Theory of evolution:
- Evolution is the process leading to the formation of new species from pre-existing species over time, as a result of changes in gene pools from generation to generation, due to reproductive isolation.

Species:
- Species refer to organisms with similar morphological, physiological, biochemical and behavioural features, which can interbreed to produce fertile offspring, and are reproductively isolated from other species.
- Reproductive isolation plays a major role in the formation of new species.

Reproductive isolation:
- Reproductive isolation among species means two populations of different species cannot breed naturally.
- Even if the reproductively isolated species are artificially bred, it results in either no offspring or infertile offspring like mules.
- There are different ways by which reproductive isolation takes place.
  - Prezygotic (before a zygote is formed) isolating mechanism includes:
    - Behavioural isolation – Individuals not recognising one another as potential mates or not responding to mating behaviour.
    - Mechanical isolation – Individuals being physically unable to mate due to the structure of reproductive organs being different in different species.
    - Gamete isolation – Inability of a male gamete to fuse with a female gamete even after the occurrence of mating or pollination.. For example, the incompatibility of pollen and stigma in plants.
    - Seasonal isolation – Different mating periods in different animal species and different gamete production times in different plant species.
  - Postzygotic isolating mechanism includes:
    - Failure of cell division in the zygote
    - Non-viable offspring
    - Viable but sterile offspring
Speciation:
- Speciation is the evolutionary process by which populations evolve to become new and distinct species.
- An example of evolution leading to speciation is Darwin’s finches. Darwin’s finches are considered evidence of ongoing evolution.
- Speciation may occur as a result of genetic isolation by:
  - Geographical separation (Allopatric speciation)
  - Ecological and behavioural separation (Sympatric speciation)

Geographical separation (Allopatric speciation):
- Geographical separation means separation of populations in different geographical landmarks in the course of time, due to change in geographic features, such as distance, rivers, and earthquakes.
- The geographic features which separate landmarks are also called geographical barriers.
  - For example: The Colorado River in the United States gradually split the Grand Canyon. Here, the Colorado River acted as a geographic barrier, separating the populations on either side and isolating them from each other.
- Speciation that occurs due to geographical separation is also known as allopatric speciation.
- In Greek, ‘patric’ means ‘homeland’, and ‘allo’ means ‘other’. So, ‘allopatric’ means ‘other homeland’.

Example of allopatric speciation – Darwin’s finches:
- Charles Darwin went on a voyage to travel around the world. The voyage stopped on the Galapagos islands after surveying the coast of South America.
- On the islands, he noticed similar-looking birds with some morphological differences. The birds resembled the mainland birds of the coast of South America.
- Darwin then collected these different bird species from different islands of the Galapagos and brought them back home. He compared them with the mainland bird species found in the American continent.
- As a result, he found that some features like body sizes, claw shapes and beak shapes were different in different species.
- For instance, he observed that insect eaters had long and thin beaks with sharp claws to catch insects. Meanwhile, nut eaters had large and thick beaks to crack open nuts.
- He also found that not a single island species completely matched with the other island species.
- They all had evolved into new species with different morphological features.
- The mainland finches might have been blown away by a big storm or wind and they landed on those islands.
- Since the distance between the islands and the mainland was very large, they could not return to the mainland. Similarly, because the distance between the islands was huge, they could not fly to other islands either. So, the birds on different islands became totally isolated. Here, the ocean acted as the geographical barrier.
- Different islands had different vegetation and climatic conditions. After isolation, the finches began to adapt to the islands’ unique environments.
- As a result of selection pressure, alleles suiting the external conditions were selected for the finches in different islands. This altered the gene pool from the original species.
- Mutations and genetic drifts also occurred in small populations, which further changed the gene pool.
- So, over time, the morphological, physiological and behavioural features of the island finch populations became so different from the mainland species that they could no longer interbreed.
- Over many generations, new species had evolved as a result of geographical barriers in different islands.
- Hence, this kind of speciation, occurring through geographical separation, is known as allopatric speciation.

Ecological and behavioural separation (Sympatric speciation):
- Speciation occurring as a result of ecological and behavioural separation is called sympatric speciation.
- In Greek, ‘sym’ means ‘same’ and ‘patric’ means ‘homeland’, so ‘sympatric’ means ‘same homeland’.
- Sympatric speciation usually occurs through polyploidy, and can easily be observed in plant species.

