Microevolution: How Does a Population Evolve?

Chapter 16

The Evolution of Evolution.Blending inheritance

Inheritance of Acquired Characteristics

Genetics alone causes evolution

Modern synthesis – evolution is due to natural selection working on inherited traits

• Population genetics

• Microevolution – changes in the frequency of the alleles of genes in a population.– Industrial melanism

• Macroevolution - the process by which species of organisms originate, change and go extinct.

• What is the source of variation within a population? – Either point mutations of genes or

chromosomal mutations

• If there is only one choice (allele) for a gene, the population is homozygous for that gene.

• If there are two or more choices (alleles) for a gene, the population is polymorphic for that gene.

• If the members of a population come in two or more forms, the population is polymorphic.

• Most human traits are polygenic – controlled by many genes

– These traits vary smoothly and continuously within a population.

– The graph of these traits is a bell curve.

• In a changing environment, highly variable populations evolve more rapidly than less variable populations.

• The factors that determine the genetic variability of a population are:

1. The rate at which mutations accumulate in the DNA.

2. The rate at which changes spread through a population (reproductive rate).

3. The rate at which deleterious mutations are eliminated from a population by natural selection.

How much genetic variation is there?

• In humans about 25% of all proteins have an alternate form which is present in at least 5% of the population.

• In humans about 7 % of our genes are heterozygous.– Invertebrates -13%– Plants -17%– Drosophila – 25%

• Remember: Natural selection works only on the Phenotype which is an interaction of the genotype and the influences of the environment (basically what the individual looks like).

• Genetic variation is fuel for evolution

• Yet, natural selection favors those traits best suited to the environment and weeds out the rest.

• All of the genes of all the individuals in a population is called the gene pool.

• Hardy-Weinberg principle: sexual reproduction by itself does not change the frequencies of alleles within a population. Genotype frequencies stay the same from generation to generation as long as certain conditions are met.

• Hardy-Weinberg equilibrium:

• p + q = 1 and p2 + 2pq + q2 = 1

Conditions:1. There was random mating

2. There is a large population size

3. There are no mutations

4. There is no breeding with other populations

5. There is no selection, either natural or artificial

• In reality, these conditions are hardly ever met, but it gives us a standard against which to measure evolution.

• Of all the conditions mentioned, only natural selection leads to adaptive change.

• The rest cause changes in gene frequency which may or may not be adaptive.

1) Random mating• Only practiced by organism which release

gametes on the wind or in the water.

• Assortative mating – based on choice– May be without regard to one’s phenotype– Positive assortative mating – choose

individuals like ourselves• Inbreeding – increases the incidence of

recessive disorders, leading to a less healthy population

– Negative assortative mating - outbreeding

2) Large population size• Random drift or genetic drift is a change

in the allele frequency due to random events. This is more likely in a small pop.

• Founder effect –a small subset of a population founds a new population.

• Bottleneck effect – the population is reduced to a few individuals by some random disaster or harsh selection pressure (such as over hunting).

• Causes new mutations to spread or be removed.

3) No Mutations

• Does not happen in nature.

• The DNA copying mechanism is nearly perfect, but mistakes are made.

• These mistakes result in new phenotypes which are subjected to selection and the basis for adaptive change,

4) No interbreeding between populations

• Gene flow occurs as the result of interbreeding between two populations.

• Individuals immigrate and bring new alleles into the population.

• It increases the variation within a population.

• It makes adjacent populations more alike.

4) No selection• Natural selection

– Harmful genes are selected against– Useful genes accumulate

Types of Natural Selection

• Directional selection – selects for one end of the bell curve

Types of Natural Selection

• Stabilizing selection – the extremes of a population are selected against and the average is favored.

Types of Natural Selection

• Disruptive selection – selects for extremes and against the average.

Disruptive selection

Taste good

Taste bad

Sexual selection• Male competition

– Male competes against other males for territory, or access to females

– Anything that gives him an advantage makes him more likely to pass on his genes

• Female selection ( or male selection)– Leads to sexual dimorphism– Male must prove he is genetically good enough– Plumage, gifts, nesting site or mating rituals

• Natural selection can also encourage genetic variation when different alleles of a gene are equally useful.– Different local environment– One allele is better at a certain time of year– Balanced polymorphism– Sometimes the superiority of the heterozygote

may maintain a high incidence of an allele which is harmful to the homozygote • Sickle cell anemia and malaria

Blue = malaria

Red = sickle cell anemia

Purple = overlap