Macroevolution Flashcards
Macroevolution
The variation of allele frequencies at or above the level of species over geological time, resulting in the divergence of taxonomic groups, in which the descendant is in a different taxonomic group to the ancestor.
Speciation
The formation of a new species following an event that splits a lineage and involves some form of reproductive isolation of populations.
Main forms of speciation
allopatric- where a population is separated by a physical barrier (ocean, river, mountain range, desert, deforestation) causing geographic (physical) or spatial (large distance) isolation. (is the most common form). No gene flow results
sympatric- where organisms live in the same habitat / niche (there is no physical barrier/ geographic isolation) but don’t breed with each other (little gene flow). (This may result from behavioural or temporal isolation (eg. mismatch of breeding behaviour / season).
parapatric- where organisms live in distinct contiguous populations that can interbreed in peripheral regions (no geographic isolation). Is low gene flow normally between individuals that live close to each other (mating is not random - with neighbours). Often multiple niches develop along a gradient as environmental factors change. Is the rarest form of speciation).
Reproductive Isolation
Occurs when two or more sections of a population are unable to breed together (for various reasons eg. geographical barriers, timing of reproductive receptiveness, physical incompatibility). This inability of organisms in different populations of the same species to breed leads to reduced gene flow (mixing of genes) between these populations.
Eventually the ability for members of both populations to breed with each other and produce fertile offspring disappears (and new species result).
Forms of reproductive isolation
geographic isolation (eg. environmental disasters or habitat fragmentation can physically separate populations)
spatial isolation (eg. individuals from a population become separated by large distances)
mechanical isolation (eg. physically difficult to breed together)
temporal isolation (eg. timing of fertility or breeding seasons don’t align between populations)
behavioural isolation (eg. mating dance or calls don’t align between populations)
Major types of macroevolution
Divergent Evolution- is the differentiation of distinctly different species from a common ancestral species.
Convergent Evolution- is the evolution through natural selection of similar features in unrelated groups of organisms (with no recent common ancestor). This is often due to similar selecting pressures acting on each group.
Parallel Evolution- occurs when related species that share a recent common ancestor evolve similar features independently of each other due to similar selecting pressures acting on each group.
Coevolution- occurs when two species that interact closely exert selection pressures on each other and evolve together in a reciprocal response to selection pressures.
Two major models depicting different paces of evolution
Punctuated Equilibruim is where a lot of evolutionary changes occurs in a short period of time often leading to speciation events (following long periods of time with no change)
Phyletic Gradualism is where evolutionary change is very gradual and adaptations accumulate very slowly (there is no clear line between ancestoral species and their descendants)
Genetic drift
Random changes (not resulting from selection pressures) in allele frequencies in a population is called genetic drift. Genetic Drift is more pronounced in small populations than large populations.
Note: Genetic drift is not the same as Gene flow. Geneflow occurs between two different populations (via immigration, emigration and interbreeding) whereas genetic drift often occurs within the same population. Genetic Drift occurs within the species level so is linked to microevolution (but could lead eventually, in combination with reproductive isolation, to macroevolution).
Two effects that result in genetic drift: Founder effect and Bottleneck effect.
Founder Effect
When a small number of a population become isolated. ‘Founder groups’ may exhibit limited variation in alleles and may not reflect the allele frequencies seen in the original population (hence, genetic drift occurs).
Bottleneck effect
When a large population is dramatically reduced in size, thus reducing its genetic diversity. As population size is lower, reproductive pairing is limited leading to inbreeding and reduced genetic variation. The smaller the population the greater the genetic ‘bottleneck effect’ is (hence, genetic drift occurs).