Examples of sympatric speciation – Polyploid organisms:
- A polyploid is an organism with more than two complete sets of chromosomes in its cells.
- This condition occurs if meiosis goes wrong during gamete formation, so that a gamete ends up with two sets of chromosomes instead of one.
- Tetraploid:
  - If two such gametes fuse, then the zygote gets four complete sets of chromosomes, which are said to be tetraploid. Tetraploids formed in this way are often sterile.
  - However, the tetraploid organism may be able to grow perfectly well and reproduce asexually through mitosis.
  - This happens quite often in plants but rarely in animals, largely because most animals do not reproduce asexually by mitosis like plants do.
- Triploid:
  - If a tetraploid plant manages to produce gametes, they will be diploid gametes.
  - If one of these fuses with a haploid gamete from the normal, diploid plant, then the resulting zygote will be triploid.
  - Triploid plants have three sets of chromosomes, so they will not produce gametes because they cannot share the three sets of chromosomes evenly between the daughter cells.
  - However, the triploid plant itself may be able to grow normally.
  - An example of triploid plants is seedless bananas.
    - The cultivated banana is believed to have been derived from a cross between the diploid species Musa acuminata and the tetraploid species Musa balbisiana. Both of them bear seeds.
    - The original parents, diploid and tetraploid plants, cannot interbreed naturally to produce the triploid offspring.
    - So, the diploid plant, triploid offspring and tetraploid plant, all are considered to be different species.
    - Even though triploid offspring have been produced in just one generation, they are considered a new species, since they cannot interbreed.
- Autotetraploid and allotetraploid:
  - If a polyploid contains four sets of chromosomes all from the same species, then it is said to be autopolyploid or specifically autotetraploid.
  - If the two sets of chromosomes are obtained from one species and the other two from another closely related species, the polyploid is called allopolyploid or specifically allotetraploid.
  - So the two chromosomes from one species pair up in twos with each other, and the two chromosomes from the other species also pair up in twos to form a total of two separate pairs.
  - Such kind of pairing up for meiosis is unlikely in autotetraploids because being originated from the same species, all chromosomes tend to pair up in fours, as a single group.
  - So it is much more likely that meiosis can occur successfully in the case of allotetraploids. And, they may be able to produce plenty of fertile gametes.
  - The allopolyploids and autopolyploids cannot interbreed with individuals from their parent species, so they both are considered new species.
- Allopolyploidy in cordgrass (Spartina anglica):
  - Cordgrass is a vigorous grass that grows in salt marshes.
  - Previously, the species of Spartina that grew in England was S. maritima.
  - In 1829, a different species called S. alterniflora was imported from America.
  - After that, S. maritima and S. alterniflora hybridised, producing a new species called S. townsendii.
  - S. townsendii is a diploid plant, with one set of chromosomes from S. maritima and one set from S. alterniflora.
  - It is sterile because the two sets of chromosomes from its parents cannot pair up, so it cannot undergo meiosis successfully.
  - Also, S. townsendii cannot interbreed with either of its two parents, making it a different species.
  - Although it is sterile, it is able to spread rapidly, reproducing asexually.
  - Around 1892, faulty cell division in S. townsendii produced cells with double its number of chromosomes.
  - As a result, an allotetraploid plant was produced, from the fusion of two abnormal diploid gametes from S. townsendii, acquiring two sets of chromosomes from S. maritima, and two sets from S. alterniflora.
  - These chromosomes can now pair up with each other, two and two, during meiosis. So this tetraploid plant is fertile.
  - This fertile tetraploid plant has been named S. anglica.
  - It is more vigorous than any of the other three species.
- Sympatric speciation in animals (Apple maggot):
  - The apple maggot is an insect that lays its eggs inside the apple, causing it to rot.
  - The apple maggot originally laid its eggs inside hawthorn fruits, which is a close relative of the apple.
  - After apples were introduced to North America in the 19^th century, a different type of maggot developed that laid its eggs only in apples.
  - But the original hawthorn species still only lays its eggs in hawthorns.
  - These two types of maggots do not interbreed but also are not different species yet.
  - It is believed that they are undergoing the process of sympatric speciation.

Evolutionary relationships between species:
- The evolutionary relationships between species can be shown from DNA sequence data as DNA molecules are used for the identification of species.
- DNA analysis is a more reliable method of classification than anatomy and morphology.
- Molecular features such as DNA can validate the differences and similarities between different species.
- DNA molecules are passed on from one generation to the next, and during that process, they may undergo different types of mutations.
  - The two types of DNA in organisms are nuclear DNA and mitochondrial DNA.
  - In the classification of organisms, a nucleotide sequence from the mitochondrial DNA is used.
  - Mitochondrial DNA is circular and cannot undergo any form of crossing over.
  - As it is only acquired from the female line, the changes in the nucleotide sequence can only be caused by mutation.
  - Unlike nuclear DNA, mitochondrial DNA is not protected by histone proteins.
  - Thus, oxidative phosphorylation in the mitochondria can produce forms of oxygen that act as mutagens.
  - As mitochondrial DNA has a small nucleotide sequence, it mutates faster than nuclear DNA.
- When any two species have a common ancestor, the greater the difference between them, the larger the number of mutations in their DNA.
- So, two distantly related species will have greater differences in their DNA sequence.
- In contrast, two closely related species will have smaller differences in their DNA sequence because they share recent common ancestors.

Molecular clock hypothesis:
- This hypothesis assumes a constant rate of mutation over time.
- So, the greater the number of differences in the sequence of nucleotides, the earlier those individuals shared a common ancestor.
- And calibration can be done by comparing nucleotide sequences of species whose date of speciation can be estimated from fossil evidence.

17.04 - Evolution

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