Speciation, Reproductive Isolation, and Patterns of Evolution AP Biology Evolution IV

Speciation Species: a group of individuals capable of interbreeding Speciation: the formation of new species (3 main types) –Allopatric Speciation –Sympatric Speciation –Adaptive Radiation

Allopatric Speciation

Allopatric Speciation Occurs when a population is divided by a geographic barrier –Barriers: mountains, rivers, regions excluding vital resources (water, food) areas covered with volcanic lava Interbreeding between populations not possible (reproductive isolation) Gene frequencies can diverge due to natural selection, mutation, and genetic drift

Sympatric Speciation apple maggot flies apples hawthorns

Sympatric Speciation The formation of new species without the presence of a geographic barrier Occurs by one of 3 ways: –Balanced Polymorphism –Polyploidy –Hybridization

Balanced Polymorphism Suppose a population of insects possesses a polymorphism for color. Each color provides a camouflage to a different substrate (rock, tree stump, etc..). When not camouflaged, they are eaten Thus, only insects with the same color can associate and mate Similarly colored insects are reproductively isolated…so gene pools can diverge.

Polyploidy More than 2 sets of chromosomes found in diploid (2n) cells. Often occurs in plants (occasionally animals) where triploid (3n), tetraploid (4n) and higher chromosome numbers exist. Caused by nondisjunction in meiosis Tetraploid individuals will continue to produce diploid gametes – making them reproductively isolated very quickly.

Hybridization Occurs when 2 different forms of a species mate and produce offspring along a geographic boundary called a hybrid zone. The genetic variation of the hybrids is greater than that of either parent This permits hybrids to adapt to environmental conditions beyond the range of either parent. Hybrids can eventually diverge from parent forms when faced with selective pressures

Adaptive Radiation

Relatively rapid evolution of many species from a single ancestor. Ancestral species colonizes an area where diverse geographic or ecological conditions are available for colonization. –i.e. many available ecological niches for a population to spread into. –Examples: Darwin’s Finches, Australian Marsupials

Patterns of Evolution Evolution can take place along the lines of the following patterns: –Divergent Evolution –Convergent Evolution –Parallel Evolution –Coevolution

Divergent Evolution Describes two or more species that originate from a common ancestor. This may happen as a result of allopatric or sympatric speciation or by adaptive radiation

Convergent Evolution Describes two unrelated species that share similar traits. Similar traits arise because each species has independently adapted to a similar niche These traits are called: Analogous Traits

Convergent Examples Sharks, porpoises, and penguins have torpedo- shaped bodies with peripheral fins. These traits arise as a result of adaptations each species has made to aquatic life…not due to a common ancestor. The eyes of squids and vertebrates are physically and functionally similar. However they are not from a recent common ancestor, both evolved independently to perform similar functions.

Parallel Evolution Describes two related species or two related lineages that have made similar evolutionary changes after their divergence from a common ancestor Example: –Species from two groups of mammals, the marsupial mammals and the placental mammals, have independently evolved similar adaptations when ancestors encountered comparable environments

Coevolution Describes the evolution of one species in response to new adaptations that appear in another species An example: evolutionary arms race between predators and prey…or –Plants and plant eating insects –Pollinators and flowering plants –Pathogens and animal immune systems