Natural selection and evolution

During the 17th century, most people believed in creationism, which considered that life was created. However, naturalists particularly Linnaeus (1707 -1778), who undertook systematic classification of organisms, began to notice similarities in groups of living things that seemed to be related. These similarities looked like a family tree. Fossil record showed that the species had not remained static but had undergone changes. Evidence of extinctions, like those of the dinosaurs and the dodo was a clear proof that changes had taken place.

Lamarck (1809) proposed the theory of inheritance by acquired characteristics. For example, the giraffes stretched their necks to reach food in tall trees, and their offspring inherited stretched necks. This theory is now known to be wrong.

Charles Darwin (1859) published a book on the origin of species by natural selection. Another naturalist called Alfred Wallace arrived at a similar findings as Darwin’s.

Natural selection is based on the following premises, namely:

1.   Populations including their offspring have inheritable characteristics that can be passed from generation to generation.

2. With time, individuals in a population tend to overproduce resulting in competition for food resources.

3. Individuals with more favourable genes out-compete the others and increase their numbers. This is known as ‘survival for the fittest.’

 4. Individuals with less favourable genes decline in numbers, eventually becoming extinct.

  Evidence in Support of Natural Selection

(a) The Giraffe’s Long Neck

Darwin explained the giraffe's long neck as follows. In a population of giraffe-like animals, there would be random genetic variation in neck length. In an environment where there were trees and bushes, the long-necked animals would be better adapted and would compete more favourably for nutrients than their short-necked relatives and would therefore live long and produce more offspring passing on their favourable traits to them. With successive generations, the numbers of long-necked giraffes would increase while the numbers of the short-necked giraffes would decrease. Eventually the population would consist only of long-necked giraffes. 

 Long necked giraffes have been favoured by natural selection over short-necked giraffes

(b) Artificial Selection

Darwin used the analogy of selective breeding (or artificial selection) to explain natural selection. In selective breeding, useful characteristics are usually chosen by humans, and only those individuals with the best characteristics are used for breeding. In this way, species can be changed over a long period. All domesticated species of animals and plants have been selectively bred like this, often for thousands of years, so that most of the animals and plants we are most familiar with today are not really natural and are nothing like their wild relatives (if any still exist). 

Types of Natural Selection

1. Directional Selection

Directional selection is due to a change in the environment. This creates a selective pressure for the species to change in response to the changes in the environment. Directional selection can be illustrated by industrial melanism, bacterial resistance to antibiotics and resistance to pesticides. 

(a) Peppered Moth (Briston betularia)

 This is not only an adaptation but also an ongoing evolutionary process.  The peppered moth Briston betularia has undergone directional change because of environmental changes. Before the industrial revolution in Britain, light coloured moths were well camouflaged from bird predators against the pale bark of birch trees, while rare mutant dark moths were easily spotted and eaten by birds.

 During the industrial revolution in the 19th century, birch woods near industrial centres became black with soot pollution. Black moths blended well with the new environment and had a selective advantage over the light coloured moths which could easily be seen by birds against the black background. The black moths increased while light coloured moths declined.

  Pollution of the environment by industrialization favoured dark-coloured moths at the expense of light-coloured moths

 (b) Bacteria Resistance to Antibiotics

Antibiotics kill bacteria, but occasionally a chance mutant appears that is resistant to that antibiotic. In an environment where the antibiotic is often present, this mutant has an enormous selective advantage since all the normal (wild type) bacteria are killed leaving the mutant cell free to reproduce and colonise the whole environment without any competition.

 (c) Pesticide Resistance

 Warfarin is a pesticide that kills rats and other pests. When it was first used against rats, some rat population already had mutant individuals that showed resistance to warfarin. These mutant rats needed a diet rich in vitamin K which was not readily available. So they were always eliminated from the population. When warfarin was introduced, it affected majority non-resistant rats, leaving the resistant ones to reproduce and multiply. After some time the population of the rats consisted mainly of individuals that were resistant to warfarin.

2. Stabilising (normalising) Selection

When selection operates against individuals at the two ends of the distribution curve for a polygenic trait, the process is known as stabilising selection. Usually when a trait in a population is subjected to two or more opposing directional pressures operating at the same time, the pressures select against the two extremes of the distribution curve. In a population of plants, for example, plants that are too tall to resist winds and those of the same species that are so short that they are shaded by other plants and cannot get enough sunlight will be selected against, while those plants falling in the middle will be favoured.

                                           Three types of selection pressures

Another example of stabilising selection can be seen in the birth weight of humans. Usually the heaviest and lightest babies have the highest mortality and are less likely to survive to reproduce and pass on their alleles. Therefore, stabilising selection reduces extreme variations in the population.

