The Evolution of Populations

Chapter 23

What is the difference between a gene and an allele?Quick refresher-

A gene is the basic instruction, a sequence of DNA, an allele is one variant of that instruction.Example:1.The cat fell from the roof.2.The cat fell off the roof.3.The snake fell from the roof.

All 3 sentences are the equivalent of a gene, they represent different alleles.Sentences 1 &2 have nearly identical meanings, however sentence 3 has a very different meaning.

Genetic variation makes evolution possible Mutations are the only source of new genes and new alleles.

Only mutations in gametes can be passed to offspring.

Point mutations are changes in one base in a gene. They can have significant impact on phenotype, as in sickle-cell disease.

Chromosomal mutations delete, disrupt, duplicate, or rearrange many loci at once. They are usually harmful.

Most of the genetic variations within a population are due to the sexual recombination of alleles that already exist in a population.

Sexual reproduction rearranges alleles into new combinations in every genera tion. Recall there are three mechanisms for this shuffling of alleles:

Crossing over during prophase I of meiosis. Independent assortment of chromosomes during meiosis Fertilization (223 X 223 different possible combinations for human sperm and

egg)

Population Genetics

Population genetics is the study of how populations change genetically over time.

Population: A group of individuals of the same species that live in the same area and interbreed, producing fertile offspring.

Gene pool: All of the alleles at all loci in all the members of a population.

In diploid species, each individual has two alleles for a particular gene, and the individual may be either heterozygous or homozygous.

If all members of a population are homozygous for the same allele, the al lele is said to be fixed.

Gene Pools

A gene pool is the combined genetic information of all the members of a particular population Recall that a population is a collection of individuals of the

same species in a given area which share a common group of genes

A gene pool typically contains 2 or more alleles (or forms of certain genes)

The relative frequency of an allele is the number of times that allele occurs in a gene pool compared to the number of times other alleles occur.

Population Genetics

Hardy-Weinberg Principle – states that allele frequencies tend to remain constant in populations unless something happens OTHER THAN Mendelian segregation and sexual recombination. This situation in which allele frequencies remain constant is

called genetic equilibrium If allele frequencies do not change, the population will not

evolve Hardy-Weinberg is a mathematical model that

describes the changes in allele frequencies in a population Allows us to predict allele and genotype frequencies in

subsequent generations (testable)

Hardy-Weinberg Theorem

Model conditions required to maintain genetic equilibrium from generation to generation:

1. Random mating population2. Large population size 3. No emigration or immigration (no movement into or out of the

population)4. No mutations5. No natural selection (all genotypes have an equal chance of

survival and reproduction) If all 5 conditions are met, there should be NO

EVOLUTION Describes a NON-EVOLVING POPULATION

G.H. Hardymathematician

Hardy-Weinberg Equation Let p= frequency of allele A Let q= frequency of allele a Let p2= frequency of genotype AA Let 2pq= frequency of genotype Aa Let q2= frequency of genotype aa

p2+2pq+q2=1 (genotype frequencies) p+q=1 (allele frequencies)

The Hardy-Weinberg Equation can show that evolution IS OCCURING within a population

W. Weinbergphysician

Causes of Microevolution Natural selection, genetic drift, and gene flow can

alter allele frequencies in a population and cause MOST evolutionary changes.

Microevolution –Evolution on its smallest scale. Generation to generation change in a population’s allele frequencies

Three main causes: 1. genetic drift2. natural selection3. gene flow

Genetic Drift

Genetic drift is the unpredictable fluctuation in allele frequencies from one generation to the next. The smaller the population, the greater the chance is for genetic drift. This is a random, non- adaptive change in allele frequencies.

