Evolution of Populations

23.1 – Mutation & sexual reproduction produce genetic variation that makes evolution

possible1) Microevolution

Change in the allele frequencies of a population over generationsEvolution on the smallest scale

2) MutationsThe only source of NEW genes & NEW allelesOnly mutations in cell lines that produce gametes can be passed on to offspring

Types of Mutations

A) Point MutationChange in one base in a geneCan impact phenotype

Sickle cell anemia

B) Chromosomal MutationDelete, disrupt, duplicate, or rearrange many loci at once

Most are harmful, but not always

3) Variations due to sexual reproduction

Rearranges alleles into new combinations in every generation3 mechanisms for this shuffling:

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1) Crossing overDuring Prophase I of meiosis

2) Independent assortmentDuring meiosis (223 different combinations possible)

3) Fertilization223 x 223 for sperm and egg

23.2: The Hardy-Weinberg equation can be used to test

whether a population is evolving

Population geneticsStudy of how populations change genetically over time

PopulationGroup of individuals of the same species that live in the same areaInterbreed & produce fertile offspring

Gene poolAll of the alleles at all loci in all the members of a populationIn diploids, each individual has 2 alleles for a gene & the individual can be heterozygous or homozygousIf all are homozygous for an allele, the allele is FIXED – only one allele exists at the locus in the populationThe greater the # of FIXED alleles, the lower the species’ diversity

Hardy-WeinbergUsed to describe a population that is NOT evolvingFrequencies of alleles & genes in a gene pool will remain constant over generations

5 Conditions for Hardy-Weinberg

1) No mutations

2) Random mating

3) No natural selection

4) The population size must be large (no genetic drift)

5) No gene flow (Emigration, immigration, transfer of pollen, etc.)

If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, thenp2 + 2pq + q2 = 1where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

Practice

Suppose in a plant population that red flowers (R) are dominant to white flowers (r). In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait?

23.3 – Natural Selection, genetic drift, & gene flow can alter allele frequencies in a

population

Mutations can alter gene frequency, but are rare

3 major factors alter allelic frequencies1) Natural selection

Alleles are passed to the next generation in proportions different from their frequencies to the present generation

Those that are better suited produce more offspring than those that are not

2) Genetic DriftUnpredictable fluctuation in frequencies from one generation to the next

The smaller the population, the greater chance

Random & nonadaptiveA) Founder effect = individuals are isolated and establish a new population – gene pool is not reflective of the source population

B) Bottleneck effect = a sudden change in the environment reduces population size – survivors have a gene pool that no longer reflects original

1. Genetic drift is significant in small populations

2. Genetic drift causes allele frequencies to change at random

3. Genetic drift can lead to a loss of genetic variation within populations

4. Genetic drift can cause harmful alleles to become fixed

3) Gene FlowPopulations loses or gains alleles by genetic additions or subtractions

Results from movement of fertile individuals or gametes

Reduces the genetic differences between populations, makes populations more similar

23.4 Natural Selection is the only mechanism that consistently causes

adaptive evolution

Relative fitnessThe contribution an organism makes to the gene pool of the next generation relative to the contributions of the other members

Does NOT indicate strength or size

Measured by reproductive success

Natural selection acts more directly on the phenotype and indirectly on the genotype

Can alter the frequency distribution of heritable traits in 3 ways:

1) Directional selection

2) Disruptive selection

3) Stabilizing selection

1) Directional selectionIndividuals with one extreme of a phenotypic range are favored, shifting the curve toward this extreme

Example: Large black bears survived periods of extreme cold better than small ones, so they became more common during glacial periods

2) Disruptive SelectionOccurs when conditions favor individuals on both extremes of a phenotypic range rather than individuals with intermediate phenotypes

Example: A population has individuals with either large beaks or small beaks, but few with intermediate – apparently the intermediate beak size is not efficient in cracking either the large or small seeds that are available

3) Stabilizing SelectionActs against both extreme phenotypes and favors intermediate variations

Example: Birth weights of most humans lie in a narrow range, as those babies who are very large or very small have higher mortality rates

How is genetic variation preserved in a population?

DiploidyCapable of hiding genetic variation (recessive alleles) from selection

Heterozygote advantageIndividuals that are heterozygous at a certain locus have an advantage for survival

Sickle cell anemia – homozygous for normal hemoglobin are more susceptible to malaria, homozygous recessive have sickle-cell, but those that are heterozygotes are protected from malaria and sickle-cell

Why Natural Selection cannot produce perfect organisms:

1) Selection can only edit existing variations

2) Evolution is limited by historical constraints

3) Adaptations are often compromises

4) Chance, natural selection, & the environment interact