Chapter 16: Evolution of Populations

16.1 Genes and Variation Variation and Gene Pools Gene pool – all genes in a population of organisms Allele frequency – the number of times the allele occurs in a gene pool In genetic terms, evolution is any change in the frequency of alleles in a population.

Gene Pool for Fur Color in Mice When scientists determine whether a population is evolving, they may look at the sum of the population’s alleles, or its gene pool. This diagram shows the gene pool for fur color in a population of mice. 

Sources of Genetic Variation Mutations Any change in a sequence of DNA Gene Shuffling Crossing-over Sexual reproduction Random arrangement of chromosomes in Metaphase I of meiosis

Single-Gene and Polygenic Traits Single Gene Traits – traits controlled by one gene that has two alleles Example: Widow’s Peak – AA or Aa - have widow’s peak aa - have no widow’s peak

In humans, a single gene with two alleles controls whether a person has a widow’s peak (left) or does not have a widow’s peak (right). As a result, only two phenotypes are possible. The number of phenotypes a given trait has is determined by how many genes control the trait.

Polygenic Traits – traits controlled by two or more genes Example: Height in humans

Bell shaped curve is normal distribution The graph below shows the distribution of phenotypes that would be expected for a trait if many genes contributed to the trait.

16.2 Evolution as Genetic Change If an individual dies without reproducing, it does not contribute its alleles to population’s gene pool. If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency. Populations, not individual organisms, can evolve over time. Photo credit: ©MURRAY, PATTI/Animals Animals Enterprises

Natural selection can affect the distributions of phenotypes in three ways: Directional Selection Phenotypes shift toward homozygous dominant or homozygous recessive Example: Darwin’s Finches The finches had beaks of different sizes to eat different food. What if the supply of small seeds disappeared only leaving the large, hard seeds? Those finches with big beaks would survive causing a shift to that phenotype

Small Medium Large Beak Size Directional selection occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. In this example, a population of seed-eating birds experiences directional selection when a food shortage causes the supply of small seeds to run low. The dotted line shows the original distribution of beak sizes. The solid line shows how the distribution of beak sizes would change as a result of selection.

Stabilizing Selection When intermediate forms (heterozygote) of a trait are favored and alleles that specify extreme forms (homozygote) are eliminated from a population Example: Darwin’s Finches What if the supply of seeds was mostly medium size seeds?  

Small Medium Large Beak Size Number of Birds in Population Beak Size In this example of stabilizing selection, human babies born at an average mass are more likely to survive than babies born either much smaller or much larger than average. Small Medium Large Beak Size

Disruptive Selection Forms at both extremes of the range of variation Extremes are favored and the intermediate form is selected against Example: Darwin’s Finches What would happen if the supply of medium seeds disappeared?

Example: Darwin’s finches What would happen if the supply of medium seeds disappeared? In this example of disruptive selection, average-sized seeds become less common, and larger and smaller seeds become more common. As a result, the bird population splits into two subgroups specializing in eating different-sized seeds. Small Medium Large Beak Size

Genetic Drift Genetic Drift – a random change in allele frequencies over the generations Genetic drift has a greater effect on small populations.

Founder Effect Occurs when allele frequencies in a group of migrating individuals are by chance not the same as that of their original population

Example: beetles Two small groups of different beetles leave the population.

These two small groups start their own population.

The two new populations are genetically different from the original population. Population A Population B

In small populations, individuals that carry a particular allele may have more descendants than other individuals. Over time, a series of chance occurrences of this type can cause an allele to become more common in a population. This model demonstrates how two small groups from a large, diverse population could produce new populations that differ from the original group.

Bottleneck Occurs when the population undergoes a dramatic decrease in size. Causes: Natural catastrophes Predation Disease

Evolution Versus Genetic Equilibrium 1908 Hardy and Weinberg independently suggested a scheme whereby evolution could be viewed as changes in the frequency of alleles in a population of organisms Hardy-Weinberg – allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. Genetic Equilibrium – When allele frequencies remain constant

5 conditions are required to maintain genetic equilibrium from generation to generation: There must be random mating Population must be very large There can be no movement of genes into or out of the population No mutations No natural selection – all genotypes must have an equal rate of survival and reproduction

16.3 The Process of Speciation Natural selection and chance events can change the relative frequencies of alleles in a population and lead to speciation. Speciation – formation of a new species Species – a group of organisms that breed with one another and produce fertile offspring.

Isolating Mechanisms As new species evolve, populations become reproductively isolated from each other. When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred.

3 Types of Reproductive Isolation Behavioral Isolation Occurs when a species does not recognize another species as a mating partner because it does not perform the correct courtship rituals, display the proper visual signals, sing the correct mating songs or release the proper chemicals

2. Geographic Isolation Occurs when two populations are separated by geographic barriers such as rivers or mountains.

3. Temporal Isolation Occurs when two species mate or flower during different seasons or at different times of the day

17.4 Patterns of Evolution Macroevolution – large-scale evolutionary patterns and processes that occur over long periods of time. Extinction More than 99% of all species that have ever lived are now extinct What effects have mass extinctions had on the history of life? Mass extinctions have: Provided ecological opportunities for organisms that survived Resulted in bursts of evolution that produced many new species

Divergent Evolution Two or more species that originate from a common ancestor. Adaptive radiation – a type of divergent evolution – the process by which a species evolves into several different species The disappearance of dinosaurs then resulted in the adaptive radiation of mammals.

time A time B time C time D parent species time

Convergent Evolution Convergent evolution – the process by which unrelated organisms come to resemble one another. Convergent evolution has resulted in sharks, dolphins, seals, and penguins.

Coevolution Coevolution – the process by which two species evolve in response to changes in each other over time. Example: predator prey