Can and does lead to allele fixation Allele fixation means that a population changes

(evolves) from many alleles represented, to only 1 allele represented

Consequences of Genetic Drift Consequences of genetic drift:

Fixation of alleles Effect of chance is different from population to

population Small populations are effected by genetic drift more

often than larger ones Given enough time, even in large populations genetic

drift can have an effect Genetic drift reduces variability in populations by

reducing heterozygosity REAL WORLD EXAMPLES OF GENETIC DRIFT:

1. The Bottleneck Effect

2. The Founder Effect

Real World Examples of Genetic Drift

The Bottleneck Effect Occurs when only a few

individuals survive a random event, resulting in a shift in allele frequencies within the population

Small population sizes facilitate inbreeding and genetic drift, both of which decrease genetic variation

Reduces genetically variability because at least some alleles are likely to be lost from gene pool

Figure 23.5 The bottleneck effect: an analogy

Real World Examples of Genetic DriftFounder effect: A few individuals become isolated from a larger popu lation and establish a new population whose gene pool is not reflective of the source populationThis small population size means that the colony may have: •reduced genetic variation from the original population. •a non-random sample of the genes in the original population. For example, the Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists. Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington’s disease, (which causes nerve cells in certain parts of the brain waste away, or degenerate.) because those original Dutch colonists just happened to carry that gene with unusually high frequency. This effect is easy to recognize in genetic diseases.

Founding Population A

Founding Population B

Descendants

The Founder Effect

Gene Flow

Gene flow can also change allele frequencies Gene flow is the physical flow of alleles into or

out of a population. Immigration – alleles coming in (added) Emigration – alleles moving out (lost)

Gene flow counteracts differences that arise through mutation, natural selection, and genetic drift.

Gene flow helps keep separated populations genetically similar – reduces differences between populations

Natural selection is the only mechanism that consistently causes adaptive evolution

Relative fitness refers to the contribution an organism makes to the gene pool of the next generation relative to the contributions of other members. Fitness does not indicate strength or size. It is measured only by reproductive success.

Natural selection acts more directly on the phenotype and indirectly on the genotype and can alter the frequency distribution of heritable traits in three ways.

Examples1. Directional selection Example: Large black bears survived periods of extreme cold better than smaller ones, and so became more common during glacial periods.

2. Disruptive selection Example: A population has individuals with ei ther large beaks or small beaks, but few with the intermediate beak size. Apparently the intermediate beak size is not efficient in cracking either the large or small seeds that are common.

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

Directional selection

Shifts the overall makeup of the population by favoring variants that are at one extreme of the distribution.

In this case, darker mice are favored because they live among dark rocks, and a darker fur color conceals them from predators.

Disruptive selection Favors variants

at both ends of the distribution.

These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage.

Stabilizing selection

Removes extreme variants from the population and preserves intermediate types.

If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against

Sexual Selection

Sexual selection is a form of natural selection in which individuals with certain in herited characteristics are more likely than other individuals to obtain mates. It can result in sexual dimorphism, a difference between the two sexes in secondary sex ual characteristics such as differences in size, color, ornamentation, and behavior.

Figure 23.16x1 Sexual selection and the evolution of male appearance

Types of Sexual Selection

Intrasexual Selection means selction within the same sex – typically males Individuals of one sex compete directly for mates of the

opposite sex Often it is based on rituals and displays that don’t risk

injury

Intersexual Selection is also called “mate choice” – typically females Female choice is typically based on showiness of the

male’s appearance and/or behavior Males will often weight the attraction of predators versus

the attraction of mates

How is genetic variation preserved in a population?

Diploidy: (refers to organisms carrying genes in pairs) Because most eukaryotes are diploid, they are capable of hiding genetic variation (recessive alleles) from selection.

Heterozygote advantage: Individuals who are heterozygous at a certain locus have an advantage for survival.

Example: sickle-cell disease, individuals homozygous for normal hemoglobin are more susceptible to malaria, whereas homozygous recessive individuals suf fer from the complications of sickle-cell disease. Heterozygotes benefit from protection from malaria and do not have sickle-cell disease, so the mutant allele remains relatively common.