 3. Disruptive (divergent) Selection

It can happen that a polygenic character of a population is subject to two or more directional pressures towards two extremes of the population distribution curve. Let us take a population of birds showing a range of beak lengths. Suppose that as time passes the birds with short beaks and those with longest beaks have a feeding advantage over those birds with intermediate beaks. This might happen if there is some change in the population of plants that produce fruits suitable for intermediate beaks, or if a population of birds that is more efficient in harvesting such fruits immigrate and become established in the area.

The effect of such population pressures, in the short time anyway, would be to divide the population into short and long beaks, thus disrupting the smooth curve of the phenotypes.

EVOLUTION

Evolution is the process of genetic change in populations from generation to generation. Evolution is due to natural selection that acts on the inheritable characteristics within the population.

Evolution occurs at the population level. A population is a group of individuals of the same species living in same place at the same time. A biological species is a group of populations whose individuals have the same potential to interbreed and produce fertile offspring. Each species is concentrated in a geographical area. One population may be isolated from others of the same species. If the isolated individuals interbreed only rarely with those in other populations, there will be very little exchange of genes and with the passage of time the individuals may become separate species. Populations may be isolated by barriers such as water, land mass, or mountain ranges.

Microevolution

The total collection of genes in a population at any time is called a gene pool. The gene pool is the reservoir from which members of the next generation of that population derive their genes. It consists of all the alleles in all the individuals making up a population. For most gene loci, there are only two or more alleles in the gene pool, and individuals may either be homozygous or heterozygous for each locus.

 For example, in plants, there are two alleles for height. One allele determines shortness and one allele determines tallness. The relative frequencies of these alleles change over time, so that one allele, say the allele for tallness, may become more common than the allele for shortness. This means that over a very long time the population of the plants in an area will consist mainly of tall plants. Such a small change in the gene pool is called microevolution.

Causes of Evolution

(a) Genetic drift

Evolution may occur as a result of genetic drift which is a change in the gene pool of a small population as a result of chance. In a population that is not evolving, the gene pool will remain the same from one generation to the next. In a large population of, let us say, 10000 individuals, a chance change in the gene pool may not show any significant impact on the gene pool in subsequent generations. However, if the population of organisms is small, a chance event may have large effect on the gene pool of the next generation.

Consider, a hypothetical situation involving 10 individuals in a population. Assume that 3 of the individuals have genotype RR, two genotype Rr and five genotype rr. If 3 of these individuals are eaten by the prey and their genotype was RR, the effect will be marked decrease in the frequency of the R allele, with a corresponding increase in the frequency of r allele. The change in the relative frequencies of allele R and allele r has occurred by chance, not by natural selection. Genetic drift operates in populations that have 100 or less individuals.

(b) Gene flow

Gene flow is the gain or loss of alleles from a population by movement of individual gametes. Gene flow is due to migrations of individuals of a population. Emigration of fertile individuals causes a loss of alleles from the gene pool and immigration constitutes a gain of alleles. Both emigration and immigration affect the frequencies of alleles in the gene pool. 

(c) Mutation

A mutation is a change in organism’s DNA, resulting in creation of new alleles. Gene mutations are rare indeed, occurring only once in 10⁵ and 10⁶ gametes. As a result gene mutation has very little impact on a large population in a generation. Over the long term, however, mutation is vital for evolution since it is the only process that produces new alleles.

(d) Random Mating

Random mating implies that there is no preferential selection of mates. In other words, there is no choice of who to mate with. This allows new alleles to be introduced to gene pool.

On the other hand, the selection of mates other than by chance is called nonrandom mating and appears to be universal among humans. Nonrandom mating tends to occur among neighbours, school mates, people of the same colour, and even people of the ‘same’ height in the population.

(e) Natural selection

This is a differential success in reproduction. Individuals in the population who are more resistant to disease, more efficient in finding food or mates, leave more offspring than the others. This would disturb the genetic equilibrium, causing the frequency of the alleles of the less successful individuals to decline in the gene pool. In this way natural selection results in the accumulation and maintenance of the traits that adapt the population to its environment.
Evidences for evolution

(a) Fossil Record

Fossils are remains of animals and plants that have been preserved between layers of sedimentary rocks for millions of years. They also include impressions of whole or parts of animals and plants. Deeper layers of rock that sedimented much earlier have remains of simple primitive organisms while the more recent rocks contain organisms that are more complex.

                                                        Impressions of an insect 

 Paleontologists have shown that the earliest organisms found in rocks date back to about 3.6 billion years ago. These simple organisms known as prokaryotes have no membrane-bound nuclei. They consisted of bacteria and cynophytes. Eukaryotic organisms that is, nucleated single celled organisms with organelles, appeared about 1½ billion years after the prokaryotes.

