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Grade 12 Life Sciences Evolution

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Questions of origins of earth and life on it have been on the minds of humans since prehistoric times. Many of us are also concerned with questions of origin: How old is the planet earth? How long has life been on earth? How did life arise on earth? How did a certain animal species come into existence? Answers for these questions come from scientific inquiry. In this chapter we will study some aspects of organic evolution. Evolution refers to the processes that have transformed life on earth from its earliest forms to the vast diversity that we observe today. Evolutionary change is based mainly on the interactions between populations of organisms and their environments. Whenever we say or hear the word evolution, name of Darwin comes in our mind immediately. In fact, he was the first person who argued from evidence that species were not specially created in their present forms, rather they had evolved from ancestral species. He also proposed a mechanism for evolution, which he termed Natural Selection.

CONCEPT OF EVOLUTION VS SPECIAL CREATION

In a bid to explain the cause of diversity of life and interrelationship among living organisms, two schools of thought emerged in the earlier 19th century. Creationists believed in the Theory of Special Creation, whereas evolutionists believed in the Theory of Natural Selection. According to the theory of special creation, all living things came into existence in their present forms especially and specifically created by Nature. Among the scientists who believed in divine creation was Carolus Linnaeus (1707-1778).

EVOLUTION FROM PROKARYOTES TO EUKARYOTES

One of the speculations trying to explain the origin of life is that it may have begun deep in the oceans, in underwater hot springs called hydrothermal vents. These vents could have supplied the energy and raw materials (for the origin and survival of early life forms. A group of bacteria, called archaebacteria that tolerate temperatures up to 120°C and seem to have undergone less evolutionary change than any other living species supports this vent hypothesis.

The nutrients produced in the primitive environment would have limited early life., If life were to continue, another source of nutrients was needed. Photosynthesis probably freed living organisms from a dwindling supply of nutrients. The first photosynthetic organisms probably used hydrogen sulfide as a source of hydrogen for reducing carbon dioxide to sugars. Later, water served this same purpose, and oxygen liberated by photosynthetic reactions began to accumulate in the atmosphere. Earth and its atmosphere slowly began to change.

Ozone in the upper atmosphere began to filter ultraviolet radiation from the sun, the reducing atmosphere slowly became an oxidizing atmosphere, and at least some living organisms began to utilize oxygen. About 420 million years ago, enough protec tive ozone had built up to make life on land possible. Ironically, the change from a reducing atmosphere to an oxidizing atmosphere also meant that life could no longer arise abiotically. The first cells were most likely very simple prokaryotic forms. The prokaryotes may have arisen more than 3 billion years ago. Eukaryotes are thought to have first appeared about 1 billion years ago. The eukaryotic cell might have evolved when a large anaerobic (living without oxygen) amoeboid prokaryote ing ested small aerobic (living with oxygen) bacteria and stabilized them instead of digesting them. This idea is known as the endosymbiont hypothesis (Fig..) and was first proposed by Lynn Margulis. According to this hypothesis, the aerobic bacteria developed into mitochondria, which are the sites of aerobic respiration and most energy conversion in eukaryotic cells. The possession of these mitochondria like endosymbionts brought the advantage of aerobic respiration to the host.

Flagella (whiplike structures) may have arisen through the ingestion of prokaryotes similar to spiral-shaped bacteria called spirochetes. Ingestion of prokaryotes that resembled present-day cyanobacteria could have led to the endosymbiotic development of chloroplasts in plants.

Another hypothesis for the evolution of eukaryotic cells proposes that the prokaryotic cell membrane invaginated (folded inward) to enclose copies of its genetic material (Fig..). This invagination resulted in the formation of several double membrane - bound entities (organelles) in a single cell. These entities could then have evolved into the eukaryotic mitochondrion, nucleus, chloroplast etc.

Whatever the exact mechanism for the evolution of the eukaryotic cell might be, the formation of the eukaryotic cell led to a dramatic increase in the complexity and diversity of life-forms on the earth. At first, these newly formed eukaryotic cells existed only by themselves. Later, however, some probably evolved into multicellular organisms in which various cells became specialized into tissues, which, in turn, formed organs for many different functions. These multicellular forms then adapted themselves to life in a great variety of environments.

