Download Biology 3201 Conditions of Hardy-Weinberg and Speciation Things

Biology 3201
Conditions of Hardy-Weinberg and Speciation
Things that disrupt Hardy-Weinberg equilibrium p. 688-696
- These conditions will cause allele frequencies in a population to change:
1. Mutation - If a mutation occurs in the DNA of a gamete, it can be passed on to future generations.
The mutation can be advantageous, disadvantageous or neutral. If the mutation is advantageous, it can
result in individuals producing a large number of offspring as a result of Natural Selection. This
changes allele frequencies in the population. E.g. Antibiotic-resistant bacteria.
2. Genetic Drift - In small populations, allele frequencies can change drastically by chance alone.
a. Bottle Neck Effect - In a small population certain alleles can become over-represented, while
others can become under-represented, by chance alone.
b. Founder Effect - When small numbers of individuals colonize a new area, the small group
will probably not contain all of the genes represented in the original larger population.
3. Gene Flow - Movement of genes into or out of a gene pool. This can reduce differences between
populations that were caused by isolation and genetic drift. E.g. Humans migration.
4. Non-random mating
a. Inbreeding - Mating between closely related partners. This does NOT change allele
frequencies (p nor q). It just causes MORE homozygous individuals in a population and fewer
pq individuals
b. Assortative Mating - Individuals choose partners with similar phenotype. E.g. Toads select
similar sized mates.
5. Natural Selection - Selective forces such as predation and competition work on populations, and
therefore some individuals are more likely to survive and reproduce more than others. This causes a
change in allele frequencies. There are 3 ways natural selection can affect gene frequencies in a
population:
a. Stabilizing Selection - Favours intermediate phenotypes and acts against extreme variants.
This reduces variation and improves adaptation of the population where the environment
remains relatively constant.. E.g. When black, white and grey colours are possible, grey is
favoured.
b. Directional Selection - Favours the phenotypes at one extreme over the other. E.g. Black or
white are favoured.
c. Disruptive Selection - Favours the extremes of a phenotypic range over the intermediate
types. E.g. Black and white are favoured over grey.
6. Sexual Selection - Selection for mating based, in general, on competition between males and choices
made by females. E.g. Birds choosing to mate with the brightest coloured mates.
Formation of New Species p. 702-703
- Adaptation - Any trait that enhances an organism’s fitness or that increases its chance of survival and
probability of successful reproduction. Adaptation is a product of natural selection.
- Species - A population that can interbreed and produce viable, fertile offspring.
- Speciation - The formation of a species. There are 2 ways by which speciation can occur:
1. Transformation - The accumulation of changes over a long period of time such that one species is
transformed into another species. E.g. Human evolution, antibiotic resistant bacteria.
2. Divergence - 1 or more species arise from a parent species that continues to exist. This causes an
increase in biological diversity. E.g. Finches of the Galápagos Islands.
Biological Barriers to Reproduction p.708-711
- Species must remain reproductively isolated (not mate) in order to remain distinct. Barriers to prevent
interbreeding include:
1. Geographical Barriers - Examples: rivers, mountains. These keep species separated.
2. Biological Barriers - Keep species reproductively isolated even when they exist in the same area.
These barriers can act before or after fertilization. These include:
A) Prezygotic Barriers - Hinder mating between species or prevent fertilization of the ova. Prezygotic
barriers include:
i) Behavioural Isolation - Species specific behaviours or signals that prevent interbreeding
between closely related species. E.g. Meadow larks have species specific songs that allow them
to recognize individuals of their own species.
ii) Habitat Isolation - Closely related species live in the same general region but occupy
different habitats or niches and rarely encounter each other. E.g. 2 species of garter snake: one
lives in open areas, the other lives near water.
iii) Temporal Isolation - Timing barriers. E.g. Species of the giant silk worm moth fly and
mate at different times during the day.
iv) Mechanical Isolation - Closely related species may attempt to mate but cannot achieve
fertilization because they are anatomically incompatible. E.g. Genitals of some species of insects
fit like a lock and key. Different species’ genitals do not fit together.
v) Gametic Isolation - Example 1: The sperm of one species may not be able to survive in the
reproductive tract of another species. Example 2: Pollen grains of one species may fail to
germinate on the stigma of another species.
B) Post – Zygotic Barriers - Post – fertilization barriers that prevent the hybrid zygote from
developing into normal, fertile individuals.
i) Hybrid inviability - Genetic incompatibility of interbred species may stop the development
of the zygote at some stage. E.g. Artificially created hybrid embryo (sheep x goat) dies early in
its development.
ii) Hybrid Sterility - Produce hybrid offspring that are sterile. Chromosomes in the 2 parents
differ so meiosis fails to produce normal gametes. E.g. Liger.
iii) Hybrid Breakdown - First generation hybrids are viable and fertile, but when their
offspring mate with each other or with one of the original parent species, the offspring are sterile
or weak.
How New Species Form (Speciation) p.714-722
1. Sympatric speciation - Term used when populations become reproductively isolated, even when
they are not geographically isolated. E.g. Chromosome changes, non-random mating.
2. Polyploidy - extra sets of chromosomes can lead to sterility. An error occurring during meiosis
causes the chromosomes not to separate so that gametes are 2n. A new species can be produced two 2n
gametes fuse. An individual that is 3n would be sterile because during meiosis, chromosomes would be
unable to pair up properly. This would lead to gametes that were not viable.
3. Allopatric speciation - population is split into 2 or more groups by a geographical barrier. They
become so distinct that they cannot reproduce if brought back together. Natural selection, mutation,
gene flow and genetic drift cause gene frequencies to diverge.
