The Evolution of Populations and Speciation

The Evolution of Populations and Speciation Modern Biology Chapter 16

Genetic Equilibrium Modern Biology 16-1

Variation

How does variation arise? Mutations changes in the structure of an organism’s DNA/ genes genes are the source of variation rates of mutation are quite variable may have positive and negative affects

How does variation arise? gene shuffling when genes are combined in different ways, different results occur Independent assortment and crossing over shuffle genes How common is genetic variation? Only about 5% of all vertebrate genes have 2 or more alleles

Genes, fitness and adaptation Fitness: success in passing genes on to the next generation Adaptation: any genetically controlled trait that increases an organism’s ability to pass along copies of its genes

Genes, fitness and adaptation Genes also define species species: a group of similar looking organisms that can breed with one another and produce fertile offspring reproductive isolation: idea that members of different species cannot reproduce gene pool: group of alleles available to a population

Reproductive Isolation Once upon a time certain fruit flies found mates on rotting bananas and ignored the rotting mangoes. • Two flies were blown away to an island where there were no bananas, so they found each other on mangoes. • When the flies from the mango island were blown back to the main land, they found mates on mangoes while the original flies still preferred bananas, hence reproductive isolation.

Speciation • The branching points on this partial Drosophila phylogeny represent speciation events that happened in the past.

The scene: a population of wild fruit flies is minding its own business on several bunches of rotting bananas, cheerfully laying their eggs in the mushy fruit.

Disaster strikes: A hurricane washes the bananas and the immature fruit flies they contain out to sea. The banana bunch washes up on an island off the coast of the mainland. The fruit flies mature and emerge onto the lonely island. The two portions of the population, mainland and island, are now too far apart for gene flow to unite them. At this point, speciation has not occurred — mainland and island fruit flies can mate and produce healthy offspring.

The populations diverge: Ecological conditions are slightly different on the island, and the island population evolves under different selective pressures and experiences different random events than the mainland population does. Morphology, food preferences, and courtship displays change over the course of many generations of natural selection.

So we meet again: When another storm reintroduces the island flies to the mainland, they will not readily mate with the mainland flies since they've evolved different courtship behaviors. The few that do mate with the mainland flies, produce unviable eggs because of other genetic differences between the two populations. The lineage has split now that genes cannot flow between the populations.

Genes, fitness and adaptation Swimming in the gene pool without mutation or natural selection the frequency of alleles in the gene pool remains fairly constant genes get resorted, but the frequency does not change

Modern Biology 16-2 Disruption of Genetic Equilibrium

Natural Selection • The idea that organisms better suited to their environments will survive to reproduce more successfully than organisms less well adapted.

Types of natural selection • Stabilizing selection • Selection in which the extremes are selected against

Types of natural selection • Directional selection • Selection against one of the extremes

Types of natural selection • Disruptive selection • Selection against the middle

Formation of Species Modern Biology 16-3

The birth of a new species • speciation: the formation of new species

The birth of a new species • for speciation to occur, a population must evolve enough genetic changes so that breeding cannot occur between the groups (why dogs are all one species)

The birth of a new species • usually occurs when a physical barrier comes between different populations, barrier may be removed later • barriers include plate tectonics, mountain ranges rising, a busy road

Animation

The pace of evolution • gradualism: a slow and steady change over long periods of time • Darwin believed this means of evolution • Evidence in horse evolution

The pace of evolution • punctuated equilibrium: species remain unchanged for a long time and then periods of evolution occur quickly • evidence in • occurs after mass extinctions

Darwin’s finches • 13 different species of finches within the Galapagos islands • all evolved from one common ancestor

Darwin’s finches • island theory: different conditions will create different pressures

Darwin’s finches • beak shapes are very different, as are body structures • different diets have caused the evolution of different shaped beaks • seed eaters have sturdy relatively short beaks for breaking open hard seed coats • insect eaters have long thin beaks for grabbing grubs in tree bark

A Galapagos study • looking for natural selection • birds became specialists in eating different foods • what selection does • when food is plentiful, the variation doesn’t matter much • when food is scarce, the variants best able to cope have a higher fitness

Evolving Differences • Organisms that are more closely related tend to have more traits in common • Not as true as once thought • Example: sharks and whales • All vertebrates have many similar characteristics: backbone, 4 limbs, etc.

Evolving Differences • Adaptive radiation: occurs when an organism or group of organisms colonizes a new area and competition drives changes toward organisms more suited to that environment • Homologous structures become less similar • Example: Darwin’s finches

The Evolution of Populations and Speciation