Among the examples Lamarck cited were the blacksmith developing a bigger bicep in the arm that works the hammer and giraffe stretching its neck to new lengths in pursuit of leaves to eat. The second idea Lamarck adopted, was called the inheritance of acquired characteristics. In this concept of heredity, the modifications an organism acquires during its lifetime can be passed along to its offspring e. the long neck of the giraffe, Lamarck reasoned, evolved gradually as the cumulative product of a great many generations of ancestors stretching higher and higher. However, now we know that acquired characteristics cannot be inherited. Charles Darwin

Charles Darwin was born in Shrewsbury, in Western England, in 1809. He joined the expedition on Beagle to South American coastline. He observed and collected thousands of specimens of diverse fauna and flora of South America. He noticed that the fauna and flora of the different regions of the continent had a definite South American stamp, very distinct from the life forms of Europe. Furthermore, the South American fossils that Darwin found, though clearly different from modem species, were distinctly South American in their resemblance to the living plants and animals of that continent.

A particularly puzzling case of geographical distribution was the fauna of the Galapagos islands. Most of the animal species on the Galapagos live nowhere else in the world, although they resemble species living on the South American mainland. It was as though the islands were colonized by plants and animals that strayed from the South American mainland and then diversified on the different islands. Among the birds Darwin collected on the Galapagos were 13 types of finches that, although quite similar, seemed to be different species. Some were unique to individual islands, while other species were distributed on two or more islands that were close together.

After returning to Great Britain in 1836, Darwin perceived the origin of new species and adaptations as closely related processes. A new species would arise from an ancestral form by the gradual accumulation of adaptations to different environments, separated from original habitat by geographical barriers. Over many generations, the two populations could become dissimilar enough to be designated as separate species. This is apparently what happened to the Galapagos finches.

By the early 1840s, Darwin had worked out the major features of his theory of natural selection as the mechanism of evolution. In 1844, Darwin wrote a long essay on the origin of species and natural selection.

But before it could be published Alfred Wallace, a young naturalist working in the East Indies developed a theory of natural selection essentially identical to Darwin’s. Wallace’s paper, along with extracts from Darwin’s unpublished 1844 essay, were presented to the Linnaean Society of London on July 1, 1858. Darwin quickly finished The Origin of Species and published it the next year. In this book Darwin developed two main points:

1. Descent with Modification: Darwin believed in perceived unity in life, with all organisms related through descent from some common ancestor that lived in the remote past. In the Darwinian view, the history of life is like a tree, with multiple branching and rebranching from a common trunk all the way to the tips of the living twigs, symbolic of the current diversity of organisms. At each fork of the evolutionary tree is an ancestor common to all lines of evolution branching from that fork.

2. Natural Selection and Adaptation: Darwin suggested that populations of individual species become better adapted to their local environments through natural selection. Darwin’s theory of natural selection was based on the following observations.

Production of more individuals than the environment can support, leads to a struggle for existence among individuals of a population, with only a fraction of offspring surviving each generation.
Survival in the struggle for existence is not random but depends in part on the hereditary constitution of the surviving individuals. Those individuals whose inherited characteristics fit them best to their environment are likely to leave more offspring than the less fit individuals.
This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favourable characteristics accumulating over the generations thus leading to the evolution of a new species.

Fossils are either the actual remains or - traces of organisms that lived in ancient geological times. The organism may be embedded in sand, resin or ice, or an impression or cast is made of the body parts, the tissue being replaced or petrified by silica or calcium carbonate minerals. Most fossils are found in sedimentary rocks.

The Fossil Record: The succession of fossil forms is a strong evidence in favour of evolution. It provides a visual record in a complete series showing the evolution of an organism. For instance, evidence from biochemistry, molecular biology, and cell biology places prokaryotes as the ancestors of all life and predicts that bacteria should precede all eukaryotic life in the fossil record. Indeed, the oldest known fossils are prokaryotes. Another example is the chronological appearance of the different classes of vertebrate animals in the fossil record. Fossil fishes, the earliest vertebrates, with amphibians next, followed by reptiles, then mammals and birds. This sequence is consistent with the history of vertebrate descent. The evolution of horse provides an example of such a history.

Comparative Anatomy : Anatomical similarities between species grouped in the same taxonomic category bring another support to the theory of the Descent with modification. For example, the same skeletal elements make up the forelimbs of human, cats, whales, bats, and all other mammals, although these appendages have very different functions.

