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Unit 17 - Speciation

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by

Nick Budziszewski

on 26 September 2013

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Transcript of Unit 17 - Speciation

Speciation
Mommy, where do new species come from?
Speciation results in the formation of new species.
Before we go any further, we should probably define what a species is. Too bad it's hard to agree on a definition!
Everyone knows that the
morphological species concept
is best! Organisms that look alike must be related, while organisms that look differently must not!
Fools! The
biological species concept
is best. Everyone knows that the dividing line between two species is whether they can successfully reproduce or not!
But what about asexual organisms? How can we tell what species they are if they are only mating with themselves? Clearly, the
lineage species concept
is best, since it looks at the evolutionary relationships between an organism and its ancestors!
No matter what model you decide to use, speciation occurs through similar mechanisms: isolation of some kind. The longer groups of organisms are kept apart, the more opportunities there are for genetic incompatibilities to arise.

Eventually, these genetic differences will be so great that the groups will be considered separate species.
Speciation occurs in two major ways:

1.
allopatric speciation

2.
sympatric speciation
Allopatric - "other home"

In other words, allopatric speciation occurs because of geographic isolation.

Allopatric speciation is the main way speciation occurs in most groups of organisms.
Sympatric Speciation - "same home"
Speciation usually takes a long time! How can new species develop if they aren't isolated geographically and have lots of chances to mate?





GOOD QUESTION, I'm happy you asked!
apple maggot fly
Sometimes disruptive selection is the cause of sympatric speciation.
hawthorn fruit
apples
Other times, polyploidy is the cause.
Duplications of chromosomes can lead to incompatibilities when it comes time to reproduce.

This process is especially common in plants.
No matter how speciation occurs, there is always some form of isolating mechanism at its heart.
Mechanical Isolation
Temporal Isolation
Behavioral Isolation
Habitat Isolation
Gametic Isolation
All of the previous forms of isolation were
prezygotic
-- they happen before offspring are formed.

Some isolating mechanisms are
postzygotic
-- they usually involve the hybrid babies not doing well. Maybe they don't develop properly, or have less fitness, or are infertile.
Full transcript