4. Adaptive radiation - Diversification of a common ancestral species into a variety of species, all of
which are differently adapted. E.g. Crossbill species have different sized beaks. Small beaks for species
that feeds on soft larch cones, medium-sized beaks for species that feeds on spruce cones and
large-sized beaks for species that feeds on hard pine cones.
5. Divergent evolution - Species that were once similar to an ancestral species diverge or become
increasingly distinct. E.g. Reptiles and birds.
6. Convergent evolution - Similar traits arise because each species has independently adapted to
similar environmental conditions. The species do not share a common ancestor. E.g. Birds and bats,
sharks and whales.
7. Coevolution - 2 species of organisms that are tightly linked evolve together. Each population
responds to changes in the other population. E.g. Predator – prey relationships; bird pollinated plants
usually have scentless flowers which are brightly coloured because birds have a poor sense of smell but
good vision; antibiotic-resistant bacteria: bacteria divide several times an hour and are able to alter their
genetic makeup very quickly. One bacterium that has mutated and become resistant to antibiotics can
quickly multiply. People help the process by not finishing their medication so surviving bacteria can
multiply.
Biology 3201
History of Evolutionary Theory
Important Names
- Georges Cuvier (p. 650-651) - A French paleontologist working in the late 1700s and early 1800s.
He found that the deeper (or older) the rocks were, the more different from modern organisms the
fossils were. He also noticed that extinctions were common and suggested that they corresponded to
catastrophes. He said the “new” species he observed had not evolved but came in from surrounding
areas unaffected by the previous catastrophe.
- Charles Lyell (p. 655) - 1830s. Expanded on a geological hypothesis that the physical features of the
Earth took a long time to develop. He suggested that the processes operated at the same rate today as
they did in the past (i.e. it takes a long time for a river to form a canyon). This concept is called
uniformitarianism. This suggested that the Earth was older than 6000, which many in Europe
believed at the time. This view supported Darwin’s ideas.
- Thomas Malthus (p.656) - 1798. He was an English economist. He wrote about the expanding
human population in Europe. He suggested human populations were growing faster than could be
supported (i.e. by improving methods of food production) and that this would have negative
consequences to survival rate. His concepts were influential to Darwin.
- Alfred Wallace (p. 657)- 1858. A British entomologist who proposed an explanation for why there
are so many beetles. His hypothesis was essentially the same as Darwin’s.
- Charles Darwin (p.655-658) - 1859. An English naturalist who studied plants and animals while
traveling on a survey ship, the Beagle, in the 1830s. Noting similarities in many different species from
different islands he proposed a theory to explain the diversity of life. His theory was Natural
Selection.
The theory of Natural Selection states:
1. Organisms produce more offspring than can survive.
2. Organisms within a population have variation (have different phenotypes).
3. Organisms which have the characteristics that are best suited to survive, will.
4. Over time these characteristics will become common in the population.
- Darwin did not know about Mendel’s work, nor about mutations. Therefore he could not thoroughly
explain how traits were inherited nor exactly how populations could change.
- Modern evolutionary theory takes these into account.
- Jean Baptiste Lamarck (p.651) - 1809. A French naturalist who proposed that organisms changed
over time by using or disusing parts of the body. If a part of an organism was used a lot it would
become larger and offspring would be born with a larger part. If they did not use it would become
smaller. I.e. Giraffe’s necks became long because an ancestor stretched it’s neck so had babies with
longer necks.
- Lamarck’s ideas existed before Darwin’s and suggested that life began as small organisms and
advanced toward perfection.
- Darwin rejected the idea of evolution toward perfection. Instead, Darwin favoured a gradual change
over time.
- Steven J. Gould & Niles Eldridge (p. 724-725) - 1972. Two Americans proposed punctuated
equilibrium which states evolution does not happen gradually but undergoes long periods of remaining
the same (stasis) and then has short bursts of great diversity.
- This helps explain gaps in the fossil record.
- “Short burst” may also be fairly long. E.g. 50, 000 years.
Alternate Explanations for Diversity
- Chemical Evolution (p. 727-728) - 1930s. Hypothesis that organic molecules could spontaneously
form from simple inorganic compounds. This was proposed by Oparin & Haldane.
- Miller & Urey (p.727-728) - 1953. Expanded on Oparin and Haldane’s work and created organic
molecules from methane, ammonia, hydrogen, water and an electrical current.
- Heterotroph hypothesis (p.729) - Proposes that the first organisms on Earth were similar to
heterotrophic bacteria. Lack of food led to the evolution of autotrophs. Autotrophs increased the
amount of oxygen in the atmosphere and aerobic life evolved. Eventually giving rise to eukaryotic
cells.
- Symbiogenesis (p.729) - Proposes that eukaryotic cells evolved through symbiosis of prokaryotic
cells. This is supported by the presence of circular bacteria-like DNA in mitochondria and chloroplasts.
- Panspermia theory (p.728) - Proposes that life began on another planet and was carried to earth on
an asteroid. This is supported by the presence of organic molecules in some asteroids and the recent
bacteria-like fossils found on a rock from Mars in Antartica.
- GAIA theory (p.728) - Living and non-living elements of the Earth work together to maintain the
biosphere. Gaia is also sometimes used as the name of the idea that the Earth is a living superorganism
which regulates itself.
- Intelligent design (p.729) - This concept suggest that life on Earth is too complex to have developed
by chance alone. Instead it was guided by an intelligence.