The basic similarity of these forelimbs is the consequence of mammals from a common ancestor. The amis, wings, flippers, and forelegs of different mammals are variations on a common anatomical theme that has been modified for divergent functions. Similarity in characteristics resulting from common ancestry is known as homology, and such anatomical signs of evolution are called homologous structures. Comparative anatomy supports that evolution is a remodeling process in which ancestral structures that functioned in one capacity become modified as they tale on new functions. The flower parts of a flowering plant are homologous. They are considered to have evolved from leaves, to form sepals, petals, stamens and carpels

The oldest homologous structures are vestigial organs, rudimentary structures of marginal, if any, use to the organism. Vestigial organs are historical remnants of structures that had important functions in ancestors but are no longer essential presently. For instance, the skeletons of whales and some snakes retain vestiges of the pelvis and leg bones of walking ancestors, (Fig..) vermiform appendix in carnivores, ear muscles in man etc.

Fig..: The whale retains pelvic and leg bones as useless vestiges

Homologous organs are functionally different but structurally alike e. Fore limbs of man, bat, horse, whale etc. are example of divergent evolution. Analogous organs are functionally alike but structurally different e. wings of bat, birds and insects etc. are examples of convergent evolution.

Molecular Biology: Evolutionary relationships among species are reflected in their DNA and proteins—in their genes and gene products. If two species have genes and proteins with sequences of monomers that match closely, the sequences must have been copied from a common ancestor. For example, a common genetic code brings evidence that all life is related. Molecular biology has thus provided strong evidence in support of evolution as the basis for the unity and diversity of life. Similarly, taxonomically remote organisms, such as humans and bacteria, have some proteins in common. For instance, cytochrome c, a respiratory protein is found in all aerobic species. NATURAL SELECTION AND ARTIFICIAL SELECTION

Natural selection occurs through an interaction between the environment and the variability inherent in any population. Darwin found evidence in artificial selection, the breeding of domesticated plants and animals. Humans have modified other species over many generation by selecting individuals with the desired traits as breeding stock. The plants and animals we grow for food bear little resemblance to their wild ancestors. From the changes achieved by artificial selection within a relativ ely short period of time, Darvviq postulated that natural selection operating over vast spans of time could account for the entire diversity of life. Population is a group of inter-breeding individuals belonging to a particular species and sharing a common geographic area.

Natural selection can amplify or diminish only those variations that are heritable. It is noteworthy to say that adaptations that an organism acquires by its own actions are not heritable. The specifics of natural selection are regional and timely; environmental factors vary from place to place and from time to time. An adaptation in one situation may be useless or even detrimental in other circumstances. An example of natural selection in action is the evolution of antibiotic resistance in bacteria.

POPULATION, GENE POOL, ALLELE AND GENOTYPE FREQUENCIES

A population is a localized group of individuals belonging to the same species. For now, we will define a species as a group of populations that have the potential to interbreed in nature. Each species has a geographical range within which individuals are not spread out evenly, but are usually concentrated in several localized populations. A population may be isolated from others of the same species, exchanging genetic material only rarely. Such an isolation is particularly common for populations confined to widely separated islands, unconnected lakes, or mountain ranges separated by lowlands. Within a population individuals are concentrated in centers and are more likely to interbreed with members of the same population than with members of other populations. Therefore, individuals near a population center are, on average, more closely related to one another than to members of other populations.

The total aggregate of genes in a population at any one time is called the population’s gene pool. It consists of all alleles at all gene loci in all individuals of the population. For a diploid species, each locus is represented twice in the genome of an individual, who may be either homozygous or heterozygous.

Hardy-Weinberg Theorem

The frequencies of genotypes of non evolving populations are described by Hardy - Weinberg theorem. Hardy-Weinberg theorem is named for the two scientists who derived the principle independently in 1908. It states that the frequencies of alleles and genotypes in a population’s gene pool remain constant over the generations unless acted upon by agents other than sexual recombination. So shuffling of alleles due to meiosis and random fertilization has no effect on the overall genetic structure of a population. A general formula, called the Hardy- Weinberg equation is used for calculating the frequencies of alleles and genotypes in populations at equilibrium.

