B15 Flashcards
(35 cards)
In the mid-19th century Gregor Mendel carried out breeding experiments on plants. One of his observations was
that the inheritance of
each characteristic is determined by ‘units’ that are passed on to descendants unchanged.
In the early 20th century it was observed that chromosomes and Mendel’s ‘units’ behaved in similar ways. This led to the idea that
the ‘units’, now called genes, were located on chromosomes.
In the late 19th century behaviour of chromosomes during cell division
was observed.
In the mid-20th century the structure of DNA was
determined and the mechanism of gene function worked out.
This scientific work by many scientists led to the what being developed
The gene theory being developed
Mendel had shown that
The Height characterisitics of pea plants was determined by separatekg inherited “hereditary units” passed on from each parent.
- the ratios of tall and dwarf plants in the offspring shoedd rhst the unit for tall plants, T, was dominant over the unit for dwarf plants, t .
Mendel reached these three importsnt conclusions about heredity in plants:
- Characteristics in plants are determined by ‘hereditary units’
- hereditary units are passed on to offspring unchanged from hith parents (one from each)
- hereditary units can be dominant or recessive- if individual has both the dominant and recessive unit for characterisitic, the dominant is expressed
Why did people not accept Mendel’s ideas when they were first published?
- People couldn’t see units of inheritance
- so no proof of their existence.
- People were unused to studying careful records of results.
New discoveries have helped to developt Darwins theory:
- The relevant sceintific knowledge wasn’t available at the time so couldn’t give a good explanation for why new characterisitics appeared or exactly how individual orgsnisms passed on beneficial adaptions to their offspring.
- now we know that phenotype is controlled by genes. New phenotypic variations arise because of gnetic variants produced by mutations. Beneficial variations are passed on to future generations in the genes that parents contribute to their offspring.
What is speciation?
The phenotype of organisms can change so much because of natural selection that a completely new species is formed
What three reasons were the reasons that Darwins theory was controversial:
- went against religious beliefs about how life on earth developed.
- Darwin couldn’t show how these new, useful characterisitics appeared or hoe they were passed on from individual organisms to their offspring.
- There wasn’t enough evidence to convince scientists, as not many other studies had been done into how organisms change over time.
Explain what Lamarck’s different ideas were:
- argued that changes that an organism acquires during its lifetime will be passed on to its offspring
Lamarck’s hypothesis was eventually
- rejected because experiments didn’t support his hypothesis.
The discovery of genetics supported darwin’s idea because
- It provided an explanation of how organisms born with beneficial characteristics can pass them on.
- fossils helped as it allowed you to see how changes in organisms developed slowly over time.
- bacteria are able to evolve to become resultant to antibiotics supporting evolution and natural selection.
Alfred Russel Wallace independently proposed the theory of evolution by natural selection. He published joint writings with Darwin in 1858 which
prompted Darwin to publish On the Origin of Species (1859) the following year.
Wallace worked worldwide gathering evidence for evolutionary theory. He is best known for his work
on warning colouration in animals and his theory of speciation.
Alfred Wallace did much pioneering work on speciation but
more evidence over time has led to our current understanding of the theory of speciation.
What are the reasons for extinction?
- the environment changes to quickly.
- a new predator kills them all.
- a new disease kills them all.
- They can’t compete with another species for food.
- A catasrophic event happens that kills them all.
Speciation happens when
Populations of the same species change enough to become reproductively isolated - meaning they can’t interbreed to produce fertile offspring
Isolation is where
Populations of a species are separated. Can happen due to a physicsl barrier. Conditions on either side of barrier will be slightly different.
- this causes different characteristics which will become more common in each population due to natural selection operating differently on the populations.
- eventually individuals from the two different pops will have changed so much that they won’t be able to breed with one another to produce fertile offspring, they would have become infertile soecies
Name the process which results in the development of new species
Speciation
describe what is happening in the four steps that results in the development of a new species.
- populations become isolated in some way, for example by a river gorge.
- genetic variation occurs between the two populations, slightly different variations occur in the two different populations.
- Alleles for characteristics that enable organisms to survive and breed successfully in the new conditions will be selected
- natural selection occurs in both populations, but independently of each other, different variants are selected in each population.
- the populations become different either side of the isolation (river) and are now different species.
Compare the similarities and differences between Alfred Wallace and Charles Darwin in the development of ideas of speciation and evolution by natural selection.
Similarities:
- both men developed their ideas while on specimen collecting expeditions;
- both based on adaptation to change, and survival and breeding of the fittest; both ideas relied on - variation in a characteristic for a species, and inheritence of the useful characteristics through reproduction.
Differences:
- Darwin thought of ideas earlier, so had time to do experimental research to back it up;
Darwin published ideas the next year in book form (leading to it being noticed);
- Wallace focused more on speciation and understanding geographical isolation as a driver of this.
Bacteria sometimes develop
Random mutations in their DNA which can lead to changes in the bacteria’s characteristics. This can lead to antibiotic resistanr strains forkinf as the gene for antibiotic resistance becomes more common in the population. They can evolve quite quickly.