Animals with backbones appeared about 435 millions of years.  The first vertebrates dominated the earth until 65 millions of years when the more intelligent mammals came to the scene. Another group of mammals, the hominids or human-like creatures, did not appear until only 2½ millions of years.

(b) Biogeography

Biogeography is the study of the distribution of plants and animals in the world. The distribution of plants and animals is influenced by many factors, such as the genetic success of the organisms, the efficiency of the method of dispersal, the existence of natural barriers such as oceans, mountain ranges and deserts. Each of these factors will determine how widely a species will be distributed over the globe.

 (c)    Continental Drift

It is believed that about 225 millions years ago the landmass consisted of one super continent called Pangaea, which floated on the denser molten core of the earth.  Through volcanic activities, the continents drifted apart, carrying with them whatever animals and plants that existed on that particular landmass at that particular time. Fossils found on different continents are quite similar. For example, fossils of plants and animals found in South America and Africa, which are now widely separated by the Atlantic Ocean, are similar.

Australia broke away from the continent of Africa about 135 millions of years after the appearance into the scene of monotremes and marsupials but before the emergence of the more placental mammals. In those continents where the placentals appeared, the monotremes and the marsupials faced insurmountable competition with these new efficient mammals and eventually died out.

 However, the Australian monotremes and marsupials had no such competition because of the ocean, which acted as a barrier and isolated Australia from the rest of the continents. Today, Australia has mammals that consist of marsupials, bats and rodents. The bats and rodents came much later.

The split and the drifting of the continents separated animals of the same ancestry, which, after millions of years of separation, became separate species. The drifting of the continents also altered climatic conditions and hence the distribution of the organisms on the earth.

(d)Embryology

It is known that animals that are quite different as adults have embryos that are similar in appearance and structure. Such embryos have a dorsal rod called the notochord, which forms the vertebral column. Secondly, they posses gill pouches that later become functional gills in fishes and some amphibians. This led to the concepts of “ontogeny recapitulates phylogeny” which means that individual development (ontogeny) repeats the development stages of the ancestral forms (phylogeny).

(e) Homology

All four legged animals or tetrapods starting from amphibians to mammals have the same basic limb bone pattern. This type of limb is called the pentadactyl limb. In the different animals, this basic structure has been modified and adapted to different environments and ways of life.

                                   Similarities in embryonic development points to a common ancestry

When the bones of vertebrate forelimbs are examined, they show striking similarities to one another although they may be adapted to quite different functions. For example, birds and bats have modified their forelimbs into wings that are used for flight. Cats and donkeys use their limbs for locomotion while humans have adapted them for use in manual and artistic work. Such body organs sharing a common origin but adapted to different functions are homologous structures.

                  There is unity of plan between different animal limbs, indicating a common origin 

(f) Analogy

Parts of the body that do not have a common origin but have similar functions are analogous structures. An external skeleton made up of a hard chitinous material covers the insect wings. On the other hand, the wings of birds have an internal skeleton of bone, which is covered by muscle, skin and feathers. These two structures are different in origin but perform a similar task, that of flight. They are analogous. 

                                             

                    A bird’s wing is analogous to an insect wing because both are adapted to flight 

  

(g) Vestigial Structures

Vestigial structures have no function and are small or rudimentary. The human appendix and the caudal vertebrate, which are the remains of a tail, are of no use to humans but are a reminder of our common kinship with other vertebrates.      

(h) Comparative Biochemistry

The occurrence of similar molecules in a complete range of organisms suggests the existence of biochemical homology, just as it is in anatomical homology. Most of the studies that have been carried out in comparative biochemistry have involved analysis of the structure of widely distributed protein molecules such as cytochrome c and haemoglobin and recently nucleic acid molecules particularly ribosomal RNA.

Cytochrome c is a respiratory protein that is located in the mitochondria of cells and is responsible for the transfer of electrons along the respiratory pathway. Studies of a variety of organisms including bacteria, fungi, wheat, insects and vertebrates show that the protein sequences of cytochrome c is quite similar. The amino acid sequence of cytochrome c in chimpanzee and humans is identical. Similar findings have been recorded in studies of myoglobin and haemoglobin of different organisms. These findings point to a phylogenetic link between these organisms.
SPECIATION

Speciation is the emergence of new species.  Species is defined as a population or groups of populations whose members have the potential to interbreed and produce fertile offspring. Human beings, irrespective of their different races, belong to the same species because they can interbreed and produce fertile offspring. The horse and the donkey can mate and produce a viable offspring, the mule, but the mule is not fertile and cannot produce offspring. Therefore, the horse and the donkey belong to separate species.

