Do Now Explain the difference between directional and disruptive selection. Directional – one extreme of the variation of the trait is most fit Disruptive – BOTH extremes of the variation is most fit What type of selection is most common? Stabilizing selection Homework: finish book work from class

Speciation

What is Speciation? A species is defined as a group of organisms that can successfully interbreed, producing fertile offspring. Speciation – the formation of new species How does a species evolve into two new species? The gene pools of two populations must become separated. Over time genetic changes cause reproductive isolation between the populations.

Reproductive Isolation Reproductive isolation means that the organisms can no longer interbreed successfully. There are 2 main types of Reproductive Isolation: Pre-zygotic – prevents fertilization from taking place Examples: Geographical, Behavioral, Temporal Post-zygotic – prevents fertile offspring from being produced after fertilization occurs

Geographic Isolation Geographic isolation occurs when populations are physically separated from each other. Natural selection works separately on each population. Differences in environment can cause genetic differences. Over time, genetic changes can lead to formation of 2 separate species. Geographic isolation does not always lead to the formation of new species. Geographical barriers may be temporary. Geographical barriers may not affect all organisms (Ex: River would separate land animals but not necessarily birds.)

Geographic Isolation Examples Pupfish lived in a large lake in Death Valley After Ice age, the species were isolated into different ponds The environments in the ponds differet and the poputions diverged, forming two different species Abert squirrel on left, kaibab squirrel on right – used to be one species but colorado river split it over 10,000 years ago. Look similar but are now different species

Behavioral Isolation Occurs when species do not interbreed because of behavioral differences. Species do not recognize the mating calls or courtship behaviors of other species. Bird Rituals

Temporal Isolation Many species do not breed all year long and have specific mating seasons. Temporal Isolation occurs when species sharing the same environment may reproduce at different times in the year.

Post-zygotic Isolation Post-zygotic isolation prevents fertile offspring from being produced from the mating of two different species. Several different ways that post-zygotic isolations occurs: Offspring may die before birth Offspring may die before reproductive age Offspring may live, but may not be fertile

Example of Post-zygotic Isolation Horses and donkeys can produce offspring, even though they are different species. The offspring of a female horse and male donkey is called a mule. Mules are sterile and cannot reproduce.