Speciation – How Species Form Section 9.2

Species  Physiology, biochemistry, behaviour, and genetics are used to distinguish one species from another .

Species  Physiology, biochemistry, behaviour, and genetics are used to distinguish one species from another  Species – individual members can interbreed to reproduce viable and fertile offspring

Speciation  The formation of a new species from existing species .

Speciation  The formation of a new species from existing species  Occurs when some members of a sexually reproducing populations change so much that they are no longer able to produce viable, fertile offspring with members of the original population

Micro vs. Macro  Microevolution = evolution within a population  Macroevolution = formation of a new species

Reproductive Isolation ..

 2 populations may become reproductively isolated (become 2 species) over time if there is little or no gene flow between them .

Reproductive Isolation  2 populations may become reproductively isolated (become 2 species) over time if there is little or no gene flow between them  Gene flow can be prevented pre-zygotic or post-zygotic

Reproductive Isolating Mechanisms Pre-zygoticPost-zygotic Prevention of Mating Prevention of Fertilization Prevention of Hybrids Behavioural isolation Mechanical isolation Zygotic mortality Temporal isolation Gametic isolation Hybrid inviability Ecological/habit at isolation Hybrid infertility

Pre-zygotic Isolating Mechanisms (5)  Impede mating between species or  prevent fertilization of the eggs if individuals from different species attempt to mate

1. Behavioural Isolating Mechanisms  Any special signals or behaviours that are species specific prevent interbreeding with closely related species  Examples: bird songs, courtship rituals, pheromones, etc.

2. Habitat Isolating Mechanisms  Two species may live in same general region but in different habitats and therefore rarely encounter one another  Example – 2 species of North American garter snakes – one prefers open areas and avoids water while another is commonly found near water

3. Temporal Isolating Mechanisms  Separation by temporal or timing barriers  Different mating schedules (different times of day, different seasons, or in different years)  Example: orchids that bloom for a single day as a response to stimuli in the weather…1 blooms after 8 days, 1 after 9 and 1 more after 10 days

4. Mechanical Isolating Mechanisms  Failed fertilization due to incompatible anatomy  Genital anatomy is distinctive in many organisms and can be used to classify species

5. Gametic Isolating Mechanisms  If gametes from different species do meet, gametic isolation ensures they won’t fuse and form a zygote  Various techniques for various species  Example – male sperm will not survive environment of female reproductive tract of another species

Post-zygotic Isolating Mechanisms  Prevention of hybrid zygote development even if the sperm of one species successfully fertilizes the egg of another species and a zygote is formed  3 methods

1. Hybrid Inviability  Genetic incompatibility of the interbred species may stop development of the hybrid during development  Normal mitosis is prevented after fusion of the nuclei in the gametes  Zygotes of sheep and goats dies early in development…long before birth

2. Hybrid Sterility  Two species mate and produce a hybrid offspring, but the offspring will be sterile and unable to reproduce  Meiosis fails to produce normal gametes in hybrid b/c chromosomes of 2 parent species differ in number or structure  Example – Mule born of a female horse and male donkey

3. Hybrid Breakdown  First generation hybrids are fertile  Second generation hybrids are sterile or weak (even if mating with an individual from either parent species)

Types of Speciation  Sympatric  Populations living in same habitat diverge and become reproductively isolated  Allopatric  Populations are separated by a geographical barrier and then diverge genetically

Sympatric Speciation – Ex 1  Chromosomal changes (in plants) and non-random mating (in animals) alter gene flow  More common in plants than animals  A new species can be generated in one generation if a genetic change results in a reproductive barrier between the offspring and parent generations

What is non-disjunction yo?

 Non-disjunction can lead to polyploidy (extra sets of chromosomes) which can lead to speciation  A polyploidy organism has 3+ sets of chromosomes (rather than 2)

 If chromosomes don’t separate in meiosis, gametes have two sets of chromosomes (diploid instead of haploid)  If two diploid gametes fuse, the offspring will be tetraploid (4 of each chromosome)  If tetraploid survive, they can undergo meiosis and produce diploid gametes  Organism can reproduce with other tetraploids, but not parent generation b/c that would form triploids  *see page 364

Illustration – page 364

 In one generation, a reproductive barrier has been established in a population b/c gene flow was interrupted

Sympatric Speciation – Ex 2  Two species interbreed to produce a sterile offspring  Offspring is sterile but can reproduce asexually resulting in a separate population  Many plants do this: wheat, cotton, oats, potatoes

Allopatric Speciation  Population is split into 2+ isolated groups by a geographic barrier  Geographic barriers  glacier or lava flow, and fluctuations in ocean levels (creation of islands)  Gene pool of split population becomes so distinct that groups are unable to interbreed even if brought back together

 Once separated populations begin to diverge b/c of natural selection, mutation, genetic drift, and/or gene flow  Isolation doesn’t need to last forever, but must last long enough for populations to become reproductively incompatible before they are re-joined

 Small populations on the periphery are more likely to become the new species  Small populations are more susceptible to genetic drift and mutations and even natural selection through selective pressure if the environment is different.  *not all isolated populations will survive long enough to change

Darwin’s Finches - Allopatric  Birds ended up on the Galapagos islands in the past  Only birds on the island and therefore had many ecological niches to undertake (ecological role and physical distribution of a species in its environment)

 Ancestral species divided into different populations and some evolved into new species due to the various selective pressures they experienced  Main differences are genetic an beak length

Practice Questions  Page 363  #15, 16, and 18  Page 365  #19, 22, 23, and 24