Isolating mechanism and speciation in time 1

Reproductive isolation and its Mechanisms
Speciation in time: gradualism and punctuated equilibrium

1. Isolating mechanism and speciation in time Submitted by: Shilpa Kumari CUHP18ZOO23 Submitted to: Miss. Ekta Guleria

2. Isolating mechanism Definition: Any factor which prevents the interbreeding between even the related groups of living organism is called isolating mechanism. The phenomenon of inability of living organism to interbreed is called reproductive isolation. According to Meyer(1942), reproductive isolating mechanisms are the biological properties of individuals which prevent the interbreeding of naturally sympatric populations.

3. When interbreeding between subgroups of a populations, called Demes, progressively decreases and gives progressively less fertile hybrids, the various sub groups(demes) eventually become different species. Deme A Deme B Deme C Deme B Deme D Deme A Deme C Deme D Splitting of a population by a barrier

4. types of isolating mechanism Pre-mating mechanism: These prevent the copulation and fusion of gametes of interspecific crosses by: Geographic isolation: In this, organisms are separated by geographical barriers such as water bodies, mountains, deserts etc. which prevent interbreeding between them. So processes like natural selection occurs independently on both side and leads to the formation of new species. Example: Darwin’s finches on Galapagos island.

5. Spatial isolation: In this, individuals isolated by long distance are unable to breed due to their inability to cover long distance.

6. Seasonal or temporal isolation: The breeding season of the animal of one species is different from that of other species, so are reproductively isolated. Example: American toad ( Bufo americana) breeds in early part of rainy season, while Fowler’s toad( B. fowleri) breeds in later part of rainy season, so are reproductively isolated.

7. Ecological or habitat isolation: Mating inability between the organism of closely related species or sub species occupied the same region but different habitat. Example: Two species of Rana that is Rana grylio (pig frog),and R. areolata (Gopher frog) live in different habitat. So the two species show ecological isolation.

8. Ethological isolation: Different mating behavior of two species prevents interbreeding. Example: Males of different species of Sticklebacks have different mode of nest building and courtship dancing behavior.

9. Physiological isolation: Mating inability due to functional incompatibility in their mating. Example: swelling of vaginal mucous membrane of Drosophila in case of interspecific mating.

10. Mechanical isolation: This type of isolation occur due to the difference in morphological structure such as external genitalia. Example: Snails of same species have reproductive structure that align but snails from different species part other species.

11. Post- mating mechanism: If the efficient pre- mating mechanism fails then post- mating prevent the hybrid formation. These are of five types: Gametic mortality: In case of external fertilization, the sperm may fail to fertile the egg due to unfavorable climate conditions, in case of internal fertilization, the sperm of different species may be immobilized or killed in female genital tract. Example Sea cucumbers release their sperm and eggs into open water. The sperm recognize their own species through chemical marker.

12. Zygotic mortality: It result in death of zygote. Example: In sheep and goat.

13. Hybrid inviability: In this, hybrid zygotes fail to develop as the embryo formed by interspecific crosses abort during development. Example: Hybrid between the Rana pipens and R . sylvatica do not survive beyond the early gastrula stage

14. Hybrid sterility: In most of the organism hybrids are sterile due to incompatibility of chromosomes. Example: Mule produced by crosses between male ass and mare.

15. Hybrid breakdown: In certain case hybrids are fertile but progeny is weak, abnormal or sterile. In other cases first generation hybrids are viable and fertile or future generations are inviable or sterile. Example: Hybrids of lions and tigers have been obtained through zoo breeding programs while the first generation may be healthy, hybrid become weak and sterile in future generation.

16. Significance: It maintains the distinctiveness of characters in different species. It helps in accumulation of variation leading to speciation. It divides a population into sub-populations, which will differ significantly from ancestral population, helps in speciation.

17. Speciation in time Speciation occurs when gene pool of a population is somehow reproductively isolated from other population of the parent species and no longer gene flow occurs between them. On the basis of period taken in speciation there are two types of mechanisms of speciation.

18. gradualism The concept of gradualism is given by James Hutton and Charles Lyell. Gradualism states that each new species evolves gradually over long span of time. The principle idea is that the large phenotypic differences that produced the emergence of new species are due to the slow accumulation of many small genetic changes.

19. Some basis characters of gradualism Evolutionary change happens over a long period of time. Change happens in slow, gradual changes. Change is consistent over time. Changes are not noticeable over a short period of time.

20. Examples of gradualism: Certain moth in one population gradually adopt change in color and wing shape after a great deal of time, the two groups of moths develop in to entirely distinct species from one another.

21. Over a long period of time tigers develop the combination of orange and black stripes so they are better able to hide in tall grasses.

22. Ancestry of horse: Gradual increase in height. Reduction in the number of functional digit. Increase in the size of crown of molars for chewing and grazing grass.

23. Punctuated equilibrium Punctuated equilibrium is proposed by Niles Eldredge and Stephen Joy Gould. They suggest that “during punctuated equilibrium, a species exist essentially unchanged for long periods of time, during which it is in equilibrium with its environment. Theses equilibrium periods are punctuated by short period of evolutionary change, during which its features may change rapidly.

24. Some basic characters of punctuated equilibrium The phenomenon of punctuated equilibrium is often supported by the fossil records. Most speciation is cladogenesis rather than anagenesis.

25. Example: A species of worms lives in the soil in a particular climate and is in a state of stasis. Climate change cause the ph. of soil to change. The change in ph. causes some worms to die. But those that survive adopt and reproduce with new ability to withstand the ph. change in soil the species returns to stasis. Ancestry of giant panda.

26. Comparison between gradualism and punctuated equilibrium Gradualism Punctuated equilibrium Speciation occurs gradually. Speciation occurs rapidly and then a species remain unchanged. Fossils will show changes over time. Fossils showing intermediate characteristics will not necessarily be found. Ancestral species can be transformed into a new species. A sub population of the ancestral species become a new species.