9.2 Speciation: How species form SBI3U Ms. De Sousa

speciation Biological Species: population where individuals are able to interbreed to produce viable offspring. Speciation occurs when there is a dramatic change and the population of species are no longer able to interbreed. Thus resulting in the formation of a new species.

speciation Two populations of species that are reproductively isolated have little to no gene flow in between them. There are two main types of mechanisms that reproductively isolate two groups of species. 1) Pre-zygotic 2) Post-zygotic

1) pre-zygotic isolating mechanisms When species are not able to mate or when eggs cannot be fertilized. There are a total of 5 different forms of pre- zygotic isolating mechanisms

a) Behavioural isolating mechanisms When species have a specific signal or behaviour it prevents closely related species from interbreeding. Both birds are very similar in appearance. Due to the differences in their song, but these birds do not interbreed. Western Meadowhalk Eastern Meadowhalk

b) Habitat Isolating mechanism When two species live in the same area but different habitats, they rarely interact with one another. Garter snake (thamnophis sirtalis) perfers open areas. Garter snake (thamnophis ordinoides) commonly found near water.

c) Temporal isolating mechanisms Temporal conditions refers to time of day, year and seasonal. Some organisms mate at different times which limits their interaction. This is very common in flower species.

d) Mechanical isolating mechanisms Some species that are closely related do not have a compatible genital anatomy. This incompatibility prevents them from mating. This is very common in insects, for it behaves as a lock-and-key system. Plants also have different structures that prevent the pollen from fertilizing the flower.

e) Gamete isolating mechanisms This occurs when a species’ gametes are not able to meet. The gametes are not able to survive within the female’s reproductive tract which prevents the gametes form interacting.

2) post-zygotic mechanisms There are times when the sperm and egg meet, however the zygote cannot develop into a viable offpsring. As a result, species still remain distinct from one another because their genetic information does not combine. There are 3 different post-zygotic mechanisms.

a) Hybrid inviability When the genetic information is not compatible between species. This prevents the zygote from undergoing mitotic division, and the zygote cannot develop further.

b) Hybrid sterility When two species can mate and produce viable offspring. The hybrid offspring cannot undergo regular meiosis and cannot produce eggs or sperm. The hybrid offspring is thus sterile and cannot propagate its genes into the population.

c) Hybrid breakdown Two organisms are able to mate and produce viable and fertile offspring. When the hybrid species mate, their offspring are weak and sterile. This limits the amount of generations in which genes can propagate within the population.

speciation In order for speciation to occur, populations of organisms need to be genetically isolated from one another. Speciation can happen in two main ways depending on the gene flow between the populations. 1) Sympatric Speciation 2) Allopatric Speciation

Allopatric speciation Occurs when the gene flow is interrupted due to the divison of the population into subpopulations. Separation can occur due to river, water level change, geological remodeling etc. The separated gene pools evolve due to different selective pressures in their environment.

Because the population is small, allopatric speciation is more likely to cocur over a shorter period of time.

Sympatric Speciation Occurs when populations live in the same geographical area and become reproductively isolated. Because these species live in the same geographical area, there needs to be a change in their genetic information. Most of sympatric speciation is due to chromosomal changes and non-random mating.

Sympatric speciation: chromosomal change Example 1: Polyploidy Causes a reproductive barrier Error in cell division resulting in an extra set of chromosomes.

Polyploidy

polyploidy During meiosis, if the cells fail to separate properly, cells become diploid instead of haploid. There ability to self-fertilize enables these diploid cells to fuse with one another to produce tetraploid species. These polyploid cells cannot be crossed with a haploid or else it would form triploid cells which are infertile.

Sympatric speciation: non-random mating When two species interbreed they produce a sterile offspring. However, the sterile offspring is able to reproduce asexually, thus creating a new population of species. Eventually this polyploid population because of self-fertilisation.

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 envrionmental conditions.

Speed of evolutionary change Gradualism: views evolution as a slow and steady pace before divergence. Big changes occur due to an accumulation of small changes. This pattern of evolution is very rare within the fossil record.

Gradualism

Punctuated equilibrium Views evolution as long period of stasis where no change occurs, which are later interupted by periods of divergence. Species usually undergo a dramatic change when they are first isolated and then stabilize. This is the most common pattern shown in the fossil history.

Human activity and speciation Allopatric speciation may occur when humans perform the following activities Convert large stretches of wilderness into croplands Develop wilderness areas for recreation or tourism Build roads Build urban subdivisions

Human Activities and population decline The following human activities cause a bottle neck effect within the population. Unregulated hunting Habitat removal