All questions of The Theory of Evolution by Natural Selection for Grade 8 Exam

Human variability refers to the fact that there is a range of possible values for each of the numerous physical and mental characteristics of human beings. Fundamental Characteristic. Variation is, in fact, inherent in every living species.

Asexual reproduction involves a single parent. It results in offspring that are genetically identical to each other and to the parent. All prokaryotes and some eukaryotes reproduce this way.  Binary fission occurs when a parent cell splits into two identical daughter cells of the same size.

Often their most distinguishing feature is their reproductive organs, commonly called flowers. Sexual reproduction in flowering plants involves the production of male and female gametes, the transfer of the male gametes to the female ovules in a process called pollination.

Trilobite: An Arthropod

Trilobites are extinct marine arthropods that lived during the Paleozoic era, for over 270 million years. They are considered to be one of the earliest known groups of arthropods, making them a crucial part of the evolutionary history of this diverse phylum.

Characteristics of Arthropods

Arthropods are characterized by having a segmented body, an exoskeleton made of chitin, jointed appendages, and a well-developed nervous system. Trilobites possessed all these features, placing them firmly within the arthropod group.

Physical Characteristics of Trilobites

Trilobites had a hard exoskeleton that was divided into three distinct sections: the cephalon (head), the thorax (body), and the pygidium (tail). They also had multiple pairs of legs attached to their body segments, which is a key characteristic of arthropods.

Fossil Evidence

The fossil record provides ample evidence of trilobites being arthropods. Their body structure, appendages, and overall morphology closely resemble that of modern arthropods, further supporting their classification within this group.

Conclusion

In conclusion, trilobites were not reptiles, birds, or mammals but instead belonged to the diverse group of arthropods. Their unique characteristics and fossil evidence make it clear that they were an important part of the evolutionary history of arthropods.

Explanation:

When studying the layers of the Earth, scientists have noticed that the deeper layers are older than the upper layers. This is because new layers of sediment are continuously being deposited on top of older layers over time.

Relation between Fossils and Earth Layers:

Fossils are the remains or traces of organisms that lived in the past. When an organism dies, its remains can be preserved in sedimentary rock layers over time. The position of a fossil in the layers of the Earth can provide valuable information about the age of the fossil and the time of extinction of the organism.

Deeper Layer and Extinction Time:

If a fossil is found in a deeper layer of the Earth, it indicates that the organism became extinct a long time ago, as it is buried under several layers of sediment. The deeper the layer, the older the fossil is likely to be. This means that option 'C' is the correct answer - the extinction of the organism has occurred thousands of years ago.

Why Other Options are Incorrect:

Option 'A' states that the extinction of the organism has occurred recently, but this is contradictory to the fact that the fossil is found in a deeper layer. If the extinction was recent, the fossil would be found in a more shallow layer.

Option 'B' suggests that the position of the fossil in the layers of the Earth is not related to its time of extinction. However, this contradicts the principle of superposition, which states that deeper layers are older than upper layers. The position of the fossil in the layers provides important chronological information.

Option 'D' claims that the time of extinction cannot be determined. While it may be challenging to determine the exact time of extinction, the relative age can be estimated based on the position of the fossil in the layers of the Earth.

Conclusion:

In conclusion, when a fossil is found in a deeper layer of the Earth, it indicates that the organism became extinct thousands of years ago. The position of the fossil in the layers provides valuable information about the relative age of the fossil and can help scientists understand the history of life on Earth.

Explanation:

The palisade cells of a plant contain 28 chromosomes. Chromosomes are the structures that carry genetic information in the form of genes. During the process of sexual reproduction, gametes are produced, which are the reproductive cells that combine during fertilization to form a new organism.

Chromosomes in gametes:
Gametes are formed through a process called meiosis. Meiosis is a type of cell division that reduces the number of chromosomes in a cell by half. In plants, meiosis occurs in the reproductive organs, such as the flowers.

During meiosis, the diploid cells (cells with two sets of chromosomes) undergo two rounds of division, resulting in the formation of four haploid cells (cells with one set of chromosomes). These haploid cells are the gametes.

Chromosome number in gametes:
Since the plant's palisade cells contain 28 chromosomes, which are diploid cells, the gametes produced by the plant will have a haploid number of chromosomes.

The haploid number of chromosomes is half the diploid number. Therefore, the number of chromosomes in each gamete will be:

28 chromosomes / 2 = 14 chromosomes

Therefore, the correct answer is option B) 14. Each gamete produced by the plant will contain 14 chromosomes.

Summary:
The palisade cells of a plant species contain 28 chromosomes. During the process of meiosis, which is involved in gamete formation, the chromosome number is reduced by half. Therefore, each gamete produced by the plant will have 14 chromosomes.

Sexual Reproduction
- Sexual reproduction involves the fusion of gametes from two parents to produce offspring with a combination of genetic traits.
- This process ensures genetic diversity and variation within a population.
- It requires the involvement of specialized reproductive organs and mechanisms, such as fertilization and meiosis.
- In sexual reproduction, the offspring inherit traits from both parents, resulting in a unique combination of genetic material.
- The genetic variation introduced through sexual reproduction can enhance the adaptability and survival of a species in changing environments.
- Examples of sexual reproduction include the union of sperm and egg cells in animals, pollination in plants, and mating in fungi.
- It is a common method of reproduction in most multicellular organisms, including humans.
- In contrast, asexual reproduction involves the production of offspring from a single parent without the involvement of gametes or fertilization.
- Asexual reproduction leads to genetically identical offspring, limiting genetic diversity and adaptability.
Therefore, the correct answer is B: Sexual reproduction.

Introduction:
Variations in offspring refer to the differences or unique characteristics that offspring inherit from their parents. These variations are mainly introduced as a result of sexual reproduction.

Explanation:
Sexual reproduction is the process by which offspring are produced through the fusion of gametes, or reproductive cells, from two parents. This process involves the combination of genetic material from both parents, resulting in offspring that possess a unique combination of traits.

Genetic Variation:
One of the key reasons why sexual reproduction leads to variations in offspring is due to genetic recombination. During sexual reproduction, the parents contribute half of their genetic material to their offspring. This genetic material is shuffled and recombined through the process of meiosis, resulting in the production of genetically diverse gametes.

Combination of Traits:
When two gametes (sperm and egg) fuse during fertilization, they combine their genetic material to form a unique combination of traits in the offspring. Each parent contributes a different set of alleles, which are alternative forms of genes, to the offspring. This combination of alleles determines the characteristics and traits that the offspring will inherit.

Random Assortment:
Furthermore, during meiosis, the chromosomes in the parent's cells randomly assort, meaning that each gamete receives a different combination of chromosomes. This random assortment further increases genetic variation in the offspring.

Recombination:
In addition to random assortment, genetic recombination occurs during meiosis, where genetic material is exchanged between homologous chromosomes. This process further increases the genetic diversity in the offspring, as it allows for the mixing and rearrangement of alleles.

Conclusion:
In conclusion, variations in offspring are mainly introduced as a result of sexual reproduction. Through the combination of genetic material from two parents, genetic recombination, random assortment, and the exchange of genetic material during meiosis, offspring inherit a unique combination of traits that contribute to the variations observed in populations. This genetic diversity is essential for the survival and adaptation of species in changing environments.