Ch.23 - Evolution of Populations PDF

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Talks about the three major causes of evolution; genetic drift, natural selection, and genetic flow.
Professor Brinton...

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Ch.23 - Evolution of Populations The Smallest Unit of Evolution ● A common misconception is that organisms evolve during their lifetimes ● Natural selection acts on individuals, but only populations evolve Darwin’s Finches ● Galapagos Islands ● A population of medium ground finches on Daphne Major ○ Variety of food sources ○ Variability in beak size ○ Changing annual rainfall ● Researchers Rosemary & Peter Grant

Microevolution: Selection, Drift, Flo ● Microevolution is a generation-to-generation change in allele frequency in a population ○ Variation in heritable traits is a prerequisite for evolution ○ Mendel’s work on pea plants provided evidence of discrete heritable units (genes) ● Three mechanisms cause allele frequency change ○ Natural selection ○ Genetic drift ○ Gene flow Only natural selection causes adaptive evolution

Genetic Variation ● Genetic variation among individuals is caused by differences in genes or other DNA segments ○ Phenotype is the product of inherited genotype and environmental influences ● Natural selection can only act on variation with a genetic component ○ Not creating new desirable, traits ○ Single-gene traits ○ Multi-gene (polygenic) traits ○ Pleiotropic genes ○ Epistasis ○ Phenotypic plasticity

Phenotypic Plasticity ● Some phenotypic variation results from environmental influences (e.g., the acidity and aluminum content in soil affects hydrangea flower color)

Sources of Genetic Variation 1. New genes and alleles can arise by mutation or gene duplication 2. Sexual reproduction can result in genetic variation by recombining existing allele A mutation is a random change in nucleotide sequence of DNA ● Recall: Insertions, deletions, substitutions ● Recall: Silent, missense, nonsense ● Recall: Dominant vs. Recessive allele Only mutations in cells that produce gametes can be passed to offspring ● Are all mutations harmful? Beneficial? ● What is the result of mutations during an organism's lifetime? e.g. cancer

Chromosome Rearrangement

Duplication & Rearrangement of Genes Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful ● Duplication of small pieces of DNA increases genome size and is usually less harmful ● Duplicated genes can take on new functions by further mutation

Implications of Reproduction Speed Slower reproduction ● Relatively long generation times ● Mutation rates are low in animals and plants ● The average is about one mutation in every 100,000 genes per generation Quick reproduction ● Mutations accumulate quickly in prokaryotes and viruses because they have short generation times ● Mutation rates are often lower in prokaryotes and higher in viruses

Gene Pools and Allele Frequencies A population is a localized group of individuals of one species ● Species: Capable of interbreeding and producing fertile offspring ● Populations are not always geographically isolated, but individuals typically only breed with members of their own population

Loci, Alleles, & the Gene Pool Each chromosome has many loci (locations), each with allele A gene pool consists of all the alleles for all loci in a population ● > 1 allele? ○ homozygous ○ heterozygous ● Fixed locus: all individuals in a population are homozygous for the same allele

Calculating Allele Frequency Diploid: total number of alleles at a locus is the total number of individuals times 2 ● Dominant allele = two alleles for each homozygous dominant individual + one allele for each heterozygous individual ● Recessive allele = ….. By convention, if there are two alleles at a locus, p and q are used to represent their frequencies The frequency of all alleles in a population will add up to 1 (always!) ●

That is, p + q = 1

Hardy - Weinberg Equilibrium

The Hardy-Weinberg theorem: hypothetical Population that is not evolving at a particular locus ● No change occurring ● Such a population is in Hardy - Weinberg equilibrium for that locus In real populations, allele and genotype frequencies do change over time ● Most populations are evolving ● Populations can evolve at some loci, while being in Hardy - Weinberg equilibrium at other loci ● Which traits are evolving changes with time

The five conditions for non - evolving populations are rarely met in nature

Using Hardy - Weinberg Equilibrium Hardy - Weinberg equilibrium describes the constant frequency of alleles in such a non-evolving gene pool Consider, for example, the same population of 500 wildflowers and 1,000 alleles where

