IB BIO TOPIC 5.4 : EVOLUTION

Question 1

Define evolution :

Answer 1

• the cumulative change in the heritable characteristics of a population, or
• the changes in allelic frequencies in the gene pool of a population over time, as a result of natural selection, genetic drift, gene flow, and mutation pressure.

Question 2

Outline the evidence for evolution provided by the fossil record, selective breeding of domesticated animals and homologous structures.

Answer 2

• Evolution of modern horse: Equus, a case in macroevolution based on paleontology.

Question 3

Evolution of modern horse : Hyracotherium

Answer 3

• fossils from Eocene, about 40 million years ago
• 25-50 cm tall at shoulder
• small molars without crowns, which could not have withstood grinding of eating grass, but probably used for browsing succulent leaves in forests.
• 3-toed forelimbs, 4-toed hindlimbs, dog-like footpads on feet
• eyes half way between top of head and tip of nose

Question 4

Evolution of modern horse : Mesohippus

Answer 4

• fossils from Oligocene, about 30 million years ago
• larger body than Hyracotherium
• increased grinding surface of molars with crowns, which continued to grow as they were worn away (like modern horses), but probably still used for browsing
• 3-toed forelimbs, 3-toed hindlimbs

Question 5

Evolution of modern horse : Merychippus

Answer 5

• fossils from Miocene, about 20 million years ago
• larger body than Mesohippus
• still larger grinding surface of molars with crowns, which continued to grow as they were worn away (like modern horses), and probably used for grazing grasses
• 3-toed forelimbs, 3-toed hindlimbs, but with middle toe enlarged for bearing weight and the other two toes greatly reduced
• thought to be the direct ancestor of Equus and all modern horses

Question 6

Evolution of modern horse : Pliohippus

Answer 6

• fossils from Pliocene, about 5 million years ago
• larger body than Merychippus
• still larger grinding surface of molars with crowns, which continued to grow as they were worn away (like modern horses), and certainly used for grazing grasses
• 1-toed forelimbs, 1-toed hindlimbs, with single toe enlarged as hoof
• thought to be the direct ancestor of Equus and all modern horses

Question 7

Evolution of modern horse : Equus

Answer 7

• modern horses from Pleistocene, about 2 million years ago, to present
• larger body than Pliohippus
• still larger grinding surface of molars with crowns, which continued to grow as they were worn away (like modern horses), and certainly used for grazing grasses
• 1-toed forelimbs, 1-toed hindlimbs, with single toe enlarged as hoof

Question 8

Summarize the evolution of the modern horse :

Answer 8

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

Question 10

What is the Fossil Record ?

Answer 10

A fossil is the preserved remains or traces of any organism from the remote past

Fossil evidence may be either:

Direct (body fossils): Bones, teeth, shells, leaves, etc.
Indirect (trace fossils): Footprints, tooth marks, tracks, burrows, etc.

Question 11

Outline why the fossil record is useful :

Answer 11

It reveals that, over time, changes have occurred in features of organisms living on the planet (evolution)

This suggests that changes to an ancestral species was likely responsible for the appearance of subsequent species (speciation via evolution)
Furthermore, the occurrence of transitional fossils demonstrate the intermediary forms that occurred over the evolutionary pathway taken within a single genus

Question 12

Outline 2 weaknesses of the fossil record :

Answer 12

Fossilization requires a unusual combination of specific circumstances to occur, meaning there are many gaps in the fossil record
Only the hard parts of an organism are preserved and often only fragments of fossilized remains are discovered

Question 13

What is selective breeding ?

Answer 13

Selective breeding of domesticated animals is an example of artificial selection, which occurs when man directly intervenes in the breeding of animals to produce desired traits in offspring

Question 14

What are the results of selective breeding ?

Answer 14

As a result of several generations of selective breeding, domesticated breeds can show significant variation compared to the wild counterparts, demonstrating evolutionary changes in a much shorter time frame than might have occurred naturally

Question 15

Give examples of selective breeding :

Answer 15

Breeding horses for speed (race horses) versus strength and endurance (draft horses)
Breeding dogs for herding (sheepdogs), hunting (beagles) or racing (greyhounds)
Breeding cattle for increased meat production or milk
Breeding zebras in an attempt to retrieve the colouration gene from the extinct Quagga

Question 16

What are homologous structures ?

Answer 16

Comparative anatomy of groups of animals or plants shows certain structural features are basically similar, implying a common ancestry

Question 17

What is teh significsnce of homologous structures ?

Answer 17

It illustrates adaptive radiation (divergent evolution) as a similar basic plan has been adapted to suit various environmental niches
The more similar the homologous structures between two species are, the more closely related they are likely to be

Question 18

Which dilemna states that populations tend to produce more offspring than the environment can support ?

Answer 18

The Malthusian dilemma states that populations tend to multiply geometrically, while food sources multiply arithmetically
Hence populations tend to produce more offspring than the environment can support

Question 19

Explain that the consequence of the potential overproduction of offspring is a struggle for survival

Answer 19

• less resources available to other members of the population (environmental resistance)
• competition for available resources and a struggle for survival
• Result of this competition will be an increase in the mortality rate, leading to an S-curve growth rate as the population approaches the carrying capacity (K)