9 2 Speciation How Species Form SBI 3
9. 2 -Speciation: How Species Form SBI 3 U 1
What is Speciation? Recall: Microevolution: change within the frequency of alleles in a population. Speciation: the formation of a new species from an existing species. A. K. A. “macroevolution” *Note: Two different species of zebra.
Why does speciation happen? � When two populations have become reproductively isolated over time, there is “No gene flow between them” Two types of reproductive isolating mechanisms: 1. Pre-Zygotic 2. Post-Zygotic
Pre-Zygotic Isolating Mechanism � Species are not able to mate or when eggs cannot be fertilized. (A. K. A. - Pre-fertilization Barriers) � 5 pre-zygotic isolating mechanisms: -Behavioural -Habitat -Temporal -Mechanic -Gametic
1) Behavioural Isolating Mechanisms � Species have a specific signal or behaviour that prevents closely related species from interbreeding. Both birds are very similar in appearance. Due to the differences in their song, these birds do not interbreed. Western Meadowhalk Eastern Meadowhalk
2) Habitat Isolating Mechanism � Two species live in the same area but different habitats, they rarely encounter and interact with one another. Garter snake (thamnophis sirtalis) perfers open areas. Garter snake (thamnophis ordinoides) commonly found near water.
3) Temporal Isolating Mechanism � Temporal conditions refers to time of day, year and season. � Some organisms mate at different times which limits their interaction. � Very common in flower species.
4) Mechanical Isolating Mechanism � Some species are anatomically incompatible and unable to fertilize. � Ex: The genital in insects operate like a lock & key model
5) Gametic Isolating Mechanism � Occurs when a species’ gametes (egg & sperm) are not able to fuse to form a zygote � Ex: Sperm is unable to survive within female reproductive tract
Post-Zygotic Mechanisms � Sperm and egg meet, but zygote cannot develop into a viable offspring. � There are 3 post-zygotic mechanisms. -Hybrid Inviability -Hybrid Sterility -Hybrid Breakdown
1) Hybrid Inviability � When genetic information is not compatible between species. � Zygote does not undergo mitosis, and zygote cannot develop further. � E. g. hybrid embryo between sheep and goats die before birth
2) Hybrid Sterility � Two species can mate and produce hybrid offspring. (ie. Mule + donkey) � However, � Meiosis The hybrid offspring is sterile. fails to produce normal gametes in the hybrid offspring.
3) Hybrid Breakdown � Two organisms are able to mate and produce viable and fertile offspring. � However, when the hybrid species mate, ( 2 nd generation) their offspring are weak and sterile. Cotton plants produce fertile hybrids, but their offspring die as seeds
Section 9. 2 Reproductive Isolating Mechanisms SUMMARY
2 Types of Speciation 1) Sympatric Speciation 2) Allopatric Speciation
1) Sympatric Speciation � Populations in same geographical area become reproductively isolated � � � More common in plants than animals chromosomal changes in plants or non-random mating in animals alters gene flow. The result is reproductive incompatibility without geographical isolation.
2) Allopatric Speciation � Occurs when populations are separated by a geographical barrier and diverge genetically. � Gene flow is interrupted � Separation can occur due to river, water level change, geological remodelling etc.
What is Adaptive Radiation? � Type of allopatric speciation � Diversification of a common ancestral species into a variety of differently adapted species � Example: ◦ A few seed eating birds occupy an empty niche with fruit ◦ Mutation makes the birds good fruit eaters ◦ Fruit and seed eaters no longer mate
Divergent vs. Convergent Evolution Divergent Evolution: �Species that were similar to the ancestral species diverge and become increasingly distinct. Convergent Evolution: �Similar traits arise because different species have independently adapted to similar environmental conditions.
Speed of Evolutionary change Two models have been proposed to explain the speed at which evolution occurs. Gradualism • views evolutionary change as slow and steady, before and after a divergence • supported since Darwin’s time Punctuated Equilibrium • views evolutionary history as long periods of stasis, or equilibrium, that are interrupted by periods of divergence
UNIT 3 Chapter 9: Evolution and Speciation Section 9. 2 The Speed of Evolutionary Change It is now accepted that both models of evolutionary change are at work. While many species have evolved rapidly during periods in Earth’s history, the fossil record also shows very gradual change for some species over extended periods of time. Two models have been proposed to explain the speed of evolution: (A) gradualism and (B) punctuated equilibrium.
UNIT 3 Chapter 9: Evolution and Speciation Section 9. 2 Impact of Human Activities Human activities have had great impact on speciation, population decline, and extinction. Here are some examples : • conversion of wilderness into cropland • development of areas for tourism • building urban subdivisions and roads
UNIT 3 Chapter 9: Evolution and Speciation Section 9. 2 Mass Extinctions �Mass extinctions decrease the number of species. �However, the overall trend is an overall increase in species over the course of Earth’s history. Example of Mass Extinction: Cretaceous Dinosaur A global trigger, likely a large asteroid impacting Earth, caused the extinction of dinosaurs and more than 50% of the marine species.
Homework: � Upcoming Evolution Unit test- TBA � Read and make note 9. 2 � Complete pg. 373 Q #1 -4, 6, 8
- Slides: 24
