Theme 4C Flashcards
What is gene flow?
When allele and genotype frequencies change due to migration into or out of the population. It also decreases the genetic differences between populations = more similar.
What is genetic drift?
- When allele frequencies in a population change from one gen to the next by chance
- Major impact on small populations
- Usually leads to reduced genetic diversity
- Driven by founder effects and population bottlenecks
What is the founder effect?
- A few individuals colonize a distant locality and start a new population
- Colonizers only carry a few alleles, the new population will be missing alleles from other population
- Alleles that were rare in the original population, may become common
What is a population bottleneck?
- When factors such as disease, starvation, and hunting kill a large proportion of a population
- Large reduction in pop size leads to a decrease in the size of the gene pool and decreased genetic diversity
- Alleles that were rare in the original pop can be gone forever
What is directional selection?
When individuals near one end of the phenotypic spectrum have higher relative fitness. Shifts the trait away from the existing mean and toward the favoured extreme.
What is stabilizing selection?
When individuals expressing intermediate phenotypes have the highest relative fitness. Eliminates phenotypic extremes and increases the frequency of intermediate phenotypes.
What is disruptive selection?
When extreme phenotypes have higher relative fitness than intermediate phenotypes. Extreme phenotypes become more common.
What is non-random mating?
When mates are selected because they have a particular phenotype.
What is inbreeding?
Mating between individuals that are genetically related. A problem because both parents share many of the same alleles.
Why does inbreeding not cause evolution?
The allele frequencies do not change over time, it doesn’t change the proportion of alleles in a population, it simply moves them from heterozygous to both homozygous genotypes.
What is inbreeding depression?
A decline in the average fitness of inbreeding individuals in a population caused by deleterious alleles that are recessive.
The solution to inbreeding?
Outbreeding for new alleles.
What is sexual selection?
Individuals with specific traits that enhance their ability to mate with individuals of the other sex. Selection usually acts on males of a species who often have showy traits. Pushes phenotypes toward one extreme.
What is intersexual selection?
- Selection based on interactions between males and females.
- Males produce useless ornaments that females associate with health and vigor.
- This selection is likely the cause of sexual dimorphism (differences in size or appearance of males and females).
What is intrasexual selection?
Selection based on the interactions between members of the same sex. Males use their large body size, antlers, etc. to intimidate, injure, or kill rival males.
Why is diploidy important?
- Can hide recessive alleles from the action of natural selection.
- Maintains genetic variability in a population
- Heterozygous genotypes mask the harmful effect of a recessive allele by the presence of the dominant allele -> protects recessive alleles from being eliminated from the population completely
What is balancing selection?
A type of natural selection in which more than one allele is actively maintained in a population.
When is balancing selection preserved?
- When heterozygotes have a higher relative fitness
- When different alleles are favoured in different environments
- When the rarity of a phenotype provides a selective advantage.
What is heterozygote advantage?
When heterozygotes have higher relative fitness than either homozygote. Allows organisms to respond effectively to environmental variation.
What is the morphological species concept?
The idea that all individuals of a species share measurable traits that distinguish them from individuals of other species
Problems with the morphological species concept?
- Some individuals of a single species look very different in size and colouration.
- It does not help distinguish some closely related species that are nearly identical in appearance.
- Does not tell much about the evolutionary processes that produce new species.
What is a biological species concept?
- Groups of interbreeding natural populations that are reproductively isolated from other such groups.
- Fail to produce fertile offspring or do not interbreed in nature = different species
- Explains why individuals of a species generally look alike: share genetic traits that determine their appearance
Problem with the biological species concept?
Does not apply to the many forms of life that reproduce asexually because individuals do not breed and extinct organisms cannot be studied.
What is the phylogenetic species concept?
- Comprises populations that share a recent evolutionary history.
- Can be applied to any group of organisms, including extinct and asexual
What is a subspecies?
- When geographically separated populations of a species exhibit dramatic and easily identifiable phenotypic variation
- Can interbreed where their geographical distribution meet and offspring exhibit intermediate phenotypes
Rings species
- Ring-shaped geographical distribution that surrounds uninhabitable terrain
- Adjacent populations can exchange genetic material directly, but gene flow between distinct populations only occurs through the intermediary populations
What is clinal variation?
- When a species is distributed over a large, environmentally diverse area, some traits may exhibit a cline (smooth pattern of variation across a geographical gradient)
- usually results from gene flow between adjacent populations that are each adapting to slightly different conditions
- If pops are at opp ends of a cline, they may exchange v little genetic material through reproduction, therefore distinct pops may be genetically and morphologically distinct
What do prezygotic isolating mechanisms do?
Exert their effects before fertilization and the production of a zygote
What do postzygotic isolating mechanisms do?
Operate after fertilization and zygote formation
What are the five prezygotic isolation mechanisms?
- ecological
- temporal
- behavioural
- mechanical
- gametic
What is ecological isolation?
When species live in the same geographical region, but live in different habitats
What is temporal isolation?
Species living in the same habitat, but mate at different times of the day
What is behavioural isolation?
When signals (i.e birds singing) used by one species are not recognized by another
What is mechanical isolation?
Differences in the structure of reproductive organs or other body parts that may prevent individuals of different species from interbreeding
What is gametic isolation?
An incompatibility between the sperm of one species and the eggs of another may prevent fertilization
What are the three postzygotic isolating mechanisms?
- Hybrid inviability
- Hybrid sterility
- Hybrid breakdown
What is hybrid inviability?
Hybrids have two sets of instructions for development that may not interact properly for the successful completion of embryonic development, as a result they die as embryos or in early life
What is hybrid sterility?
When some hybrids between closely related species develop into healthy and vigorous adults, but may not produce functional gametes. Often results from a difference in structure or # of chromosomes. Therefore, hybrids have zero fitness.
What is hybrid breakdown?
When hybrids are healthy and fully fertile and can mate with both parents and other hybrids in the first generation. However, by the 2nd gen, hybrids tend to exhibit reduced survival or fertility.
What is allopatric speciation?
- When a physical barrier subdivides a large population or when a small population becomes separated from a species’ main geographical distribution.
- Prevents gene flow, may accumulate genetic differences that isolate them reproductively.
- Most common in large animals.
What is secondary contact?
When allopatric populations re-establish contact when the geographical barrier is breached or eliminated. Provides a test of whether the genes in the population have diverged enough to make them reproductively isolated.
What is species fusion?
If the gene pools of two populations did not differentiate much during geographical separation, the populations will interbreed and merge into one
What is sympatric speciation?
When reproductive isolation evolves between distinct subgroups that arise within one pop. Often occurs in plants through polyploidy.
What is polyploidy?
- When an individual has one or more extra copies of the entire haploid complement of chromosomes.
- Can lead to speciation because these large-scale genetic changes may prevent polyploid individuals from breeding with individuals of the parent species
What is autopolyploidization?
- Polyploidy can cause sudden speciation
- Meiosis fails and the organism produces 2n gametes
- If 2n gamete is fertilized with another 2n gamete = autopolyploid
- Autopolyploids can only mate with other autopolyploids
