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When the human embryo, for instance, is but a simple vesicle, it is an infusorian; when it has gained a liver, it is a mussel; with the appearance of the osseous system, it enters the class of fishes; and so forth, until it becomes a mammal and then a human being.
In biology, evolution is the process of change in all forms of life over generations, and evolutionary biology is the study of how evolution occurs. Biological populations evolve through genetic changes that correspond to changes in the organisms ' observable traits .
Dr. Dinesh Prasad Saklani is the director of NCERT since 2022. [2] In 2023, NCERT constituted a 19-member committee, including author and Infosys Foundation chair Sudha Murthy, singer Shankar Mahadevan, and Manjul Bhargava to finalize the curriculum, textbooks and learning material for classes 3 to 12. [4]
They have been treated formally at the rank of division Progymnospermophyta or class Progymnospermopsida (as opposite). The stratigraphically oldest known examples belong to the Middle Devonian order the Aneurophytales , with forms such as Protopteridium , in which the vegetative organs consisted of relatively loose clusters of axes. [ 2 ]
Evolution is the change in the heritable characteristics of biological populations over successive generations. [1] [2] It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, resulting in certain characteristics becoming more or less common within a population over successive generations. [3]
The mechanisms of evolution focus mainly on mutation, genetic drift, gene flow, non-random mating, and natural selection. Mutation: Mutation [12] is a change in the DNA sequence inside a gene or a chromosome of an organism. Most mutations are deleterious, or neutral; i.e. they can neither harm nor benefit, but can also be beneficial sometimes.
The history of life on Earth traces the processes by which living and extinct organisms evolved, from the earliest emergence of life to the present day. Earth formed about 4.5 billion years ago (abbreviated as Ga, for gigaannum) and evidence suggests that life emerged prior to 3.7 Ga. [1] [2] [3] The similarities among all known present-day species indicate that they have diverged through the ...
As the first stage in the evolution of sexual reproduction in all known lifeforms, isogamy is thought to have evolved just once, in a single unicellular eukaryote species, the common ancestor of all eukaryotes. It is generally accepted that isogamy is an ancestral state for anisogamy [1] [9].