Speciation

How do new species generate from pre-existing species?

Generating New Species Steps

1. Two populations become isolated from one another with no gene flow between the two

2. Divergence in the two populations because of no gene flow. New variants are introduced through random mutation OR the environment of one of the population changes and there is a difference in fitness for a variant

3. Divergence of traits that prevent interbreeding between the two species

divergence- differences accumulating over time

Reproductive Isolation Mechanisms

1. Pre-Zygotic Mechanisms

• zygote- single cell diploid

• happens before the sperm and egg cell make a zygote

a. Temporal Barrier

• temporal barrier- relating to the timing of reproduction

  ex. flowers that bloom in spring can’t mate with flowers that bloom in the fall due to a different time in reproduction of the two flower types

• species that reproduce at different times

b. Behavioral Barrier

• unique mating ritual

c. mechanical barrier

• how reproduction happens

• difference in genitalia

d. gametic barrier

• prevents fusion of gametes

2. Post-zygotic Mechanisms

   a. hybrid inviability

• embryo can’t completely develop to independence

• before birth (for humans) after birth (for animals)

b. hybrid sterility

• hybrid offspring is not capable of reproducing

Types of Speciation

How is gene flow stopped between two populations?

1. Allopatric Speciation

• geographic barrier (river, canyon, etc)

2. Sympatric Speciation

• no geographic barrier

EX. apple maggot fly

1800s european apple trees introduced, males and females prefer to live around the type of tree they hatched on. limited gene flow because they had a preference

• rare in animals

• roughly 25% of plants arose from sympatric speciation

Polyploidy

• polyploidy- an organism that has many versions (sets) of a chromosome in its genome

• sometimes generated by a mistake in mitosis or meiosis

• some plants self fertilize and can create new polyploid populations because the polyploids can’t reproduce with a plant with a different number of chromosomes

• can happen in one generation