Transcript Speciation
Theory of Evolution • A change in the allele frequencies of a population from one generation to the next Gene Pool • All the different alleles in the gene pool of a population at a given time. Processes of evolution • There are several processes by which the allele frequencies can be changed. 1. Natural Selection A process resulting in the survival of those individuals in a population that have the genotypes that make them better adapted to the environment. These individuals are able to reproduce and pass these features on to the next generation. Those less well adapted tend to be eliminated. • Understanding Evolution - Inheritance and Change - Natural selection in action - Understanding Evolution.wmv Darwin’s theory of natural selection OVERPRODUCTION • Populations tend to produce too many offspring for the environment to support which means that some will die. GENETIC VARIATION • The current conditions of the environment tend to favour particular genetic variations within the population. NATURAL SELECTION • These individuals have a greater ability to survive and/or be healthy enough to reproduce INHERITANCE • Those that do reproduce pass their “successful” genes on to the next generation .i.e. their genetic variations are inherited. REPRODUCTIVE SUCCESS • The best suited leave the most offspring. Each new generation will contain more offspring with the better suited traits, and less with the unfavourable traits. Modes of Action • Natural selection has three modes of action: 1. Stabilizing selection 2. Directional selection 3. Disruptive selection Number of Individuals Small Large Size of individuals a. Stabilizing Selection • Acts against extremes and favours the intermediate. Number of Individuals Small Large Size of individuals b. Directional Selection • Favours variants of one extreme. Number of Individuals Small Large Size of individuals c. Disruptive Selection • Favours variants of opposite extremes. Number of Individuals Small Large Size of individuals Genetic drift: The changes in allele frequencies that occur in populations as a result of RANDOM changes such as infertility or sudden death of a breeding individual. These changes in allele frequencies are significant (show up) in SMALL POPULATIONS, but are not so significant in large populations. Founder Effect • The result of starting a new population with a low number of individuals (founders) so that their gene pool may not contain the same proportion of alleles as the parent population. Gene Flow The gain or loss of alleles from a population by the movement of individuals or gametes. • Immigration increases genetic diversity • Emigration decreases genetic diversity Mutation: Change in an organism’s DNA that creates a new allele. This is the only way new information can enter a population. This will increase genetic diversity. Speciation • The formation of a new species. Species • A group of actually, or potentially, interbreeding organisms that are reproductively isolated from other groups. They produce fertile offspring. • Question 1 What is a species.docx Allopatric Speciation • Occurs when a group becomes isolated from the ancestral population by a physical barrier (geographical isolation.) • The allele frequency of this group may not represent the original and may be acted upon by different selection pressures so that, over time, they become reproductively isolated so a new species is formed. Stage 1 Parent population moves into a new environment • New niches may open up or groups may wander away from each other due to competition for resources Stage 2 Geographical isolation of parts of the population May be unable to return to main population due to the formation of physical barriers due to climate change or natural disasters. Stage 3 Different selection pressures Will favour some phenotypes over others leading to better reproductive success in these individuals (natural selection). This could lead to the formation of a subspecies. Stage 4 Reproductive isolation Over time, isolated subspecies may undergo further genetic and behavioural changes that ensure the populations remain reproductively isolated. Thus a new species is formed. Even if they are reunited, they will not be able to interbreed (an example of allopatric leading to sympatric speciation) Examples of allopatric speciation • Kaka Nestor meridionalis • Kea Nestor notabilis • Kakapo Stigops habroptilus Ancestors of this group became isolated from the remaining parrots when New Zealand broke away from Gondwana about 82 mya, resulting in a physical separation of the two groups. This is led to allopatric speciation. (See later notes for speciation that occurred later in their development) Break up of Pangea • http://highered.mcgrawhill.com/sites/0072402466/student_view 0/chapter19/animations_and_movies.ht ml# Sympatric Speciation • Speciation that takes place in geographically overlapping populations with no physical barriers separating gene pools. There are two types: • Niche isolation • Instant speciation by polyploidy Niche Isolation. • • This happens in environments with “micro-habitats”. Organisms living in micro-habitats within an area rarely come in contact with each other even though they are not physically separated. Can also occur through organisms becoming excluded from the main population because they are unable to compete so they establish themselves in a different micro-habitat. • Lack of gene flow between these two populations and the build up of genetic variation over time can result in reproductive isolation. http://evolution.berkeley.edu/e vosite/evo101/VC1eSympatric .shtml Bird lice • Pigeons are able to groom their back feathers, but cannot reach to groom their heads. Some lice phenotypes were better suited to survive in the back feathers because they were faster movers and flatter. They had long legs enabling them to hold onto the feather barbs. These features helped them to escape grooming and so were selected for in this area. Fatter, slow moving lice could only survive on the head. b. Polyploidy (Polyploids have more than 2 sets of chromosomes) Autopolyploidy occurs through nondisjunction in all chromosomes during meiosis and gives instant reproductive isolation from the parent (Instant speciation). Any offspring , if formed at all, will be sterile. Because in polyploidy the sex determining mechanism is disturbed animals rarely achieve new species status this way. Plants, however, can reproduce vegetatively and some can self- fertilise, so can achieve a breeding population. – In allopolyploidy the offspring, called “hybrids”, are usually infertile because they have different chromosome numbers and types that do not match in meiosis. – However, if the hybrids undergo nondisjunction during meiosis, this restores chromosome numbers that do match and so the species can become fertile again (called amphiploidy).This can cause sympatric speciation. Allopolyploidy Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Species A 2n = 4 Unreduced gamete with 7 chromosomes Viable fertile hybrid (allopolyploid) Meiotic error; chromosome number not reduced from 2n to n 2n = 10 Normal gamete n=3 Species B 2n = 6 Figure 24.9 Normal gamete n=3 • Question Recent Speciation in Salsify.docx • Question Leucogene Phylogeny.docx Speciation animationsummary • http://bcs.whfreeman.com/thelifewire/co ntent/chp24/2402001.html CLINES • A continuous gradation in a phenotypic characteristic within a species, associated with a change in an environmental variable across a region e.g. altitude or temperature • In some clines the difference in appearance at the extremes of the cline prevents mating and so a new species can form. Tomtits • North Is. South Is. Ring species • A special type of cline that has a circular or looped geographical distribution resulting in the two ends of the cline overlapping. Adjacent populations can interbreed but not the two ends of the ring. • Question 1 What is a species.docx