IB DP Biology Topic 5: 5.1 Evidence for evolution: Study Notes

​In the Evidence for Evolution unit we look at look at how time and many generations change the genetic make-up of species as they become adapted to new surroundings or altered conditions. One result of these changes may be the evolution of new varieties and species.

​The unit is planned to take 2 school days.

​Essential idea:

• There is overwhelming evidence for the evolution of life on Earth.

​Nature of science:

• Looking for patterns, trends and discrepancies—there are common features in the bone structure of vertebrate limbs despite their varied use. (3.1)
  • Propose a mechanism that explains the pattern found in vertebrate limb structure yet allows for the specialization of different limb functions.

5.1.U1 ​Evolution occurs when heritable characteristics of a species change. (Oxford Biology Course Companion page 242).

• Define evolution​

Evolution at its most fundamental level simply describes a change over time. In living organisms this change refers to the heritable characteristics of a species (biological evolution)

• When heritable characteristics of a species or a biological population change over successive generations
• These traits cannot be acquired over a lifetime, they are heritable traits or alleles in an organism’s DNAthe cumulative change in the heritable characteristics of a population, or
• Evolution is the change in allelic frequency in a gene pool of a population over time, as a result of natural selection, genetic drift, gene flow, and mutation pressure.

5.1.U2 ​The fossil record provides evidence for evolution. 

• Define strata and paleontology.
• Explain three pieces of evidence that fossils provide that evolution has occurred.

Something provides evidence for evolution when it demonstrates a change in characteristics from an ancestral form. The fossil record provides evidence by revealing the features of an ancestor for comparison against living descendants.

A fossil is the preserved remains or traces of any organism from the remote past. The preserved remains (body fossils) provide direct evidence of ancestral forms and include bones, teeth, shells, leaves, etc. Traces provide indirect evidence of ancestral forms and include footprints, tooth marks, burrows and faeces (coprolite). The totality of fossils, both discovered and undiscovered, is referred to as the fossil record. The fossil record shows that over time changes have occurred in the features of living organisms (evolution)

Fossil Record

• Fossils are the preserved remains of animals, plants, and other organisms from the past.
• The fossil record shows the gradual change of species over time.
• The timeline in which fossils appear are what scientists would expect, with bacteria and algae being the oldest in the fossil record. Followed later by shelled animals and trilobites, then dinosaurs and early reptiles, birds and mammals later still.

• Many fossil sequences link together present day organisms with their likely ancestors. For example present day horses and zebras are closely related to tapirs and rhinos, which are all linked back to the Hyracotherium, an animal similar to the rhinoceros​
• Whale evolution fossil record is also whale documented

Summary changes correlate with environmental changes Eocene: marshy with succulent leaves Miocene to present: grasslands spreading with drier climate* grasses favor increased grinding surface of molars harder ground favors reduction of toes, development of hoof grassland environment favors larger size, faster speed to escape predators a variety of different species of horses lived at most times

Reliability of the Fossil Record

• Interpretation of fossil record based on differential preservation, i.e., some organisms have not been found or some locations provide necessary conditions for fossilization while other areas do not
• Bias present since not all areas of the globe have been searched for fossils, e.g. Central Asia, due to inaccessibility
• Small number (e.g. one or two) of fossils found for a species so not sure if that is a typical or atypical representation
• Gaps of time or “missing links” still exist in the fossil record

5.1.U3 ​Selective breeding of domesticated animals shows that artificial selection can cause evolution. 

• Explain the process of artificial selection using selective breeding.
• Use an example to explain how selective breeding has lead to evolution in a species.

Selective breeding is a form of artificial selection, whereby man intervenes in the breeding of species to produce desired traits in offspring. By breeding members of a species with a desired trait, the trait’s frequency becomes more common in successive generations. Selective breeding provides evidence of evolution as targeted breeds can show significant variation in a (relatively) short period

• Breeding plants and animals for specific genetic traits.
• Shows a good record of recent changes in genetic characteristics over a few dozens of generations that man has selected to breed.
• For example, chickens that produce more eggs or cows that produce more milk are selected to breed, hopefully passing these traits onto next generations.
• Plants can be bred in a similar manner based on useful or beneficial characteristics breeders would like to see in the next generation of plants.
• The evolution of domesticated dogs has produced many different breeds through artificial selection

5.1.U4 ​Evolution of homologous structures by adaptive radiation explains similarities in structure when there are differences in function. 

• Contrast analogous structures and homologous structures.
• Contrast convergent evolution and adaptive radiation.
• State an example of analogous structures.
• State an example of homologous structures.
• Define vestigial structure.
• State an example of a vestigial structure

Comparative anatomy of groups of organisms may show certain structural features that are similar, implying common ancestry. Anatomical features that are similar in basic structure despite being used in different ways are called homologous structures. The more similar the homologous structures between two species are, the more closely related they are likely to be.

Homologous structures: vertebrate embryos and the pentadactyl limb.

