Chapter 21 - Phylogeny, Speciation, and Extinction 

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• Life's evolutionary history is shaped by speciation and extinction.
  • This chapter will look at how scientists trace a species' evolutionary history, the mechanisms that lead to speciation and extinction, and modern-day instances of species evolution.
• Phylogeny is the study of the evolutionary history of a species or group.
  • Phylogeny depicts ancestry, common descent, and connections among organism groupings.
  • %%Phylogenetic trees and cladograms are theories regarding the evolution of organisms across time, with phylogenetic trees reflecting the approximate timing of evolutionary events.%%
  • These evolutionary trees and cladograms are constructed using fossil morphological data and molecular clock time estimations (changes in DNA and protein sequences over time).
  • In general, data from molecular clocks is seen to be more accurate than morphological evidence.
• %%Morphological evidence from fossils reveals features that are acquired or lost over time, which may be utilized to build phylogenetic trees.%%
• Traits that are found in more than one lineage are referred to be shared features.
  • Shared derived features are present in a group of related species known as a clade, and they distinguish the clade from other organisms.
  • Shared derived features suggest homology and indication of common ancestry among species in a clade.
  • Common ancestors are represented as nodes in phylogenetic trees.
  • The closer two species are connected, the more recent their common ancestor was.
• There are two prominent ideas concerning the origins of life on Earth:

1. Inorganic materials found in Earth's early atmosphere reacted to form the building blocks of living molecules. This is the theory.

• This notion is reinforced by data from the Miller-Urey experiment, in which a mimic of Earth's early atmosphere was built in a lab and amino acids and other components of living molecules were discovered within a few weeks.

1. Another idea is that meteorites brought organic compounds (necessary for life) to Earth. Meteorites are considered to have battered early Earth. The Murchison meteorite (discovered in Australia in 1969) included sugars and over 70 distinct amino acids, lending support to this notion.

• The biological species idea, which states that a species is a group of organisms capable of interbreeding and generating viable and fruitful offspring, is the current definition of a species.

  • It is vital to highlight that the notion of a species is a human construct intended to aid in the classification of creatures.
  • As additional information about an organism is collected, the concept of which species it belongs to may change.
  • The evolution of new species is referred to as speciation. When two populations are reproductively separated from each other, speciation occurs.
  • This reproductive isolation inhibits interbreeding, and as the settings in which these separated populations evolve, new species may emerge.

• Adaptive radiation, the development of organisms into diverse species that occupy different ecological niches, can result from speciation.

  • Darwin's finches on the Galapagos Islands exhibit adaptive radiation.
  • These finch species are descended from the same species but have developed through time to occupy the same habitat.

• The rates of speciation can differ.

  • If an environment is generally stable, populations will face less selection pressure and the pace of speciation will be slower.
  • Gradualism refers to the slow and consistent rate of speciation.
  • Punctuated equilibrium refers to a protracted period of stability in a species that is broken by periods of fast evolution.

• Allopatric or sympatric speciation is possible.

  • Allopatric speciation occurs when a larger population becomes geographically isolated and smaller subgroups divide into different species over time.
  • Although sympatric speciation occurs in the same geographical region, other reasons provide reproductive barriers between members of the groups.
  • Polyploidy, the replication of additional sets of chromosomes, is one process of sympatric speciation, and it is a common route of sympatric speciation in plants.

• Plants that generate extra sets of chromosomes are frequently unable to interbreed with plants that retain the original number of chromosomes, and hence evolve into a distinct species over time. In animals, sexual selection can also result in sympatric speciation.

• Prezygotic or postzygotic reproductive obstacles can induce speciation.

  • Prezygotic barriers are impediments that inhibit the creation of a zygote, or fertilized egg. Postzygotic barriers exist after the zygote is produced and prevent it from maturing into a viable and fruitful adult organism.
  • Extinction (the extinction of all individuals of a species) has occurred throughout Earth's history, as evidenced by the fossil record.
  • The degree of genetic variety in a population can influence its capacity to adapt to environmental changes.

• More genetically varied populations are more likely to be able to adapt to changing settings because they contain more individuals who can endure shifting environmental stressors.

  • Less genetically diversified communities are more vulnerable to population decreases or extinction.
  • Human-caused ecological pressures, like as habitat degradation or overhunting, can raise extinction rates.
  • Some experts believe that the Earth is presently undergoing a sixth big extinction event as a result of human activity.
  • The variety of species is determined by the rate of speciation vs the rate of extinction.
  • Species diversity will grow if speciation rates exceed extinction rates.
  • Species diversity will decline if extinction rates surpass speciation rates.
  • While extinction might have negative repercussions, it can also clear accessible habitats and reduce biodiversity.

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