How Does Evolution Work?Individual organisms cannot

evolve. Populations of a particular species evolve.

Natural selection acts on the range of phenotypes in a population.

Microevolution occurs as the frequency of alleles in a population changes.

Evolution-What Happens?Macroevolution or Evolution occurs when there is a change in allele frequency which

produces a new species.

DefinitionsGene pool: All alleles of the

population’s genes.Allelic frequency: % of a specific

allele in the gene pool. Example: Approximately 75% have

dominant allele for tongue rolling. 25% non-rolling

Genetic Equilibrium: This exists when the frequency of alleles remains the same over generations. The population is not evolving.

When Does Evolution Occurs?Evolution results when there are Forces

that change allelic frequencies.

Forces that cause Evolution:1. Gene flow: Transport of genes by

migrating individuals.2. Nonrandom Mating: Mating based

on preferencesExample: a female may choose a mate

based on male size, color, or ability to gather food.

Forces of Evolution Continued3. Mutation: Change in DNA4. Genetic Drift: chance event

changes allelic frequencies – Greatly affect small populations such as the animals of the Galapagos Islands or Amish.

Causes of Genetic Drift1. Mating over a long time

period in a small population.

2. Little movement of males or females into or out of the population.

3 Types of Natural SelectionStabilizing selection – favors average individuals

Directional selection – favors one of the extreme variations of a trait

Disruptive selection – favors individuals with both extremes of a trait (eliminates intermediate phenotypes)

What is a Species?A population or

group of populations whose members have the ability to breed with one another and produce fertile offspring

Evolution of Species (Speciation)Significant changes in the gene pool can lead to evolution of a new species over time.

Speciation occurs when members of similar populations no longer interbreed to produce fertile offspring within their natural environment.

Artificial Speciation

Diane Dodd’s fruit fly lab, 1989

Why Don’t the Populations Interbreed?

1. Geographic isolation – physical barrier divides a population.

2. Reproductive isolation – formerly interbreeding organisms can no longer mate to produce offspring..

3. Change in niche -- Change in food source. Example Finches

1. Geographic Isolation

A physical barrier that separates a population into groups.

Can be1. Mountains or Rivers2. Islands with water in between

Darwin’s 13 finches on Galapagos3. Valleys caused by lava flow4. Roads/Highways

1. Geographical Isolation

2. Reproductive IsolationPrevents closely related species from interbreedingTimingBehaviorHabitat

TimingSimilar species have different breeding seasons

Eastern Spotted Western SpottedSkunk Skunk

Behavior

Similar species may have different courtship or mating behaviors.

Ex: Eastern & Western meadowlarks almost identical in color shape and habitat, but difference in courtship rituals differ different species

HabitatSpecies remain reproductively isolated because they are adapted to different habitats.

Ex: Stickleback fish one is a bottom feeder, one spends time in the top open layers of lakes in British Columbia, Canada

Patterns of Evolution

1. Divergent Evolution – evolution where species diverge or become less and less alike as they adapt to different environments.

2. Adaptive Radiation-ancestral species evolves into an array of species to fit diverse habitats. This is a type of divergent evolution

Both the wooly mammoth, which occupied parts of North America, and the elephant, still found in Asia and Africa are presumed to have evolved from a common ancestor.

Their geographical isolation and environmental selection pressures caused further evolution of the species.

Each, in its own location, occupies(d) a similar niche.

Patterns of EvolutionContinued2. Convergent Evolution – Unrelated species occupy similar environments in different parts of the world.

Similar pressures of natural selection lead to similar adaptations.

A Hummingbird Moth

A Humming Bird

Example of Convergent Evolution

Rhea

Emu

Ostrich

Speciation can occur quickly or slowlyGradualism – idea that species originate through a gradual accumulation of adaptations.

Punctuated equilibrium – hypothesis that speciation occurs relatively quickly, in rapid bursts, with long periods of genetic equilibrium in between.

GradualismGradual changes in species over time

Evidence of many intermediate forms in fossil records

Punctuated EquilibriumScientists

found remains of intermediate forms

Also saw that populations remained the same over large periods of time then suddenly changed

Phyletic SpeciationPhyletic speciation is a process of gradual change in a single population. The modern form of the organism differs from the original form so much that the two can be considered separate species. Phyletic speciation could be drawn as a line. Species A becomes species B, which becomes species C, etc. In the past, phyletic speciation has been proposed for human evolution and the evolution of the horse. The problem with phyletic speciation is that it would only occur if there were a gradual change in the selective regime that progressively favored the modern form. This seems an unlikely occurrence in nature and the fossil record does not support phyletic speciation for either human or horse evolution. For these reasons, natural phyletic speciation is believed to be rare. Artificial selection in domestic animals and plants approximates phyletic speciation, however. The familiarity of this sort of evolution is probably the only reason that phyletic speciation was ever considered as a hypothesis of natural speciation.

Divergent Speciation

If phyletic speciation is drawn as a line, divergent speciation has the form of a branching tree. Species A splits into species A and B. Species B may subsequently branch into species C, and so on. Species A, B and C may exist all at the same time and any of them may be ended by extinction at some point in the process. Divergent speciation is consistent with fossil evidence of biological evolution and with the known mechanisms of biological evolution

Divergent speciation, the branch points in the tree described above, results from reproductive isolation of two parts of a population. Reproductive isolation means that interbreeding between the two groups is prevented by some barrier. Once interbreeding ends, two processes cause the isolated group to become different from the parent population:

1) Genetic variation occurs independently in the two groups. Lack of interbreeding prevents sharing of these independent genetic variations. Thus, the genetic variation on which natural selection acts is different in the two groups. 2) Selection may be different for the two groups, especially if they live in different places. If selection differs, different variants will be favored in the two groups.

Over time, the two populations become sufficiently different that they can no longer interbreed even if barriers to interbreeding are removed. Speciation has occurred.