Stuff I still need to memorise Flashcards
Descent with modification
Equivalent structures with the same body plan that might perform different functions are called homologous structures or characteristics. These structures provide evidence of evolution since the same structure can be traced back to a common ancestor. Examples of homologous structures: fur on mammals
Why giraffes have long necks according to Lamarck
Giraffes have a need to reach higher to eat leaves in tall trees
-because of the law of use and disuse where if an individual uses a characteristic more it changes that characteristic to suit the use. Giraffes stretch their necks each time they feed.
-Each time their necks get a little bit longer and throughout their lives this streching means they acquire a slightly longer neck.
-When the giraffes have offspring then those offspring will inherit the acquired characteristic from their parents.
-Over time, individuals adapt to their environment due to these processes and form new species.
NATURAL SELECTION
Organisms of a population produce a large number of offspring.
There is a great deal of variation amongst the offspring of the organisms in a population.
Some organisms have favourable characteristics giving them an advantage and some do not.
When there is a change in the environmental conditions or if there is increased competition, then organisms with characteristics that make them more suited, survive, whilst organisms with characteristics that make them less suited, die.
The organisms that survive, reproduce and thus pass on the favourable characteristic to their offspring.
The next generation will therefore have a higher proportion of individuals with favourable characteristics.
The changes occurring in the population of a species over time may lead to the formation of a new species.
Similarities between natural selection and artificial selection:
Variation exists in a particular population,
that variation is inherited from parents to offspring,
and the populations change over time with certain characteristics becoming more frequent.
Differences between natural selection and artificial selection:
With natural selection, the environment acts as the selective pressure whereas artificial selection has humans determining selected traits.
With natural selection, the selected traits are advantageous in the environment whereas artificial selection selects traits that are not necessarily advantageous for survival in the wild.
Natural selection usually maintains or increases variation whereas artificial selection generally decreases variation in a population through inbreeding.
ALLOPATRIC SPECIATION
If a population of a single species
becomes separated by a geographical barrier (sea, river, mountain, lake),
then the population splits into two populations.
There is now no gene flow between the two populations.
Since each population may be exposed to different environmental conditions,
natural selection occurs independently in each of the two populations
given enough time, the individuals of the two populations become very different from each other genotypically and phenotypically.
Even if the two populations were to mix again, they will not be able to reproduce with each other due to reproductive isolating mechanisms.
They have thus become different species.
Mode of locomotion
Upright bodies expose a smaller surface area to the sun reducing the risk of overheating during daytime locomotion (hunting, gathering, and escaping predators).
At the same time, a larger body surface area is exposed to the wind which cools the body.
Aside from its energetic efficiency, bipedalism also has the advantages of raising the head and therefore allowing a wider range of vision in a grassland environment,
and of freeing the hands for carrying items/infants or for tool use.
Foramen magnum
The foramen magnum is the hole in the base of the skull through which the spinal cord passes.
In humans, the foramen magnum is positioned further forwards, facing directly downwards, which allows the human body to be oriented vertically for bipedalism. In chimpanzees and other apes, the foramen magnum is further towards the back of the skull with the spinal cord exiting at a slight angle.
Pelvis
Pelvis
Humans have a much broader pelvis giving stability when walking upright as it transfers the weight directly to the legs. Humans have a shorter and wider, more ‘bowl-shaped’ pelvis to better support the organs above.
Apes have a much narrower, longer pelvis as the weight of the upper body is supported by the forelimbs on the ground.
The human femurs attach to a much lower point on the pelvis than apes.
Cranial ridges
Modern humans do not have sagittal crests because we do not have to chew tough foods like apes do or our ancestors did. While our jaw muscles end just below the ear, in a species with a sagittal crest they would extend all the way up, giving them the extra power they need to eat.
Genetic evidence
Geneticists use mitochondrial DNA and y-chromosome DNA to study human origins and migrations since these genomes are passed unchanged from parent to offspring. At the same time because mutations (changes) to the mtDNA do occur, scientists can determine the rate at which such mutations (or markers) take place, and can then use them as a type of molecular clock to determine the age of a particular maternal mtDNA lineage. From the studies, we can conclude:
African populations of Homo sapiens have the greatest number of mtDNA markers and are thus the oldest.
The most recent common ancestor whose genetic marker is found in all living humans, must have lived in eastern Africa approximately 150 000 years ago.
Based on markers in non-African populations, scientists believe that modern humans moved out of Africa about 60 – 70 000 years ago.
Fossil and cultural evidence
Genetic evidence
Using our current understanding of DNA, its structures and properties, scientists have analysed human DNA to explore the relationships between humans across the world.
Geneticists use mitochondrial DNA and y-chromosome DNA to study human origins and migrations since these genomes are passed unchanged from parent to offspring. At the same time because mutations (changes) to the mtDNA do occur, scientists can determine the rate at which such mutations (or markers) take place, and can then use them as a type of molecular clock to determine the age of a particular maternal mtDNA lineage. From the studies, we can conclude:
African populations of Homo sapiens have the greatest number of mtDNA markers and are thus the oldest.
The most recent common ancestor whose genetic marker is found in all living humans, must have lived in eastern Africa approximately 150 000 years ago.
Based on markers in non-African populations, scientists believe that modern humans moved out of Africa about 60 – 70 000 years ago.
Fossil and Cultural Evidence
Fossil and cultural evidence
The fossil record uncovered in Africa thus far points unambiguously to the fact that modern humans originated and evolved in eastern and southern Africa. Based on the fossil record, we can state that:
Fossils from the earliest hominin species, Ardipithecines and Australopithecines, have been found only in Africa and not anywhere else.
Fossils for the earliest species of the genus Homo, Homo habilis, have also only been found in Africa.
Fossil evidence for Homo erectus indicates this species lived in eastern and southern Africa between 1,9 – 1,4 million years ago. The earliest finds in Africa are the oldest fossils of Homo erectus discovered anywhere in the world.
Finally, Homo sapiens emerged in Africa. The oldest Homo sapiens fossils were discovered in 2005 near Ethiopia’s Omo River and are dated to between 195 000 and 160 000 years ago.