The emergence of new species is due to genetic isolation caused by geographical isolation over a number of generations. Once animals are isolated, the gene pool (the sum total of all the genes possessed by all the individuals in the population) shrinks and the variation among the animals decreases. Over many generations, new individuals with different physical characteristics from those they separated from emerge.

New species can arise either by geographical isolation or reproductive isolation.

Geographical isolation (allopatric speciation)

An interbreeding population can be divided by physical barriers such as mountains, deserts, water or just a long distance. This can happen through migration or dispersal of some of the population because of catastrophic changes such as earthquakes, floods or famine or due to continental drift. If the new environment is different from the original environment, the two populations will experience different selection pressures and will evolve differently. Eventually two different species that cannot interbreed will emerge.

Sympatric (reproductive) isolation

New species arise by mutation causing reproductive incompatibility in this genetic isolation. This can happen if a mutant individual fails to mate with other individuals in the population. A new gene producing a certain hormone may cause an animal to be rejected from the mainstream group, but breeding may be possible within its own groups of variants. When this mechanism results in the production of a new species it is known as sympatric speciation.

ADAPTATION

Adaptation is any genetically controlled characteristic that increases an organism’s fitness. Adaptation increases an organism’s chances of perpetuating its genes, usually by leaving descendants. Animals that are adapted to their environment survive and reproduce. As their numbers increase, they will spread and occupy more territory.

Variation is necessary for adaptation and the kind of variation that promotes the survival of a population will be retained and eventually spread across the population. Unfavourable variations do not lead to adaptation because the organisms that acquire such traits do not survive.

Adaptive radiation

Adaptive or divergent evolution is the evolutionary splitting of species into many separate descendant species. This form of speciation is due to geographical isolation. Adaptive radiation is usually illustrated by types of finches found on the Galapagos Islands in South America. Fourteen species of finches live on the islands and all are similar in everything except in beak structure and feeding habits. All these birds are believed to be descended from a mainland finch that had a short stout beak that was adapted for crushing seeds.

 Each of these migrant species was then subjected to the process of natural selection as it became adapted to the particular conditions on the island. In time, the various populations became so genotypically different from one another that now when they are found together on the same island they do not interbreed. They developed different type of beaks to suit the kind of diet found at particular area colonised by the finch.

                                              Finches that had a common ancestor but because of isolation became different species. 
                                            They were forced to develop beaks adapted to kinds of food they found in their new areas.

Adaptation to pollination in flowering plants

Flowering plants depend on external agents for pollination. Flowers of each species are adapted in shape, structure, colour and odour to particular pollinating agents on which they depend. Both the flower and its pollinating agents have developed evolutionary adaptations that are closely linked to each other’s characteristics. This type of evolution is termed coevolution.

Thus, bees are attracted to bright colours, sweet aromatic or minty odours. Flowers pollinated by bees have showy brightly coloured petals that are usually blue or yellow. Flowers pollinated by the hummingbirds are usually red or yellow and odourless because hummingbirds have poor sense of smell.

Moths and bats are usually active at dusk and during the night. The flowers they pollinate are usually white, which open only during the late afternoon or at night. These flowers have a heavy flagrance that guides the pollinators to them.

Defensive Adaptations

Cryptic appearance

Many animals blend into their surroundings so well as to be nearly undetectable. Frequently their colour matches the background almost perfectly. In some cases, the animals even have the ability to alter the conditions of their own pigment cells and change their appearance to harmonise with their background. Often rather than match the colour of the general background the animals may resemble inanimate objects commonly found in their habitat such as leaves or twigs. When the shape or colour of an animal offers concealment against its background, it is said to have a cryptic appearance. The frog Hyla versicolor can change colour to match either a tree trunk or vegetation. This form of colour change is called cryptic colouration.  

                             Hyla versicolor changes its color to match its surroundings

Mimicry

Species not naturally protected by some unpleasant character of their own may closely resemble (mimic) in appearance and behaviour some dangerous or unpalatable species. Two types of mimicry will be considered here.

1.              Batesian mimicry

Batesian mimicry is a condition in which predators avoid a harmless or less palatable species because they associate it with one that is poisonous or harmful. The harmless species called a mimic survives by assuming the colouration of a dangerous or distasteful species. The predators avoid the harmful species usually because they taste or smell bad or secrete poisons. 

      Mullerian mimicry      

Mullerian mimicry is a phenomenon in which two different species in a community such as wasps and bees, for example, evolve a common colour pattern as a warning to would be predators of the danger they face if they attempt to eat them. The colour pattern of white and black of wasps and bees becomes associated with danger by the predator.

                      

  The wasp to the left and bee to the right develop similar color patterns that predators avoid because they associate with danger 

Questions

1(a) State five evidences for evolution

   (b) To what extent does each one support the theory of evolution?

2 (a) Explain what you understand by natural selection