Evolution of Populations Chapter 16 Evolution of Populations

16.1 – Genes and Variation Biologists studying evolution often focus on a particular population. Population - a group of individuals of the same species in a given area. Because all members of a population can interbreed, they share a common group of genes, called a gene pool. Gene Pool – the combined genetic information of all the members of a particular population.

Sources of Genetic Variation There are two main sources of genetic variation: Mutations – any change in the sequence of DNA. Gene Shuffling – results from sexual reproduction. Examples: production of gametes crossing over segregation fertilization

O. I will understand the steps of speciation. J. In a population the dominant phenotype of a certain trait occurs 91% of the time. What is the frequency of the dominant allele? The recessive allele?

Single-Gene and Polygenic Traits The number of phenotypes produced for a given trait depends on how many genes control the trait. Single-Gene Trait – a trait that is controlled by a gene that has two alleles. Results in discreet variation ;only two phenotypes Ex. – widows peak Fig. 16-3 Pg. 395

Polygenic Trait – traits that are controlled by two. or more genes Polygenic Trait – traits that are controlled by two or more genes. Results in quantitative variation; characteristics vary along a continuum Ex. – Height in humans Fig. 16-4 Pg. 396

How variation is preserved: Diploidy – 2 copies of a gene Balanced polymorphism – the ability of natural selection to maintain diversity in a population, heterozygous protection or advantage Ex. Sickle cell anemia – ss – suffers from sickle cell NS – immune to malaria NN – normal blood, susceptible to malaria Hybrid vigor – hybrid plants much hardier

16.2 – Evolution as Genetic Change Natural Selection on Single Gene Traits can lead to changes in the frequencies of alleles which then lead to evolution. Ex. – lizard survival

Natural Selection on Polygenic Traits - When traits are controlled by more than one gene, the effects of natural selection are more complex. - The effect of selection on varying characteristics in any of three ways: Directional Selection Stabilizing Selection Disruptive Selection

Directional Selection - occurs when individuals at one end of the curve have a higher fitness than individuals in the middle or at the other end. Fig. 16-6 Pg. 398

Stabilizing Selection - occurs when individuals near the center of the curve have higher fitness than individuals at either end of the curve. Fig. 16-4 Pg. 396

Disruptive Selection occurs when individuals at the upper and lower end of the curve have higher fitness than individuals near the middle. Fig. 16-8 Pg. 399

16.3 – Speciation Isolating Mechanisms As new species evolve, populations become reproductively isolated from each other. There are two modes of speciation base on how gene flow among populations is interrupted. They are: Allopatric Speciation Sympatric Speciation

Allopatric Isolation - occurs when a geographical barrier that physically isolates populations initially blocks gene flow. - Also called: Geographic Isolation

Sympatric Isolation - occurs when chromosomal changes (in Sympatric Isolation - occurs when chromosomal changes (in plants) and nonrandom mating (in animals) alter gene flow. Includes: Behavioral Isolation – occurs when two populations are capable of interbreeding but have different courtship rituals or other types of behavior. Reproductive Isolation The eastern meadowlark (left) and western meadowlark (right) have overlapping ranges. They do not interbreed, however, because they have different mating songs.

Temporal Isolation – when two or more species Temporal Isolation – when two or more species reproduce at different times. Sympatric populations become genetically isolated even though their ranges overlap.

Steps of Speciation Itext Activity (Page 409)

O. Understanding evolution of single-gene traits and polygenic traits. J. What are the steps of speciation?

Population becomes isolated from original population Population changes Population becomes reproductively isolated – the 2 populations no longer can mate together

Question: Are Darwin’s finches an example of Allopatric or Sympatric Isolation?

Speed of Speciation Gradualism - Species descended from a common ancestor gradually diverge more and more through physical changes as they acquire unique adaptations Punctuated Equilibrium - a new species changes most as it buds from a parent species, and then changes very little for the rest of its existence.