Lesson # 8: Evolution (Speciation)

Introductory Terms - In order for one population to become very different from another, they must be reproductively isolated - This means that there will no longer be a free exchange of alleles between the two populations What is a Species? Includes all members of a population that can interbreed under natural conditions Speciation Occurs when the gene pool of a population become split The formation of new species The process by which reproductively isolated populations become distinct species

-New species can form as a result of the evolution of a reproductive isolating mechanism that prevents members of two populations from interbreeding -New species can evolve under a variety of circumstances Reproductive Isolating Mechanism -Any behavioural, structural, or biochemical trait that prevents individuals of different species from reproducing successfully together Modes of Speciation

Types of Barriers Prezygotic Barriers -A reproductive isolating mechanism that prevents interspecies mating and fertilization Examples 1)Behavioural isolation 2)Temporal isolation 3)Ecological isolation 4)Mechanical isolation 5)Gametic isolation Postzygotic Barriers -A reproductive isolating mechanism that prevents reproduction in offspring from interspecies reproduction Examples 1)Zygotic mortality 2)Hybrid inviability 3)Hybrid infertility

Prezygotic Barriers 1)Behavioural Isolation -Different species use different courtship and other mating clues to find and attract a mate -Usually in the form of mating signals and rituals Examples -Male frogs of different species have unique calls that attract only females of their own species -Fireflies showing distinct patterns of flashes attract various mates

Prezygotic Barriers 2) Temporal Isolation -Different species breed at different times of the year Examples -Often used by plants because plants cannot move so they must use mechanisms such as timing to reproduce at specific times  One might flower in the early spring, and by the time that is done flowering another species might just beginning to flower (certain flowers mate in the spring, others in the fall)

Prezygotic Barriers 3) Ecological Isolation -Very similar species may occupy different habitats within a region -Species utilize different resources in the habitat -Individuals mate in their preferred habitat, and therefore do not meet individuals of other species with different ecological preferences. Example -The mountain bluebird lives at high elevations, while the eastern bluebird prefers lower elevations and does not encounter the mountain species

Prezygotic Barriers 4) Mechanical Isolation -Differences in morphological features may make two species incompatible -The structure of the genitals is so distinct that it is often used as a character in insect classification -Only one male reproductive organ will fit into the specific female reproductive tract of a species Example -Many insects have modifications on their genitalia such that the male and female parts are a perfect mechanical fit -Orchids are shaped so that only certain beetles can reach the point of pollination, and therefor pick up the pollen. That beetle will then go to another orchid of the same type to deposit the pollen

Prezygotic Barriers 5) Gametic Isolation -Male gametes may not be able to recognize and fertilize an egg of a different species -Two populations exchange sperm and egg but chemical markers prevent the eggs from being fertilized because they do not recognize them Example -Many marine animals including corals, clams, and sea cucumbers release their sperm and eggs into open water. The sperm recognizes the eggs of their own species through chemical markers on the surface of the eggs

Scenarios

Two snake types share an over – wintering burrow and eat the same diet, but mate at different optimal temperatures

Two turtles share a pond and general diet, but the female of one has a tall shell and the other a flat shell; the female of the first has a hallow in its lower shell, the male has a flat lower shell

Two sparrows share a similar diet, winter in different places, but share mating and breeding grounds with one singing from the tops of trees and one from small bushes

Two voles hibernate in different places, occasionally mate but the sperm and egg do not fuse

Two fireflies share the same diet, mate at the same time throughout the year, but create various patterns of flashes to attract their mate

Post Zygotic Barriers 1)Zygotic Mortality -Mating and fertilization are possible, but genetic differences result in a zygote that is unable to develop properly -Results in death of zygote Example -Some species of sheep and goat are able to mate, but the zygote will die 2) Hybrid Inviability -A hybrid individual develops but either dies before birth or, if born alive, cannot survive to maturity Example -When tigers and leopards are crossed, the zygote begins to develop but the pregnancy ends in a miscarriage or stillborn offspring 3) Hybrid Infertility -Hybrid offspring remain healthy and viable but are sterile Example - Mules are the sterile hybrid offspring of a horse – donkey cross

Other Mechanisms 1)Allopatric Speciation -The formation of a new species as a result of evolutionary changes following a period of geographic isolation Example -River, volcano, mountain range 2) Sympatric Speciation -The evolution of populations within the same geographic area into separate species Example -Various types of tress in a forest