Bottleneck Effect

Bottleneck Effect

• The Bottleneck Effect occurs when there is a disaster of some sort that reduces a population to a small handful,   which rarely represents the actual genetic makeup of the initial population. This leaves smaller variation among the surviving individuals.

Genetic bottlenecks are defined by the textbook as "events in which the number of individuals in a population is reduced drastically." Essentially, this is genetic drift at it's finest. It occurs when the true frequency of alleles of a certain population, is not accurately presented in the new population. There is no other way to avoid a bottleneck effect because having a certain phenotype or genotype does not matter. This can have an extreme effect on the genetic diversity of the population. To illustrate this point: if a large population, say of a bacteria, were to be growing on a surface and then one was to clean it with Lysol or the like, we would expect that not all the bacteria in the population would be killed and thus survive to pass on their genes. This dramatic population decrease would effectively cut out most of the genetic variation that existed within the population pre-Lysol attack. Now, on the same surface, we can expect that the remaining alleles frequencies have shifted dramatically, and all the forthcoming generations will be limited to this gene pool (barring the introduction of new genes to the population). This, effectively, can spread a trait like resistance through the entirety of a population in a few generations. And we know that bacteria run through generations quickly, leading to issues like MRSA.

An example of a bottleneck effect: Northern elephant seals have reduced genetic variation most likely due to being hunted. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Since then their population has rebounded to over 30,000 but the genes still carry the marks of their bottleneck. They have much less variation than a population of Southern Elephant seals that have not been hunted.

>Generation 2 an unusually small population creates a "bottleneck".

One of the most famous examples of bottleneck occurred in 1789, when the mutineers of a British ship set the captain and the crew adrift. Twenty seven adults and one baby arrived at island called Pitcairn and their descendants lived on that island for decades. This event caused a drastic reduction in the population size which decreased the genetic variation in the population (Evolution: Making Sense of Life).

• It is believed that there was a human bottleneck ~70,000 years ago, caused by the Toba catastrophe.

• The entire human population was likely reduced to 3,000-10,000 survivors in a small part of Africa.

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Classification of Hemichordata

Hemichordata

Position hemichordate in classification is doubtful. Foregut in hemichordates gives out a hollow buccal diverticulum into proboscis. This structure was earlier considered as “notochord” which is one of the fundamental characters of chordates. Based on this structure and other fundamental characters hemichordate was placed in Phylum-Chordata. But systematic position of hemichordata became doubtful when buccal diverticulumn was not considered as true notochord. Hence, hemichordata is not true chordate and it is treated as an independent invertebrate phylum.

General Characters of Hemichordata:1. Solitary and colonial, mostly tubicolous, exclusively marine.
2. Body soft, fragile, vermiform and divisible into proboscis, collar and trunk.
3. Body wall with a single-layered epidermis.
4. Coelom enterocoelous, divisible into protocoel, mesocoel and metacoel.
5. Buccal diverticulum, earlier considered as notochord, present in the proboscis.
6. Digestive tract complete; in the form of straight or U-shaped tube.
7. Gill-slits, when present, are paired and one to numerous.
8. Circulatory system simple and well developed; closed type; usually with a contractile heart vesicle and two longitudinal vessels, one dorsal and one ventral, interconnected by lateral vessels and sinuses.
9. Excretion by a single glomerulus situated in the proboscis.
10. Nervous system primitive comprising mainly of an intra-epidermal nerve plexus.
11. Reproduction mostly sexual. Sexes separate or united.Gonads one to several pairs.
12. Fertilisation external. Development mostly indirect through a free swimming tornaria larva. Direct development is also found in some forms.

Classification:-

Classification of Hemichordata:

Phylum Hemichrodata has been divided into following four classes:

Class 1. Enteropneusta:

             1. Commonly known as “acorn” or “tongue worms.”

             2. Solitary and burrowing worm-like marine animals.

             3. Body consists of proboscis, collar and trunk; collar without tentaculated arms.

            4. Alimentary canal straight; mouth and anus at opposite ends.

            5. Numerous pairs of U-shaped gill-slits.

            6. Two pairs of hepatic caeca present in the middle of the trunk.

            7. Sexes separate; gonads numerous, sac-like.

            8. Development with or without tornaria larva.

           Examples:

           Balanoglossus, Saccoglossus (= Dolichoglossus), Ptychodera.

Balanoglossus

Class 2. Pterobranchia:

            1. Sedentary, solitary or colonial, tubicolous marine animals.

           2. Proboscis with ciliated tentacles to produce ciliary feeding currents of water.

           3. Collar with two or more tentaculated arms bearing tentacles.

           4. One pair of gill-slits or none, never U- shaped.

           5. Alimentary canal U-shaped with dorsal anus situated near the mouth at the same end.

           6. Sexes separate or united; single or one pair of gonads.

           7. Development direct, may or may not include a free swimming larval stage.

           8. Asexual reproduction by budding in some.

                      Order 1. Cephalodiscida:

                        1. Solitary or several individuals living unconnected in a common gelatinous house.

                        2. Collar with several tentaculated arms.

                        3. Gill-slits single pair.

                        4. Gonads single pair.

                       Examples:

                        Cephalodiscus, Atubaria.

Cephalodiscus

                      Order 2. Rhabdopleurida:

                       1. Colonial, zooids connected by a stolon.

                       2. Collar with two tentaculated arms.

                       3. Gill-slits absent.

                       4. Gonad single.

                      Example:

                      Rhabdopleura (Single genus).

Rhabdopleura

Class 3. Planctosphaeroidea:

This class is represented by a few small rounded, transparent and pelagic larvae, supposed to be specialised tornaria of some unknown hemichordate Planctosphaera pelagica. The larva has branching arborescent ciliated bands on the surface. The alimentary canal of larva is U-shaped. The adult form is yet unknown.

Class 4. Graptolita:

These are extinct colonial hemichordates, mainly known from the fossil structures of their tubes. Each animal is housed in a zooid. These were abundant in the Ordovician and Silurian periods.

Example:

Dendrograptus.

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