For a gene locus where only two alleles occur in a population, population geneticists use the letter p to represent the frequency of one allele and the letter q to represent the frequency of the other allele. In the imaginary wildflower population, p=0 and <7=0. Note that p+q= 1; the combined frequencies of all possible alleles must account for 100% of the genes for that locus in the population. If there are only two alleles and we know the frequency of one, the frequency of other can be calculated:

Ifp + q-1, then 1-p = q, or l-q - p

When gametes combine their alleles to form zygotes, the probability of generating an AA genotype is p 2. In the wildflower population, p-0, and p 2 =0, the probability of an A sperm fertilizing an A ovum to produce an AA zygote. The frequency of individuals homozygous for the other allele (aa) is q 2 , or 0.2x0.2=0 for the wildflower population. There are two ways in which an Aa genotype can arise, depending on which parent contributes the dominant allele. Therefore, the frequency of heterozygous individuals in the population is 2pq (2x0.8x0.2=0, in our example). If we have calculated the frequencies of all possible genotypes correctly, they should add up to 1: P + 2pq+q 2 = 1 Frequency Frequency Frequency of AA of Aa plus aA of aa For our wildflowers, this is 0 + 0 + 0=1*

In fact the Hardy-Weinberg equation is a binomial expansion: (p+q) 2 or p 2 +2pq+q 2

Factors affecting gene frequency Many factors can alter gene frequency. Out of these five affect the proportion of homozygotes and heterozygotes enough to produce significant deviations form the proportion claimed by Hardy Weinberg principle. They are reflected in the table below. Table 24 Factors for evolutionary change Factor Description Mutation The ultimate source of all changes; individual mutations occur so rarely that mutation alone does not change allele frequency much. Migration A very potent agent of change, migration locally acts to prevent evolutionary changes by preventing populations that exchange members from diverging from one another. Emigration and immigration of members of a population, cause disturbance in the gene pool. Genetic drift It is the change in frequency of alleles at a locus that occurs by chance. In small populations, such fluctuations may lead to the loss of particular alleles. This may occur in a small population when a few individual fail to reproduce and then genes are lost from the population. Non-random mating Inbreeding is the most common form; it does not alter allele frequency, but lessens the proportion of heterozyote individuals. Individuals with certain genotypes sometimes mate with one another more commonly than would be expected on a random basis. This is called non-random mating, causing the frequencies of particular genotypes to differ greatly from those predicted by the 1 lardy- Weinberg principle. Selection Some individuals leave behind more progeny than others, and the rate at which they do so is affected by their inherited characteristics. This is called selection. Selection can be artificial selection or natural selection. In artificial selection, the breeders select for the desired characters. In natural selection, the environment plays this role, thus affecting the proportions of gene in a population.

An endangered species is in imminent danger of extinction throughout its range (where it lives). A threatened species is likely to become endangered in the near future. Saving species requires more than preserving a few remnant individuals. It requires a large diversity of genes within species groups to promote species survival in changing environments. This genetic diversity requires large populations of plants and animals. Preservation of endangered species depends on a multifaceted conservation plan that includes the following components:

A global system of national parks to protect large tracts of land and wildlife corridors that allow movement between natural areas.
Protected landscapes and multiple-use areas that allow controlled private activity but also retain value as a wildlife habitat.
Zoos and botanical gardens to save species whose extinction is imminent. In Pakistan. Cheetah. Tiger. Asian lion. Indian rhino. Cheer pheasant. Crocodile and Gaviul have been declared extinct. While. Indus dolphin. Blackbuck, Common leopard. Great Indian bustard. Houbara bustard. White-headed duck and Marbled teal are among the animal near to extinction. Deserts, Sub-mountianous tract and Wetlands are habitats in peril. We must protect them rapidly. Endangered species of plants have been recorded to more than 500.

Q1 Fill in the blanks.

Archaebacteria can tolerate high temperature sup to
The first eukaryote appeared about years ago.
presented the theory of the origin of species by means of Natural Selection.
developed a theory of natural selection essentially identical to Darwin’s.
are considered to be the ancestors of all life.
A respiratory protein called is found in all aerobic organisms.
Total aggregate of genes in a population at any time is called its
Hardy Weinberg theorem describes a population.
is a series of changes in the genetic composition of a population over time.
Level of classification between species and family is called.
Hardy Weinbeig equation is binomial expansion of.
An species is in imminent danger of extinction throughout its range.
A is a localized group of individuals belonging to the same species.
The first photosynthetic organisms used as source of hydrogen for reducing carbon dioxide to sugars.
published an essay on The Principle of Population’.
Multiple Choice

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Grade 12 Life Sciences Evolution

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