18.9 Sickle cell disease

18.3 Macroevolution

Macroevolution requires reproductive isolation

macroevolution: evolutionary change at or above the level of species

speciation: the splitting of one species into two or more species or the transformation of one species into a new one

18.3 Macroevolution What Is a Species?

biological species concept: a group of populations that can breed among themselves to produce fertile offspring

members of one species cannot reproduce with members of another species

members of a species have a shared gene pool

Fig. 18.10 Species concept

18.3 Macroevolution What Is a Species?, cont.

reproductive isolating mechanism: any structural, functional, or behavioral charac-teristic that prevents successful reproduction from occurring

prezygotic isolating mechanisms prevent repro-duction attempts or successful fertilization


reproductive isolation, cont.prezygotic isolating mechanisms

habitat isolation temporal isolation behavior isolation mechanical isolation gamete isolation

Fig. 18.11 Temporal isolation


reproductive isolation, cont.postzygotic isolating mechanisms prevent hybrid offspring from developing or breeding

zygote mortality hybrid sterility F2 fitness

Isolating mechanisms

18.3 Macroevolution Modes of Speciation

allopatric speciation: origin of new species between populations that are separated geographically

examples: squirrels across Grand Canyon, salamanders in CA

sympatric speciation: origin of new species in populations that overlap geographically

example: polyploid plants

Fig. 18.12 Allopatric speciation

Fig. 18.13 Sympatric speciation

Forms of speciation

18.3 Macroevolution Modes of Speciation, cont.

adaptive radiation involves many new species arising from a single ancestral species when members become adapted to different environments

particular form of allopatric speciation

examples: Galapagos finches, Hawaiian honeycreepers

Steps in adaptive radiation

Fig. 18.14 Adaptive radiation

Hawaiian honeycreepers

chapter 18 process of evolution 18.1 microevolution principles of genetics explain how populations...

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