D2 Evolution by natural selection | 33 | v4

ABSTRACT

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Evolution by natural selection

Natural selection is the constraint that natural conditions put on the size of populations, forcing individuals of the same species to compete for limited resources. Those types which use the resources most successfully in order to reproduce, pass on their genetic material to their offspring, and are selected for. Inefficient or unsuccessful types fail to reproduce as successfully and are selected against. The process is analogous to selection, by farmers, of the best types of stock and plants for breeding. This causes a gradual change in the genetic makeup of the population. The phenotypic characters which are selected for will become more frequent providing that they are genetically controlled. Relative fitness is the differential average reproductive success of individuals with different genotypes (combinations of alleles). Natural selection was first proposed as the cause of evolution by Charles Darwin, but he did not know how inheritance worked.

Darwin's observations and deductions

Darwin's observations were: (i) the potential number of descendants from any species is infinite - many more offspring are produced than can survive; (ii) this potential population growth is prevented by limited resources - populations are relatively constant in size; (iii) there are many differences between individuals which affect their ability to survive and reproduce.

Darwin deduced that, therefore: (i) there is a struggle for the limited resources; (ii) those individuals which survive and reproduce successfully will pass on to their offspring the characteristics which helped them to survive and reproduce; (iii) gradually, the inherited characteristics which help in the competition to reproduce will become more frequent in the population. This gradual change is evolution.

Modes of selection

(i) Stabilizing selection removes phenotypes which deviate too far from the optimum (norm). In a stable environment, change or deviation usually produces less fit phenotypes. (ii) Directional selection selects for a new optimum, and against one end of the range of phenotypes, causing the mean for the character to change towards the new optimum. This can occur after a change in environment, including competition with another species. Character displacement occurs when two species compete. Selection is greatest where their niches overlap, so each species is selected to be different from the other (e.g. one species becomes larger, the other smaller, reducing competition). (iii) Disruptive selection selects against the mean phenotype and favors two different optimum phenotypes. It can develop as a sexual difference (e.g. female sparrow hawks are larger than males and catch larger prey). Disruptive selection can work when there are two qualitatively different types, but there are theoretical problems with continuously variable characters because of the continued production of unfit intermediates.

Sexual selection

In polygamous species, individuals of one sex, usually the males, compete for mates. Intra-sexual selection is competitive interaction between individuals of the same sex for access to mates. Typically it is competition between males and selects for increased body size and weapons such as antlers. Inter-sexual competition is an interaction between members of opposite sexes, when one sex displays to advertise their fitness and attract mates. The other sex (typically ‘choosy’ females) select their mate on the basis of the display. This selection produces extravagant display ornamentation (e.g. the peacock's tail). These may be described as ‘handicaps’ but the terms ‘advertisements’ or ‘status symbols’ would reflect their role more accurately.

Incipient speciation

Races are incipient species. Genetic differences between separated populations can increase until they first become different races then eventually gain the status of separate species.

Selfish DNA/genes

Only copies of DNA or genes can survive through generations (phenotypes die with individuals, genotypes are disrupted at meiosis). Transposable elements are short regions of DNA (many related to retroviruses) which are copied and inserted into all the chromosomes in a cell, thereby increasing their frequency without increasing the fitness of the individual they are in. DNA sequences are selected to enhance transmission of copies of themselves, even to the detriment of other genes in the genome, or the host individual. In extreme cases DNA or a whole chromosome may show meiotic drive, distorting segregation at cell division to increase their frequency in gametes and zygotes, or causing destruction of gametes which do not carry a copy of themselves (e.g. segregation distorter in Drosophila melanogaster), reducing competition. Such selfish or parasitic DNA can reduce the phenotypic fitness of the individual carrying it, providing meiotic drive increasing its transmission exceeds fitness reduction.

Historical perspective

Charles Darwin recognized natural selection as the mechanism causing evolution in 1838, but did not publish the Origin of Species until 1859; Alfred Russel Wallace reached the same conclusion independently in 1858. Previous theories of evolution supposed that organisms improve their status by their own efforts (inheritance of acquired characteristics), a theory now known as ‘Lamarkian’ after its last great proponent. It was supposed that there was a progression from slime up to humans.