For RANDOM MATING AND NO DEATH, the frequency of genotypes can be calculated as:

p x q x 2= heterozygous

Three major causes of evolutionary change Three major factors alter allele frequencies, bringing about most evolutionary change: ● Natural selection ● Gene flow

● Genetic drift

Natural Selection Selection results in alleles being passed to the next generation in proportions that differ from those in the present generation Natural selection can cause adaptive evolution ● Traits that enhance survival or reproduction increase in frequency over time ● For example, an allele that confers resistance to DDT in fruit flies increased in frequency after DDT was used widely in agriculture

Genetic Drift Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next ● Small sample = greater the chance of random deviation ● Tends to reduce genetic variation through the random loss of alleles

Gene Flow: Immigration and Emigration Gene flow consists of the movement of alleles among populations ● Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) ○ Immigration : Movement into a population (Into) ○ Emigration : Movement out of a population (Exit) ● Gene flow can decrease or increase the fitness of a population Gene flow tends to reduce variation among populations over time

Genetic Drift: The Founder Effect The founder effect occurs when a few individuals become isolated from a larger

population Allele frequencies in the small founder population can be different from those in the larger parent population

Genetic Drift: The Bottleneck Effect The bottleneck effect is a sudden reduction in population size due to a change in the environment ● Note: The event (e.g. natural disaster) is not the cause of the change, even if it caused the bottleneck. ○ Why? ● If the population remains small, it may be further affected by genetic drift

Effects of Genetic Drift: A Summary 1. 2. 3. 4.

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

Natural Selection: A Closer Look Natural selection brings about adaptive evolution by acting on an organism ʼs phenotype

Concept: Fitness Misconceptions with “survival of the fittest” and “struggle for survival”

Relative Fitness Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals Selection favors certain genotypes by acting on the phenotypes of individuals

Directional, Disruptive, and Stabilizing Selection There are three modes of selection ● Directional selection favors individuals at one extreme end of the phenotypic range ● Disruptive selection favors individuals at both extremes of the phenotypic range ● Stabilizing selection favors intermediate variants and acts against extreme phenotypes

The Key Role of Natural Selection in Adaptive Evolution Increases the frequencies of alleles that enhance survival and reproduction ● Adaptive evolution occurs as the match between a species and its environment

increases ● Because the environment can change, adaptive evolution is a continuous process Striking adaptations have arisen by natural selection. ● The jaws of snakes allow them to swallow prey larger than their heads ● Certain octopuses can change color rapidly for camouflage

Types of Natural Selection: Sexual Selection Sexual selection is natural selection for mating success Sexual dimorphism: marked differences between the sexes in secondary sexual characteristics How do preferences evolve? The “good genes” hypothesis: Honest Indicators of Mate Quality ● Hypothesis: Preference and frequency of costly sexual traits will correlate with genetic quality or health of the individual ● Prediction: If a trait is related to genetic quality or health, both the trait and preference for that trait should increase in frequency

Intra- vs. InterIntrasexual selection is direct competition among individuals of one sex (often males) for mates of the opposite sex Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates Male showiness due to mate choice can increase a male ʼs chances of attracting a female, while decreasing his chances of survival

Balancing Selection Diploidy maintains genetic variation in the form of recessive alleles hidden from selection in heterozygotes ● Difficult to eliminate an existing allele entirely ● Not always advantageous to decrease diversity Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population Balancing selection includes ● Heterozygote advantage

● Frequency -dependent selection

Frequency - Dependent Selection In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population ● Selection favors whichever phenotype is less common in a population

Heterozygote Advantage Heterozygote advantage occurs when heterozygotes have a higher fitness than does either homozygote ● Result: Natural selection will tend to maintain multiple alleles at that locus ● More genetic variation in a population Heterozygote advantage can result from stabilizing or directional selection ● How? Hint: Does heterozygosity always result in a moderate version of a trait?

Why Natural Selection Cannot Fashion Perfect Organisms 1. 2. 3. 4.

Selection can act only on existing variations Evolution is limited by historical constraints Adaptations are often compromises Chance, natural selection, and the environment interact...