Evolution Around Us. Many of us think of evolution as something that might have happened long ago. However, by studying species around us today we can understand how life evolves over time.
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Evolution Around Us
Many of us think of evolution as something that might have happened long ago. However, by studying species around us today we can understand how life evolves over time.
In the next few slides, you will see pictures of animals and plants that may challenge your idea of what a species is. Is a species fixed, unchangeable, and well-defined or is it dynamic, changing, and “fuzzy?” See for yourself.
Do these snails represent different species or is this just color variation in one species?
Answer: These marine snails all belong to the same species, Liguus fascitus, and represent color variation in one population.
Therefore these are differences within a species.
Answer: Two species - clockwise from top left: husky, timber wolf, bulldog, arctic fox). The Arctic fox is a separate species from the other three).
Artificial selection- the selective breeding of domesticated plants and animals to encourage the occurrence of desirable traits.
Over the past 100 years dachsunds have changed as breeders have preferred sleeker, lighter built animals with shorter legs and more elegant heads. Dachsunds born with these refined traits were allowed to reproduce and pass on these traits.
Thought Question: What type of evolution does this dog show (disruptive, directional, or stabilizing)?
Gene flow – The transfer of alleles between individuals in a population from one generation to the next. A pekingese and a great dane dog might not be able to interbreed but indirectly there is gene flow between them. Therefore, they belong to the same species.
How many species of vegetables are shown here?
Answer: One - they are all the same species!
All fourvegetables are variants (varieties) of the species, Brassica oleracea.
The way people cultivated these varieties from the same species was through artificial selection.
How is this similar to natural selection?
How is it different?
Artificial Selection -
How it works:
Wild Brassica oleracea:
Breed plants with larger flowers
Yellow cauliflower (newest mutant)
Can a “mutant” trait suddenly appear, so that a farmer could use it to breed a new variety?
In the early 1970’s in Canada, a cauliflower plant was born with a mutation that causes it to store 100 times the beta-carotene (a pigment that becomes vitamin A) in its flowers.
Breed plants with larger buds
Breed plants with larger leaves
How mutation can result in a new species
In the early 1900’s botanist Hugo de Vries bred a primrose species (O. lamarckiana) to produce a mutant offspring that had 28 chromosomes instead of the normal 14. This new plant was larger and could no longer breed with the parent species. De Vries named the new species O. gigas for its larger size.
Thought Question: What error in meiosis produced this new species?
Darwin on Variation
I HAVE hitherto sometimes spoken as if the variations- so common and
multiform with organic beings under domestication, and in a lesser
degree with those under nature- were due to chance. This, of course, is
a wholly incorrect expression,but it serves to acknowledge plainly our
ignorance of the cause of each particular variation.
Thought Question: What does Darwin mean by this statement? Do we have a better idea of what causes variation today? Explain.
How many species of squirrel are shown?
Answers: Clockwise from top left: gray squirrel, gray squirrel (black variant), red squirrel, gray squirrel (albino). The red squirrel at bottom right is a different species from the other 3.
Do these squirrels belong to the same species or different species?
Answer: These two species of squirrels inhabit opposite rims of the Grand Canyon.
Thought Question: What type of isolation most likely led to reproductive isolation and genetic divergence between these two squirrels?
Reproductive Isolation Leads to Genetic Divergence
When the Portuguese first settled the island of Madeira during the 15th century they inadvertently brought house mice (Mus musculus) with them. Mountains divide the island and several populations of mice have evolved in isolation from one another.
The chromosome number of the house mouse is 20 (2n = 20). How is the chromosome number in these mice different?
Thought Question: How could crossing-over in meiosis alter chromosome number?
Based on the appearance of these meadowlarks, do you think these birds belong to one species or two?
Answer: The eastern meadowlark (Sturnella magna, left) and the western meadowlark (Sturnella neglecta) have very similar body shapes and colorations, but they represent different species. Their songs are distinct, a behavioral difference that helps prevent interbreeding between the two species.
(a) Normal light
(b) Monochromatic orange light
(a) Under normal light these two different species of cichlids from Lake Victoria will breed with individuals that show the proper color pattern. (b) But when monochromatic lighting is used under laboratory conditions the two species color preference is undetectable and individuals will mate indiscriminately.
Do you think the shorter-winged, flightless cormorants on the right belong to the same species to the cormorant on the left?
Answers: Althoughindividuals in a species of cormorants vary in wing size, the Galapagos cormorants on the right (different species) have much smaller wings than their mainland relatives. In other words, this is not variation within a species.
Thought Question: What are the costs/benefits of having large wings? On the Galapagos, do you think the costs outweigh the benefits? Explain how this could this have led to these cormorants evolving smaller wings?
Imagine an 8 foot, white-furred animal with six fingers, six toes, and pink eyes. What is it?
A human! How is this so? For an explanation, read the next slide.
(Drawing courtesy of Christine Yoo, 2004)
If the people shown below were bred together, over several generations a person could be born with all the traits described on the previous slide. Although this person would appear unusual, they of course are still human. But if over time these people were isolated, such as on an island, they would not be able to breed with other people and would become so genetically distinct that we might consider them a separate species.
A Human in the Future?
If two species share a recent common ancestor, they may still be able to produce “hybrid” offspring. These hybrids however are almost always infertile.
“Why…has the production of hybrids been permitted? To grant species the special power of producing hybrids, and then to stop this further propagation by different degrees of sterility, seems a strange arrangement.”- Charles Darwin
Darwin on the “Special Creation” of Species
Thought Question: Who do you think Darwin was addressing this point to? Why do you think he felt the need to make this point?
Different species show many similarities between them:
Below are pictured different species of frogs, lizards, and birds. However, notice the similarities among each. In some cases subspecies within a species are shown. These similarities are not coincidental. It is because they share a common ancestor.
More ape-like + most ancient
More human-like + most recent
Thought Question: Humans and Chimpanzees are 98% genetically identical. If people share a common ancestor with apes, then one would expect to find a range of intermediate forms that link us together. Given this possibility, how might genetic variation and directional selection be used to interpret the fossil data above?
1) Variation exists in every species
Is this micro or macro evolution? Why?
Variation within a species
2) Reproductive isolation between two populations in a species
Is this micro or macro evolution? Why?
Two subspecies genetically distinct, but still part of the same species