• homologous structures: various different structures sharing the same fundamental plan

• derived from a similar embryonic origin
• variations on the basic structure allow different functions
• adaptive radiation = permitting exploitation of different ways of life

• suggests divergence from a common ancestor
• common ancestor was a land animal e.g. shrew, with short five-toed limb with not specific specialization
• Modern mammal species evolved by modification of limbs to a wide variety of habitats
• Habitats can be terrestrial, aquatic and air
• i.e. running (horse) digging (moles), flying (bats) swimming ( whales) and grasping (monkey)

5.1.U5 ​Populations of a species can gradually diverge into separate species by evolution. 

• Describe the process of gradual speciation.

​Within a population of any given species there will be genetic variation (i.e. variation which is inheritable). Typically this variation will be continuous and follow a normal distribution curve as the rate of change is gradual and cumulative
​

• Within a population there is genetic variation
• If two populations of the same species become separated so that they do not reproduce or interbreed because they become separated by geographical boundaries; for example one group migrates to an island or they became separated by a mountain range, then natural selection will act differently on those two separate populations
• Over time, these populations change so that they are recognizably different and can or do not interbreed if they were to merge together again​

5.1.U6 ​Continuous variation across the geographical range of related populations matches the concept of gradual divergence. 

• Explain how continuous variation across geographical ranges is evidence of evolutionary change.
• State an example of recognizably different populations of the same species across a geographical range.

The degree of divergence between geographically separated populations will gradually increase the longer they are separated. As the genetic divergence between the related populations increase, their genetic compatibility consequently decreases. Eventually, the two populations will diverge to an extent where they can no longer interbreed if returned to a shared environment. This process is called speciation
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• When populations diverge over time and are separated, one would expect these populations to be in different stages of variation or divergence and not all separate distinct organisms right away or all the same unchanged species
• Darwin gave many of these examples that showed populations that are slightly different, but are not clearly separate species
• Examples of this are the Lava lizards and finches of Galapagos, and the Spiny Sticklebacks of BC

​Biogeography is the study of species distributions. It examines how species have been distributed across different places at different times. The distribution of species shows a very clear pattern. More similar species tend to be found closer to one another geographically. The distribution of many animals and plants across different continents can be explained by continental drift (the movement tectonic plates).

The continents were once all joined together in one giant super-continent. About 200-180 million years ago the southern half called Gondwanaland broke away. This would later split into what we now know as Antarctica, Africa, Australia, South America and India.

5.1.A1 ​Development of melanistic insects in polluted areas. 

• Explain how natural selection leads to changes in the melanistic variety of insects in polluted areas.

Peppered moths (Biston betularia) exist in two distinct polymorphic forms – a light colouration and a darker melanic variant

• In an unpolluted environment, the trees are covered by a pale-coloured lichen, which provides camouflage for the lighter moth
• In a polluted environment, sulphur dioxide kills the lichen while soot blackens the bark, providing camouflage for the dark moth

In pre-industrial revolution tree trunks in forests around major cities were light grayish-green due to presence of lichens. Most peppered moths in area are light colored with dark spots. Prior to 1850, no dark (melanic) form of moth was ever collected.

In post industrial revolution tree trunks in forests become dark due to covering with soot. (i.e. coal-based industries). After 1850, dark form of peppered moth were collected. Over the next 45 years, dark forms of moth became more common.

Melanic form of moth accounts for 95% of peppered moth population around many industrial cities in England. The mottled form accounts for largest percentage of population in non-industrial areas. This evidence shows that both formes of the peppered moth existed in the population. Brds prey upon both forms of moth when resting on tree trunks. The increase of one form over the other form (e.g. dark over light) due to bird predation (i.e. selective pressure) is an example of natural selection. Increased prevalence of melanic form of peppered moth during the Industrial Revolution is an example of a directional selection following a change in environmental conditions.

5.1.A2 ​Comparison of the pentadactyl limb of mammals, birds, amphibians and reptiles with different methods of locomotion. 
​

• Define pentadactyl limb.
• List the bone structures present in the pentadactyl limb.
• Identify pentadactyl limb structures in diagrams of amphibians, reptiles, birds and mammals.
• Relate differences in pentadactyl limb structures to differences in limb function.

Comparative anatomy of groups of organisms may show certain structural features that are similar, implying common ancestry. Anatomical features that are similar in basic structure despite being used in different ways are called homologous structures. The more similar the homologous structures between two species are, the more closely related they are likely to be

• basic structure: the forelimbs of all tetrapods (amphibians, reptiles, birds, mammals) have the same basic pattern of 5 metacarpals and 5 phalanges arising from the same embryological structures because development is determined by many shared genes from a common a ancestor
• bats: metacarpals and phalanges elongated to support wing
• humans: slender metacarpals and phalanges with opposable thumb
• elephants: metacarpals and phalanges short and stout to support weight similar to a column

Vertebrate embryos:

• basic structure:vertebrate embryos share many similarities of body shape during early embryological development (zygote, blastocyst, body segment development., limb bud stage),
• diverging to species specific development in later embryological stages; common developmental plan arises from a set of genes inherited from a common ancestor
• salamander, chicken, pig, monkey and human all similar through limb bud stage
• differential growth produces species specific patterns during late fetal development
• gill pouches disappear during chicken, pig, monkey, and human fetal stages, but remain in salamander until adulthood
• tail remains in salamander, chicken, pig, and monkey throughout development, but is lost during human fetal development